Specific cation effects on water structure at the air-water and air-octadecanol monolayer-water interfaces

The nature of water structure at the air-water interface and under insoluble monolayers at the air-water interface has been examined by standard surface tension and Langmuir trough techniques. Surface tension, equilibrium spreading pressure and surface pressure-molecular area data have been collected as a function of the concentration (up to ca. 4 M) of a range of alkali and alkali metal chlorides, ammonium chloride and tetramethyl ammonium chloride (TMA). The monolayers were spread from ultrahigh purity n-octadecanol. The thermodynamic information on the adsorption density of the salts at the air-water and air-monolayer-water interfaces has been examined in terms of various classifications of “structure making” and “structure breaking” ions. It was found that the salt, normally excluded from the air-water interface, is not negatively adsorbed at the interface covered with n-octadecanol. At equivalent bulk concentrations less mechanical work is involved in transferring structure breakers into the interface compared with structure makers. The salts examined cause a marked expansion of the low pressure (high area) liquid region of the pressure-area isotherm of n-octadecanol. The extent of this expansion is directly proportional to the elevation of the surface tension of water induced by the salts. The expansion decreases (at 4 M) and with Cl- as the reference anion as Mg2+>Ca2+>Li+>Na+>K+>Rb+≃NH4+>Cs+≃TMA+