Investigation of adsorption of surfactant at the air-water interface with quantum chemistry method

Abstract

Density functional theory (DFT) of quantum chemistry was used to optimize the configuration of the anionic surfactant complexes CH 3 (CH 2 ) 7 OSO− 3 (H 2 O) n (n=0-6) and calculate their molecular frequencies at the B3LYP/6-311+G * level. The interaction of CH 3 (CH 2 ) 7 OSO − 3 with 1 to 6 water molecules was investigated at the air-water interface with DFT. The results revealed that the hydration shell was formed in the form of H-bond between the hydrophilic group of CH 3 (CH 2 ) 7 OSO − 3 and 6 waters. The strength of H-bonds belongs to medium. Binding free energy revealed that the hydration shell was stable. The increase of the number of water molecules will cause increases of the total charge of hydrophilic group and S10-O9-C8 bond angle, but decreases of the alkyl chain length and the bond lengths of S10-O11, S10-O12 as well as S10-O13, respectively. anionic surfactant, quantum chemistry method, density functional theory A surfactant molecule has an amphiphilic structure and a tendency to escape from solution. Therefore, surfactants can concentrate from the solution and adsorb each other together to form a parallel single molecular layer easily at the interface when dissolved in water. This is a speculation from molecule structure of surfactant [1] . By measuring the interfacial tension and using the Gibbs adsorption equation, we can conduct experiment analysis of the status of adsorption of surfactants at the air-water interface [1,2] . And by using molecular dynamics simulation, the adsorption of surfactants at the interface also can be predicted [3][4][5] . But neither of the above described the changes of molecular structure of surfactants adsorbed at the interface and the essential of interaction between surfactant and solvent at the molecular level. Because of the complicated molecular structure of the surfactant, which generally consists of from dozens of atoms to hundreds of atoms and possesses the amphiphilic structure, and very little work has been reported on the adsorption of the surfactant at the interface with quantum chemistry method so far. Ryszard and his co-workers [6] investigated the interaction of an alkyl ammonium surfactant with only three water molecules. Yan et al. [7] investigated the characteristics of electronic structure for surfactants in solution by using Onsager model. But neither of them described the mechanism of interaction between surfactant and solvent at the molecular level. Hence, if the interaction of surfactant with solvent at the air/water interface can be investigated with quantum chemistry method, it will not only provide theoretical reference for explaining the adsorption of surfactant at the interface, but also enlarge the application of the quantum chemistry method in surfactant area.In the present work, with the help of quantum chemistry, the interactions of a widely used anionic surfactant,