Salt Effects On the Activity Coefficient of Organic Acids in Methanol/Water System.

Numerous organic chemicals are continuously introduced into the soil environment in large quantities. Prediction of sorption organic chemicals by soil is essential for evaluating the risk of these compounds to the environment.  However, the existing sorption models of organic acids to variable charged soils in water-cosolvent mixture are unpredictable for organic acids carrying carboxylic acid group.  In order to explain the effect of acidic functional group on organic acids regarding the sorption, the assumptions are made; the activity coefficient of organic acids in mixed solvent system does not follow the trend predicted cosolvent sorption model because the presence of strong salts in soil solution can either increase or decrease the solubility of organic acids.  From solubility measurements on benzoic acid with carboxylic acid functional group and 2,4,6-TCP with hydroxyl functional group in methanol/water mixtures at room temperature, the salting constant (K^s) was determined in methanol/water solution 0M, 5mM, 1M, and 2M calcium chloride and in soil solution (K-1, K-3) extracted from methanol/water solution of 0M, 5 mM, 1 M, and 2 M CaCl₂ matrix. K^s values of 2,4,6-TCP was higher than that of benzoic acid in aqueous phase because polarity of benzoic acid is smaller than that of 2,4,6-TCP. And K^s value was higher in aqueous phase than that in neat methanol. Also, solubility of organic acids decreases as the concentration of salts increases. The reason is that salt effect is dependent on the polarity of both the solute and the salt. Activity coefficient calculated from a given salt solution decrease (i.e. solubility increases) in an exponential way with increasing volume fraction of methanol. Therefore, the solubility of organic acids as methanol volume fraction regarding salting out effect from soils is not the factors which influence the positive relationship of sorption and methanol volume fraction. Further efforts are required to explain the interaction between functional group of organic acids and the soil surface, such as ion exchange, cation bridging, hydrogen bonding, and van der Waals forces in methanol/water mixture system.