Stagnant Layer Conduction in Surfactant-Stabilized Hexadecane Emulsion Systems Measured by Electroacoustics

Abstract

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.

Formation and surface-stabilizing contributions to bare nanoemulsions created with negligible surface charge

“…The intensity increase is a result of a reordering of the interfacial hexadecane. Similar to the free OD behavior, this effect becomes noticeable with surfactant concentrations of 10 μM and ZP values near −50 mV, near the ZP values common in the literature for bare nanoemulsion samples (7,8,10,13,25). We also observed these behaviors for nanoemulsions prepared with deuterated potassium laurate, a carboxylic acid-based surfactant (SI Appendix).…”

“…With regards to contributing factors previously invoked to explain the nanoemulsions stabilization of those with ZP measurements in the range of between −50 and −80 mV, most assume the surface charge is a contributing factor in some form or another (5)(6)(7)(8)(9)(10). In the case of the hydroxide ion as a contributing factor, it is possible there are small amounts of hydroxide ions at the LCNE interface that could be contributing to the small ZPs measured.…”

“…Although most nanoemulsions are stabilized by various surfactant systems, recent studies report nanoemulsions stabilized without added surfactants (5)(6)(7)(8). Such reports beg the question as to what interfacial molecular factors are responsible for such stabilization.…”

Electroacoustic and ultrasonic attenuation measurements of droplet size and ζ-potential of alkane-in-water emulsions: effects of oil solubility and composition

“…The functions G * and f * are complex quantities describing the inertial (size) and surface conduction (K s ) effects on the mobility, respectively. Recent work 13,14 has highlighted the importance of considering the conduction of ions confined to the so-called stagnant layer (additional surface or stagnant layer conductance K i s ), and the effect it has on the interpretation of the dynamic mobility and hence z-potential. Sizing by electroacoustics was done by varying the surface conductivity and zeta potential to obtain the best fit to the dynamic mobility spectrum while closely matching the size distribution obtained from attenuation.…”

Australian Colloid and Interface Symposium Special Issue

“…Gentle et al [10] report the formation of an interdigitated bilayer of porphyrins in cast films; Vanderlick et al [11] describes the use of peptides to control drug delivery by leakage from vesicles; and the preparation of cell membrane-mimetic polymer structures is reported by Stenzel and Davis. [12] The remaining papers cover diverse aspects of surfactant science including liquid crystals (Cassidy and Warr), [13] emulsion structure (Beattie et al), [14] sonochemistry (Grieser et al), [15] and some unusual features of liquids and solutions confined near surfaces (Kaneko et al [16] and Kurihara et al). [17] You will find Stephen Hyde's essay [18] as thoughtprovoking as his conference presentation.…”