Experimental and theoretical study of adsorption of synthesized amino acid core derived surfactants at an air/water interface

Polish National Science Centre, European Union, ICSC PAS

Partial financial support from the Polish National Science Centre grant number 2019/33/B/NZ7/01608 & 2020/38/E/ST8/00173 is acknowledged with gratitude. The European Union Erasmus+ programme (project number: 2018-1-PL01-KA103-047692) is acknowledged for providing scholarship (financial support) for the research/mobility/traineeship. PW and JZ acknowledges partial financial support of the project by the statutory research fund of ICSC PAS. We also wish to thank John Belanger for proofreading this manuscript.

The adsorption characteristics of amino acid surfactants, synthesized as substances with different volumes and hydrophilic head properties, have been previously described experimentally, without robust theoretical explanation. A theoretical model enabling the characterization of the adsorption behavior and physicochemical properties of this type of biodegradable surfactants, based on molecular structure, would be beneficial for assessment of their usefulness in colloids and interface science in comparison with typical surface-active substances. In this paper, the adsorption behaviour of synthesized amino acid surfactants at the liquid/gas interface was analyzed experimentally (by surface tension measurements using two independent techniques) and theoretically by means of an elaborate model, considering the volume of the surfactant hydrophilic “head” and its ionization degree. It was shown that the adsorption behavior of the synthesized compounds can be successfully described by the proposed model, including the Helfand–Frisch–Lebowitz isotherm based on the equation of state of 2D hard disk-like particles, with molecular properties of surfactant particles obtained using molecular dynamics simulations (MDS). Model parameters allow for direct comparison of physicochemical properties of synthesized amino acid surfactants with other ionic and non-ionic surface-active substances. Furthermore, it was revealed that intermolecular hydrogen bonds allow the formation of surfactant dimers with high surface activity.