Adsorption of atmospheric gases at the air-water interface. 4: The influence of salts

We have measured the standard free energies and standard enthalpies of adsorption as well as saturated surface coverages for hexanoic acid and I-propanol adsorbed at the air-aqueous interface of sodium chloride (NaCl) and ammonium sulfate ((NH4)(2)SO4) Solutions Of concentrations between 0 and 4 mol L-1. Temperature-dependent surface tension measurements are used to obtain DeltaG(aq-->0)(0) and DeltaH(aq-->0)(0); using known Henry's law constants and estimated values of the salting-out coefficient we calculate the gas-interface adsorption parameters DeltaG(g-->0)(0) and DeltaH(g-->0)(0). A small (similar to1-2 kJ mol(-1)) decrease in DeltaG(aq-->0)(0) with increasing salt concentration is accompanied by an increase (similar to10-30%) in saturated surface coverage, consistent with the expected "salting-out" behavior. Adsorption from the gas phase becomes somewhat less favorable with increasing salt concentration for both organic species. The standard enthalpy of adsorption from the gas phase decreases with increasing salt concentration for hexanoic acid, but not for I-propanol. This may be a consequence of acid dimer formation at the interface becoming important at higher coverages.