Pressure-induced structural changes in water-in-propane microemulsions

Abstract

The effect of pressure and temperature on the stability and structure of low-density water-in-oil microemulsions in propane (ρ≈ 0.48 → 0.56 g cm^–3) has been studied with three different surfactants Aerosol-OT (anionic), DDAB (cationic) and C₁₂E₅(non-ionic). Small-angle neutron scattering (SANS) measurements were made using a specially constructed high-pressure optical cell (maximum pressure 500 bar). For all three systems, the phase behaviour with pressure follows a general pattern; at high pressures (400 bar) stable transparent L₂ microemulsion phases form, whilst at lower pressures (e.g. 70 bar for the AOT system) a phase instability boundary is reached. The transition pressure and nature of the phase separation depend both on the temperature and surfactant type. SANS measurements followed the droplet sizes and interactions on approaching the phase boundary at a constant temperature. For ionic surfactants AOT and DDAB at 25 °C and [water]/[surfactant]w= 20 the mean droplet radius of a Schultz distribution, R_av≈ 40 Å and is essentially independent of pressure. A small increase (ca. 14%) in radius is observed with DDAB on decreasing pressure from 400 to 35 bar where R_av≈ 38 and 42 Å, respectively. For these two surfactants the phase transition is the haze point, driven by strong attractive droplet interactions. However, for the C₁₂E₅ microemulsion a solubilisation phase boundary is encountered at cA. 120 bar and 55 °C. In this case there is no evidence for strong pressure-dependent interactions at the transition and the polydispersity is higher than for the ionic surfactants. However, in contrast to DDAB, R_av for C₁₂E₅ decreases at lower pressures, cA. 70 Å bar at 400 bar and 54 Å at 120 bar. This observation is consistent with a decrease in the concentration of monomer non-ionic surfactant (c.µ.c.) at lower pressures. The results demonstrate that the mechanisms of pressure-induced phase transitions in propane-continuous microemulsions depend on surfactant type.