Microemulsion of Mixed Chlorinated Solvents Using Food Grade (Edible) Surfactants BOR-JIER SHIAU,* ,† DAVID A. SABATINI, † JEFFREY H. HARWELL, ‡ AND DE QUANG VU ‡ School of Civil Engineering and Environmental Science and School of Chem ical Engineering and Materials Science, The Institute for Applied Surfactant Research, The University of Oklahoma, Norman, Oklahoma 73019 Ground water contamination frequently consists of mixed chlorinated solvents [e.g., tetrachloroethylene (PCE), trichloroethylene (TCE), and trans-1,2- dichloroethylene (DCE)]. In this research, mixtures of the food grade (edible) surfactants bis(2- ethylhexyl) sodium sulfosuccinate (AOT) and sodium mono- and dimethylnaphthalene sulfonate (SMDNS) were used in the formation of middle-phase micro- emulsions for mixed chlorinated solvents. Micro- emulsions of binary (e.g., PCE/TCE, PCE/DCE, DCE/ TCE) and ternary (PCE/TCE/DCE) chlorinated solvent systems were evaluated. Several empirical cor- relations were used for describing and/or predicting the phase behavior of the resulting middle-phase microemulsions (e.g.,the ideal mixing rule or the nonideal regular mixing theory). The ideal mixing rule provided a good approximation for binary and ternary systems,but experimental deviations from the predictions were significant enough to affect the optimal sur- factant system. Nonideal regular mixing theory dem- onstrated much better predictive capabilities than ideal mixing for the binary and ternary systems. The recognition of nonideal mixing behavior and the resulting predictive correlations will be valuable in the design of groundwater remediation scenarios when surfactants are used for remediation of mixed chlorinated solvents. Introduction In a prior study (1), we demonstrated that mixtures of a food grade surfactant and hydrotrope produced classical Winsor type III (middle-phase) microemulsions for three chlorinated solvents [i.e., tetrachloroethylene (PCE), trichlo- roethylene (TCE),and trans-1,2-dichloroethylene (1,2-DCE orDCE)]. Utilization ofmiddle-phase microemulsionshas the potentialto significantlyimprove the efficiencyofpump- and-treat remediation for residual/free phases of chlori- nated solvents. Groundwater contamination commonly consistsoftwoormorecontaminants(2),and the surfactant formulation necessary for achieving a middle-phase mi- croemulsion may be dramatically different for each com- ponent(3). In our prior research (1),pure (neat)chlorinated solvent systems were studied;this data is inadequate when addressing residual and/or free-phase systems comprised of mixed chlorinated solvents. For mixed chlorinated solvents, the solution behavior likely depends on the oil composition; that is, properties of the mixed chlorinated solvent system should be between that of the pure compounds. The screening process for obtaining middle-phase microemulsions for the myriad ofpossible solvent systems (even for binary and ternary systems) will be extremely time-consuming compared with that of single solvent systems. Thus, it will be virtually impossible to conduct the screening process for all possible contaminant com- positions. Similar difficulties have been realized in en- hanced oilrecoverywhere crude oils typicallyhave dozens of components. In order to reduce the screening efforts forobtainingmiddle-phase microemulsionswith crude oils, empirical correlations capable of predicting the phase behavior ofmulticomponent systemshave been developed (4-11). The applicability of these correlations for mixed chlorinated solventsisobviouslyofinterest. Ifviable,these correlations can significantly reduce laboratory work and thusthe costsassociated with design ofsurfactant-enhanced subsurface remediation systems. The hypotheses for this study are as follows: (1) the molarcompositionsofmixed chlorinated solventswillaffect formation of middle-phase microemulsions and (2) cor- relations can be derived from ideal mixing or nonideal mixing rules for predicting surfactant compositions neces- sary to achieve middle-phase microemulsion for mixed chlorinated solvents. The objectives of this study are to determine the optimal surfactant/cosurfactant concentra- tions for binary and ternary chlorinated solvent mixtures and to compare the experimental results with model predictions. Theory Background. Previous research in surfactant-enhanced oil recovery demonstrated that optimal middle-phase microemulsions can solubilize large quantities of oil and produce ultralowinterfacialtensions(IFTs)(ca. < 10 -3 dyn/ cm between the excess phases and the surfactant rich middle phase). Reed and Healy(9)showedthattheoptimal surfactant formulation for maximizing oil recovery occurs when the IFTs between the excess oiland water phases and the surfactant-rich phase are equal. At this point in the three-phase region, equal amounts of oil and water are dissolved in the middle-phase system. Salager et al. (6) defined the optimal salinity as the midpoint of the salinity range for which the system exhibits three phases [com- parable to the definition of Reed and Healy (9)]. These criteria are very useful because they permit the screening ofmicroemulsion systemsusingsimple laboratorytests.In our previous study (1), we define the optimal cosurfactant *Correspondingauthortelephone: (405)325-4257;fax: (405)325- 4217; e-mail address: Bjshiau@mailhost.ecn.uoknor.edu. † School of Civil Engineering and Environmental Science. ‡ School of Chemical Engineering and Materials Science. Environ. Sci. Technol. 1996, 30, 97-103 0013-936X/96/0930-0097$12.00/0  1995 American Chemical Society VOL. 30, NO. 1, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 97