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.

Surface and Interfacial Properties of Octanoic Acid–Octylamine Mixtures in Isooctane–Water Systems: Influence of Acid: Amine Molar Ratio and Aqueous Phase pH

“…For organic acids there are reports that IFT (a) passes through a minimum as the pH increases (3,25), (b) decreases continuously with increasing pH (26,(29)(30)(31), and (c) remains unchanged as the pH increases (27). The local minimum in IFT observed by Rudin and Wasan (25) and Lord et al (3) was attributed to the simultaneous adsorption of the anionic and neutral forms of the acid at the interface.…”

“…Lord et al (3) studied the oxylene-Octac-water system, using an Octac concentration that was twice as high and a NaCl concentration that was 10 times higher than the ones employed in the present study. It has been demonstrated that increasing the concentration of the interfacially active component and counterions in the system lowers the IFT further (3,27). Figure 6a shows that the 1 : 1 mixtures reduce the IFT significantly more than the individual components.…”

“…Aside from its fundamental importance, interest in the topic is, also due to its significance within applications like liquid-liquid extraction (1,2), and as a model system for the study of processes related to subsurface transport of organic waste (3,4), wettability alteration (5), adhesion (6), etc.…”

Partition Coefficients and Interfacial Activity for Polar Components in Oil/Water Model Systems

“…In a study of an o-xylene/water/octanoic acid system, Lord et al (12) found that the acid form (HA) of the octanoic acid molecule was more surface active than the octanoate form (A Ϫ ). In addition, Rudin and Wasan (11) have observed that unionized oleic acid by itself is surface active.…”

“…The distribution coefficient is found to be strongly pH dependent. The decrease in IFT between acidic oil solutions and water of increasing pH has also been investigated by several authors (12,(15)(16)(17)(18)(19). The effect is observed to approximately correspond to the dissociation of the acidic functional groups with increasing pH.…”