Effects of solidification of the oil phase on the structure of colloidal dispersions in cyclohexane

The liquid-to-solid transition of the alkane-continuous phase of a dilute surfactant-stabilized particle or droplet dispersion can be induced in a reversible manner without destabilizing the colloid by pressure and/or temperature changes. The structural changes have been studied by small-angle neutron scattering (SANS) over a range of pressure (1-600 bar) and temperatures (3-20-degrees-C). The SANS results indicate that there are different levels of structure in the solidified system in which a solid alkane coexists with fluid cluster domains. The clusters show large-scale structural correlations of order 5-50 mum; within these clusters the particles are in close contact, so that under certain conditions, e.g. high pressure, the stabilizing surfactant layers of adjacent particles are interdigitated. The distance between particle centers, and therefore the degree of surfactant interdigitation, can be readily varied by the application of pressure. An interpretation of the SANS results is given in terms of the effects of temperature and pressure upon the osmotic pressure of the concentrated solution of particles/droplets. The analysis provides an estimate of the interparticle pair potential energy between adjacent particles in a cluster as a function of separation.