Article pubs.acs.org/Langmuir

Predicting the pKa and Stability of Organic Acids and Bases at an Oil−Water Interface M. P. Andersson,* M. H. M. Olsson, and S. L. S. Stipp Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen OE, Denmark ABSTRACT: We have used density functional theory and the implicit solvent model, COSMO-RS, to investigate how the acidity constant, pKa, of organic acids and bases adsorbed at the organic compound−aqueous solution interface changes, compared to its value in the aqueous phase. The pKa determine the surface charge density of the molecules that accumulate at the fluid−fluid interface. We have estimated the pKa 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 pKa for acids is shifted by ∼1 pH unit to higher values compared to the bulk water pKa, whereas they are shifted to lower values by a similar amount for bases. Because this pKa shift is similar in magnitude for each of the molecules studied, we propose that the pKa for molecules at a water−organic compound interface can easily be predicted by adding a small shift to the aqueous pKa. 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 pKa depends on the degree of deprotonation through direct hydrogen bonding between protonated and deprotonated acidic headgroups.



or water from another rock formation.13 The extra oil that can be produced by this method varies from reservoir to reservoir, but it has been observed that polar compounds, such as acids and bases, have to be present in the crude oil for an effect to be realized. Part of the mechanism for low-salinity water flooding is proposed to be a result of expansion in the electric double layer on mineral surfaces when the salinity is less than 10% that of seawater, namely,

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, pKa, of organic acids and ba...
980KB Sizes 0 Downloads 3 Views