What is the origin of conductivity in water-poor reverse micelles?

Abstract

Conductivity of water in oil microemulsions as well as reverse micelles of anionic surfactants depend on cations as charge transporters. We first use the versatile molecular system toluene/diethylhexylphosphate H_xNa_1−xDEHP/water to investigate the domains in the phase prism in which four molecular mechanisms of conductivity are identified. The reduced molar conductivity varies over six orders of magnitude. In the regime of “reverse micelles”, where all water in the organic phase is bound as first layer of hydration of head-groups, the dismutation mechanism, discovered by HF Eicke, dominates. In the w/o microemulsion region, we identify three more conductivity regimes occurring in different regions of the phase diagram. Beyond the dynamic and static percolation, we identify also a more elusive regime: the curvature frustration regime is characterized by a decrease in molar conductivity observed upon addition of water. This anti-percolation regime is due to curved film packing frustration that is at the origin of an increase of tortuosity. The HDEHP/toluene/water system is the first molecular system for which the four conductivity regimes can be easily observed at room temperature. We also identify the last three conductivity regimes in a microemulsion based on AOT. The single-phase inversion channel, studied as a function of temperature, is limited by Winsor II and Winsor I phase separation. In this domain, the three regimes that can be found are dynamic percolation, anti-percolation as well as static percolation. Therefore, we propose that all four different mechanisms are found in ternary w/o microemulsions containing cations as charge carriers.