Criteria for Structuring Surfactants To Maximize Solubilization of Oil and Water: Part 1-Commercial Nonionics

Abstract

Abstract The phase behavior of nonionic surfactants having the same hydrophilic/lipophilic balance (HLB) but differing molecular weights has been studied. It is shown that the optimal alkane carbon number (ACN) depends on the HLB, but that increasing the hydrophobe molecular weight narrows the middle phase region, increases the solubilization parameter, and decreases the interfacial tension (IFT). We found that the width of the three-phase region is in simple inverse proportion to the solubilization parameter at optimal salinity and that the multiple of IFT times the square of the solubilization is a constant. We also found it possible to synthesize nonionics that rival anionics in the properties mentioned above. Introduction There is increasing evidence that the phase behavior of surfactant/oil/brine systems and the efficiency of oil recovery with micellar solutions are connected intimately. For instance, laboratory core floods have shown that surfactant systems exhibit maximum oil recovery at the optimal salinity. The concept of optimal salinity, introduced by Healy and Reed, is especially useful because it pen-nits screening of surfactant systems by relatively simple experiments requiring the observation of the number and the types of phases that coexist at equilibrium when surfactant/oil/brine mixtures are blended. Optimal salinity, defined as that middle-phase microemulsion system containing equal volumes of oil and water, is not difficult to determine, and, thus, conditions for the most efficient surfactant system can be established. It is now well known that many different surfactant systems have the same optimal salinity. Further, it generally has been assumed, but not definitely established by laboratory experiments that the preferred surfactant system, selected from a group of systems having the same optimal salinity, will be that which solubilizes the largest volume of oil and brine per unit mass of surfactant. We do not necessarily subscribe to this simple view. since there are many factors other than solubilization (such as surfactatant retention) that may influence oil recovery efficiency however, all other factors being equal, it is reasonable to attempt to maximize solubilization, especially because it has been found synonymous with minimal IFT's-an equally important factor governing effectiveness of oil recovery. This paper seeks to identify some surfactant structural features that will lead to increased solubilization and decreased IFT. We have addressed this important question in past publications but have met with only limited success. The difficulty has been that changing the surfactant structure dictates that a second corresponding change be made so that the resulting system would remain optimal. For instance, one can increase the length of the hydrocarbon tail of the surfactant molecule and at the same time compensate for this change either by decreasing the amount of hydrophobic alcohol added to the system or by decreasing the salinity of the system. The results obtained in this manner have remained difficult to interpret because all changes can and most often do alter the solubilization of oil and water in the middle-phase microemulsion. Therefore, it was not possible to separate that pan of the resulting solubilization change caused strictly by the modification of the surfactant structure. In the study discussed here, we made compensating changes in the surfactant structure, keeping all other variables fixed. For nonionic surfactants, compensating changes can be made in several ways. SPEJ P. 743^