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TABLE 11: Net Charge Distributions (in e ) within the Isobutene-Na' Complex" b y ab Initio STO-3G Calculations system molecular region or atom" I I1 11' c, c* N a+
isobutene isolated -0.05 0.05 isobutene-Na+total minimum -0.04 0.15 isobutene-Na+local minimum -0.16 0.19 Net charge with respect to the positively charged sodium. Cf. Figure 1. of the final hydrocarbon C1 amounts to -0.40 nm. Optimizing the bond length of the double bond does not alter these results qualitatively. The complex shows a local minimum with the ion in the molecular plane (R, = 0.264 nm with optimized RW = 0.131 nm) and a stable minimum above the double bond (R, = 0.235 nm and R = -0.38 nm with an optimized double-bond length of 0.1327 nm). These two arrangements differ energetically by about 48.7 kJ/mol. The charge distribution within the isolated isobutene as well as the charge shift and charge transfer in interaction with the sodium ion (Table 11) yielded by our ab initio calculations agree well with the PCILO and CNDOI2 results of the earlier paperlo emphasizing the interpretation given there. But the higher charge transfer to the sodium ion within the total minimum shows that the chargetransfer interaction is stereospecifically favored above the C=C double bond, whereby in molecular region I (CH2 group) the net charge remains nearly constant and the charge transfer to the sodium ion is delivered mainly by molecular region I1 (C, and the two CH3 groups). This charge transfer comes especially from the two CH3groups
0.01 0.06 0.05
-0.15 -0.20 -0.19
0.03 0.03 0.09
-O.llb -0.03b
(region 11'). For the local minimum (Na+in front of the isobutene) an essential smaller charge transfer to the sodium ion may be regarded but a stronger charge shift within the isobutene toward molecular region I results, indicating a stronger polarization of the isobutene due to the complexation as in the case before.
Conclusions The stabilization of the isobutene-Naf complex is yielded by a cooperation of charge-transfer and electrostatic interactions. On the one hand, the calculations of the molecular electrostatic potential by PCILO and CND0/2 confirm the electrostatic favoring of the arrangement of the sodium ion in the region in front of the final carbon atom of the double bond. On the other hand, the favored chargetransfer interaction above the double bond is more strongly reflected by ab initio and CNDOI2 than by PCILO, which explains a favored arrangement above the double bond. Altogether, a stable fixation of the sodium ion above the double bond, displaced toward the final carbon, may be assumed.
Visual Evldence Regarding the Nature of Hemimicelles through Surface Solubilization of Plnacyanol Chloride
Charles C. Nunn,+ Robert S. Schechter, and William H. Wade' The Departments of Chemical Engineering and Chemistry, The University of Texas, Austin, Texas 78712 (Received: November 10, 1981; In Final Form: Merch 24, 1982)
Dye coadsorbed with surfactant molecules on a solid substrate exhibit a color which is characteristic of a hydrocarbon environment thereby providing visual evidence for the existence of aggregated surfactant molecules, called hemimicelles, which form when surfactants adsorb.
In 1956, Fuerstenau' coined the term hemimicelle to describe the two-dimensional aggregates he proposed to be responsible for the rise through several orders of magnitude in the observed adsorption of surfactant molecules on certain mineral oxide surfaces from aqueous solution. This rise takes place over such a narrow concentration range that the formation of hemimicelles can be envisioned as a two-dimensional analogue of miscellization in the bulk. A wide variety of surfactant adsorption 'Current address: School of Chemistry, The University, Cantock's Close, Bristol BS8 l T S , England.
systems have been found for which this wort of behavior occur^^^^ but the presence of the sudden rise in adsorption (1) Fuerstenau, D. W. J. Phys. Chem., 1966, 60, 981. (2) Rosen, M. J. "Surfactants and Interfacial Phenomena"; Wiley: New York, (1978); p 32 ff. (3) Scamehorn, J. F.: Schechter. R. S.: Wade, W. H. J. Colloid Interface Sci., in press. (4) Nunn, C. C.; Schechter, R. S.; Wade, W. H., invited paper presented at the ACS Symposium on Enhanced Oil Recovery, 181st National Meeting of the American Chemical Society, Spring 1981, Atlanta, GA. To appear in ACS Monograph. (5) Koganovskii, A. M.; Klimenko, N. A.; Tryasorukova,A. A. Colloid J. (U.S.S.R.) 1974,36, 790. Koganovskii, A. M.; Klimenko, N. A. Ibid. 1974,36,790. Koganovskii, A. M.; Klimenko, N. A. Ibid. 1976,38,149.
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Additions and Corrections
remains the primary evidence for hemimicelle formation, and little is known of their structure. Coadsorption of nonpolar oils with surfactants has been reported4s5and it is tempting to postulate that it is solubilized in hemimicelles, by analogy to micellar behavior. I t had been observed by Stigter et al.6 that orange OT is coadsorbed with a cationic surfactant on glass. They suggested that the dye is located between a double layer of surfactant and coined the term “surface solubilization” to describe the phenomenon. In thisnote, a very simple experiment is described which gives visual evidence that dye molecules can be coadsorbed with surfactant on an alumina surface into an environment similar to that in which they are solubilized into micelles in the bulk. The compound studied was pinacyanol chloride, a dye of the cyanine class, which has been extensively used for anionic surfactant cmc determinations based on the fact that pinacyanol dissolved in surfactant solutions below the cmc shows a red color, while above the cmc, and when dissolved in organic solvents, a blue color is formed. In spite of the well-known perturbation of the cmc caused by the presence of the dye7 , it is clear that formation of the blue color indicates the solubilization of the dye into the hydrocarbon-like interior of micelles. In the present study, a series of solutions was prepared, each containing a constant amount of pinacyanol ( M) and various concentrations of an anionic surfactant, sodium p-(1-propylnony1)benenesulfonate. These solutions (8 mL) were contacted with 40 mg of y-alumina (described in ref 4),and the equilibrium concentration of surfactant was determined by use of a tritium-labeled tracer. Surfactant adsorption was found to be unchanged from that determined in the absence of pinacyanol.*
Those systems whose equilibrium surfactant concentrations were below the cmc but in which substantial surfactant adsorption occured (1molecule/ 100 A2or greater) showed perfectly clear supernatant solutions, while the solid was dyed the blue color characteristic of pinacyanol solubilized in micelles. If the colorless supernatant was removed, and pinacyanol added to it, the red color characteristic of premicellar solutions was ovserved, verifying that the solution in equilibrium with the alumina was below the cmc. In systems having surfactant concentrations above the cmc, the blue color was observed both on the solid and in the solution, indicating a partitioning of pinacyanol between micelles and the adsorbed layer. These observations clearly indicate that the pinacyanol is adsorbed into a micelle-like environment. The fact that the solutions were clear in systems below the cmc indicates that essentially all the pinacyanol partitions into the adsorbed layer. Incremental additions of pinacyanol caused the blue color of the solid to deepen, the solution remaining clear, until a point was reached where the solubilizing capacity of the adsorbed layer was exceeded, at which point the solution took on the characteristic premicellar red color. Preliminary experiments using spectrophotometric analysis indicated that pinacyanol was not significantly adsorbed on alumina in the absence of surfactant (as might be expected for a cationic dye on the positive alumina surface). Thus, the concept of surface solubilization is justified. Further experimentation is required to establish the structure of hemimicelles; however, the results reported here prove conclusively that surface aggregates exist and the bilayered structures proposed by Scamehorn et al.3 seem a reasonable configuration.
(6) Stigter, D.; Williams, R. J.; Mysels, K. J. J. Phys. Chem. 1955,59, 330. (7) Mukerjee, P.; Mysels, K. J. J. Amr. Chem. SOC.1955, 77,2937. (8)Nunn, C.C., Ph.D. Dissertation,The University of Texas at Austin, 1981.
Acknowledgment. The authors thank the following for their support of this research the Department of Energy, the Robert A. Welch Foundation, Chevron, Gulf,Tenneco, Arco, Witco, Shell, Mobil, Elf-Aquitane, Union Oil, Texaco, Exxon, Sun, Amoco, British Petroleum, Conoco, and Getty.
ADDITIONS AND CORRECTIONS 1982, Volume 86
Haruo Shizuka* and Hidesumi Obuchi: Anion-Induced Quenching of Aromatic Ketones by Nanosecond Laser Photolysis. Page 1299. In Table I, the q / p s value for benzophenone should be 3.1.
