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 (Zn) 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 Zn 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 Zn exhibits a marked concentration dependency. This phenomenon underscores the complex nature of the involved interfacial assembly and the dual role of both C12 and Zn 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 Zn ions. These integrated methodologies demonstrate that the Zn 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 Zn at interfaces in various chemical engineering and environmental management applications.applications.