Adsorption of Atmospheric Gases at the Air−Water Interface. 3:  Methylamines

The surface-bound state of methylamines adsorbed at the air−water boundary has been studied using a combination of equilibrium surface tension measurements and ab initio quantum calculations. Methylamine, dimethylamine, and trimethylamine all adsorb at the water surface, with saturated coverages roughly corresponding to the average concentration of “free” surface hydrogens. The standard enthalpies of adsorption from the gas phase show a correlation with the standard enthalpies of solvation, implying that partial solvation takes place at the air−water boundary. Comparison of experimental adsorption enthalpies and ab initio amine−water binding enthalpies suggests that the binding at the surface consists primarily of a single hydrogen bond to a surface water molecule possessing a “free” hydrogen.