Surface tension behavior of aqueous solutions of a propoxylated surfactant and interfacial tension behavior against a crude oil

Equilibrium interfacial tensions (EIFTs) are important in determining the oil recovery efficiency in surfactant-based enhanced oil recovery (EOR) processes, in which ultralow IFT values (less than 10(-2) mN m(-1)) are often needed. The dynamic IFTs (DIFTs) for un-pre-equilibrated drops in contact with aqueous solutions and the adsorption mechanisms at the oil-water interface are important for the initial stages of EOR and for screening various surfactants for field applications. The IFT behavior of a commercial anionic surfactant, termed S13D, a single-chain propoxylated sodium sulfate salt has been studied. The synthetic brine used (9700 ppm total dissolved solids) was similar to that in an actual reservoir of oil, from which purified crude oil samples were prepared. The dynamic and equilibrium surface tensions (DSTs and ESTs) and the IFTs were measured at 24 degrees C, with the emerging bubble/drop method, or the spinning bubble/drop method. The DSTs for surfactant concentrations from 0.1 to 10,000 ppm by weight have a simpler adsorption mechanism than the DIFTs, involving only diffusion from the aqueous phase and adsorption/desorption. Surface and interfacial tension relaxation tests after surface area perturbation were performed for establishing the validity of the ESTs and EIFTs. The critical micelle concentration (cmc) was 12 ppm in water and 1 ppm in brine. The lowest observed EIFT was 1 mN m(-1) with water and ultralow, 2x 10(-3) mN m(-1), with brine. The equilibration timescales were shorter with brine than with water, for both DSTs and DIFTs, and slightly longer for DIFTs, because evidently the adsorbed surfactant desorbed and diffused away from the interface, for partitioning in the oil phase. The results suggest that the IFTs were associated with the typical adsorbed soluble monolayers at the oil-water interface, and that solubilization effects did not affect the measured DIFTs and EIFTs.