Colloids volume restriction effects

The volume restriction effect as discussed in this paper was proposed several years ago by Asakura and Oosawa (12,13). Their theory accounted for the instability observed in mixtures of colloidal particles and free polymer molecules. Such mixed systems have been investigated experimentally for decades (14-16). However, the work of Asakura and Oosawa did not receive much attention until recently (17,18). A few years ago, Vrij (19) treated the volume restriction effect independently, and also observed phase separation in a microemulsion with added polymer. Recently, DeHek and Vrij (20) have reported phase separation in non-aqueous systems containing hydrophilic silica particles and polymer molecules. The results have been treated quite well in terms of a "hard-sphere-cavity" model. Sperry (21) has also used a hard-sphere approximation in a quantitative model for the volume restriction flocculation of latex by water-soluble polymers. 

Fig. 3 Steric stabilization of lyophobic colloidal particles. The particles repel one another because of volume restriction and osmotic pressure effects.

Protective agents can act in several ways. They can increase double layer repulsion if they have ionizable groups. The adsorbed layers can lower the effective Hamaker constant. An adsorbed film may necessitate desorption before particles can approach closely enough for van der Waals forces to cause attraction, or approaching particles may simply cause adsorbed molecules to become restricted in their freedom of motion (volume restriction). The use of natural and synthetic polymers to stabilize aqueous colloidal dispersions is technologically important, with research in this area being focused on adsorption and steric stabilization [75-80]. 

Clearly, there are also numerous other areas which could be included in a chapter devoted to the application of surface and colloid chemistry in pharmacy, in particular relating to the surface properties of dry formulations, such as spray or freeze-dried powders, wettability of drug crystals, etc. Flowever, in order to harmonize with the scope of the volume as a whole, these aspects of surface chemistry in pharmacy will not be covered here. Furthermore, even with the restriction of covering only wet systems, the aim of the present chapter is to illustrate important and general effects, rather than to provide a complete coverage of this vast field. 

Fig. 10. Calculated free energy barrier for homogeneous crystal nucleation of hard-sphere colloids. The results are shown for three values of the volume fraction. The drawn curves are fits to the CNT-expression Eq. (1). For the identification of solid like particles we used the techniques described before. The cutoff for the local environment was set to Vq = 1.4

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