Temperature-dependent behavior of C14-C18 alkyl surfactants with ionizable and neutral head groups at the water-decane interface.

HYPOTHESIS: Surfactants stabilize liquid-liquid interfaces, and their temperature-dependent behavior is highly relevant for applications involving emulsions. While temperature-induced phase transitions of interfacial films are well known, their detection depends strongly on the applied experimental technique. We hypothesize that combining interfacial shear rheology with a temperature-dependent thermodynamic description based on the Gibbs adsorption framework enables a more sensitive and quantitative identification of such transitions. Furthermore, we expect that molecular architecture, specifically head-group chemistry and alkyl chain length, governs the mechanical stability and temperature-induced phase behavior of surfactant films.

EXPERIMENTS: Temperature-dependent interfacial shear rheology and pendant drop tensiometry were performed for a series of alkyl-chain surfactants with systematic variation of head group (amine, alcohol, acid) and chain length (C14, C16, C18) over a range of 15-80 °C. Interfacial tension data obtained from pendant drop measurements were analyzed using a newly derived temperature-dependent adsorption equation to extract adsorption free energies ∆Gads and identify phase transition temperatures.

FINDINGS: Interfacial shear rheology reveals pronounced temperature-induced transitions in surfactant films, reflected by a loss of elasticity and changes in interfacial structure. The transition temperature depends strongly on molecular architecture, with systematic variations in head group and chain length. In contrast, pendant drop tensiometry captures these changes only partially, indicating that interfacial tension alone does not fully reflect structural rearrangements. The combined thermodynamic and rheological analysis highlights the importance of interfacial mechanics for understanding temperature-dependent phase behavior at liquid-liquid interfaces.