Temperature-Dependent Behavior of C14-C18 Alkyl Surfactants with Ionizable and Neutral Head Groups at the Water-Decane Interface

Surfactants stabilize liquid-liquid interfaces, where adsorption is highly temperature-dependent. Interfacial shear rheology and pendant drop tensiometry were combined to study surfactant monolayers at the n-decane-water interface. Shear rheology revealed a temperature-driven phase transition, characterized by a loss of elasticity and a transition to a gas-like state. Pendant drop experiments confirmed this behavior, showing rising interfacial tension followed by plateau formation. Thermodynamic calculations based on a temperature-dependent Gibbs adsorption equation demonstrated that the adsorption free energy (ΔG ads ) is the dominant factor controlling the adsorption state. Phase transition temperatures determined by both methods were strongly influenced by chain length and head group chemistry: longer chains enhanced temperature stability as both chain-chain and chain-solvent interactions intensified and positively charged head groups exhibited the highest interfacial ordering. Together, these results link interfacial thermodynamics and mechanics, offering new insights into surfactant-driven phase stability and emulsion control.