Variations in Structure Explain the Viscometric Behavior of AOT Microemulsions at Low Water/AOT Molar Ratios

TL;DR

This study investigates how structural variations in AOT microemulsions at low water/AOT ratios influence their viscosity behavior, revealing that elongated, cylindrical droplet structures are an intrinsic feature affecting viscosity maxima.

Contribution

It demonstrates that non-spherical, cylindrical droplet structures are inherently formed in AOT microemulsions at low molar ratios, explaining viscosity variations through structural analysis.

Findings

Cylindrical droplet structures are observed at high concentration regimes.

The viscosity maximum correlates with the formation of elongated droplets.

Salt addition suppresses the viscosity maximum and alters droplet structure.

Abstract

The viscosity of AOT/water/decane water-in-oil microemulsions exhibits a well-known maximum as a function of water/AOT molar ratio, which is usually attributed to increased attractions among nearly spherical droplets. The maximum can be removed by adding salt or by changing the oil to CCl$_4$. Systematic small-angle X-ray scattering (SAXS) measurements have been used to monitor the structure of the microemulsion droplets in the composition regime where the maximum appears. On increasing the droplet concentration, the scattering intensity is found to scale with the inverse of the wavevector, a behavior which is consistent with cylindrical structures. The inverse wavevector scaling is not observed when the molar ratio is changed, moving the system away from the value corresponding to the viscosity maximum. It is also not present in the scattering from systems containing enough added salt to essentially eliminate the viscosity maximum. An asymptotic analysis of the SAXS data, complemented by some quantitative modeling, is consistent with cylindrical growth of droplets as their concentration is increased. Such elongated structures are familiar from related AOT systems in which the sodium counterion has been exchanged for a divalent one. However, the results of this study suggest that the formation of non-spherical aggregates at low molar ratios is an intrinsic property of AOT.