Water-Diesel Microemulsions Stabilized by an Anionic Extended Surfactant and a Cationic Hydrotrope Ibrahim Kayali, 1 Khawla Qamhieh, 1 Ulf Olsson, 2 Lada Bemert, 3 and Reinhard Strey 3 1 College of Science and Technology, Al Quds University, Jerusalem, Palestine 2 Division of Physical Chemistry 1, Chemical Center, Lund University, Lund, Sweden 3 Institute for Physical Chemistry, University of Koln, Koln, Germany Alcohol-free microemulsions were formulated using mixtures of extended surfactant (C 12-14 - PO 14 -EO 2 SO 4 Na), sodium dodecyl benzene sulfonic acid and cationic hydrotropes with equal amounts of water and diesel. The cationic hydrotropes had short hydrocarbon or propylene oxide chain. The formulation included sodium carbonate to convert naphthenic acids in diesel to soaps. The phase behavior at ambient temperature of oil-free mixtures as a function of NaCl concen- tration was investigated. Visual inspection as well as cross polarizers were used to detect ani- sotropy. The microemulsion fish phase diagram and solubilization ratios for diesel and brine in the middle phases were determined. The minimum surfactant concentration needed to initiate middle phase formation was 0.10 wt%. Salinity scans revealed that optimal salinity can be adjusted according to the hydrophilic/ lipophilic nature of the hydrotrope used. Interfacial tension measurements using a spinning drop tensiometer showed a minimum value of 0.0015 mN/m between middle phase microemulsion and excess brine and a value of 0.032 mN/m between diesel and brine. Keywords Cationic and anionic surfactants, extended surfactant, middle phase microemul- sions, ultra low interfacial tension INTRODUCTION Microemulsions—optically isotropic and thermodyna- mically stable mixtures of oil, water and surfactant [1] — offer several advantages for use in various fields of applied technologies. The high solubilization capacity and ease of formation together with the long term stability make microemulsions very attractive for enhanced oil recovery (EOR), surfactant enhanced aquifer remediation (SEAR), alternative fuel, detergency, pharmaceutical and cosmetic preparations plus many other applications. [2–4] Microemulsions can form in situ with surfactant present either in water and oil phases or both when brought into contact, as is the case in soil remediation, SEAR and EOR methods developed to remove residual oil left. The high surfactant concentration required to formulate a microemulsion, usually 20% or more, remains a major concern for the user. In addition, much of the work on microemulsions formed with sulfonates and sulfates anionic surfactants has employed short-chain alcohols as co-surfactants and as co-solvent in order to decrease surfactant film rigidity, thus, promoting microemulsion for- mation. Alcohol can also reduce the time needed for equili- bration to be reached in multi phase systems. However for applications like SEAR and EOR, alcohol may be undesir- able to use. For example, when foam is to be used to improve sweep efficiency, short-chain alcohol may act to destabilize this foam. [5] Also, alcohols volatility, flammabil- ity and toxicity constitute major problems for the users. [6] Alcohol-free microemulsions could be formed using surfactants with branched hydrocarbon chains and=or ethylene or propylene oxide chains. Branching can increase solubilization of high molecular weight oils by making pen- etration much easier. [7] The presence of propylene oxide chain will lengthen the hydrophobic portion of the surfac- tant molecule, thus, also increasing solubilization of oils. [8] Another approach for formulating alcohol-free microe- mulsion is by using mixed anionic and cationic surfactants. Previous studies showed such mixtures to form the anisotropic liquid crystal rather than forming the isotropic Received 17 February 2011; accepted 9 March 2011. The authors would like to thank George Smith and his group (Huntsman Corporation, Houston, Texas) for kindly providing the extended surfactant samples, and Peter Schwab, Evonic Gold- schmidt, Germany, for providing us with Variquat samples. Financial support from the Swedish Research Council is grate- fully acknowledged. Address correspondence to Ibrahim Kayali, College of Science and Technology, Al Quds University, P.O. Box 20002, Jerusalem 91202, Palestine. E-mail: i_kayali@yahoo.com Journal of Dispersion Science and Technology, 33:516–520, 2012 Copyright # Taylor & Francis Group, LLC ISSN: 0193-2691 print=1532-2351 online DOI: 10.1080/01932691.2011.574924 516