Thesis
Reference
Importance of complexation processes on the fate, reactivity and transport of manufactured nanoparticles in aquatic systems
LOOSLI, Frédéric
Abstract
The rapid development of nanotechnology and the increased use of engineered nanoparticles (ENPs) in consumer products will result in the occurrence of nanomaterials in the environment. The present thesis (compilation of publications) is dealing with the investigation of ENP stability, fate and impact in aquatic systems. Influence of important water physico-chemical properties (pH, ionic strength, divalent electrolytes) but also the presence of natural organic matter (NOM) and surfactants on the stability of TiO2 ENPs has been systematically studied. A special interest was given to the stability of already formed ENP agglomerates. Presence of NOM, at environmental concentrations, was shown to promote significant disagglomeration which is a key result for the risk assessment associated to ENPs.
Interactions mechanisms between ENPs and NOM were also investigated by calorimetry for the first time in environmental nanoscience and opened a new insight into the understanding of ENP behavior in presence of NOM.
LOOSLI, Frédéric. Importance of complexation processes on the fate, reactivity and transport of manufactured nanoparticles in aquatic systems. Thèse de doctorat : Univ.
Genève, 2015, no. Sc. 4811
URN : urn:nbn:ch:unige-748565
DOI : 10.13097/archive-ouverte/unige:74856
Available at:
http://archive-ouverte.unige.ch/unige:74856
Disclaimer: layout of this document may differ from the published version.
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UNIVERSITÉ DE GENÈVE FACULTÉ DES SCIENCES
Section des Sciences de la Terre et de l’Environnement Docteur Serge STOLL
Importance of Complexation Processes on the Fate, Reactivity and Transport of Manufactured
Nanoparticles in Aquatic Systems
THÈSE
présentée à la Faculté des Sciences de l’Université de Genève pour obtenir le grade de Docteur ès Sciences, mention Interdisciplinaire.
par
Frédéric LOOSLI de
Sumiswald (BE)
Thèse N°4811
GENÈVE
Atelier d'impression ReproMail 2015
Table of contents
Acknowledgements
iSummary in french
iiiChapter I - Introduction and thesis objectives 1
I.1 General introduction 3
I.2 Nanoparticle transformation in aquatic systems 3
I.3 Colloids and aquagenic compounds 4
I.4 Interaction mechanisms between (nano)particles and natural organic matter 7 I.5 Thesis objectives 9 I.6 List of papers 10
I.7 References 10
Chapter II - Introduction to colloidal stability theory and instrumental methods 17
II.1 DLVO theory 19II.2 Instrumental methods 24
II.2.1 Coulter Counter 24
II.2.2 Dynamic light scattering and laser Doppler velocimetry 25 II.2.3 Electron microscopy 27
II.2.4 Isothermal tiration calorimetry 28
II.3 References 29
Chapter III - Adsorption of TiO
2nanoparticles at the surface of micron-sized latex particles. pH and concentration effects on suspension stability. 31
III Introduction 33
III Main results 33
III References 35
Paper I 37
Chapter IV - Influence of natural organic matter on the agglomeration and disagglomeration of TiO
2nanoparticles. Effect of pH and NOM concentration on nanoparticle stability. 49
IV Introduction 51IV Main results 51
IV References 53
Paper II 55
Paper III 69
Chapter V - Effect of electrolyte valency, concentration and pH on the destabilization of engineered TiO
2nanoparticles. Influence of alginate concentration on agglomerate partial restabilization in presence of divalent electrolyte. 79
V Introduction 81
V Main results 82
V References 83
Paper IV 85
Paper V 95
Chapter VI - Towards a thermodynamic quantification of the interactions between nanoparticles and natural organic matter.
How isothermal titration calorimetry can help to predict agglomeration mechanisms? 107
VI Introduction 109
VI Main results 109
VI References 110
Paper VI 111
Chapter VII - Effect of sodium dodecyl sulfate concentration, pH and divalent electrolytes on the stability of manufactured TiO
2nanoparticles. 123
Chapter VIII - Conclusions and perspectives 145
Annexes 149
Annex table of contents 151
Annex 1 - Referring to the Supporting Information of Paper III (Chapter IV) 153 Annex 2 - Referring to the Supporting Information of Paper IV (Chapter V) 155 Annex 3 - Referring to the Supporting Information of Paper V (Chapter V) 161 Annex 4 - Referring to the Supporting Information of Paper VI (Chapter VI) 163
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Acknowledgements
Je voudrais tout d'abord remercier Serge Stoll d'avoir accepté de me prendre comme doctorant dans son groupe de recherche. Les nombreuses discussions scientifiques que nous avons eu, m'ont permis de pleinement développer mes connaissances dans ce milieu passionnant qu'est la chimie des colloïdes. Je voudrais également le remercier pour son soutien au quotidien et pour sa bonne humeur permanente qui permettent, selon moi, un épanouissement personnel privilégié et une avancée optimale des recherches. Merci aussi pour sa grande disponibilité.
Ce fut également un plaisir de partager avec lui des discussions non professionnelles.
Je tiens également à remercier toutes les personnes composant mon jury de thèse, non seulement d'avoir contribué scientifiquement et techniquement à la réalisation de mon travail de thèse, mais également d'avoir pris le temps de le lire, de l'évaluer de façon critique ainsi que de se déplacer jusqu'à Genève pour la soutenance. Par ordre alphabétique Daniel Ariztegui (Université de Genève), Jean-François Berret (MSC, Université Denis Diderot), Jérome Labille (CEREGE, Aix-Marseille Université), Philippe Le Coustumer (Université Bordeaux 1 et 3) et Corinne Nardin (Université de Genève), je vous en remercie énormément.
Merci également à Leticia Vitorazi, pour son aide et sa gentillesse lors des expériences ITC que j'ai effectué à Paris.
Merci aux amis et collègues de Forel, plus particulièrement Arnaud, Elena et Fabrice, avec qui j'ai passé de très agréables moments lors de ces quatre dernières années.
Merci au Fonds National Suisse (200010_152647 et 200021_13240) pour son support financier.
Finalement, je tiens à remercier tout particulièrement ma famille (LF, Archi, Yannick, Sam, Little monster, Lucette, Robert, Josy, Pierre, Pinpin) et surtout mes parents pour tout.
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Summary in french
Dans le but de développer des produits innovants avec des propriétés uniques, les nanoparticules manufacturées (NPs) sont de plus en plus utilisées dans le domaine industriel.
Ces particules sont souvent définies comme des matériaux avec au moins deux dimensions externes comprises entre 1 et 100 nm. La particularité de ces matériaux provient de leur importante surface spécifique et donc de la grande proportion d'atomes présents à leur surface.
Les NPs possèdent des propriétés optiques, électroniques ainsi que des propriétés de surface qui ont révolutionné le domaine industriel. Elles sont en effet utilisées dans des domaines variés comme la confection de textile et de peinture, dans le domaine de la biomédecine et également dans la production d'équipements électroniques et de biosenseurs. L'étude de la stabilité des NPs en présence de divers composés ainsi que des processus et les mécanismes d'interactions avec ces composés sont primordiaux pour des raisons industrielles lors de la confection de produits mais également pour estimer le transport, la réactivité et la toxicité potentielle des NPs susceptibles de diffuser dans les systèmes aquatiques naturels et les organismes vivants.
La stabilité des NPs est fortement influencée par les propriétés physico-chimiques du milieu dans lequel elles sont présentes mais également des composants présents dans le milieu.
Mon travail de thèse s'est principalement orienté sur l'étude des phénomènes de complexation entre des NPs de dioxyde de titane et des macromolécules chargées. Dans un premier temps le comportement de NPs de TiO2 en présence de colloïdes synthétiques et leurs effets sur la stabilité des particules colloïdales ont été étudiés. Puis plusieurs études ont été consacrées à la stabilité des NPs de TiO2 en présence de polyélectrolytes naturels. L'effet et l'importance, sur la stabilité des NPs, de certain paramètres physico-chimiques du milieu de dispersion tels que le pH, la force ionique, la concentration et la valence des électrolytes mais également l'influence de la présence et concentration de composés aquagéniques ont été sujet d'étude.
Finalement la quantification des énergies en jeu lors des interactions entre les NPs de TiO2 et deux classes importantes de matière organique naturelle a été effectuée à l'aide de la titration calorimétrique isotherme.
Lors d'une première étude nous nous sommes intéressés à l'adsorption de NPs de TiO2 à la surface de microparticules de latex. L'influence de la concentration en TiO2 sur la stabilité de particules de latex a été étudiée en déterminant la modification de la charge de surface, les cinétiques d'agglomération ainsi que la morphologie des agglomérats formés. L'interaction entre les particules de latex et les NPs de TiO2 a été étudiée pour trois scénarii électrostatiques différents. En effet, les latex sont fortement négativement chargés sur tout le domaine de pH étudié (3 à 11) dû à la présence de groupes fonctionnels sulfate alors que la charge de surface des TiO2 est dépendante du pH. Pour des pH inférieurs à 5.2, leur surface est fortement positivement chargée et les NPs sont stables. Dans le domaine 5.2<pH<7.2 les forces de van der Waals sont prédominantes et les NPs sont déstabilisées. Le point de charge nulle (PCN)
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des NPs de TiO2 correspond ici à un pH de 6.2. A ce pH les agglomérats formés ont une taille plus conséquente. Pour des pH supérieurs à 7.2 les NPs sont stables et fortement négativement chargées. Nous avons donc décidé de travailler à deux valeurs de pH auxquels le TiO2 est stable (pH < pH PCN, TiO2 et pH > pH PCN, TiO2) ainsi qu'à celle où il y a la plus forte déstabilisation (pH = pH PCN, TiO2). C'est bien entendu pour un pH < pH PCN, TiO2 que la présence des NPs a fortement modifié la charge de surface des particules de latex car les NPs s'adsorbent sur les latex ce qui engendre une diminution importante du potentiel zeta (ζ) des latex. Pour une certaine concentration en TiO2 la charge des latex est neutralisée ce qui correspond au point isoélectrique (PIE). Pour des concentrations en TiO2 plus importantes la charge est inversée. Pour les deux autres pH la variation du potentiel ζ du latex en fonction de la concentration en TiO2 n'indique pas d'interaction (observations confirmées par images obtenues par microscopie électrique à balayage (MEB)). Ensuite les cinétiques d'agglomération des particules de latex en fonction de la concentration en TiO2 pour pH < pH
PCN, TiO2 ont été déterminées. Les constantes cinétiques sont dépendantes de la concentration en NPs et les constantes les plus élevées correspondent à des concentrations proche du PIE.
La déstabilisation des particules de latex est observée même pour des valeurs de potentiel ζ relativement importantes, et largement supérieures aux valeurs estimées lors de calculs théoriques (basés sur la théorie concernant la stabilité des particules colloïdales proposée par Derjaguin, Landau, Verwey et Overbeek (DLVO). Ceci indique que la neutralisation de charge n'est pas le seul mécanisme impliqué dans la déstabilisation des particules de latex.
Une contribution attractive supplémentaire s'explique par une distribution hétérogène et la présence de "patch" (NPs individuelles et petits agglomérats de NPs) de TiO2 à la surface des latex (confirmées par images MEB). Finalement la détermination de la dimension fractale des agglomérats formés pour des concentrations de TiO2 correspondant au PIE révèle une agglomération limitée par la diffusion des particules.
Après cette première étude qui peut être une aide précieuse pour la fabrication de film de latex dopé avec des NPs, nous nous sommes alors intéressés à la stabilité des NPs de TiO2 en présence de matière organique naturelle (MON), ceci dans le but d'évaluer le potentiel transport et transformations possibles des NPs présentes dans les systèmes aquatiques.
L'influence de la concentration de deux classes importantes de MON (substances humiques et substances exopolymériques) sur la stabilité de NPs de TiO2 à pH < pH PCN, TiO2, pH = pH PCN, TiO2 et pH > pH PCN, TiO2 a été étudiée en déterminant la variation de la taille ainsi que de la charge des NPs en fonction des concentrations en MON. Les rôles de l'architecture et de la charge structurelle de la MON sont également discutés. L'alginate est un biopolymère linéaire alors que les acides humiques sont considérés comme des macromolécules globulaires hétérogènes et semi-rigides. Tous deux sont négativement chargés pour des pH compris entre 3 et 11. Pour pH < pH PCN, TiO2, la concentration en MON influence fortement la stabilité des NPs due à son adsorption. Le domaine de déstabilisation (en terme de potentiel ζ et concentration respective) est dépendant de la nature de la MON. Pour l'alginate ce domaine va de +28 à -28 mV alors que pour les acides humiques il est entre +20 et -10 mV. De plus, la concentration en acides humiques requise pour neutraliser la charge de surface des NPs de
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TiO2 est supérieure à celle en alginate (1.40 ± 0.23 contre 1.25 ± 0.21, respectivement, en terme de rapport de charges entre MON et TiO2). De telles différences peuvent être reliées à la différence d'architecture et propriétés chimiques entre ces deux composés. Les acides humiques présentent des encombrements stériques plus importants que l'alginate qui est un polymère semi-flexible avec une densité de charges homogène.
Pour les expériences à pH > pH PCN, TiO2 les NPs de TiO2 ainsi que la MON sont négativement chargées. Quand l'alginate est en présence des NPs aucune interaction n'est observée (la concentration en alginate n'influence ni la taille ni la charge de surface des NPs qui demeure constante). En présence d'acides humiques une adsorption limitée est observée dû à des interactions de type van der Waals.
L'influence de la concentration en MON sur la stabilité des NPs de TiO2 a été étudiée à pH = pH PCN, TiO2. A ce pH, les nanoparticules forment de larges agglomérats et la MON, une fois adsorbée à la surface des agglomérats provoque une inversion de charge et une désagglomération de part les répulsions électrostatiques et les encombrements stériques générés par les molécules adsorbées à la surface des NPs. Ce processus est dépendant de la concentration en MON mais également des propriétés de cette dernière. En présence d'alginate, la désagglomération est plus rapide que dans le cas des acides humiques (45 min contre 24 h) en raison des propriétés de l'alginate. Comme l'alginate est plus flexible et la distribution de charges plus homogène il a tendance à interpénétrer plus facilement les agglomérats et faciliter leurs fragmentation. La taille des fragments obtenus est cependant plus faible en présence d'acides humiques (250 nm contre 500 nm) dû aux répulsions électrostatiques plus importantes ainsi qu'à une stabilisation stérique plus efficace.
Les points importants d'un point de vue environnemental sont: i) la MON permet la fragmentation d'agglomérats de NPs de TiO2 pour des concentrations relativement faibles (et environnementales et dont pour des rapports de concentration NPs sur MON supérieurs à 50) en MON. ii) l'architecture et les propriétés physico-chimiques de la MON influencent la déstabilisation des NPs de TiO2 mais également les cinétiques et la taille des fragments obtenus lors du processus de désagglomération. iii) seulement une désagglomératoin partielle mais significative est observée (confirmée par images TEM).
La composition du milieu de dispersion jouant un rôle très important sur la stabilité des NPs, l'étude de l'influence de la valence et de la concentration en électrolyte sur la déstabilisation de NPs de TiO2 a été menée à pH < pH PCN, TiO2 et pH > pH PCN, TiO2. Pour ce faire, la variation des tailles des NPs en fonction du temps, pour différentes concentrations en NaCl et CaCl2, a été mesurée pour calculer les vitesses d'agglomération et les probabilités de collage. Ceci permet la détermination graphique des concentrations critiques de coagulation (CCC). La CCC représente la concentration minimum en électrolyte qui induit une agglomération limitée par la diffusion des NPs. Des mesures de la mobilité électrophorétique ont également été effectuées pour une meilleure compréhension de l'influence des mécanismes d'écrantage de charges et d'adsorption spécifique sur la déstabilisation des NPs. Le valence des contre-ions ainsi que la solvatation de ces derniers jouent un rôle important sur l'efficacité de la déstabilisation des NPs et donc sur les valeurs de CCC obtenues. En effet, les ions Ca2+ étant
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divalents, ils s'adsorbent de façon spécifique à la surface des NPs pour un pH > pH PCN, TiO2 (NPs négativement chargées) et modifient fortement la charge de surface des NPs par une forte diminution, voire inversion du ζ potentiel. La CCC obtenue dans ce cas (6 × 10-5 M) est nettement inférieure à la CCC au même pH en présence de NaCl (0.28 M, Na+ contre-ions) ainsi que celle à pH < pH PCN, TiO2 en présence de CaCl2 (0.021 M, Cl- contre-ions).
L'hydratation des contre-ions influence également la déstabilisation des NPs. En effet, pour les expériences faites en présence de NaCl, les valeurs de CCC sont différentes pour pH < pH
PCN, TiO2 (0.040 M) et pH > pH PCN, TiO2 (0.028 M) dues à la nature des contre-ions impliqués.
Na+, ayant un pouvoir structurant supérieur aux Cl- depart sa plus grande solvatation, et donc une capacité d'écrantage supérieure (donc CCC inférieure pour pH > pH PCN, TiO2).
Les valeurs expérimentales de potentiel ζ déterminées lors de la précédente étude de déstabilisation des NPs en fonction de la concentration en électrolytes ont ensuite été utilisées comme paramètres d'entrée pour des calculs de DLVO. Pour des concentrations supérieures ou égales aux CCC expérimentales, les forces électrostatiques répulsives sont contrebalancées par les interactions attractives du type van der Waals. Cette théorie sur la stabilité des particules colloïdales décrit bien la déstabilisation des NPs de TiO2 à l'exception des expériences faites à pH > pH PCN, TiO2 en présence de CaCl2. Ceci à cause du pontage cationique qui n'est pas pris en compte dans le modèle de DLVO.
L'étude de l'influence de la concentration en alginate sur la stabilité d'agglomérats de NPs formés à un pH et une dureté représentative de celle du lac Léman a été étudiée (pH 8.2, [Ca2+] = 45 mg L-1 et [Mg2+] = 5 mg L-1). Dans de telles conditions physico-chimiques les NPs forment de larges agglomérats (supérieurs au micromètre) positivement chargés et l'adsorption d'alginate induit une rapide (60 min) et significative fragmentation (fragments de 400 nm pour [alginate] ≥ 8 mg L-1) des agglomérats due à des effets stériques et aux répulsions électrostatiques.
Pour avoir de plus amples informations sur les mécanismes d'agglomération ainsi que sur les énergies mises en jeu lors de l'interaction entre les NPs et la MON, la quantification thermodynamique des interactions entre le TiO2 et l'alginate a été déterminée. Des titrations de TiO2 dans une solution d'alginate et vice versa (pour étudier l'effet de l'ordre de mélange) ont été effectuées et les interactions quantifiées par titration calorimétrique isotherme. Ces interactions sont, quelque soit l'ordre de mélange, enthalpiquement favorables (avec des enthalpies négatives de l'ordre de -8 kJ mol L-1, processus exothermique) et les réactions principalement guidées par un important gain d'entropie (-TΔS < -30 kJ mol-1). Par contre la stœchiométrie de réaction se révèle être, elle, fortement influencée par l'ordre de mélange.
Quand l'alginate est titré avec des NPs, elle est d'environ 0.58 dû au pontage des NPs avec l'alginate, et lorsque le TiO2 est ajouté à une solution d'alginate, elle est aux alentours de 0.90 (NPs individuellement enrobées). Ces différents mécanismes sont également en bonne corrélation avec les valeurs de changement d'entropie qui sont supérieures dans le cas du pontage dû à la moindre perte entropique conformationelle des biopolymères. Les variations de tailles et ζ potentiel ont été également déterminées, par des techniques de diffusion de
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lumière, indiquant qu'aucune interaction n'a lieu après saturation des sites de surface (inversion de charge observée) et la formation de larges agglomérats (précipitation).
L'influence de la stabilité de NPs de TiO2 en présence d'agent tensio-actif anionique (sodium dodecyl sulfate (SDS)) a également été étudiée car ces produits sont non seulement souvent utilisés comme agents stabilisants dans la production de dispersion de NPs, mais sont également présents dans les systèmes aquatiques dû à leur importantes utilisations comme détergents et agents de nettoyage.
L'influence de la concentration en SDS sur la stabilité de NPs de TiO2 à pH < pH PCN, TiO2 indique que l'adsorption de SDS modifie fortement la charge de surface des NPs (diminution voire l'inversion de la charge de surface). Bien que la présence de SDS implique la formation d'agglomérats plus larges que lorsque les NPs, seules, sont déstabilisées par variation de pH, le domaine de déstabilisation en terme de ζ potentiel est identique (+20 à -20 mV). Cette différence de taille des agglomérats est due aux interactions hydrophobes entre les molécules adsorbées sur la surface des NPs.
Ensuite l'effet du pH sur des complexes TiO2-SDS stables formés à pH < pH PCN, TiO2 indique que seul les complexes formés avec un large excès de SDS restent stables quelque soit le pH.
La présence de SDS modifie cependant la variation des charges de surface des NPs en fonction du pH (différent PIE car déprotonation plus difficile). La stabilité de ces complexes (toujours formés à pH < pH PCN, TiO2) a également été étudiée dans des conditions de pH et de dureté représentatives du lac Léman. La présence de cations divalents influence de manière importante la désorption des molécules de SDS (formation de complexes SDS-ions divalents) mais également la charge de surface des NPs due à leur adsorption spécifique. La concentration en SDS change l'équilibre complexe entre phénomènes de désorption, de complexation et de modification de la charge de surface. Seule une concentration élevée en SDS préserve les NPs de l'agglomération dans de telles conditions.
Chapter I
Introduction and thesis objectives:
Nanoparticles behavior in aquatic systems. Stability,
fate and transformation of nanoparticles in presence of
aquagenic compounds
3 I.1 General introduction
Due to their very unique properties, nanomaterials are employed in numerous industrial applications. Nanomaterials, as suggested by the European commission (2011/696/EU), are defined as natural or manufactured material having at least 50% of the particles, for a number size distribution, with one or more external dimension in the range 1-100 nm. On the other hand nanoparticles (NPs) are nanomaterials with 2 or 3 external dimensions in this nano-range according to ASTM International (E2456-06/2012).
As the size of materials is decreasing, their specific surface area, and thus percentage of atoms located at their surface, is drastically increasing. The excess energy at their surface induces reconformation and rearrangement of the surface atoms which give them very unique physicochemical catalytic properties (Auffan et al. 2009, Dai 2002, Jain et al. 2008). To adjust the NP properties and reactivity for specific industrial applications, nanomaterials are synthesized and functionalized. Such materials are named engineered nanoparticles (ENPs), as suggested by the European Commission (1363/2013).
ENPs are thus widely used in textile, cosmetic industry but also in biomedicine and for the development of electronic devices and biosensors (Chen and Mao 2007, Lu et al. 2007, Luo et al. 2006).
ENPs are produced in large quantities to respond to the industrial needs (Hendren et al. 2011, Piccinno et al. 2012, Schmid and Riediker 2008) and may thus enter environmental aquatic systems through industrial discharge, surface runoff but also because of the lack of efficiency in removing them in wastewater treatment plants (Batley et al. 2013, Seitz et al. 2012).
One of the major challenge associated to the environmental risk of nanomaterial is to evaluate their transformation and fate once entering aquatic systems (Ju-Nam and Lead 2008, Klaine et al. 2008, Lead and Wilkinson 2006).
Once in aquatic systems ENP stability is strongly dependent on water physicochemical properties (French et al. 2009, Gallego-Urrea et al. 2014, Guzman et al. 2006, Mukherjee and Weaver 2010, Shih et al. 2012), presence and properties of aquagenic compounds but also on the ENP intrinsic properties (Baalousha et al. 2008, Belen Romanello and Fidalgo de Cortalezzi 2013, Erhayem and Sohn 2014, Lee et al. 2011).
I.2 Nanoparticle transformation in aquatic systems
The behavior and possible transformation of ENPs will modify their fate and residence time in environmental water systems (Christian et al. 2008, Horst et al. 2012, Klaine et al. 2008).
Indeed if ENPs are stable they will remain in the water column and be transported over long distances, whereas if being destabilized they will be rapidly eliminated from the water column by sedimentation. The stability of ENPs is strongly influenced by the water properties (pH, ionic strength, electrolytes valency, etc), the ENP intrinsic properties (surface chemistry and charge, size, shape) and the presence of aquagenic compounds (such as colloids). Possible
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transformations and related physicochemical processes in both freshwaters and marine waters are represented in Fig. 1.
Fig. 1: Influence of water properties (ionic strength) and the presence of aquagenic compounds on the transformations and stability of ENP entering aquatic systems (Loosli et al. 2014).
At low ionic strength, such as in freshwater, ENPs will enter aquatic systems as isolated or agglomerated particles and interact with aquagenic compounds such as humic and exopolymeric substances. Such interactions will lead to the formation of agglomerates which, if large enough, will sediment. In such conditions, electrostatic interactions will be predominant and result in agglomerate formation via polymer bridging or patches agglomeration mechanisms. Nevertheless, due to steric effects and electrostatic repulsions between polymers coating the ENPs, aquagenic compounds will, in some circumstances, prevent the ENP agglomeration or even promote the fragmentation of ENP agglomerates. As a result ENPs will remain longer times in the water column. At high ionic strength water, such as in marine water, the ENP surface charges and the aquagenic compound structural charges will be screened and lead to the formation of large agglomerates which will be rapidly eliminated from the water column.
I.3 Colloids and aquagenic compounds
Colloids are defined as entities that have supramolecular structures and properties but are small enough not to sediment rapidly (within days) through the water column. Their size is thus typically ranging from 1 nm to 1 μm. Natural colloids can be organic or inorganic compounds (Fig. 2) and their amount and distribution strongly dependent on the water bio- geo-chemical origin and property.
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Fig. 2: Schematic representation of some of the important organic and inorganic compound classes found in natural aquatic systems. Colloids are defined as compounds with sizes in the 1-1'000 nm range.
Among organic colloids an important class is represented by the natural organic matter (NOM). NOM can be divided into two main fractions: the humic and non humic substances.
The larger fraction is represented by humic substances. Indeed humic substances constitute the major fraction with up to 30-50% of surface water organic matter (Thurman and Malcolm 1981). Based on their solubility properties, humic substances are divided in three classes.
Humin which are insoluble in water, humic acids which are soluble for pH>3 and fulvic acid which are soluble at any pH (Piccolo 2001). Humics are formed from plant and animal residues through humification processes (such as reactions involving lignin, polyphenol and sugar amine condensation) (Jones and Bryan 1998). They are mainly composed of carbon, oxygen, hydrogen and nitrogen in a smaller amount (Cook and Langford 1998, Piccolo 2002, Roger et al. 2010). Humics are large and complex supramolecular assemblies of covalently linked aromatic and aliphatic residues mainly composed of carboxyl, phenolic and alkoxy functional groups. As the residues result from long residence times and to the exposure of oxygen and sunlight radiation they are stable and should not undergo further degradation unless specific exposition to special chemical agents or conditions.
The real structure of humics has not been yet clearly determined. Hypothetical chemical structures of fulvic and humic acids as suggested by Buffle and Stevenson, respectively, are shown in Fig. 3 (Buffle et al. 1977, Stevenson 1982).
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Fig. 3: Fulvic (a) and humic (b) acids chemical structures suggested by Buffle (1977) and Stevenson (1982)
Their molecular weight is in the order of hundred of Da to tens of kDa (Baalousha et al. 2005, Leenheer et al. 2001, Piccolo 2001) and their respective size ranging from few to hundred of nanometers (Balnois et al. 1999, Hosse and Wilkinson 2001). They have the tendency to form agglomerates weakly bounded by hydrophobic interactions and H-bonding, especially at high concentration, high ionic strength and acidic pH (Conte and Piccolo 1999, Leenheer and Croue 2003).
Humics are chemical heterogeneous and polydispersed macromolecules with high density (Hosse and Wilkinson 2001) and long lifetime (hundreds of years). Due to their high degree of branching (Schulten et al. 1998) and important degree of hydration (Buffle 1988) they are much more less flexible than linear polysaccharides and behave more as small rigid globules.
Non humic substances such as polysaccharides, aminosugars, proteins, DNA, etc, are generally produced in the water column from the exudation or degradation of phytoplankton and bacteria (Leenheer and Croue 2003). They often exhibit lower charge density than humic substances and have less pronounced hydrophobic character (Buffle 1988). Fibrilar polysaccharides and peptidoglycans are released from phytoplankton as exudates or cell compounds. Polysaccharides represent from 10 to 30% of the natural organic matter present in surface freshwater (Buffle et al. 1998, Wilkinson et al. 1997) and up to 50% of the total NOM in saline surface water (Engel et al. 2004, Wells 1998). A representation of the chemical structure of alginate, a linear and negatively charged block copolymer, is shown in Fig. 4.
Fig. 4: Chemical structure of sodium alginate, a representative linear exopolymeric substance composed of mannuronic (M) and guluronic (G) acid monomers taken from (RSC consulted the 20.05.2015).
Alginate is a natural polysaccharide found in aquatic systems (Davis et al. 2003, Gombotz and Wee 1998) produced by brown algae species (Gregor et al. 1996).
7
I.4 Interaction mechanisms between (nano)particles and natural organic matter
When (nano)particles interact with NOM the stability of the particles is strongly dependent on the particle intrinsic properties (size, surface charge, etc), the NOM physicochemical properties (structural charge, molecular weight, etc) and concentration (i.e. influence of the particle surface coverage, osmotic pressure, etc) and the water systems properties (pH, ionic strength, etc) (Colvin 2003, Ju-Nam and Lead 2008, Klaine et al. 2008, Nowack and Bucheli 2007, Wiesner et al. 2006).
At low NOM concentration, the adsorption of NOM onto the (nano)particle surface is expected to induce the destabilization of the (nano)particles especially by charge neutralization and bridging mechanisms whereas high NOM concentration usually tend to stabilize the (nano)particles through electrostatic repulsions (after charge inversion) and steric restabilization (Elimelech et al. 1995, Gregory 1987, 1996). Parameters such as the affinity of NOM for the (nano)particle surface, its chemical structure, electronic properties and molecular weight but also the (nano)particle-water interface conditions have key role on the resulting (nano)particle stability (Gregory 1973). All this parameter will strongly influence the layer thickness and the conformation adopted by the NOM adsorbed on the (nano)particle surface (coating).
Charge neutralization is often the predominant mechanism at low NOM concentration when NOM adsorption onto the oppositely charged (nano)particle strongly reduces the electrostatic repulsions between the (nano)particles (electrophoretic mobility close to zero) (Fig. 5a).
Fig. 5: Schematic representation of (nano)particles destabilization by charge neutralization (a) and patch mechanisms (b).
In presence of (nano)particles having a relatively low surface charge, highly structural charged NOM is known to induce local charge heterogeneity. Such "patches" interact with regions of opposite charge from different (nano)particles and promote the agglomeration via patch mechanisms (Fig. 5b). In such a destabilization mechanism the overall (nano)particle surface charge can differ from neutrality.
8
Destabilization of (nano)particles in the presence of NOM is also possible due to bridging effects. The NOM properties and concentration strongly influence the bridging mechanism (Fig. 6a).
To optimize such mechanism NOM should indeed attach several points and be large enough to have free loops and tails to bind other (nano)particles. Such mechanism can be achieved via extended and linear NOM conformations such as polysaccharides. The (nano)particle surface coverage is also playing essential role and the highest probability of bridging is found for half surface coverage (Healy and Lamer 1964). Such mechanism can be largely influenced by several factors. At high ionic strength the (nano)particle surface charge is partially screened and the contact between (nano)particles is facilitated due to the reduction of the electrostatic repulsions. High ionic strength also modifies the NOM conformation (to coils) and reduces the NOM rigidity hence decreasing the importance of bridging mechanism. Optimal NOM bridging mechanism occurs for intermediate (nano)particle charge densities and ionic strength (Matsumoto and Adachi 1998).
Fig. 6: Schematic representation of polymer bridging and depletion agglomeration mechanisms.
Destabilization mechanism of (nano)particles in presence of NOM can also be induced when the conformational entropic restrictions of NOM are not compensated by the energy of adsorption (Jenkins and Snowden 1996). Such agglomeration mechanism, named depletion agglomeration mechanism (Fig. 6b), occurs when NOM has a stronger affinity for the water than for the (nano)particle surface, which can be uncoated or already covered by NOM.
An osmotic pressure, induced when NOM is excluded from the space between (nano)particles, promotes here (nano)particle agglomerate formation. Such scenario is favored for high molecular weight and high concentration of NOM and for (nano)particle diameters less than the effective NOM diameter. (Nano)particle and NOM sizes are key parameters influencing depletion agglomeration mechanism (Burns et al. 2000, 2002, Otsubo 1996, Sperry et al. 1981).
9
Presence of NOM can not only promote the (nano)particle agglomeration but also favor their stabilization. Indeed when (nano)particles are in the presence of a relatively high amount of NOM, electrostatic and steric interactions between the coated (nano)particles can prevent agglomerates formation or promote the agglomerate fragmentation (Fig. 7).
Fig. 7: (Nano)particle steric effects and electrostatic repulsions.
Steric stabilizations depend on the NOM conformational, structural and electronic properties but also on the way they are coating the (nano)particle. If the NOM layer is thick enough it will not only sterically stabilize the (nano)particle. If the NOM carries structural charge, it will prevent the agglomeration due to important electrostatic repulsions between the NOM coating the (nano)particles. High NOM concentration is needed to undergo steric restabilization, but not to high on another hand, as non adsorbed NOM are favoring depletion agglomeration mechanisms.
I.5 Thesis objectives
The main objective of this thesis work is to evaluate the stability of TiO2 ENPs in the presence of two important natural organic matter compounds (humic and exopolymeric substances) by increasing step by step the dispersion complexity (chemical composition, component number, etc). The influence of water pH, ionic strength, electrolyte valency and concentration as well as the NOM concentration were systematically investigated.
Quantification of the energy involved during interaction process between ENPs and natural organic matter was also investigated using isothermal titration calorimetry. Such a novel approach is expected to have a high potential to contribute to a better understanding of the behavior of ENPs in the presence of NOM. Indeed such quantitative information is often missing when ENP interactions with aquagenic compounds have to be investigated.
Experimental results obtained will permit a better understanding of the interaction processes and mechanisms regulating the stability of ENPs in aquatic systems which constitutes an important step in the current knowledge on risk assessment associated to ENPs in the environment.
10
This thesis work was supported by the Swiss National Foundation (200010_152647 and 200021_13240).
I.6 List of papers
Loosli, F., and Stoll, S., 2012, Adsorption of TiO2 nanoparticles at the surface of micron-sized latex particles. pH and concentration effects on suspension stability, published in Journal of Colloid Science and Biotechnology, v. 1, no. 1, p. 113-121
Loosli, F., Le Coustumer, P., and Stoll, S., 2013, TiO2 nanoparticles aggregation and disaggregation in presence of alginate and Suwannee River humic acids. pH and concentration effects on nanoparticle stability, published in Water Research, v. 47, no. 16, p.
6052-6063
Loosli, F., Le Coustumer, P., and Stoll, S., 2014, Effect of natural organic matter on the disagglomeration of manufactured TiO2 nanoparticles, published in Environmental Science:
Nano, v. 1 , p. 154-160
Loosli, F., Le Coustumer, P., and Stoll, S., 2015, Effect of electrolyte valency, alginate concentration and pH on engineered TiO2 nanoparticle stability in aqueous solution, published in Science of the Total Environment, DOI: 10.1016/j.scitotenv.2015.02.037
Loosli, F., Le Coustumer, P., and Stoll, S., 2015, Impact of alginate concentration on the stability of agglomerates made of TiO2 engineered nanoparticles: Water hardness and pH effect, published in Journal of Nanoparticle Research, v. 17 , p. 44
Loosli, F., Vitorazi, L., Berret, J.F., and Stoll, S., 2015, Towards a better understanding on agglomeration mechanisms and thermodynamic properties of TiO2 nanoparticles with natural organic matter, published in Water Research, v. 80 , p. 139-148
Not published yet:
Loosli, F., Vitorazi, L., Berret, J.F., and Stoll, S., 2015, Isothermal titration calorimetry as a powerful tool to quantify TiO2-Humic acids interactions, will be submitted to Environmental Science: Nano, (at the end of June)
Loosli, F. and Stoll, S., 2015, Effect of sodium dodecyl sulfate concentration, pH and divalent electrolytes on the stability of manufactured TiO2 nanoparticles. will be submitted to Journal of Colloid and Interface Science (first draft)
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Behavior, fate, bioavailability, and effects. Environmental Toxicology and Chemistry 27(9), 1825-1851.
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Chapter II
Introduction to colloid stability theory and
instrumental methods
19 II.1 DLVO theory
A theoretical model on the colloid stability was developed by Derjaguin, Landau, Verwey and Overbeek (DLVO) to understand the interactions between colloidal particles and their agglomeration behavior. Derjaguin developed first the principal ideas (Derjaguin 1939) which were extended by a collaboration article with Landau (Derjaguin and Landau 1941) and then publicized in a book by Verwey and Overbeek (Verwey and Overbeek 1948).
The theory was first developed for the interaction between identical particles (symmetric system) which can be seen as the agglomeration of identical particles (homoagglomeration).
Then the theory was extended for the interaction between two different particles and agglomeration of different particles (heteroagglomeration). In the case of particles with very large size disparity the interaction process was similar to the deposition of particle to a planar substrate. In their theory they also developed the calculation of agglomeration rates.
The force F(h) between two colloidal particles with a separation distance h, as suggested by the Derjaguin approximation (Elimelech et al. 1995, Russel et al. 1989), is related to the free energy of two plates VP(h) per unit area.
2 1 where reff is the effective radius which is equal to:
2 where r1 and r2 correspond to the radius of the two particles involved in the interaction process.
The potential energy of interaction, V(h), can thus be calculated by integrating the force between the colloidal particles as shown in Eq. 3.
3 The free energy per unit area V(h) is approximated in the DLVO colloid stability theory by being equal to the sum of the van der Waals (VvdW) and the double layer (Vdl) interactions (Eq. 4).
4
The van der Waals interactions are due to the interaction of rotating and fluctuating dipoles (permanent or induced) of atoms and molecules. They refer to Keesom, Debye and London forces and are most of the time attractive intermolecular interactions. Van der Waals forces, for the interaction between two spherical particles, can be modeled when the separation
20
distance is very small ((h/r) << 1) with Eq. 5 which is a good approximation when considering the van der Waals forces as being short range interactions,
6 5
and where A12 is the Hamaker constant which is dependent on the material properties and defined as
6
where B12 is the London constant for the interaction between material 1 and 2, whereas N1
and N2 represent the number of molecules per unit volume in the material 1 and 2, respectively.
The second interaction taken into account in the DLVO theory is the double layer interaction which are due to the overlap of the diffuse part of the charged particle double layers. A very important assumption made here is that the interaction between charged particles do not depend on the surface potential, but on the potential at the shear plane called the zeta (ζ) potential. Indeed we assume that the ζ potential coincides with the Stern plane, which represents the closest approach of the solution counterions to the surface. The main advantage of such assumption is that the ζ potential can be determined experimentally. Moreover the ζ potential, which is lower than the surface potential, is more relevant than the surface potential as the double layer interaction are predominantly determined by the particle diffuse layers.
The energy of interaction between two spherical particles, exhibiting a low ζ potential value (<50 mV) ζ1 and ζ2, which are at a separation distance equal to h and assuming that the potential in the region of the minimum is equal to the sum of the contribution of isolated particle (linear superposition approximation) is given in Eq. 7,
4 7
where εr and ε0 represent the water relative permittivity and the permittivity of vacuum, respectively, and κ the Debye-Hückel parameter which is the inverse of the Debye length defined as being equal to
2 8 where kB represent the Boltzmann constant, T the absolute temperature, q the elementary charge, NA the Avogadro number and I the solution ionic strength. The ionic strength is equal to
21 1
2 9 where zi represent the valence of the ion i and ci its molar concentration.
The electrostatic double layer interactions (Eq. 7) when working at high ionic strength are decreasing rapidly and are thus relatively short range interactions whereas at low electrolyte concentration they are long range interactions.
The total energy of interaction being the sum of the van der Waals and double layer interactions, V(h) can be approximated for spherical particles by
4 ,
6 10 Three different electrostatic scenarios can be described. The first situation consists in the interaction between identical particles (same size and ζ potential) and refers to a symmetric situation which represents homoagglomeration processes. Then asymmetric situation which represents particle heteroagglomeration process is referring to two important electrostatic scenarios. One is considering the interaction between two particles of similar sizes but opposite charges and the second the interaction between a charged and an uncharged particle.
Symmetric situation referring to homoagglomeration
When combining van der Waals and double layer interactions and looking at the potential energy diagram which represent here the interaction energy between two identical particles as a function of separation distance (Fig. 8), the curve profile is dictated by the van der Waals forces at small distance whereas the double layer forces are predominant for larger distances and low ionic strength.
Fig. 8: Calculated interaction energy as a function of distance separation for a symmetric situation.
The DLVO force profile results in a deep attractive well called primary minimum for small separation distances. At larger separation distances, after passing through a maximum, which occurs for separation distance comparable to the Debye-Hückel length, the profile subsequently passes through a shallow minimum called the secondary minimum.
Depending on the particle charge properties and the distance of separation between them, particles can remain stable if electrostatic repulsion forces are predominant over van der
22
Waals attractive forces. Nevertheless when the interaction profile is attractive, particles approach each other and when being into contact, they stick together and promote the formation of dimers and trimers. Then, as long as the agglomeration process is going on, the particles form larger clusters of particles resulting in particle unstability and sedimentation through the reaction medium.
The particle suspension stability can be quantified by considering the kinetics of formation between particles to form dimers, as shown by
→ 11 where A refers to individual particle monomer and A2 to dimer agglomerates. In Smolukovski theory the investigation of the formation of dimers in the early stage of agglomeration is considered and the dimer rate of formation is given by the rate law
2 12 where N1 and N2 represent the number concentration of particle monomers and dimers respectively, t the time, and k the agglomeration rate coefficient.
At high electrolyte concentration, agglomerate formation rate is almost equal to the diffusion controlled rate as the double layer interactions have only minor effects. Such an agglomeration regime is referred to diffusion controlled agglomeration. At low electrolyte concentration the agglomeration rates are lower due to the electrostatic double layer repulsions, particles having to pass through an energy barrier, and the regime is in this case called a reaction controlled agglomeration. So when the concentration of electrolyte decreases, the agglomeration rate decreases due to the energy barrier resulting in double layer repulsions. The transition between the two regimes is relatively sharp and is referred to the critical coagulation concentration (CCC) which is an important characteristic when investigating agglomeration process as it provides an estimation of the particle stability. The CCC values are strongly influenced by the electrolyte concentration and valency due to the importance of the ionic strength on the exponential decay of the electrostatic interaction (Eqs.
7 and 8)) but also on the specific adsorption of multivalent ions and their influence on the ζ potential value. The CCC shift towards lower values is also reflected by the Schulze-Hardy rule with a dependence roughly following
∝ 1
13 where z is the valence of the electrolyte counterions.
The agglomeration rates are not significantly influenced by the secondary minimum at the exception of interaction between large particle where the energy depth of this secondary minimum can be rather important.
23
Asymmetric situations referring to heteroagglomeration
When investigating the interaction process between two different particles, especially particles with different ζ potentials as the size difference only modifies the effective radius, we can have depending on the particle charge density two important cases which are particles with same charge density but with opposite charges, and interaction between a charged and an uncharged particle.
For oppositely charged particles both electrostatic interactions between the double layers and van der Waals interactions are attractive. Double layer interactions are strongly decreasing when the electrolyte concentrations are high due to the screening charge processes.
For the interaction between charged and uncharged particles the diffuse layer of the charged particle is influenced by the approach of the uncharged particle which provokes its compression. Here the charge regulation effects are extremely important in comparison to charge regulation effects during the interaction between two charged particles.
For asymmetric situations, agglomeration processes may lead to possible scenarios: two homoagglomeration processes which lead to two different particle dimers AA or BB. One heteroagglomeration process which lead to asymmetric dimers AB.
→ 14 → 15 → 16 Description of short-time kinetic of agglomeration and stability ratio
For a one-component colloidal dispersion the agglomeration state is usually described in term of cluster-size distribution, ci(t), i.e., the concentration of clusters composed by i particles.
In dilute systems, the Smoluchowski agglomeration (Eq. 17) equation can quite successfully describe the time evolution of agglomeration processes (Lopez-Lopez et al. 2009, von Smoluchowski 1917).
1
2 , 17
where kij represents the absolute agglomeration rate constant controlling the reaction between the i- and j-mer. If the agglomeration process starts with an imposed monomeric concentration, the monomer-monomer (formation of dimer) agglomeration rate, k11, is the most relevant kinetic constant.
For a two-component (A and B) colloidal dispersion, the formation of dimers can lead to the formation of A-A, B-B or A-B, and each of these reaction is characterized by its own absolute agglomeration rate constant. Indeed A-A and B-B dimer formation are characterized by the absolute homoagglomeration rate constant kAA and kBB, respectively, whereas the formation
24
of dimer A-B is characterized by the absolute heteroagglomeration rate constant kAB. By assuming that the frequency of encounters between two particle is only dependent on the relative particle concentration (Hogg et al. 1966) an effective dimer formation rate constant, keff, can be defined as
1 2 1 18
where x represent the relative concentration of A type, being equal to
19
Once the kinetic rate constants, in the early stage of the agglomeration process, are determined, the calculation of the attachment efficiency (α), which is the inverse of the stability ratio (W), can be obtained
1
20 where kslow and kfast refer to determination of kinetic rate constant for concentration below and above the CCC, respectively.
II.2 Instrumental methods II.2.1 Coulter Counter
The Coulter counter is a technique to determine the size distribution of particles by measuring the variation of impedance with electrodes situated before and after an orifice in which particles are going through as shown in Fig. 9.
Fig. 9: Schematic overview of a Coulter Counter.
25
This variation of impedance is proportional to the volume displaced by the particle passing through the aperture (Eq. 21) and the time dependent intensity of the pulse measured by the instrument is then related to the equivalent spherical size of the particle.
∆ 8
3 1 21 where ΔR represent the change of resistance (similar to impedance but for direct current), d the diameter of a sphere, D the diameter of the aperture, and ρ0 and ρ the resistivity of the fluid in the absence of particle within the orifice and the resistivity in the presence of a particle within the orifice, respectively.
This high resolution and accuracy counting technique has the benefit to be unaffected by the particle intrinsic properties (shape, refractive index, etc) and able to provide size distribution for particles in the range 0.4 to 1'200 μm (by using tubes with different aperture diameters).
This technique was used in this work to investigate the influence of TiO2 ENP concentration on the agglomeration kinetic rate of latex particle (1 μm) by measuring the decrease number of latex particle monomers as a function of time (Chapter III).
II.2.2 Dynamic light scattering and laser Doppler velocimetry
Dynamic light scattering (DLS) is an instrumental technique to determine the size distribution of particles ranging from 1 nm to 10 μm. The time dependent fluctuation of the intensity of the light scattered by the particles undergoing Brownian motion is measured. The particle translational diffusion coefficient (D), which is determined by the help of a correlation function, is then related to the particle hydrodynamic diameter (dh) via the Stokes-Einstein equation (Eq. 22) with assumption of spherical particle and no interaction.
3 22 where kB represent the Boltzmann constant, T the absolute temperature and η0 the solvant dynamic viscosity. The schematic overview of a DLS instrument is shown in Fig. 10.
Fig. 10: Schematic view of a DLS instrument.
