EFFECTS OF ADDITIVES ON THE INTERNAL DYNAMICS AND PROPERTIES OF WATER AOT ISOOCTANE MICROEMULSIONS

The influence of several additives, frequently present in reactions carried out in microemulsions, upon the properties of the system water/AOT/isooctane has been investigated. Electrolytes (sets, HCI, NaOH, guanidinium chloride) all hinder the electrical percolation phenomenon and decrease both the microemulsion viscosity and its capacity of water solubilization. Their effect seems due to the association of cations to the interface. This association increases the natural negative curvature of the surfactant, hinders the exchange of materials between droplets, increases the interfacial rigidity, and decreases attractive interactions among droplets. The efficiency of several electrolytes in promoting these effects is discussed. A similar behavior is observed when microemulsion water is replaced by D2O; this is related to the lower zero-point energy of the hydrogen bonds established between solvating water and AOT for the deuterated solvent. Another group of additives is formed by ureas, thioureas, formamides, and ethylene glycol. All these additives favor the electrical percolation phenomenon, which can be reached at unusually low temperatures with moderate concentrations of additives. These additives also increase microemulsion viscosity and reduce its water solubilizing capacity. Comparison of the H-1 NMR chemical shifts corresponding to microemulsion water under different conditions, in the presence and absence of these additives, indicates that the additives used do not significantly alter the estate of water in the microemulsion. In addition, the trends observed when comparing ureas with thioureas or when increasing the degree of methylation of the additives also suggest that their effect is related to the interfacial association of the additives to the AOT head groups. Binding of the additives to the interfacial layer reduces the curvature of the surfactant, thereby favoring the opening of the AOT layers during interdroplet collisions. This association also results in a greater disorder in the surfactant environment, which could help to promote attractive interdroplet attractions. Finally, the influence of several amines was also investigated but is more confused. Whereas morpholine favors electrical conductivity, other amines (dimethylamine, pyrrolidine, etc.) hinder electrical percolation.