CSIRO PUBLISHING www.publish.csiro.au/journals/ajc Aust. J. Chem. 2003, 56, 1081–1089 Stagnant Layer Conduction in Surfactant-Stabilized Hexadecane Emulsion Systems Measured by Electroacoustics Alex M. Djerdjev, A James K. Beattie A,B and Robert J. Hunter A A School of Chemistry, University of Sydney, Sydney 2006,Australia. B Author to whom correspondence should be addressed (e-mail: j.beattie@chem.usyd.edu.au). Previously reported zeta-potentials calculated from the electroacoustic behaviour of sodium dodecyl sulfate (SDS) stabilized hexadecane emulsion droplets show certain anomalies. These can be resolved when electrical conduction in the stagnant layer behind the shear plane is included in the analysis. If stagnant layer conduction is ignored the addition of salt causes the apparent droplet size to increase and the magnitude of the zeta-potential to show a maximum. When stagnant layer conduction is included the dynamic mobility spectra can be fitted to a constant size distribution independent of the salt concentration with zeta-potentials that decrease as expected with increasing electrolyte concentration. Increasing SDS concentration, before the homogenization process, causes a decrease in droplet size and an increase in the total surface conductance to a constant value corresponding to the saturation of the surface with SDS. It is shown that the surface conductance and particle size distribution of hexadecane at any given volume fraction are functions of the concentration of SDS and the oil volume fraction. The zeta-potential changes log-linearly with added electrolyte and is independent of the SDS concentration or oil volume fraction used during the emulsification process. Manuscript received: 21 January 2003. Final version: 17 July 2003. Introduction When electrokinetic measurements are made at low salt con- centration in systems with a reasonably high surface charge (say greater than 5 μC cm −2 ) the calculated zeta-potentials so obtained are often lower in magnitude than is expected. Since the effect decreases in significance with increase in the concentration of indifferent electrolyte, the usual result is a maximum in the plot of |ζ| against indifferent electro- lyte concentration. [1–4] The maximum has been explained in terms of co-ion adsorption and surface hairiness resulting in a shift (away from the surface) of the shear plane at low con- ductivity. Co-ion adsorption of halides at the inner Helmholtz plane of polystyrene sulfate surfaces does not explain the maximum in |ζ|. [5–7] In some cases, [8] it can be the result of the selection of the wrong root to the equation for mobil- ity as a function of zeta-potential. That situation can now be unambiguously resolved using electroacoustic measurements because the high-frequency dynamic mobility so obtained does not suffer from this ambiguity. [9] There are, however, a significant number of situations in which the maximum is best explained as the result of the occurrence of electrical conduction processes in the stagnant layer behind the shear plane. The process is most obvious at low electrolyte concentrations and has two effects—the enhanced conduction lowers the electrophoretic mobility for a given zeta-potential and, if not properly taken into account, results in a spuriously low value for |ζ|. Also, conduction measurements sample charge, which is not sampled by the normal electrokinetic techniques. The disparity between estimates of zeta-potential from conduction experiments and the more standard electrokinetic methods (electrophoresis and streaming potential for example) can then become quite marked (as much as 300% or so). Although it cannot be assumed to be present in every sys- tem, and indeed it is not, conduction in the stagnant layer behind the shear plane has been inferred in a wide range of systems, including some latices, [5,10] clays, [11,12] oxides, [13] and Gram-positive bacteria. [14] Recently, [15] it was shown by high-frequency conduction and electroacoustic measure- ments that stagnant layer conduction was also present in emulsion systems of perfluoromethyldecalin and hexadecane stabilized with SDS. Its effect on the dynamic mobility, par- ticularly at low conductivity, was significant and adversely affected the interpretation of the results unless it was taken into account. Fortunately, that study [15] showed that conduc- tion behind the shear plane can be detected and corrected for by the use of electroacoustics alone. A recent electroacoustic study from this laboratory [16] on hexadecane emulsion systems between 5 and 50 vol-% reported unusual size and zeta-potential behaviour. At con- stant SDS concentration the hexadecane droplets appeared to increase in size with volume fraction, whereas the more vis- cous sunflower oil droplets decreased in size with increasing volume fraction. The zeta-potential was also found to become © CSIRO 2003 10.1071/CH03013 0004-9425/03/101081