INFLUENCE OF ORGANIC-ACID SOLUTION CHEMISTRY ON SUBSURFACE TRANSPORT-PROPERTIES .1. SURFACE AND INTERFACIAL-TENSION

Typically, the migration of multiple fluids in the subsurface is model ed as if it were independent of aqueous phase composition. However, so lution conditions including pH, concentration of surface-active solute s, and ionic strength may impact the interfacial tension and the wetta bility of a system, which in turn may markedly affect subsurface trans port. This study, presented in two parts, investigates the effects of solution chemistry upon surface tension, interfacial tension, wettabil ity, and the subsurface transport property of capillary pressure versu s saturation. In this part, the changes in air-water surface tension a nd o-xylene-water interfacial tension due to the presence of the surfa ce-active solute octanoic acid were measured as a function of pH, conc entration, and ionic strength. The interfacial tension depended only o n the concentration and speciation of the octanoic acid and the aqueou s phase, which displayed a strong dependence on pH. At the air-water i nterface, 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 hi gh pH effected greater interfacial tension lowering because of the par titioning of the neutral form into the o-xylene.