Effect of alcohols in AOT reverse micelles. A heat capacity and light scattering study

Apparent molar heat capacities at 25 degrees C of a series of 1-alcohols and three branched alcohols were determined in mixtures of the type alcohol + (p wt % AOT + n-decane) at R = 0 and 10, R being the water-surfactant molar ratio [W]/[S] For methanol and 1-hexanol, the measurements were done for different p values as a function of alcohol concentration. For all the other alcohols p = 5. Heat capacities for the binary (AOT + nC(10)) and the ternary (AOT + W + nC(10)) mixtures, as well as for 1-hexanol in a 5 wt % solution of dioctyl succinate, were also measured at 25 degrees C. For all alcohols + AOT + n-decane, kinematic viscosities and dynamic light scattering (DLS) were measured at 25 degrees C for R = 10 and p = 13 as a function of alcohol concentration. DLS was also measured for the ternary mixture AOT + W + nC(10) with R = 10 at 25 degrees C. A reasonable molecular picture of the alcohol-AOT interactions in the presence and absence of reverse micelles emerges from the experimental DLS and heat capacity results, the latter having been analyzed within the Treszczanowicz-Kehiaian model framework. In the absence of reverse micelles, ail alcohols form complexes with the free AOT molecules in the solution, a process that competes with the alcohol self-association. The alcohol-AOT complex is most probably formed via an interaction between the hydroxyl group of the alcohol and the ionic head of AOT. When reverse micelles are present, two behaviors were found: (i) Methanol and ethanol are located in the micelle water pool; at low AOT concentration these alcohols only interact with water, but at higher AOT concentration they also form a complex with AOT molecules at the micellar interface. (ii) For butanol, longer I-alcohols, and the three branched alcohols studied here two different processes occur: AOT molecules are withdrawn from the micelles to be complexed with alcohol molecules in the bulk of the solution, and alcohol molecules penetrate the micellar shell, where they also form a complex with AOT.