Nanoclay interaction with charged species: from fundamentals to functional materials

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Thesis

Reference

Nanoclay interaction with charged species: from fundamentals to functional materials

PAVLOVIC, Marko

Abstract

In this thesis, interaction of layered double hydroxide (LDH) nanoparticles with various charged species were investigated. Moreover, functional materials with enzymatic activities were prepared. While the Chapter 1 represents the introduction, experimental background and outline of the thesis, the Chapter 2 focuses on ion specific effect on colloidal stability and charging properties of the LDH material. It was observed that different monovalent salts influence the electrophoretic mobilities and the colloidal stability of LDH nanoparticles in a different extent. The main reason for this phenomenon is the variation of the counter-ion hydration, as it had been predicted by the Hofmeister series. Multivalent ions showed more pronounced effect on charging and aggregation of LDHs according to the Schulze-Hardy rule.

However, such ion specific effects were more pronounced than predicted by these theories due to strong adsorption of the counter-ions on the surface of the LDH particles. In the Chapter 3, the influence of a copolymer adsorption on the colloidal behavior of LDH was studied. Coating was performed at various ionic [...]

PAVLOVIC, Marko. Nanoclay interaction with charged species: from fundamentals to functional materials. Thèse de doctorat : Univ. Genève, 2018, no. Sc. 5222

DOI : 10.13097/archive-ouverte/unige:105952 URN : urn:nbn:ch:unige-1059525

Available at:

http://archive-ouverte.unige.ch/unige:105952

Disclaimer: layout of this document may differ from the published version.

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UNIVERSITÉ DE GENÈVE FACULTÉ DES SCIENCES Section de chimie et biochimie

Département de chimie minérale et analytique Professeur Michal Borkovec Dr. Istvan Szilagyi

Nanoclay interaction with charged species: from fundamentals to functional materials

THÈSE

présentée à la Faculté des sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention chimie

par

Marko PAVLOVIĆ

de

Novi Sad (Serbie)

Thèse N^o xxx

GENÈVE Atelier ReproMail

2018

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Abstract

In this thesis, interaction of layered double hydroxide (LDH) nanoparticles with various charged species were investigated. Moreover, functional materials with enzymatic activities were prepared.

While the Chapter 1 represents the introduction, experimental background and outline of the thesis, the Chapter 2 focuses on ion specific effect on colloidal stability and charging properties of the LDH material. It was observed that different monovalent salts influence the electrophoretic mobilities and the colloidal stability of LDH nanoparticles in a different extent. The main reason for this phenomenon is the variation of the counter-ion hydration, as it had been predicted by the Hofmeister series. Multivalent ions showed more pronounced effect on charging and aggregation of LDHs according to the Schulze-Hardy rule. However, such ion specific effects were more pronounced than predicted by these theories due to strong adsorption of the counter-ions on the surface of the LDH particles.

In the Chapter 3, the influence of a copolymer adsorption on the colloidal behavior of LDH was studied. Coating was performed at various ionic strengths to clarify the nature of interparticle interactions. The negatively charged copolymer was able to induce charge neutralization followed eventually by significant charge inversion of the LDHs. This charge reversal led to increased surface charge density and to stable dispersions even at elevated salt concentrations.

Interaction of LDH platelets with oppositely charged latex spheres was probed in mobility measurements and by dynamic light scattering in the Chapter 4. In this case, LDH behaves as a polyelectrolyte and strongly adsorbs on the surface of the latex particles causing charge inversion. Charge neutralization occurred at a certain value of LDH dose inducing rapid particle aggregation in the dispersion. Aggregation in these samples was faster than the diffusion-limited aggregation, which was a proof of additional attractive force (patch-charge force). Latex particles were coated by two types of LDHs with a different anion intercalated among layers, however, this difference did not influence the outcome of the experiments.

Preparation of LDH-DNA hybrid nanocomposite was performed in the Chapter 5. Two different approaches were examined, namely, adsorption due to the opposite charge of the

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support and the guest molecule and covalent bonding after functionalization of the support and activation of the DNA. Successful immobilization was proved by IR spectroscopy and SEM-EDX measurements.

Chapter 6 concerns functionalization of LDH clay with biocompatible polyelectrolyte, heparin. It was able to induce charge inversion, which corresponds to typical U-shaped stability curves. In comparison to the bare particles, the coated ones exhibited multiple times improved colloidal stability. Difference in the morphology of the bare and coated nanoparticles was not observed. This was the initial step towards preparation of enzymatically active, highly stable, functional material.

The two final chapters with results (Chapter 7 and Chapter 8) are focused on the preparation of functional materials. Namely, superoxide dismutase (SOD) and horseradish peroxidase (HRP) were adsorbed separately on LDH platelets. LDH-SOD was afterwards coated with heparin in order to make the material resistant against salt-induced aggregation. While for HRP immobilization, heparin adsorption had to be performed initially, in order to introduce sufficient negative charge to the particles, which enables successful enzyme adsorption via electrostatic interaction (HRP has a net positive charge at neutral pH). Importantly, both enzymes kept their native conformation, which was reflected as a preserved enzymatic activity upon immobilization. Materials were also stable at elevated ionic strength (above 150 mM, which is the one in blood) and changes in morphology of the particles upon functionalization were not observed.

The last chapter of the present thesis is concerned with the conclusions of the work.

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Résumé

Dans cette thèse, l'interaction des nanoparticules d'hydroxyde double couche (LDH) avec diverses espèces chargées a été étudiée. De plus, des matériaux fonctionnels à activité enzymatique ont été préparés.

Alors que le premier chapitre représente l'introduction, le contexte expérimental et les grandes lignes de la thèse, le deuxième chapitre se concentre sur l'effet spécifique des ions sur la stabilité colloïdale et les propriétés de charge du matériau LDH. Il a été observé que différents sels monovalents influencent les mobilités électrophorétiques et la stabilité colloïdale des nanoparticules de LDH dans une mesure différente. La raison principale de ce phénomène est la variation de l'hydratation des contre-ion, comme l'avait prédit la série Hofmeister. Les ions multivalents ont montré un effet plus prononcé sur la charge et l'agrégation des LDH selon la règle de Schulze-Hardy. Cependant, ces effets ioniques spécifiques étaient plus prononcés que prévu par ces théories en raison de la forte adsorption des contre-ions à la surface des particules de LDH.

Dans le troisième chapitre, l'influence de l'adsorption d'un copolymère sur le comportement colloïdal de la LDH a été étudiée. Le revêtement a été effectué à différentes intensités ioniques pour clarifier la nature des interactions entre les particules. Le copolymère chargé négativement a été capable d'induire une neutralisation de charge suivie éventuellement d'une inversion de charge significative des LDH. Cette inversion de charge a conduit à une augmentation de la densité de charge de surface et à des dispersions stables, même à des concentrations élevées de sel.

L'interaction des plaquettes LDH avec des sphères de latex chargées en sens inverse a été sondée lors de mesures de mobilité et par diffusion dynamique de la lumière dans le quatrième chapitre. Dans ce cas, la LDH se comporte comme un polyélectrolyte et s'adsorbe fortement à la surface des particules de latex, ce qui provoque l'inversion de charge. La neutralisation de la charge s'est produite à une certaine valeur de la dose de LDH induisant une agrégation rapide des particules dans la dispersion. L'agrégation dans ces échantillons était plus rapide que l'agrégation limitée par diffusion, ce qui était une preuve de force d'attraction supplémentaire (force de charge de patch). Les particules de latex ont été

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enrobées par deux types de LDH avec un anion différent intercalé entre les couches, mais cette différence n'a pas influencé le résultat des expériences.

La préparation du nanocomposite hybride LDH-ADN a été réalisée dans le cinquième chapitre. Deux approches différentes ont été examinées, à savoir l'adsorption due à la charge opposée du support et de la molécule invitée et la liaison covalente après fonctionnalisation du support et activation de l'ADN. Le succès de l'immobilisation a été prouvé par spectroscopie IR et par des mesures SEM-EDX.

Le sixième chapitre concerne la fonctionnalisation de l'argile LDH avec des polyélectrolytes biocompatibles, l'héparine. Il a été capable d'induire une inversion de charge, ce qui correspond aux courbes typiques de stabilité en forme de U. Par rapport aux particules nues, les particules enrobées présentaient une stabilité colloïdale plusieurs fois supérieure. On n'a pas observé de différence dans la morphologie des nanoparticules nues et des nanoparticules enrobées. C'était la première étape vers la préparation d'un matériau enzymatiquement actif, hautement stable et fonctionnel.

Les deux derniers chapitres sont axés sur la préparation des matériaux fonctionnels. En effet, la superoxyde dismutase (SOD) et la peroxydase de raifort (HRP) ont été adsorbées séparément sur les plaquettes de LDH. La LDH-SOD a ensuite été recouverte d'héparine afin de rendre le matériau résistant à l'agrégation induite par le sel. Alors que pour l'immobilisation HRP, l'adsorption de l'héparine devait être réalisée initialement, afin d'introduire une charge négative suffisante aux particules, ce qui permet une adsorption enzymatique réussie par interaction électrostatique (HRP a une charge positive nette à pH neutre). Il est important de noter que les deux enzymes ont conservé leur conformation native, ce qui s'est reflété comme une activité enzymatique préservée lors de l'immobilisation.

Les matériaux étaient également stables à une force ionique élevée (supérieure à 150 mM, qui est celle du sang) et les changements de morphologie des particules lors de la fonctionnalisation n'ont pas été observés.

Le dernier chapitre de la présente thèse porte sur les conclusions des travaux.

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Chapter 1

1. Introduction

Chemists in the nineteenth century were puzzled by the behavior of the hydrated alumina and some natural polymers in water. As a consequence, all these compounds were put together in the group of ‘colloids’ (meaning ‘glue-like’), named by Wolfgang Ostwald. New field of chemistry, colloid and interface science, dealing with colloidal dispersion was born and introduced by Thomas Graham in 1861.^1,2

The main difference between colloids and molecules in solutions is that their interaction energy is higher than k_BT, where k_B is the Boltzmann constant and T is a temperature in K. If the interaction among colloidal particles is repulsive, particles can float freely in the dispersant. In contrast, van der Waals attraction can occur, which causes particles aggregation. On the other hand, solutions are always leaning towards disorder, due to high entropic contribution as a consequence of interactions among molecules that are in the order of the k_BT in energy. Colloidal gold was the material that helped Faraday to understand the stabilization process. He observed that by adding simple salt, repulsion due to alike charge of the particles can be screened, which causes the change of the suspension color from red (single particle absorption) to blue (absorption of the aggregates).¹ Derjaguin, Landau, Verwey, and Overbeek came up with the theory that explained the colloidal stability as a result of the interplay between van der Waals attractive and Coulomb repulsive forces (classical DLVO theory). This type of limited stability at higher salt concentration was a limiting factor for many future applications. Inspired by the ancient Egyptians, who used Arabic gum (long chain polysaccharide) to coat carbon grains and obtain stable dispersions, colloidal particles were functionalized with many different polymers. Beside the expected increased colloidal stability, by using some functional (bio)polymers or combination of (bio)polymers, various functional materials were prepared. Many of these materials are already part of our everyday life, however, as possibilities for making new materials are endless, new functional materials are produced and investigated further.

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1.1. Particle dispersions

Particle dispersions or colloids consist of solid or liquid particles that are dispersed in a liquid medium. One might say that colloid is a fluid dispersion that can be either natural or synthetic. From the perspective of size, dispersed particles are ranging from a few nanometers to micrometers, which means that they are able to form systems similar to the atomic and molecular ones, while at the same time, they are big enough to be observed by an optical microscope. They possess very broad compositional (e.g., metals, clays, polymers) and morphological (e.g., spherical, rod-like, platelets and hollow structure) diversity.³

Due to previously mentioned high diversity of colloids, they are present in numerous biological systems, in addition to many industrial applications. These applications are mostly related to water purification (Figure 1a)⁴, paper making (Figure 1b)⁵, food (Figure 1c)¹ and paints formulation (Figure 1d)⁶ as well as delivery systems (Figure 1e)⁷ in biomedical science. Depending on the type of application, both stable suspensions and aggregation of the colloidal particles can be desired. Moreover, owing to the fact that properties of colloidal particles can be easily tuned, these particles often serve as building blocks for diverse functional materials through the process of self-assembly (Figure 1f).^8,9

Figure 1. Applications of colloidal particles in different fields.

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Moreover, particle dispersions are also used in the preparation of novel sensors¹⁰ and catalytic systems¹¹. Inducing particle aggregation in suspensions (coagulation) is an important tool for water purification. This is an efficient way to remove all solid particles that are suspended in the wastewater by addition of natural polymers in appropriate dose that will cause the formation of larger sized aggregates, which can be then easily removed by filtration.⁴ In similar manner, in the papermaking procedure large quantity of cellulose fibers in combination with filler particles are present in the suspension. In order to obtain the raw material, cellulose fibers need to be aggregated. This is achieved by the addition of positively charged flocculants that can reduce the charge of fibers and induce aggregation.⁵ Moreover, different types of fillers, namely, natural clays, silica, titania, or other particles are added to the suspension prior to the coagulation process in order to tune mechanical and optical properties of the final product. In contrast to these applications, in the food and paint industries, there is a high demand for stable colloidal suspensions. Development of colloidal science facilitated the replacement of toxic organic solvents in the paint industries by water.

Discovery of novel systems for the stabilization of pigments in aqueous medium led to the water-based paints. Similarly, many biocompatible particles and polymers are added to food products due to their ability to accumulate on the water-oil interface in the case of emulsions, or on the water-air interface for foams.^12-14 The most complex application from the perspective of stability is in material science¹⁵ and in drug delivery systems¹⁶, where aggregation needs to be controlled. In the latter case, stable suspensions are required in biofluids, because aggregation of the delivery particles can cause blood clot and thrombosis for instance.¹⁷ As it is obvious from the previously described applications, colloidal stability is an important concept that needs to be well understood. Stabilization or/and coagulation can be accomplished only under appropriate conditions. Colloidal stability depends crucially on a size, composition and surface charge of the particles. In addition, dispersants play an important role. Elevated salt concentrations can often lead to aggregation, while the addition of charged polymers (so-called polyelectrolytes) can have twofold effect on the stability depending on the applied dose. This effect can be induced by modifying surface charge or introducing additional forces. Furthermore, type of solvent and pH of medium can also influence stability of the dispersions by modifying interparticle forces that are responsible for aggregation process. It is important to mention that depending on the composition and properties of aggregates, they can sediment, cream or form gels.

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1.1.1. Colloidal stability

It was evident from many experiments throughout past centuries that neutral particles tend to attract each other and sediment. It was understood that gravitational forces wouldn’t be sufficient for this process. In the middle of the previous century, colloidal stability of particles was successfully described by the DLVO theory. This theory was initially proposed by Boris Derjaguin, before it was further developed together with Lev Landau.^18,19 There was also a tremendous contribution to the further improvement of this theory by Evert Verwey and Jan Overbeek.²⁰ Main statement of previously mentioned scientists was that colloidal stability of particles crucially depends on the interplay between attractive van der Waals forces (London forces) and double-layer forces (Coulomb forces). In other words, total energy interaction (V(h)) between two identical particles at distance h equals the sum of electrical double layer potential (V_EDL(h)) and van der Waals potential energy (V_vdW(h)):

( ) _EDL( ) _vdW( )

V h V h V h (1)

However, it is important to note that there is a need for additional terms in the previous equation (1) to describe potential between two particles, if some non-DLVO forces exist.

These forces can be either attractive or repulsive and they include hydrophobic interaction,²¹ depletion forces,²² ion-ion correlation²³ or steric repulsion²⁴ for instance.

( ) _EDL( ) _vdW( ) _{non DLVO}( )

V h V h V h V _ h (2)

The van der Waals interaction potential can be calculated by employing the Derjaguin approximation. This approximation relates forces between to spherical bodies to the one between two plates. Nevertheless, it is valid only in the case when particle size is significantly larger than the distance between particles (R h). If this is true, van der Waals interaction potential is given as:

( ) 12 vdW 12 V h RH

  h ⁽³⁾

where R is radius of the particle and H₁₂ is an effective Hamaker constant for particles 1 in medium 2. This constant solely depends on the composition of the particle and the type of medium.

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Double layer repulsion forces for the particles of high electrostatic potential can be calculated by solving the complex Poisson-Boltzmann equation.²⁵ This type of interaction can be estimated well by utilization of Debye-Huckel approximation in the case of particles of low surface charge density. Final equation for double layer potential is simplified and can be expressed as a function of electrostatic potential (_D):

2

( ) 2 0 ^kh

EDL D

V h    R e^ (4)

where ₀ and  are vacuum permittivity and dielectric constant of water, respectively. As a consequence, entire interaction energy can be represented by the sum of the two previously mentioned components given in equations 3 and 4:

2 12

0 12

( ) 2 _D ^kh

V RH

R e

h     ^  h ⁽⁵⁾

Interaction potential as a function of the distance between particles is also given in Figure 2.

Figure 2. Interaction potential between charged colloidal particles. Interaction potential represents the sum of double layer and van der Walls interaction potentials. Force is given

by dashed line.

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Depending on the interplay between these two potential, energy barrier (V_max) rises at certain separation. This barrier plays a major role in the colloidal stability of the particle dispersion.

As it is well known, particles undergo thermal (Brownian) motion in dispersions. If salt concentration of the surrounding particles is low, large energy barrier will develop. This is normally too high in energy to be overcome by thermal motion and particles are separated at larger distances. In other words, stable colloidal suspensions are obtained. If the ionic strength in the dispersion is high, ions will be able to screen particles charge, which will cause a decrease in the extent of double layer repulsion and, more importantly, vanishing of the energy barrier. Consequently, particles will reach primary minimum and form dimers (unstable suspension).

Once interaction energy is known, aggregation process can be described by an aggregation rate (k), which can be calculated by the Fuchs formula²⁶:

1

0 2

4 ( ) ( )

3 (2 ) exp

B

k T B h V h

k dh

R R h k T



 

   

 



    ⁽⁶⁾

where B(h) is the hydrodynamic resistance function, η is the viscosity of water. This function is a consequence of the hindrance of the diffusion as a result of a hydrodynamic flow of the neighboring particle. Nevertheless, when particles do not interact, one obtains the relation developed by Smoluchowski to quantify the diffusion-limited rate of aggregation.

17 3 1

8 1.2 10

3 k TB

k m s



 

   (7)

where k_B is the Boltzmann constant. Numerical result (Smoluchowski rate) is valid at 25 °C and water as dispersant. This type of fast aggregation occurs only in the case when salt concentration is high, therefore, energy barrier is negligible or non-exiting. In real-life situation, even at elevated ionic strength, weak repulsion forces are present, therefore energy barrier prevents the success of all particle collision leading to dimer formation, and only slow aggregation occurs. More about this issue will be discussed in a following part of this chapter regarding the measurement of colloidal stability.

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1.1.2. Investigating colloidal stability

Due to the large importance of colloidal dispersions in applications and in fundamental research, different techniques have been developed to study colloidal stability. They are probing aggregation process together with electrokinetic behavior of particles.

Electrophoresis is proved to be an important method to study charging properties of particles, which can further give us an idea about stability of these particles. On the other hand aggregation rates can be directly measured by static and dynamic light scattering (DLS)²⁷and turbidity measurements²⁸.

Microscopic methods, namely scanning electron (SEM) and transmission electron (TEM) microscopies are employed in order to look into the morphology of the particles or aggregates. Therefore, it can tell us more about the structure of the aggregates or individual particles. The main disadvantage is related to a long, time-consuming process of treating the acquired micrographs (significant number of particles has to be processed in order to obtain size and a polydispersity with a decent error). Moreover, if cryo technique is not applied prior to experiments, there is a possibility of the formation of `false` aggregates or sensitive particle deformation due to the drying process. Another commonly used method to study particle aggregation is the turbidity measurements by spectrophotometry. This is a simple method that follows changes in transmitted light due to the light scattered (turbidimetry) or absorbed (absorbance) by particles in suspension. It is a well-known, simple and non- destructive method, but, at the same time, it has only low sensitivity due to a need for high particle concentration. Only elevated concentration of colloidal particles can give enough change in transmittance upon aggregation. This fact makes spectroscopy not suitable for detection of low-ranked aggregates and therefore not suitable for measuring aggregation kinetics. In contrast, DLS and SLS are very effective experimental approaches for measuring hydrodynamic radius, polydispersity, aggregation kinetics, in addition to the form factor (that gives us information about the structure of the particles). More about DLS will be given in the following section.

1.1.2.1. Dynamic light scattering

DLS is a suitable, non-destructive technique that is able to measure colloidal stability of the dispersions and is based on two characteristics of the dispersions, namely, Tyndall effect

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(scattering of light) and Brownian motion (thermal motion of particles).²⁹ It is based on the fluctuations (that are related to the particles size) of the scattered light.

First, let us discuss the particle aggregation process. In the case when attractive forces overcome repulsive ones, initially dimers form. Subsequently, formation of higher order aggregates occurs, however, only the initial stages of aggregation will be explored in the following part (Figure 3). Dimer (AA) formation that is connected to the loss of monomers (A) can be expressed by the following equation:

1 ²

2

AA

A

dN kN

dt  (8)

where N_A and N_AA are number concentrations of monomers and dimers, respectively, t is time and k is the aggregation rate constant.

If there is no stabilizing force between particles and they undergo Brownian motion, aggregation is controlled only by their diffusion. In that case, aggregation rate constant is given by Smoluchowski’s rate (equation (7)). There is a possibility to calculate absolute aggregation rate by employing simultaneously SLS and DLS³⁰, but focus will be made on the determination of apparent rate constant (kDLS) by DLS measurements. This can be estimated as:

0

1 ( , )

( , 0)

DLS

t

dR q t

k R q dt _

 

   ⁽⁹⁾

where R(q,0) and R(q,t) represent initial hydrodynamic radius (radius of the particle together with the hydrodynamically stagnant layer moving together with the particle) and the one after a short period of time, t is time and q is the scattering vector:

4 sin( ) 2

q n 

  ⁽¹⁰⁾

Figure 3. Illustration of stable suspension, early and late aggregation stages.

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where n is refractive index of the sample, λ is a wavelength of light and θ is a scattering angle.

By comparing apparent rate constant (

kDLS) of the actual measurement with the one from the fast, diffusion-limited rate (_k_DLS^fast), stability ratio (W) can be obtained:

0,

0

( , ) ( , )

fast

t fast DLS

DLS

t

dR q t k dt

W k dR q t

dt



 

 

 

 

 

 

 

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If W equals unity, the aggregation is diffusion-limited and hence, fast aggregation occurs. High stability ratio values show stable colloidal dispersions while, values between 1 and 100 correspond to slow aggregation (Figure 4). Stability ratio value can be also interpreted as a necessary number of particle collisions that will lead to dimer formation (W of 1 corresponds to the case when each collision has enough energy to overcome the barrier and form aggregate of the first order). W values can be measured at different ionic strength or

polyelectrolyte doses. Dose or salt concentration at which transition from fast aggregation (W

= 1) to stable dispersion (W > 1) occurs is called critical coagulation concentration (CCC). It represents an important parameter of

any colloidal system and it can be partially predicted by DLVO theory.

One can realize at this point that information about colloidal stability can be obtained if we measure hydrodynamic radius of particles by DLS and its evolution in time.

Let us discuss the basic setup and the theory that lies behind this powerful

experimental technique. A typical DLS device (Figure 5) is composed of a laser that gives coherent, monochromatic light. This light hits particles from the dispersion, which makes

Figure 4. Illustration of different suspension stabilities.

Figure 5. DLS setup with obtained fluctuation in scattering intensity.

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them behaving as a novel source of illumination as a consequence of the scattering.³¹ Although, most of the electromagnetic waves pass through the sample unchanged, some part is scattered and caught by the detector. Due to the interference of scattered waves with particles, the intensity fluctuates in time as the relative positions of the particles change. This gives as a result bright and dark spots in the scattering pattern, or so-called speckles.

Sufficient resolution is crucial for the efficient photodetector. This parameter is given by the spatial coherence factor (B)³²:

1 1 ^ap

coh

B A

A





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where A_ap stands for aperture area of the photodetector and A_coh is a coherence area or an area of a single speckle. Coherence area critically depends on a distance of a detector from the scattering object (r), width of the laser beam (x) and a wavelength of the light (λ).

2 2 coh 2

A r

x



  (13)

In order to achieve satisfactory high values of the coherence area, easiest way is to use an appropriate pin holes (aperture) in front of the photodetector. In other words, we have to decrease aperture area, while keeping the coherence area sufficiently high.²⁹ Previously mentioned fluctuations in the intensity of the scattered light depend on the diffusion coefficient (D) of the particles that is directly related to the hydrodynamic radius (R_h).²⁹ In other words, small particles have higher values of diffusion coefficient and therefore will give faster fluctuation. Quantitative information about these fluctuations can be obtained by employing the autocorrelation technique. In order to explain autocorrelation function of the varying scattering intensity let us consider the intensity at certain time, I(0), and the intensity after a sort delay I(0+τ). For these two intensities, one can write the intensity correlation function as³²:

' 0

(0) ( ) lim ^T ( ) ( )

x

I I  I t I t  d

 



 ⁽¹⁴⁾

where t stands for an actual time, τ for the time delay and T’ is the total measurement time. It is evident that when τ approaches 0 the entire function equals I² , while if it goes to infinity function has a value of I ². The normalized intensity correlation function can be written as:

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2

(0) ( ) ( ) I I

g I

   ⁽¹⁵⁾

On one hand, intensity correlation function was obtained and, on the other hand, there is also an electric-field correlation function, which gives a correlation between the positions of the particles at different τ:

1 2

( ) exp( )

g   Dq (16)

where D stands for the diffusion coefficient. The intensity correlation function, g²(τ), and electric-field correlation function, g¹(τ) are related by the Siegert relation³³:

2 1 2

( ) 1 ( ( ))

g  A B g   ⁽¹⁷⁾

where A is a baseline and B is a coherence factor. In this way, we are able to calculate diffusion coefficient of a monodisperse colloidal particles, which directly related to the hydrodynamic radius (Rh) by the Stokes-Einstein equation:

6

B h

R k T

D

 (18)

where T is a temperature in K, η is viscosity of the medium. It is important to mention that in order to obtain diffusion coefficient from the Siegert equation we need to know the scattering vector that comes from the DLS setup (equation (10)).³⁴ In the Figure 6 below, correlation functions of differently sized particles are given.

Figure 6. Scattering intensities and correlation functions for two types of particles.

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As it can be seen, fluctuations of the scattered intensity are different, which directly causes different decays of the correlation functions and different hydrodynamic radii.

1.1.2.2. Electrophoresis

While DLS determines the size of colloidal particles, it does not provide any information about the charging properties. These properties can be found from electrophoresis. Let us first elaborate on the main theory and the corresponding phenomena of charged particles in electrolyte solution.

Charged particles that are suspended in a salt solution are surrounded by both, positive and negative, types of ions. Spatial distribution of these ions (point-like charges) in a vicinity of the particle surface will rise to the formation of electrical double layer (EDL)³⁵. If particles are positively charged, negative ions (that behave as counter-ions) will accumulate in the diffuse layer close to the surface. Their concentration will gradually decrease going from the surface to the bulk concentration. In contrast, most of cations (in this case acting as co-ins) will be expelled from the vicinity of the particle surface, and their concentration will gradually increase starting from zero next to the surface to the bulk concentration.²⁵ In addition, there is also a possibility of counter-ion adsorption, which can subsequently lead to charge neutralization and/or overcharging.^36-38 Formation of EDL is displayed in Figure 7. As it can be

seen in this figure, we need to define three different potentials. Surface potential (ψ0) corresponds to the potential at the surface that is in a contact with ions. Potential at the diffuse part of EDL is called diffuse-layer potential (ψD).Finally, zeta potential (ζ) represents the potential at the slip plane, or in other words, it is a difference in potential between the dispersion medium and the stationary layer of liquid attached to the particle.^39-41Zeta potential depends on the ionic strength of the medium, but it is always smaller than diffuse-layer potential.

Figure 7. Electrical double layer (a), charge density (b)

and potential profile (c) for positively charged surface.

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Poisson-Boltzmann theory, describes the potential profile as a function of the distance from the surface in the EDL. Let us discuss the simple situation with charged surface in a salt solution. Electrostatic potential (ψ) can be calculated by knowing charge density per unit of volume (ρ):^35,42

2 2

0

d dz

 

   (19)

If one assumes that surface is positively charged and anions act as counter-ions, charge density can be derived by the following equation:

( )

q c c

 ^ ^ (20)

whereby c⁺ and c^- are concentrations of cations and anions, respectively, and the charge of a single ion can be obtained by:

qze (21)

where e is the elementary charge, z in the ion valence. In order to get concentrations of ions one need to employ chemical potential of ions.⁴²

(0) k T_B ln(c ) q

__  ^   (22)

In equation (22), _⁽⁰⁾stands for the reference chemical potential of ions. In equilibrium state, far away from the surface, where ion concentration is equal to bulk concentration, chemical potential is constant, which represents a good way to eliminate reference potential and obtain ion concentration as dependence of bulk concentration (cB).

B

q k T

c c eB

 

  (23)

By insertion of equation (23) for ion concentrations into the one for charge density, equation (19), one obtains its final form:

2 2

0

( ^B ^B )

q q

k T k T

cB

d e e

dz

 





 

 

  (24)

Due to high complexity of the previous differential equation solution, approximations can be introduced. On one hand, for particles of sufficiently low charge (lower than 25 mV) Debye-

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Huckel theory represents a good approximation. This theory helps us to calculate one important particle feature, namely, surface charge density (σ):

0 0

   (25)

where κ stands for the inverse Debye screening length that critically depends on ionic strength (I) as follows:⁴³

2

0

2 _A

B

N e I

 k T

   (26)

whereby NA is Avogadro’s number and I is ionic strength. On the other hand, Grahame model³⁵ was developed to calculate surface charge density from potential in case of charged planar surfaces in electrolyte solution. It is more suitable for highly charged surfaces in comparison to the Debye-Huckel model and leads to the following form:

0

2 asinh( )

2

B

k T e

e k T

 

   (27)

It is obvious from the previous equations (27) that precious information regarding surface charge density can be gained if one knows the surface potential. However, due to the fact that there is no experimental technique, which can probe this electrostatic potential directly, one can use electrokinetic (zeta) potential to acquire σ. Electrophoresis has been widely utilized investigation method to probe electrophoretic mobility (and therefore zeta potential) of charged particles in the presence of different coagulation or stabilizing agents.^44-48 If charged particles are placed in an external electric field, they will start moving with a certain velocity (v) towards the oppositely charged electrode. This velocity is proportional to the electrophoretic mobility (u) and depends on the magnitude of the established electric field (E).

vuE (28)

Velocity of particles can be detected by light scattering. As a consequence, electrophoretic mobility can be determined as a ratio between the charge (Q) and friction coefficient (f)⁴³:

6

Q ze

u^ f ^ R ⁽²⁹⁾

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19

After we gained information about u, we have to convert it to zeta potential. However, there are few different models that can achieve this (Figure 8). In order to choose the correct one, we need to take into account their charge, size of the particles and ionic strength in dispersion.

When ionic strength is high, which causes small Debye screening length (low potential

50 mV

  ) and large particles (R 1) we can use Smoluchowski model to obtain ζ:

0

u

 ^  ⁽³⁰⁾

Oppositely, if R 1, the recommended equation is Huckel:

0

2 3

u

 ^   ⁽³¹⁾

Finally, in the situation between, when R1, the best estimation is given by Henry’s model^39,49:

0

2 ( )

3

u f R

 

   ⁽³²⁾

where f(κR) is called Henry’s function, which can vary from 1 to 1.5 depending on the ionic strength. However, it is always applicable for low potentials. In the case of high potentials some other models, like the one developed by O’Brien-White, have to be used.⁴³

Finally, understanding charging properties of the particles can help us understand, and even improve, thier colloidal stabiltiy, which is ussually a precondition for utilization of the dispesion in any applications.

Figure 8. Comparison between two different models for obtaining zeta potential from electrophoretic mobility.

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1.2. Colloidal particles

In Chapter 1.1, colloidal particles have already been discussed in brief. However, here I will give a more elaborative description of particles that my PhD was based on, namely, layered double hydroxides (LDHs), with an overview regarding their composition, synthesis and application.

LDHs are a large group of anion-intercalated inorganic materials, which are also known as anionic clays.⁵⁰ They are widely distributed in nature, however, not to the extent of cationic clays.^51-54 Discovery of LDH happened in 1842 in Sweden. It was the most common type of these clays, hydrotalcite (Mg6Al2(OH)16CO3·4H2O). In spite of widespread presence of LDHs in nature, due to hard purification process and ease of synthesis based on the controlled co- precipitation, most of the studied LDH particles were synthetic. Increasing interest for anionic clays was observed since the end of the sixties when most of the LDH publications were concerned with the description of their structure, different physical properties, novel synthetic approaches and several new ideas for their application. Results obtained by these studies, together with the proofs of their biocompatibility^55,56, were crucial for the increasing interest in LDHs.

Let us first discuss the structure of LDH structure is based on octahedrally coordinated metal cations, as in the structure of brucite, Mg(OH)2.⁵⁷ These octahedral units share their edges in order to form brucite-like structure. However, the main difference between brucite and LDH is the isomorphous substitution of some divalent metal cations (M²⁺) with trivalent (M³⁺) ones. In the case of hydrotalcite, it is the substitution of Mg²⁺ ions with Al³⁺, which causes positive net structural charge. In order to maintain net neutrality, anions are present in the interlayer space, together with some water molecules (Figure 9). Taking into account all previous facts about its structure, general formula of an LDH clay material can be expressed as [M^2+(1-x)M^3+(x)(OH)₂]^x+[X^m-_x/m×nH₂O]^x-. M²⁺usually corresponds to Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ or Zn²⁺, while M³⁺stands for Al³⁺, Mn³⁺, Fe³⁺, Co³⁺, Ni³⁺, Cr³⁺, Ga³⁺. This means that trivalent and divalent metal cations found in LDH structure mainly belong to the third and fourth periods of the periodic table. This is generally due to the ionic radii that can compose this type of structure. Another important aspect is the value of x (trivalent metal ratio). Generally speaking, it corresponds approximately to 0.2 ≤ x ≤ 0.4, but there are some exceptions of this rule. Trivalent metal ratios (x) larger than 0.4 can lead to strong

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electrostatic repulsion between neighboring layers. In contrast, lower values than 0.2 cause a collapse of the lamellar structure due to the high distance between the interlayer anions.^50,58 Interlamellar space can contain large variety of anions and differently charged and not

charged moieties. One major characteristic of LDHs is that in most cases only weak bonding occurs between these interlamellar ions or molecules and the layers. Therefore, LDHs have high ion exchange capacity, and there is even a possibility of exchanging anions with large drug molecule⁵⁹ or polymeric compounds⁶⁰ of negative charge. In this case, intercalation can be proved by the increase in the h parameter. Most commonly, due to the very high affinity, CO₃^2- is present in interlayer gap. That is the reason why synthesis of any other intercalated LDH has to be done under nitrogen atmosphere, without any traces of carbon-dioxide (especially during synthesis at elevated pH values).⁶¹ Here are some of the anions commonly found between the cationic layers:

1. halides

2. non-metal oxoanions

Figure 9. Schematic representation of different levels of LDH structure with all important structural parameters.

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3. oxo- and polyoxometallate anions

4. anionic complexes of transition metal ions 5. organic anions

6. anionic polymeric structures

If one discuses the synthesis of LDH clays, there are several approaches, like co- precipitation⁶², urea method⁶³, anion-exchange method⁶⁴, reconstruction method⁶⁵. Among all of them, synthesis by co-precipitation is the most commonly used method, mainly due to the high versatility of LDH materials that can be prepared by a relatively simple procedure.

Co-precipitation is performed by the addition of two metallic salts mixture into water at a certain pH (where hydrolysis of the metal cations is suppressed) under vigorous stirring.

Second solution containing base is added in adequate amount in order to maintain desired pH.

The mechanism of the formation of brucite-like layer structure by co-precipitation is based on the condensation of metal hexa-aquo complex. Beside pH, there are several more important parameters for the successful synthesis:

1. temperature of the reaction mixture

2. concentration of the mixed metallic salts solution 3. concentration of the base solution

4. rate of addition of reactants to the reaction mixture (drop-by-drop or ‘flash’ co- precipitation)

5. aging of the obtained material

Most of the precipitation occurs at room temperature, however, elevated temperature can, in some cases, induce improvement in crystallization. Rate of addition can also influence crystallinity of the final product. While adding drop-by-drop is the safest way for obtaining high crystallinity, it can cause high polydispersity index (PDI) which makes DLS measurements difficult. Because of that, ‘flash’ co-precipitation (mixing reactants as fast as possible) is often performed, which gives particles with a better size distribution. This issue can also be overcome afterward, with a hydrothermal treatment. This treatment significantly improves the crystallinity of the LDH. It is based on the heating of the LDH dispersion in a closed steel chamber, at high temperature (below the temperature of LDH decomposition) in the presence of water vapors that cause increase in pressure.⁵⁰ Regarding the pH of the

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precipitation, we can differentiate coprecipitation at constant and at variable pH.

Nevertheless, in both of these methods it is very important that pH is higher than the pH, at which more soluble hydroxide precipitates. To conclude this part about synthesis, there are many operating conditions in the coprecipitation process that have to be addressed for each system.

Layered double hydroxides have many advantageous properties leading to many possible applications. One of their widely exploited features is the ability to undergo delamination (Figure 10).^66-71 This can cause highly transparent LDH dispersion of particles with a thickness of only a few atomic layers. These thin platelets can be further used for the formation of nanocomposites or building block for many different organic-inorganic or inorganic-inorganic hybrid materials.⁷² Numerous new processes have been developed for exfoliation due to difficulty of this procedure in comparison to the same one for cationic clays, like montmorillonite or laponite.⁷³ This is explained by the combination of high charge density of LDH layers that causes strong electrostatic attraction and possible strong hydrogen interlamellar bonding. Delamination procedures include surfactants or organic solvents (e.g., formamide, butanol, CCl₄, etc.), however there were some performed in a water medium.⁷⁴ It is important to mention that ultrathin anionic clay monolayers can be obtained also by ‘bottom-up’ approaches.⁷⁵ Exfoliated LDHs have found application in many fields in which accessibility to the inner surfaces is required. Namely, they are used in preparation of thin films, synthesis of catalysts, development of different hybrid magnets or bioinorganic materials and synthesis of electrode material.⁷⁶

Another interesting property of this clay material is the so-called ‘memory effect’. It is first described by Miyata in 1980 as a reconstruction of the original LDH structure from calcined

Figure 10. Schematic representation of delamination process.

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clay by hydration. After heating LDH particles, they undergo different steps of decomposition:

1. 25-250 ºC → dehydration and dehydroxylation of layers 2. 250-350 ºC → interlayer anion decomposition

3. 350-550 ºC → collapsing of the layers 4. 550- ºC → crystallization of new phases

Rehydration is possible after moderate calcinations, prior to the crystallization of the new, mixed oxides phase. If rehydration is performed in the presence of different anions, like sebacic acid, intercalation of this anion can occur.⁷⁷ This is a widely used technique for anion intercalation between LDH layers (Figure 11).

Combination of previously described properties together with high compositional diversity ensured application of LDH materials in diverse fields. Among all, the most important ones are related to catalysis or use as catalyst support, photo- and electrochemistry, gene and drug delivery as well as ion-exchange and novel material design.⁷⁶

In the area of catalysis, we need to distinguish the use of LDH as catalyst support from use as a solid base catalyst.⁷⁸ Regarding the first one, differently composed LDH platelets served initially as a support for Ziegler catalysts for olefin polymerization.⁷⁹ Some more recent utilization is related to immobilization of vanadium oxide catalyst on calcined MgAl-LDHs for oxidative dehydrogenation of butane⁸⁰ and synthesis of isobutyraldehyde⁸¹. With the year of 2000, LDH was proved as an efficient support for noble metal catalysts, as Pd(0). This was achieved by ion-exchange with PdCl₄^2-, followed by subsequent reduction.^82,83 In a similar manner, clay material containing rhodium metal for Heck, Suzuki and Stille reactions of haloarenes was prepared.⁸⁴ However, besides supporting different metal complexes, Figure 11. Calcination of LDH at elevated temperature followed by rehydration (memory

effect).

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immobilization of biocatalysts, namely enzymes, has been a widely investigated field. More about enzymes and enzyme immobilization will be discussed in one of the next chapters.

LDHs, due to their ability to provide Bronsted type base sites, are studied for the purpose of replacing homogenous catalysts with more environmentally friendly and recyclable ones.

This includes formation of C C and CCbonds by Michael addition⁸⁵, aldol⁸⁶ and Claisen–Schmidt condensations⁸⁷, Knoevenagel condensations⁸⁸ and Henry reaction⁸⁵. In addition, many studies have been conducted in order to implement LDH in the formation of

C O ⁸⁹, NO⁹⁰ and S O ^91,92 bonds.

Application of LDH nanoparticles as a novel delivery system is a very promising area. While drug delivery followed by the controlled release is a widely investigated field for a longer period of time, recent interest for the gene delivery by LDH has also risen.⁹³ Initially, many different anionic molecules including drugs were intercalated into these clays by the ion- exchange process, namely nucleotides, enzymes, ATP and amino acids.⁹⁴ Concerning gene delivery, mostly short-sized nucleic acid were delivered for the purpose of gene therapy.⁹⁵ For both of these applications, biocompatibility of the support material is of the vital importance.⁹⁶

Finally, high diversity makes these hydroxides even more interesting for materials science, where numerous different composite and hybrid materials are being made. Many different types of polymers were intercalated or immobilized on the surface of the particles, including dendrimers.⁹⁷ However, one of the most promising fields is the conjugation of clays with different enzymes that can be applied for biomedical purposes but also to prepare functionalized electrodes for novel sensors.⁹⁸ LDH can be also combined with various metal nanoparticles. Interestingly, in contrast to everything previously described, LDH sheets can also be adsorbed on big polymeric particles, like silica particles, which can serve as a good approach for production of new, highly porous material with many advantageous properties.⁹⁹ These examples represent only a small portion of all possible future application. However, interest for these particles is growing and new ideas for novel utilizations of LDH nanoclays are being born every day.

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1.3. Interaction with charged species

Tuning colloidal stability of particles in dispersion is an essential point in the majority of applications. Accordingly, during the removal of anion in wastewater treatment, in drug and gene delivery, food and pain applications, stable primary particle dispersion is required. This can be achieved by the functionalization with different (bio)polymeric structures or (bio)macromolecules. It’s is also important to have knowledge about the environment of the particles because high ionic strength can destabilize particles as it was explained by DLVO theory. There is also a possibility of influencing the aggregation behavior by adsorption of some multivalent ions or some specific interaction of non-DLVO origin. Differently charged species can demonstrate various types of interaction with colloidal particles, or in the case of my PhD work, with LDH particles.

1.3.1. Mono- and multivalent ions

Influence of monovalent electrolytes on colloidal stability has been widely studied in the past decades. DLVO theory gave us great insight into this issue. It predicts that at low salt concentration, particle will possess thick electrical double layer that will prevent particles from aggregation due to electrostatic repulsion upon approach. In contrast, high electrolyte levels will decrease double layer repulsion and van der Waals forces will predominate and particles will tend to aggregate. However, the part where DLVO theory fails is the incapability to differentiate between monovalent salts according to their chemical composition and hydrophobicity. In other words, there is no ion specificity in this theory and it predicts the same CCC values for all monovalent salts. In this case, some specific interaction has to be included with a possibility of ion adsorption on the surface of particles.^100-102

As it has been shown in many cases, presence of the ions of the same valence can induce different CCCs.^44,103,104 This can be rather well explained by Hofmeister series, which puts ions in the order according to their affinity to surfaces and to their hydration level.¹⁰⁵ The effect was for the first time observed by Franz Hofmeister, who investigated the influence of series of salt on the solubility of protein and their structure.¹⁰⁶ Nevertheless, the series is applicable also on particle aggregation, however, with an exclusion of higher valence ions.

These ions have a very strong effect, that can be only explained by the Schulze-Hardy rule^107,108 to be discussed later. Entire Hofmeister series can be separated in four different

Nanoclay interaction with charged species: from fundamentals to functional materials

Thesis

Reference

Nanoclay interaction with charged species: from fundamentals to functional materials

Nanoclay interaction with charged species: from fundamentals to functional materials

THÈSE

Marko PAVLOVIĆ

Contents

Abstract

Résumé

Chapter 1

1. Introduction

1.1. Particle dispersions





1.2. Colloidal particles

1.3. Interaction with charged species