Microemulsions Containing Brine, SDS, Pentanol or Heptanol, and Dodecane or Hexane: Structural Information Derived from Electrochemical and Fluorescent Probe Studies

Abstract

Three microemulsion systems containing brine, sodium dodecylsulfate, pentanol or heptanol, and dodecane or hexane have been investigated. Results of experiments including conductivity and viscosity measurements, electrochemical diffusion coefficients and fluorescent probe studies have been gathered and compared in order to gain additional understanding of the microemulsion structure. The results show a good correlation between the information provided by each technique. The diffusion coefficients of hydrophilic hydroquinone and hydrophobic ferrocene, obtained from the Levich equation at the rotating disc electrode, vary as the self-diffusion coefficients of water and dodecane, respectively and are consistent with the results obtained by other workers from tracer or NMR self-diffusion studies. The systematic fluorescence analysis of the polarity sensed by pyrene and the microviscosity experienced by dipyrenylpropane provides insight into the microscopic properties of the oil domains in which the fluorescent probes are assumed to be located. The properties of microemulsions belonging to the large monophasic area of the brine/SDS/pentanol/dodecane system give evidence for the well known O/W, bicontinuous and W/O structures; on the contrary, the microemulsions of the small lens included in the oil-rich region of the same pseudo-ternary phase diagram exhibit less defined properties which are reminiscent of structures with simultaneously water and oil continuous phases. In the W/O area of the brine/SDS/heptanol/dodecane system, the effective viscosity experienced by dipyrenylpropane is much lower than the bulk viscosity of the microemulsions. Similarly, the diffusion coefficients of ferrocene, measured by electrochemistry, correlate through the Stokes-Einstein equation, with the microviscosity derived from the fluorescence experiments. Microemulsions of the lenticular area of the brine/SDS/pentanol/hexane system seem to undergo the same structural transitions as microemulsions of systems which present a single monophasic area.