DYNAMICS OF THE DEVELOPMENT OF STRUCTURES IN COLLOIDS AND BROWNIAN MOTION

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DYNAMICS OF THE DEVELOPMENT OF

STRUCTURES IN COLLOIDS AND BROWNIAN

MOTION

H. Übelhack, F. Wittmann

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, Dkcembre 1976, page C6-269

DYNAMICS OF THE DEVELOPMENT OF STRUCTURES

IN

COLLOIDS AND BROWNIAN MOTION

H. J. UBELHACK and F. H. WITTMANN

Abteilung fiir Werkstoffphysik, Technical University of Munich, Germany

RBsumB. - Le mouvement d'une particule selon Brown dans une suspension cause un Blargisse- ment de la ligne d'absorption resonante. Au moyen de cet effet le developpement des structures thixotropiques a et6 6tudi6. La ligne de resonance devient plus Btroite selon une fonction exponen- tielle et on re~oit un 6quilibre qui est dependant de la concentration du systkme. I1 est demontr6 que le developpement de la structure thixotropique peut 8tre lie avec la viscosite des couches minces de l'eau entre les particules. Un avantage essentiel de la spectroscopie de Mossbauer est le fait que l'influence du temps peut 6tre 6tudiB sans dkranger le systkme.

Abstract.

-

The development of thixotropic gel-like structures in concentrated suspensions has been studied by observing the broadening of the resonance lines of the Mossbauer spectrum caused by the diffusive Brownian motion of the suspended particles. An exponential decay of the line width to a concentration dependent equilibrium value was found. The gradual build-up of structures in the thin separating water films being responsible for the complex stabilisation of thixotropic systems thus could be characterized by the actual viscosity of the water films. An essential advantage of the method described here is the non-destructive way in which the time dependent process is studied.

1. Introduction.

-

Near certain solid water inter- faces an ordered structure of adsorbed water films is built up [l]. In colloidal systems such as concentrated hydrosols or suspensions this phenomenon may contri- bute to a complex stabilizing interaction leading to a more or less rigid gel like structure. This structure may be destroyed by mechanical treatment thus liquifying the system again. Systems exhibiting this behaviour are usually called thixotropic systems. The specific rheo- logical characteristics of thixotropic materials are of great interest in many areas of applied technology e. g. in paint technology.

The time necessary for the rearrangement of thixotropic structures depends very much on the actual system and may vary between some seconds and several days. If the transition from the disordered to the ordered state is slow enough it can be studied in a nondestructive way with the help of Miissbauer spectroscopy. During the transition period the time dependence of the viscosity of the thin water films separating the suspended colloidal particles strongly influences Brownian Motion which can be observed by a corresponding broadening of the resonance lines of the Mossbauer spectrum.

2. Interaction between colloidal particles.

-

The complex interaction of hydrophilic colloidal particles suspended in water consists of a superposition of several basic types of interactions. Major types of interaction may be mentioned here : a) particles are

attracted by van der Waals forczs which depznd on the distance between interacting surfaces, the surface energy, and the dielectric properties of the separating liquid, b) another attractive interaction is created by fluctuating chains of H-bonds within separating ordered films, and c) particles immersed in water will be charged by dissolution or adsorption processes and the surface charge will be compensated by a diffusive layer of ions. The electrically charged particles experience Coulomb interaction as soon as diffusive layers overlap. d) Near the solid-water interface water molecules become orientated by surface forces and structured water films are built up which may be as thick as 100

A.

The chemical potential of water molecules within these films will be lower than in bulk water and hence the film tends to expand creating a repulsive force. The combi- nation of type c) and d) interactions is generally called disjoiningpressure [2]. e) In addition to above mention- ed interactions in colloids some types of colloidal particles may be linked by primary bonds which in most cases will lead to the formation of a stable three dimen- sional network.

A quantitative description of the complex interaction of combinations of type a), c) and d) is presented within the DLVO-theory (Derjaguin, Landau, Verwey, Overbek) [3,4]. More detailed studies of single compo- nents of the total interaction may be found in the literature. The present paper is mainly concerned with the structural component d ) of the disjoining pressure and the formation of chains of H-bonds b).

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C6-270 H. J.

ELH HACK

AND F. WITTMANN

3. Brownian motion. - The resonance lines of a Mossbauer spectrum are broadened if colloidal particles being used as source or as absorber perform Brownian Motion. The Brownian Motion may be characterized by a diffusion coefficient D which governs the line broadening [5].

If the diffusive motion of Brownian particles is govern- ed by the Einstein-Stokes relation the diffusion constant D can be expressed as a function of particle radius R and the viscosity q of the liquid in which they are immersed :

Viscous flow of liquids may be described as an activa- tion energy controlled process [6]. From the change of the linewidth a change of activation energy Q can be evaluated :

4. Experimental.

-

In a suspension of hydration products of cements calcium-ferrite hydrates (CaOFe,O, .(H,O),) having a mean radius of about 100 ,.& are fixed to bigger agglomerations (103-104 ,.&)

and they may serve as marker particles for FeS7 Moss- bauer spectroscopy. The Mossbauer spectrum of the marker particles consists of a quadrupol split line

(IS = 0.62 mm/s against Na,Fe(CN),

.

N0.2 H,O,

AEQ = 0.7 mmls). Spectra have been recorded using a constant acceleration drive system with a ~o~~ in Pt source. The line width has been determined by a fit with two Lorentzian lines.

Two samples with different waterlhydrate ratio have been studied :

-

suspension (A) with 60 Vol

%

water and 40

%

hydrates and

-

suspension (B) with 67 Vol

%

water and 33

%

hydrates.

The agglomerations (microgels) are known to have 50

%

microporosity. Therefore a corresponding pro- portion of the water is taken up by the gel and a remai- ning 20 Vol

%

for suspension (a) and 37 Vol

%

for suspension (B) can be considered t o be free water. Hence from geometrical considerations it becomes clear that the particles are nearly in contact and are separated by very thin water films onIy.

The samples have been sealed in a PVC foil to pre- vent loss of water during the experiment. Before testing they have been stored for a week so that mechanical equilibrium could be reached. At t = 0 the thixotropic structure has been destroyed by mechanical treatment.

A

pronounced and immediate increase of the line width

could be observed. The equilibrium value was reached again within some days (see Fig. 1 and 2). The equilibrium value of line width

r

has been found to be different for the two samples under investigation. The

FIG. 1.

-

Time dependent change of the width of the resonance lines as measured on sample (A). At t = 0 the ordered structure

has been mechanically destroyed.

FIG. 2.

-

Time dependent change of the width of the resonance lines as measured on sample (B). At t = 0 the ordered structure

has been mechanically destroyed.

characteristic time for the exponential decay was nearly the same for both thixotropic suspensions, i. e. 1.7 days for suspension (A) and 2 days for suspen- sion (B). The extension of the measurements to higher water contents (80 Vol

X)

shows, that this applies only to the concentrated systems. In dilute suspensions the undisturbed state is reached more slowly because geometrical rearrangement becomes increasingly important.

5. Discussion.

-

In colloidal suspensions with a low waterlsolid ratio the thin liquid films separating individual gel particles may exhibit a high degree of ordering. These structured layers are often formed in the vicinity of solid surfaces. By mechanical treatment relative movement of colloidal particles may be caused.

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DYNAMICS OF THE DEVELOPMENT OF STRUCTURES IN COLLOIDS AND BROWNIAN MOTION C6-271

and in this way Brownian motion is increasingly hindered 171.

It has been shown that Mossbauer spectroscopy may be used to study the rearrangement of thixotropic colloidal structures [8, 91. The systems studied in this investigation, once being disturbed, reach their equilibrium position according to an exponential law. The characteristic relaxation time depends on the water content. As may have been anticipated thicker liquid films require more time to become rearranged. The major advantage of the approach described here is the non-destructive way in which the gradual decrease -

of Brownian motion in collodial systems can beystudied.

nG, 3. - Increase of the activation energy of viscous flow

As above viscous may be as function of time after the thixotropic structure has been

rized by introducing a n activation energy. As the degree mechanically destroyed.

of order within thin liquid films increases the activation energy increases as well. The average potential barrier

of the elementary process of viscous flow increases. By way are shown on figure 3. It may be concluded that applying eq. (3) the time-dependent ncrease of activa- thixotropic systems can be successfully investigated tion energy may be calculated. Results obtained in this with the help of Mossbauer spectroscopy.

References

[l] DROST-HANSEN, W., Ind. Eng. Chem. 61 (1969) 10. [5] SINGWI, K. S., SJOLANDER, A., Phys. Rev. 120, Nr. 4 (1960)

. .

[2] DERJAGUIN, B. V., J. Coll. Interf. Sci. 49 (1974) 249. 1093.

[6] FRENKEL, J. I., Kinetic Theory of Liquids (Dover) 1945. [3] DERJAGUIN, B. V., LANDAU, L., Acta Phys. Chim. USSR 14 _l71_HAUSER,_{E. A., Xolloid-Z.}₉₈_{(1929) 57.}

(1941) 633. _[S]_HAN~EL,_{D. and SEVSEK,}_F.,_A_{study of thixotropic 8-FeOOH} [41 VERWEY, E. J. W., OVERBECK, J. Th. G.,] Theory 'of the Sta- by Mossbauer Effect (to be published).

bflity of Lyophobic Colloids (Elsevier, Amsterdam) [9] ~ ~ B E L H A C K , H. J., PhD-Thesis, Technical University Munich