Abstract
An increase in temperature typically leads to a decrease in the interfacial tension of a water/oil interface. The addition of surfactants to the system can complicate the situation significantly, i.e., the interfacial tension can increase or decrease with an increasing temperature. For most concentrations of the two studied surfactants, the cationic tetradecyl trimethyl ammonium bromide (TTAB) and the nonionic tridecyl dimethyl phosphine oxide (C13DMPO), the measured interfacial tension of the aqueous mixed surfactant solutions against hexane increases when the temperature decreases between 30 °C and 20 °C. However, with a further temperature decrease between 20 °C and 15 °C, the reverse effect has also been observed at some concentrations, i.e., a decrease of interfacial tension. Additionally, the corresponding dilational interfacial visco-elasticity shows some discrepant temperature effects, depending on the bulk concentration and oscillation frequency. The experiments have been performed with a capillary pressure tensiometer under the conditions of micro-gravity. The reason for the positive and negative interfacial tension and visco-elasticity gradients, respectively, within certain ranges of the temperature, concentration and mixing ratios, are discussed on the basis of all available parameters, such as the solubility and partitioning of the surfactants in the two liquid phases and the oscillation frequency.
Highlights
Fundamental knowledge on the adsorption of surfactants at water/oil interfaces is of eminent importance for many technological processes, such as oil recovery [1], food processing [2] and the production of pharmaceutics and cosmetics [3]
There are quite a number of studies on the temperature dependence of surfactant adsorption layers at the water/air interface; only very few investigations deal with water/oil interfaces
Das and Das studied the adsorption layers of alkyl trimethyl ammonium bromides at the solution–air interface at temperatures between 25 ◦C and 50 ◦C, and they explained their results on the micellization process in terms of enthalpy–entropy compensation effects without peculiarities [8]
Summary
Fundamental knowledge on the adsorption of surfactants at water/oil interfaces is of eminent importance for many technological processes, such as oil recovery [1], food processing [2] and the production of pharmaceutics and cosmetics [3]. While the impact of surfactant bulk concentrations and the mixing ratio on the adsorbed amounts is rather easy to explain via the Gibbs fundamental equation [5], the effect of temperature can be quite complicated. The increase in T should result in an increased molar area and in less adsorbed molecules at the interface This should lead to a decrease in interfacial tension, according to the Gibbs equation. The temperature dependence of the interfacial tension is much more complex, as discussed by El-Batanoney et al in [12] Investigations on this topic have been published recently [13,14], including Molecular Dynamics simulations [15], many questions could not be answered so far
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