The effect of volatile compounds' molecular structure on mass transfer at air-water interface

A temporal and spatial coupling diffusion model based on volume of fluid (VOF) method was developed without chemical reaction. VOF is a numerical technique acquiring and tracking the free surface of water flow. Molecular structure of compounds was characterized by enlarging topological index of molecule W*. The relationships of mass transfer coefficient Schmidt Numbers and W*were developed. Based on the relationships of enlarged topological indices of molecule W*and Henry's constant and Schmidt Numbers, the temporal and spatial coupling diffusion model was applied to predict concentration distributions of ethanol, benzene, hexanal and 2, 2, 4-trimethlypentane after point source leakage. The predicted results were well consistent with the experimental results. Schmidt Numbers of alkylbenzene, alcohol, aldehyde and hydrocarbon are in linear relations with the enlarged topological index of molecule W*0.25. Schmidt Numbers increase with the decline of the molecule distance. Schmidt Numbers of alkylbenzene, alcohol, aldehyde and hydrocarbon in water are 600 times as high as those of in the air. For the compounds with the same topological index of molecule W*, Schmidt Numbers increase with the molecular weights. At the initial stage of leakage, the effect of Henry's constant was stronger than Schmidt Numbers. The volatilization of compounds increases with Henry's constant. The average declining rate of the peak concentration of ethanol in water is only 45.1% as high as that of benzene. At the later stage of leakage, the effect of Schmidt Numbers was stronger than Henry's constant. The volatilization of compounds increased with the reduction of Schmidt Numbers. The decline rate of the peak concentration of benzene drops. The declining rate of the peak concentration of ethanol is 120% higher than that of benzene. The topological index of molecule W*of hexanal is 16.27 times of that of 2, 2, 4-trimethlypentane. However, Henry's constant and Schmidt Numbers of hexanal and 2, 2, 4-trimethlypentane were close to each other, the processes of volatilization were similar.