Surfactant-free oil/water and bicontinuous microemulsion composed of benzene,ethanol and water

Available online at www. sciencedirect.comCHINES EScienceDirectCHEMICALLETTERSEL SEVIERChinese Chemical Letters 21 (2010) 880- -883www.elsevier.com/locate/ccletSurfactant-free oil/water and bicontinuous microemulsioncomposed of benzene, ethanol and waterFei Song", Jie Xub, Wan Guo Hou a,b.*a Key Laboratory for Colloid and Ilnterface Chemistry of Education Ministry, Shandong University, Jinan 250100, Chinab College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, ChinaReceived 13 November 2009AbstractGenerally, a microemulsion consists of oil, water, surfactant and sometimes cosurfactant. Herein, we report a novel surfactant-free microemulsion (denoted as SFME) composed of benzene, water and ethanol without the amphiphilic molecular structure oftraditional surfactant. The phase behavior of the ternary system was investigated, finding that there were a single-phase region andatwo-phase region in ternary phase diagram. The electrical conductivity measurement was employed to investigate the microregionof the single-phase region, and a bicontinuous microregion and a benzene-in-water (O/W) microemulsion microregion wereidentified, which was confirmed by freeze-fracture transmission electron microscopy (FF-TEM) observations. The sizes of themicroemulsion droplets are in the range of 20- -50 nm.C 2010 Wan Guo Hou. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.Keywords: Surfactant-free microemulsion; Phase behavior; Conductivity; FF-TEMSince their discovery by Hoar and Schulman [1] in 1943, microemulsions have received the attention of a number ofresearchers concerned with the theoretical and application aspects of their formation and stabilization [2]. The term“microemulsion”was coined by Schulman and Stoechnius [3] in 1959, now it has been widely accepted thatmicroemulsion is an optically isotropic, transparent, and thermodynamically stable dispersion formed by two or moreimmiscible liquids, which are stabilized by an adsorbed surface-active molecule film at the liquid- liquid interface.Generally, a microemulsion consists of oil, water, surfactant and sometimes cosurfactant [3], and it is believed that thesurfactant is the important component in stabilizing these systems. However, Smith et al. [4] in 1977 reported an oil-continuous (or W/O) microemulsion composed of hexane, 2-propanol and water, and this ternary system wasconsidered as surfactan-free (or detergentless) microemulsion (denoted as SFME) because no traditional surfactantinvolved in the system. Subsequently, the SFMEs have attracted much attention [5-13], their many physical propertieswere measured, and it was found that the phase and tension behavior of the systems were similar to those observedwith surfactant-based microemulsion systems. Up to now, all SFMEs reported in literatures [4 -13] are water-in-oil(W/O) systems. Herein, we report oil-in-water (O/W) and bicontinuous SFMEs composed of benzene, water andethanol., Jinan 250100, China.E-mail address: wghou@ sdu.edu.cn (W.G. Hou).中国煤化工CNMH G1001-8417/$ - see front matter C 2010 Wan Guo Hou. Published by Elsevier B.V. onAll rights reserved.doi:10. 101 6/.cclet.2010.01.029F Song et al./Chinese Chemical Letters 21 (2010) 880- 883881thanolbaSingle-phase/Two-phase\^EWaterFig. 1. Phase diagram of benzene/ethanol/water ternary system at 25 °C. Line a and line b with RB/E = 15 and 25, respectively, are chosen for theconductivity examinations and sample points A and B are the samples chosen for the FF-TEM study. I and II micoregions are water in-benzene andbicontinuous microemulsion regions, respectively.The ternary phase diagram of the system at 25.0土0.2 °C was constructed on the basis of physical properties ofvarious compositions of benzene, ethanol and water. Mixtures of benzene and water with various volume ratios ofbenzene to water (RB/w) were prepared in dry test tubes, titrated with ethanol under stirring. The ethanol volume atwhich the mixtures became clear was noted. The entire experimental procedure was repeated three times and anaverage value was used to plot the phase diagram, see Fig. 1. The region marked with “single-phase”includestransparent compositions, the region marked with“two-phase”includes turbid compositions when freshly prepared oragitated and these compositions eventually break into two phases.he electrical conductivity measurement is the most widely used simple technique for examining thmicrostructures and their structural changes in surfactant-based microemulsion systems [14-16]. For traditionalaqueous microemulsions, Clausse et al. [17] demonstrated that with increasing water content, the microemulsionelectrical conductivity (K) changes in the following four successive stages: (1) the initial nonlinear increase of Kreveals the existence of a percolation phenomenon that may be attributed to inverse microdroplet aggregation; (2) thenext linear increase is due to the formation of aqueous microdomains which results from the partial fusion of clusteredinverse microdroplets, which suggests that a W/O microemulsion is formed in this low water content region; (3) thethird nonlinear increase of K indicates that the medium undergoes a gradual structural transition and a bicontinuousmicrostructure is formed due to the progressive growth and interconnection of the aqueous microdomains; (4) the finaldecrease of K with increase of water content corresponds to the appearance of water-continuous microemulsion, thatis, an O/W microemulsion is formed at high water content, and the progressive decrease of K results from theprogressive decrease of the concentration of the O/W microemulsion droplets. On the basis of conductivitymeasurement and ultracentrifugation examination, three subregions corresponding to W/O microemulsion, small H-bonded aggregates of water and alcohol, and normal ternary solution, respectively were identified by Smith et al. [4] inthe single-phase region composed of hexane, 2-propanol and water.The variations of electrical conductivity K as a function of the water volume fraction (φw) along dilution lines atdifferent Re/B (volume ratio of ethanol/benzene) values were determined in this study. As two typical examples, Fig. 2shows the variations of K as a function of φw along dilution lines a and b (Fig. 1). As can be seen, the curves may bedivided into four parts by the dash lines, this variation trend of K is similar to that observed with surfactant-basedmicroemulsion systems. According to the standpoints of Smith [4] and Clausse [17], we believe that the initial increaseof K with increasing φw corresponds to normal ternary solution; the next increase may be interpreted as theconsequence of the formation of small H-bonded aggregates of water and中国煤化工zene medium; thethird increase of K reveals that the medium undergoes a structural trans-Is, owing to theprogressive growth and interconnection of the H-bonded aggregates diYHCNMHGhal decrease of Kindicates the formation of O/W microemulsions at high φw, and the decrease of K is due to the successive decrease of882E Song et al. /Chinese Chemical Letters 21 (2010) 880 -8832.2|-R。g=15|2.0 -1.8-1.4-10.8.0.6-0.4.0.00.0.61.0ψw2.5.b-R=252.0-1.5-0.5.).0+.2.6中wFig. 2. Variations of the electrical conductivity K as a function of the water volume fraction (φw) for the benzene/ethanol/water ternary systems atdifferent RB/e values (along line a and b in Fig. 1). The curves were divided into four parts by the dash lines.the concentration of O/W microemulsion droplets with the increasing φw. The conductivity curves in Fig. 2 evidentlyillustrate the presence of four different types of microstructure: normal ternary solution, small H-bonded aggregates,bicontinuous and O/W microemulsions. By repeating the experiment for other samples with different RE/B values, fourtypes of microregions can be determined. The subregions of bicontinuous and O/W microemulsions identified werealso marked in Fig. 1.100mm200nm中国煤化工HCNMH GFig. 3. FF-TEM pictures of the surfactant free microemulsion, a and b correspond to the samples chosen in Fig. 1.F Song et al./Chinese Chemical Leters 21 (2010) 880 -883883Freeze-fracture transmission electron microscopy (FF-TEM) is one of the powerful tools to characterize the shapeand size of the aggregates in microemulsions. In order to examine the presence of the microstructures of bicontinuousand O/W microemulsions, FF-TEM observations were performed for the two test samples A and B (Fig. 1) with thevolume ratios of benzene/ethanol/water of 1.5:0.1:14.4 and 1.5:0.1:3.73, respectively. The FF-TEM images of samplesA and B are shown in Fig. 3. The spherical droplets can be clearly observed for sample A in O/W microemulsionmicroregion, and most of the droplets are in the size range of 20- 50 nm. However, no clear droplets are observed forsample B in the bicontinuous microregion, only some traces of the so-called“sponge”structure may be found, this isbecause both water and oil are continuous phases [18].AcknowledgmentsThis research was supported by the National Natural Science Foundation of China (No. 20953003), the NaturalScience Foundation of Shandong Province of China (No. Z2008B08 and ZR2009BZ001) and Taishan ScholarFoundation of Shandong Province of China (No. ts20070713).References[1] TP. Hoar, J.H. Schulman, Nature 152 (1943) 102.[2] S.P. Moulik, B.K. Paul, Adv. Colloid Inerface Sci. 78 (1998) 99.[3] J.H. Schulman, W. Stoeckenius, L.M. Prince, J. Phys. Chem. 63 (1959) 1677.[4] G.D. Smith, C.E. Donelan, R.E. Barden, J. Colloid Interface Sci. 60 (1977) 488.[5] B.A. Kelser, D. Varie, R.E. Barden, et al. J. Phys. Chem. 83 (1979) 1276.[6] NF. Borys, S.L. Holt, R.E. Barden, J. Colloid Interface Sci. 71 (1979) 526.[7] B.M. Knickerbocker, C.V. Pesheck, L.E. Serlven, et al. J. Phys. Chem.83 (1979) 1984.[8] G. Lund, S.L. Holt, J. Am. Oil Chem. Soc. (1980) 264.[9] J. Lara, G. Perron, JE. Desnoyers, J. Phys. Chem. 85 (1981) 1600.[10] B.M. Knickerbocker, C.V. Pesheck, HT. Davls, et al. J. Phys. Chem.86 (1982) 393.[11] JE. Puig, D.L. Hemker, A. Guta, et al. J. Phys. Chem. 91 (1987) 1137.[12] YL. Khmelnitsky, A. van Hoek, C. Veeger, et al. J. Phys. Chem. 93 (1989) 872.[13] M. Zoumpanioti, M. Karali, A. Xenakis, et al. Enzyme Microb. Technol. 39 (2006) 531.[14] M.D. Angelo, D. Fioretto, G. Onori, etal. Phys. Rev. E 54 (1996) 993.[15] ZJ. Yu, R.D. Neuman, Langmuir 11 (1995) 1081.[16] S.K. Mehta, X.x. Kavaljit, K. Bala, Phys. Rev. E 59 (1999) 4317.[17] M. Clausse, A. Zradba, L. Nicolas-Morgantini, Microemusions Systems, Dekker, New York, 1987, p. 387.[18] H. Hoffmann, C. Thunig, U. Munkert, et al. Langmuir 8 (1992) 2629. .中国煤化工MYHCNMH G .