Solubility of organic acids in various methanol and salt concentrations: The implication on organic acid sorption in a cosolvent system

Abstract

The well-known cosolvency-induced sorption model is not applicable to predict the sorption of carboxylic acids in cosolvent system. To investigate the phenomenon, sorption and solubility of chlorinated phenols (2,4-dichlorophenol (2,4-DCP) and 2,4,6-trichlorophenol (2,4,6-TCP)) and carboxylic acids (benzoic acid and 2,4-dichlorophenoxyacetic acid (2,4-D)) were measured in soil–methanol mixture with various ionic strengths. The sorption (Km) of chlorinated phenols was explained by a cosolvency-induced sorption model; the inverse log-linear relationship between the Km and methanol volume fraction (fc). However, the Km of carboxylic acids increased with increasing fc. This discrepancy was attributed to the effect of the carboxylic moiety. To explain the effect, solubility was measured for benzoic acid and 2,4,6-TCP from various liquid conditions. For both solutes, the cosolvency power (σ) increased with CaCl2 concentrations and the salting constant (Ks) became smaller as fc increased. However, the σ value at a given salt concentration and the Ks value at a given fc were greater for 2,4,6-TCP than for benzoic acid, both of which were due to the greater hydrophobicity of the former. Overall, the solubility profiles of the both solutes on combination of fc and CaCl2 concentration evidenced no specific role of the carboxylic moiety. Therefore, it can be reasonably concluded that the positive relationship between Km and fc for carboxylic organic acid can be attributed to the modification of the activity coefficient occurred in the solid phase, which cannot be traceable by cosolvency-based model.