Colloidal Aggregation in a Binary Mixture near Its Demixing Temperature
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COLLOIDAL AGGREGATION IN A BINARY MIXTURE NEAR ITS DEMIXING TEMPERATURE MICHAEL L. BROIDE* AND DANIEL BEYSENS** *Lewis & Clark College, Department of Physics, Portland, OR 97219, USA. "**Service de Physique de rEtat Condensd, Centre d'Etudes de Saclay, F-91191 Gif-sur-Yvette Cedex, France. ABSTRACT . We have investigated temperature-induced aggregation of silica colloids in a binary mixture of 2,6-lutidine and water. Aggregation occurs in the one-phase region of the binary mixture and can be reversed by cooling the sample. Using low-angle light scattering, we have determined the growth kinetics and aggregate structure for this system. We find that the time dependent scattered intensity is well described by I(q, t) = M(t)/[1 + (qlR(t)) 2/10] 2 , where the weight-average cluster mass MI(t) - t, and the average cluster radius R(t) - t1/ 3. Our results indicate that this experimental system generates compact clusters, M - 0 3 , and that the rate of aggregation is diffusion limited. The dense, non-fractal, structure we observe suggests that the bonds between the silica spheres are not rigid, and supports the hypothesis that aggregation is induced by the adsorption of lutidine on the silica surface. INTRODUCTION Colloidal aggregation plays a role in many chemical, physical and biological processes. Generally, aggregation is induced by changing the pH or salt concentration of the colloidal suspension [1]. For a wide range of experimental systems, the structure of the aggregates is fractal. That is, the aggregate's mass is related to its radius by M - Rdf, where df is the fractal dimension. Two limiting regimes of cluster growth have been identified [2]: Diffusion-limited cluster aggregation (DLCA), which occurs when every collision between diffusing clusters results in the formation of a bond, and reaction-limited cluster aggregation (RLCA), which occurs when only a very small fraction of collisions between clusters results in the formation of a bond. For DLCA, df = 1.75, and for RLCA, df = 2.10. RLCA generates denser clusters than DLCA because the aggregates are able to interpenetrate one another more fully before forming a bond. We stress, however, that even for the extreme case of RLCA, the clusters are ramified, since df is well below 3, the dimension for a dense-packed structure. In this paper, we describe an experimental system which, in contrast to previously studied systems, generates dense-packed aggregates. The system consists of silica microspheres suspended in a binary mixture of lutidine and water. As has been previously demonstrated [3], the spheres aggregate in the one-phase region of the binary mixture above a threshold temperature Ta, which depends on lutidine concentration. The same phenomenon has also been observed using polystyrene spheres [4]. A distinguishing feature of this aggregation process is that it is reversible: if the temperature is reduced below Ta, the spheres completely disaggregate. The exact mechanism that triggers aggregation is still not known, and little theoretical work has been don
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