A Correlation for Phase Behavior of Nonionic Surfactants M. BOURREL, 1 J. L. SALAGER, 2 R. S. SCHECHTER, AND W. H. WADE Departments of Chemistry and Petroleum Engineering, University of Texas at Austin, Austin, Texas 78712 Received November 20, 1979; accepted January 30, 1980 In previous studies, the s ystematics of variation of phase behavior of oil/water/surfactant systems have been detailed for anionic surfactants. The present study details the behavior of such systems with nonionic surfactants, including the variables: temperature, alcohol type and concentration, added electrolyte, ethylene oxide number of the surfactant, alkane molecular weight for the oil phase, and the hydrophobic molecular weight for the surfactant. INTRODUCTION Waterflooding is a standard technique for improving the efficiency of oil recovery from a reservoir; yet, half the oil remains thermodynamically trapped as dispersed droplets. If the interfacial tension between oil and water can be reduced sufficiently, then additional oil droplets may be dis- placed (1). Surfactant solutions have been formulated to attain these required ultralow interfacial tensions (2). Furthermore, it has been shown that these tensions are often ob- served for conditions such that the oil/water/ surfactant mixtures separate into three distinct phases (3), and recent studies (4, 5) have documented the equivalency of these two approaches. When three-phase systems form, the sur- factant is located largely in the phase of density intermediate between that of water and oil and hence is called the middle phase. Winsor (6) called such systems with the thermodynamically stable middle-phase microemulsion in equilibrium with both excess oil and water phases Type III. 1 Present address: Societe Nationale Elf Aquitaine, Lacq, France. 2 Present address: Universidad de los Andes, Merida, Venezuela. Type I systems are those for which the surfactant is in the aqueous phase (micro- emulsion in equilibrium with excess oil) and Type II systems consist of microemulsion (surfactant in oil) in equilibrium with excess water. Of course, in each system residual surfactant exists in all the various equi- librium phases. It is possible to effect a I~III~II transition (or the reverse) by changing any one of a number of system variables. For the case of anionic surfactants, extensive studies of this transition have been con- ducted and a correlation between the salinity, oil, water/oil ratio, alcohol type and concentration, surfactant structure, and temperature has been established (5). All of these parameters change the relative affinity of the surfactant for the oil and water and therefore tend to influence the system type. The correlation shows that the system is particularly sensitive to the salinity and often a small increase of this parameter can promote the I ~ III ~ II transition. Temperature, on the other hand, does not strongly influence anionic sur- factant partitioning (5). A similar quantita- tive study of nonionic phase behavior has not yet been reported. However, a first qualitative study of the effect of some of 451 Journal of Colloid and Interface Science, Vol. 75, No. 2, June 1980 0021-9797/80/060451-11502.00/0 Copyright© 1980 by Academic Press,Inc. All rightsof reproduction in any formreserved.