Influence of Organic Acid Solution Chemistry on Subsurface Transport Properties. 1. Surface and Interfacial Tension

Abstract

Typically, the migration of multiple fluids in the subsurface is modeled as if it were independent of aqueous phase composition. However, solution conditions including pH, concentration of surface-active solutes, and ionic strength may impact the interfacial tension and the wettability of a system, which in turn may markedly affect subsurface transport. This study, presented in two parts, investigates the effects of solution chemistry upon surface tension, interfacial tension, wettability, and the subsurface transport property of capillary pressure versus saturation. In this part, the changes in air−water surface tension and o-xylene−water interfacial tension due to the presence of the surface-active solute octanoic acid were measured as a function of pH, concentration, and ionic strength. The interfacial tension depended only on the concentration and speciation of the octanoic acid and the aqueous phase, which displayed a strong dependence on pH. At the air−water interface, the neutral acid form, prevalent at low pH, was found to be more surface-active than the anionic form. However, in the two-liquid systems with fixed organic acid mass, the anionic form prevalent at high pH effected greater interfacial tension lowering because of the partitioning of the neutral form into the o-xylene.