Low Frequency Dielectric Spectroscopic Analysis of Reverse Microemulsions

Abstract

Low frequency dielectric spectroscopy in the MHz down to 0.1 mHz range, was used to investigate and characterise ternary reverse microemulsions, micellar systems and their individual components. Anionic surfactant sodium bis (2-ethylhexyl) sulfosuccinate (AOT), oil isopropyl myristate (IPM) and deionised water were chosen as they readily form reverse microemulsions and have been investigated with other physicochemical techniques. Low frequency dielectric spectroscopy is a non-invasive technique, able to probe the electrical properties of materials without isotopic labelling. The dielectric response of the microemulsions stabilised with 20% w/w AOT displayed a small anomalous increase in the conductance and a decrease in the capacitance to negative values at frequencies below the characteristic intracluster frequency ω1, but above the commencement Maxwell-Wagner interface charging process. Spectroscopic measurements of the individual components showed IPM exhibited similar anomalous characteristics at frequencies below 1Hz. Increases in conductance are indicative of structural change, and negative capacitances have been reported to occur in animal cells such as neurons undergoing changes in membrane potential during ion flux. Hence the response of IPM displayed characteristics of biological membranes able to undergo structural changes resulting in an increased conductance in specific frequency ranges. The response of the anomalous features were fitted using the Dissado-Hill cluster model adapted to include the linearised frequency dependant conductance term G(ω). This term replaced the static conductance component in the bulk parallel conductance–capacitor pair of bulk–barrier series circuit model of the samples investigated. The anomalous features, together with the changing intracluster relaxation time τ1 were used to track the micellar to microemulsion transition as the molar concentration ratio W0=[H2O]/[AOT] was increased 0<W0<26. The presence of microemulsion aggregates indicated by the slowing down of the τ1 response time at W0>9 was confirmed by the detection of particles with radii 12 to 29 nm using light scattering.

Date of Award	2017
Original language	English
Awarding Institution	King's College London
Supervisor	Margaret Lawrence (Supervisor) & Richard Harvey (Supervisor)