Predicting the pKa and Stability of Organic Acids and Bases at an Oil-Water Interface

We have used density functional theory and the implicit solvent model, COSMO-RS, to investigate how the acidity constant, pK(a) of organic acids and bases adsorbed at the organic compound aqueous solution interface changes, compared to its value in the aqueous phase. The pK(a) determine the surface charge density of the molecules that accumulate at the fluid fluid interface. We have estimated the pK(a) by comparing the stability of the protonated and unprotonated forms of a series of molecules in the bulk aqueous solution and at an interface where parts of each molecule reside in the hydrophobic phase and the rest remains in the hydrophilic phase. We found that the pK(a) for acids is shifted by similar to 1 pH unit to higher values compared to the-bulk water plc, whereas they are shifted to lower values by a similar amount for bases. Because this pK(a) shift is similar in magnitude for each of the molecules studied, we propose that the pK(a) for molecules at a water organic compound interface can easily be predicted by adding a small shift to the aqueous pK(a). This shift is general and correlates with the functional group. We also found that the relative composition of molecules at the fluid fluid interface is not the same as in the bulk. For example, species such as carboxylic acids are enriched at the interface, where they can dominate surface properties, even when they are a modest component in the bulk fluid. For high surface concentrations of carboxylic acid groups at an interface, such as a self-assembled monolayer, we have demonstrated that the pK(a) depends on the degree of deprotonation through direct hydrogen bonding between protonated and deprotonated acidic headgroups.