RATE OF WATER-UPTAKE BY WATER-IN-OIL MICROEMULSIONS IN RELATION WITH THE PROPERTIES OF THE AMPHIPHILIC FILM

We examine in this work the possible correlations between the rate of water solubilization in inverse microemulsion systems (AOT/n-decane/H2 O, with different additives) and the fluidity or rigidity of the dropl et amphiphilic film. Conductivity measurements are used to characteriz e the existence (or absence) of droplet percolation. The stopped-flow technique with turbidity detection has allowed us to perform fast inje ction of small amounts of water and to follow the kinetics of water up take. A careful control of the flow rate was found necessary to obtain relevant data. A stirred-cell with variable stirring rates was also u sed for additional measurements. Different additives known for their e ffect on the droplet behavior (either favoring or retarding droplet co alescence) were considered in this study: NaCl (0-0.3 M); POEG (with m olecular weights 2000 and 10000); n-alkanols (from l-butanol to l-deca nol). The turbidity changes with time were best fitted with biexponent ial functions, where the fast process characterized by a first-order r ate constant k(1) (s(-1)) was assumed to control the collisions betwee n the dispersed water droplets and the water-in-oil microemulsion drop lets. The significance of this rate constant is analyzed, taking into account the part played by the distance from the phase boundary. The r esults demonstrate a systematic tendency for hi to be larger for the p ercolating systems compared to the nonpercolating ones. The possible r elation of k(1) with the amphiphilic film rigidity is discussed.