Deciphering Zn(II)-Carboxylic Acid Interactions: Tailoring Oil/Water Interfaces and Surfactant Efficiency

This study elucidates the synergistic interactions between dodecanoic acid (C12) and zinc ions (Zn²⁺) at oil/water interfaces, a critical phenomenon for understanding the intricate dynamics of surfactant systems. Interfacial tension (IFT) measurements, performed via pendant drop tensiometry, reveal that the pronounced affinity of C12 for the oil/water interface causes an approximate 35% reduction in the IFT (from 50 to about 32 mN/m). However, introducing Zn²⁺ ions with C12 created an IFT decrease to approximately 22 mN/m, representing an overall reduction of nearly 55%, indicative of their interactions that substantially enhance interfacial adsorption and promote molecular ordering. The stoichiometric relationship between C12 and Zn²⁺ exhibits a marked concentration dependency. This phenomenon underscores the complex nature of the involved interfacial assembly and the dual role of both C12 and Zn²⁺ in modulating the physicochemical properties of the interface, which has been supported by the complementary density functional theory (DFT) and COSMO-RS calculations. Moreover, vibrational sum frequency generation (VSFG) spectroscopy corroborates the experimental findings by detecting high-order alkane chain arrangements induced by the Zn²⁺ ions. These integrated methodologies demonstrate that the Zn²⁺ ion's role varies, depending on the surface coverage by C12, and causes a more ordered interfacial film under controlled conditions, optimizing the reduction of IFT. Our research introduces a promising approach for creating advanced surfactant systems, emphasizing the intricate role of metal cations like Zn²⁺ at interfaces in various chemical engineering and environmental management applications.applications.