The ionic strength dependent zeta potential at the surface of hexadecane droplets in water and the corresponding interfacial adsorption of surfactants

Abstract

An anomalous maximum in the ionic strength dependent electrophoretic mobility curves has been observed in previous reports from particles dispersed in colloids. This maximum has been considered anomalous because it is contradictory with the Gouy-Chapman model. The existence of such a maximum has been attributed to specific ionic adsorption, a hairy layer at the surface, or the effect of the anomalous change of surface conductivity in different studies. It was also pointed out that the O'Brien-White approach based on the Gouy-Chapman model could be used to understand this maximum in electrophoretic mobility curves and lead to understandable zeta potential curves. This implied that the observed maximum was actually not "anomalous". In this work we report our simulation of ionic strength dependent zeta potential curves based on the O'Brien-White approach and experimental studies of the ionic strength dependent electrophoretic mobility of the hexadecane droplets in the hexadecane-water emulsions at different pH or in the presence of sodium dodecyl sulphate at varied concentrations. In some cases, the simulation shows that the calculation with the O'Brien-White approach does change the trend in the concerned ionic strength dependent curves. However, the simulation in some other cases also leads to similar trends in the ionic strength dependent electrophoretic mobility curves and zeta potential curves. In the experiments, both the existence and non-existence of such a maximum were observed and demonstrated to be system dependent. The corresponding molecular structure of the oil-water interface was then discussed with the analyses of the zeta potential curves and second harmonic generation signals recorded at the hexadecane-water interface.

Order-Disorder transition of carboxyl terminated chains in polydiacetylenes vesicles probed by second harmonic generation and two-photon fluorescence

“…The geometric constrains [58−60] from the continuous order-disorder transition of the spatial conformation of the interfacial carboxyl chains sterically hinder the electrostatic attraction interaction between the positively charged D289 molecules and the negatively charged surface of PDAs vesicles due to the decrease in the adsorption density of D289 molecules at the surface of PDAs vesicles with the increased UV irradiation. Besides, the chain-chain interaction is known to significantly influence the molecular energetics at the interfaces [40,44]. The interaction between the D289 molecules and the carboxyl chains of PDAs may be weakened by the ragged distortion and interleaving of the carboxyl chains in chromatic transitions from monomer to blue-to-purple-to-red forms as shown in FIG.…”

“…During the experiments the size of the vesicle kept relatively stable with standard deviation of the average diameter less than 10%, and the polydispersity index (PDI) less than 0.2. Zeta potential of vesicles was determined by the mobility values according to the Smoluchowski equation, which is normally applied at cases other than very low ionic strength [40,49].…”

Nucleic acids complexation with cationic elastin-like polypeptides: Stoichiometry and stability of nano-assemblies

“…After each measurement, the sample was transferred back into the initial solution (to keep a nearly constant volume of ELP/DNA complex solution) before the next addition of ELP. The electrophoretic mobility of the particles was measured by the instrument and converted into ζpotential value using the classical Smoluchowski expression [66]. In a set of experiments, Sodium dodecyl sulfate (SDS) was used to test the stability of DNA/ELP complexes [68].…”

Dynamic mobility of surfactant-stabilized nano-drops: unifying equilibrium thermodynamics, electrokinetics and Marangoni effects

“…More recently, the possibility of emulsion-stabilized drops having the higher of two ζ -potentials predicted by the standard electrokinetic model was rejected on the basis that they are unphysically high (Yang et al 2017). However, such a claim is Dynamic mobility of surfactant-stabilized nano-drops 895 A14-3 controversial, as other electrokinetic studies have expressly identified such interfaces as bearing significantly higher charge (Hunter & O'Brien 1997).…”