J. Phys. Chem. 1985,89, 363-365 nm in solution 5. In thioproperazine iodine, the CTS band appears between 297 and 299 nm and a wide low-intensity band appears at 297 nm in solution 4. Besides this, a very weak peak a t 340 nm appears which may be due to 1; (the triiodide ion). In chloropromazine-iodine, the CTC band appears between 297 and 298 nm but several weak intensity peaks appear at 303, 317, and 331 nm due to perturbation of the unstable state of 13-. In prochloroparazine-iodine, the CTC band appears between 297 and 299 nm in different solutions but a sharp peak of low intensity at 348 nm indicates the formation of the I< ion to a slight extent, suggesting some concentration of the ionic structure of CTC. In promazineiodine, the CTC band lies between 292 and 294 nm. The band becomes wide with increasing drug concentration. It shows the complete absence of I< or perturbation peaks. The iodine 1-1 band does not appear in any spectra indicating complete complexation with the donor. The possibilities of a 2:l monomer and 1:1 dimer to [D'A-1, [D:A]+ and [D:A]- are ruled out. However, benzene does not prove to be a good solvent for UV studies of donor-acceptor systems. It shows some weak interactions with the donor-acceptor system. A conclusion in favor of a 1:1 molecular complex is drawn and a "No-Bond" CTC structure predominates over other unstable structures. It is noticed that energy of the CTC band lies in 292-298-nm region and does not appreciably differ from one system to another. Also the AHo values are practically constant in the donor-acceptor systems studied. The drug activity primarily
363
arises from the hydrogen bond acceptor feature on the acceptor m ~ l e c u l e and ' ~ does not completely depend on the AHo values in the above systems. However, the AHo values in an aqueous medium will dominate to give rise to drug activity as has been suggested by Van De Vorst in his paper. The charge transfer mechanismI4 lies between depolarization excitation and hyperpolarization inhibition depending upon the donor molecule, transferring the electron at the cell surface from outside or inside, respectively. Acknowledgment. The authors are thankful to CST (Lucknow, India), for support of this research and to the staff of the college for providing the facilities. Supplementary Material Available: Detailed tables including P, e, and d for all donor-acceptor systems studied at 30 and 40 OC and a qualitative analysis of the UV spectra of each system, including all solutions (6 pages). Ordering information is available on any current masthead page. Registry No. Pericyazine, 2622-26-6; thioproperazine, 3 16-81-4; chloropromazine, 50-53-3; prochloroperazine,58-38-8; promazine, 5840-2; iodine, 7553-56-2;pericyazine.12,88787-13-7;thioproperazineI,, 17036-07-6; chloropromazine-12, 16025-83-5; prochloroperazine~I,, 17036-06-5; promazineI,, 88787-14-8. (13) Nash, T; Allison, A. C. Biochem. Pharmacol. 1963, 12, 601. (14) Karreman, G.; Isenberg, I.; Szent-Gyorgi,A. Science 1959,130, 1191.
Free Energy, Enthalpy, and Entropy Changes during the Formatlon of a n-Hexadecane/Potasslum Stearate/Water/l-Pentanol Microemulsion System Henri L. Rosano* and George B. Lyons Department of Chemistry, City University of New York, City College, New York, New York 10031 (Received: April 5, 1984; In Final Form: June 21, 1984)
n-Hexadecane/potassium stearate/water emulsions were titrated to clarity with 1-pentanol. The volume of the aqueous phase was varied from 10 to 60 mL. The mole fractions 1-pentanol/potassium stearate and l-pentanol/H20 were determined at various temperatures (25-50 "C). At 30 OC,the free energy and the enthalpy changes accompanying alcohol adsorption during microemulsion formation were found to be -14.8 and +8.74 kJ/mol. The relatively small negative free energy change following 1-pentanoladsorption at the oil-water interface may explain why the method of preparation affects the final results. The entropy change of formation was calculated to be 77.8 J K mol-'. It was concluded that when these systems are produced they are basically entropy driven.
Introduction Mixtures of an oil and water in the presence of a surface-active agent usually form coarse emulsions which are optically opaque, or nearly so, and usually separate on standing. In some cases, transparent mixtures of oil and water can be prepared with the proper combination of surface-active materials. The terms microemulsion,' swollen micellar solution,* micellar e m ~ l s i o n , ~ , ~ middle phase: unstable micro emulsion^,^ and spontaneous transparent emulsions6 have been used to describe these systems. Some confusion about the definition of the exact meaning of the term microemulsion still exists. FribergI3 suggested that thermodynamic stability, although generally accepted, is not always a valid criterion. Studies with these systems as well as experience (1) Stoeckenius, W.; Schulman, J. H.; Prince, L. M. Kolloid-Z.1960,169, 170. (2) Shinoda, K.; Kunieda, H. J . Colloid Interface Sci. 1973, 42, 381. (3) Adamson, A. W. J. Colloid Interface Sci. 1969, 29, 261. (4) Robbins, M. L. In "Micellization, Solubilization, and Microemulsions"; Mittal, K. L., Ed.; Plenum Pres: New York, 1977; Vol. 2, pp 713-753. See also: Scriven, L. E. Ibid. pp 877-893. ( 5 ) Rosano, H. L.; Lan, T.; Weiss, A.; Whittam, J. H.; Gerbacia, W. E. F. J . Phys. Chem. 1981,85, 468-473. ( 6 ) Hoar, T. P.; Schulman, J. H. Nature (London) 1943, 152, 102.
with technical microemulsions have demonstrated that a great number of systems do not have thermodynamic ~ t a b i l i t y . ~ - ~ ~ J ~ Podzimek and FribergI4 concluded that for o/w microemulsions with high oil content, thermodynamic stability appears to be an exception. The influence of the type and composition of surfactants, cosurfactants, and oil structure on system formation has been studied. One variable controlling droplet size was the quantity of surfactant present with a particular cosurfactant.17 Furthermore, sedimentation measurements imply that the cross-sectional area of the surfactant at the interface is constant regardless of the volume (7) Gerbacia, W. E.; Rosano, H. L.; Zajac, M. J . Am. Oil Chem. SOC. 1976, 53,
101-104.
(8) Rosano, H. L.; Lan, T.; Weiss, A.; Gerbacia, W. E. F.; Whittam, J. H. J . Colloid Interface Sci. 1979, 72,, 233-244. (9) Rosano, H. L.; Jon, D.; Whittam, J. H. J . Am. Oil Chem. SOC.1982, 59, 36C-363. (10) Rosano, H. L. US.Patent 4 146499, 1979. (1 1) Gerbacia, W.; Rosano, H.L.; Whittman, J. H. "Colloid and Interface Science"; Kerker, M.,Ed.; Academic Press: New York, 1976; Vol. 11, pp 245-256. (12) Overbeek, J. Th. Discuss. Faraday SOC.1978, 65, 7-19. (13) Friberg, S. E. Colloids Surf. 1982, 4, 201. (14) Podzimek, M.; Friberg, S. E. J . Dispersion Sei. Techno/. 1980, I , 34. (15) Rosano, H. L. J . SOC.Cosmef.Chem. 1974, 25,609-619.
0022-3654185 12089-0363SO1SO10 Q 1985 American Chemical Society
364
The Journal of Physical Chemistry, Vol. 89, No. 2, 1985
Rosano and Lyons
TABLE I: Stearic Acid (2.30 X IO" molVn-Hexadecane (2.30mL)/0.375 M KOH (Variable Volume) Titrated with I-Pentanol" temp, mL of intercept, mL mL of 1-pentanol/mL mol of 1-pentanol/ X,b x 4c, OC n-hexadecane of laentanol of aaueous Dhase X 10' mole K stear X. 103 kJ/mol 25 30 30 35 40 43 45 50
2.30 1.15 2.30 2.30 2.30 2.30 2.30 2.30
1.040 1 0.9010 0.8947 1.1353 1.188 0.8815 1.2675 1.4989
1.4150 1.1375 1.3058 1.1561 0.9771 1.7259 1.1647 1.7259
4.1709 4.0607 3.587 4.524 4.7638 3.5346 5.0826 6.0104
0.8066 0.8024 0.7820 0.8199 0.8265 0.7795 0.8356 0.8574
2.8819 1.8883 2.1678 1.9192 1.6242 1.8652 1.8535 1.8506
-13.978 -15.264 -14.854 -15.532 -16.240 -14.749 -16.178 -16.509
"AG30= -14.8 k J mol-'; 4H,o = +8.7 k J mol-'; S = 77.8 J K-' mol-'
of the dispersed It is important to notice that in the particular case of a coarse emulsion n-hexadecanelstearic acid/KOH solution titrated with a series of 52 alcohols, only 1-pentanol and four other alcohols were able to produce a transparent system. As noticed by Tadro~:~the most significant difference between macro- and microemulsions lies in the fact that putting work into macroemulsions or increasing the surfactant concentration usually improves their stability; but this is not the case with microemulsions which appear to be dependent for their formation on specific interactions among the constituent molecules and with the interface. If these interactions are not realized, neither the work input nor increasing the surfactant concentration will produce a microemulsion. On the other hand, once the conditions are right, spontaneous formation occurs and no mechanical work is required. With the system discussed in this paper (n-hexadecanelpotassium stearate/H20/ 1-pentanol) we reported that the system will clear spontaneously when the 1-pentanol was added dropwise to the coarse emulsion. If the equivalent volume of 1-pentanol necessary to clear the system by titration was predistributed between the oil and water phases no clearing was observed. When all 1-pentanol was added to the aqueous phase the system finally ~ is then postulated that the cleared after 5 min of ~ t i r r i n g . It titration procedure was the method tp use since it produced spontaneous clearing. Moreover, it was assumed that the system was in an equilibrium thermodynamic state. When preparing microemulsions by the "point" (titration) method various changes are often noted. Sometimes, there are dramatic viscosity increases just before the mixture becomes transparent. Also, mixtures may progress from lactescent to clear, rapidly or slowly, as drops of cosurfactant are added. These observations appear to indicate that the initial macroemulsion may be transformed into a Scriven26 bicontinuous structure which eventually results into a microemulsion system.
Experimental Section The n-hexadecane, stearic acid, KOH, and I-pentanol were all mol/mL reagent grade. Stearic acidln-hexadecane 1.0 X mixtures were placed into a round-bottom flask connected to a condenser and heated until a clear solution was obtained. nmol of stearic acid Hexadecane (2.3 mL) containing 2.3 X was then pipetted (with a hot pipet) into a variable volume of a 0.375 N KOH aqueous solution which had been placed in a 200-mL water-jacketed beaker. A Fisher Teflon magnetic stirrer was used to maintain gentle stirring during the titration. The system was titrated with 1-pentanol (the cosurfactant) by use of a microburet (0.02 mL). A Bausch and Lomb Spectronic 20 spectrometer at 510 nm was used during the titration with 1pentanol. As 1-pentanol was added the system remained opaque until the percent transmittance increased abruptly. Eventually, addition of 1-pentanol will produce a decrease in transmittance. (See Figure 1 of ref 5.) Microemulsions of potassium stearic acidln-hexadsanelwater/ 1-pentanol were prepared by the same procedure but the volume of the aqueous KOH solution was increased from 10 to 60 mL. The same procedure was repeated at various temperatures from 25 to 50 "C. ~~
(16) Rosano, H. L.; Peiser, R. C.; Eydt, A. Rev. Fr. Corps Gras 1969, No. 4, 249-257
Results At a given temperature and for a given volume of the aqueous phase the minimum volume of 1-pentanol needed to obtain maximum percent transmittance was determined from the titration curve. Graphs of milliliters of alcohol vs. volume of aqueous KOH gave straight lines. A linear regression program was used to fit the data: slope (mL of 1-pentanol/mL of aqueous phase) and intercept (mL of alcohol at 0 mL of KOH) were determined. Table I lists the results. From the slope and intercept, the mole fraction of alcohol in the interphase and in the aqueous phase were calculated. The free energy change per mole for the adsorption of alcohol was calculated from the formula AG, = -RT In Xai/Xab where X,' and Xabare the mole fraction of 1-pentanol in the interphase and the continuous aqueous phase, respectively. The variation of AG, vs. temperature allowed the determination of the entropy of formation of these systems (again with a regression method) since d(AG,)/aT = -S. The results are tabulated in Table I. From Table I, the entropy change accompanying alcohol adsorption was calculated by the linear regression method to be 77.8 J K mol-'. The interpolated AG and AHvalues at 30 O C are -14.8 and +8.74 kJ/mol, respectively. In one experiment the volume of n-hexadecane was reduced from 2.3 to 1.15 mL. It was found that the volume of 1-pentanol to produce an abrupt rise in percent light transmittance was very similar in both cases. In a previous publicationS it was reported that, when the microemulsion has been formed, excess short-chain alcohol finds its way inside the oil droplet, eventually increasing its radius and decreasing transmittance. It is concluded that the alcohol at the opaque/clear transition state is predominantly in the oil/water interphase at the point where the percent transmittance increases abruptly. From simple geometric considerations the formation of a microemulsion system corresponds to an explosive breaking up of macroemulsion droplets. For example, a droplet with a 120-nm radium will break into 1728 microdroplets with a 10-nm radius. This is why it is desirable to prepare a fine emulsion initially before titration with the cosurfactant. From the above considerations the main factors involved in the preparation of a microemulsion system seems to be (1) the very low interfacial tension during the large increase of the interfacial area accompanying the transformation of coarse droplets into microdroplets, (2) an overall decrease in free energy accompanying the adsorption of the cosurfactant at the oillwater interface, and (3) the formation of a flexible oil/surfactant/cosurfactant interface as evidenced by the positive value found for the entropy. Recently, di Meglio et al. l9 using spin-labeling techniques have studied the system sodium dodecyl SUlfate/H20/C6Hl2/ 1-pentanol just needed to obtain a clear system. These authors concluded that the resolution of these systems into a microemulsion phase depends on the flexibility of the interface.'* In Table I, it can (17) Rosano, H. L.; Weiss, A,; Gerbacia, W. E. "Proceedings of the 7th International Congress on Surface Active Substances, Moscow, Sept 1976"; 1977, Vol. 1, p 453. (18) Danielsson, I.; Lindman, B. Colloid Sur/. 1981, 3, 391-392. (19) di Meglio, J. M.; Dvolaitzky, M.; Ober, R.; Taupin, C. J . Phys., Left. (Orsay, Fr.) 1983, 44, L229-L234.
J. Phys. Chem. 1985,89, 365-369
I
I
I
1
I O
10
30
ML
OF
I 40
hQUEOU5
1
I
50
60
PHASE
1175 M
* Konl
Figure 1. Plot of 2.30 mL of n-hexadecane/2.30 X lo-’ mol of stearic acfd/a variable volume of 0.375 M KOH titrated to clarity with I-pentan01 at 35 “e:intercept, 1.1353 mL of 1-pentanol; slope, 1.1561 x lo-* mL of 1-pentanol/mL of aqueous volume (0.375 M KOH).
be seen that the ratio of the number of moles of 1-pentanol to potassium stearate varies from 3.6:l to 6:l.From film penetration into monolayer experiments it is well-known that the presence of alcohols in the film produces low interfacial tension and an expanded monolayer.23 The mutual solubility of 1-pentanol and 0.375 N KOH was determined by the Hill and Malisoff method.24 At 27 OC,the mole fraction of 1-pentanol in 0.375 N KOH was found to be 8.73 X At this temperature, the mole fraction of 1-pentanol from the slope of the curve in ~i~~~~1 is approximately 2X Let us now assume that the difference [6.73X mol] between these two values corresponds to 1-pentanol associated
365
to 2.3 X mol of potassium stearate and is responsible for the formation of the dispersed oil phase. Interestingly, this corresponds to a 3:1 ratio of moles of 1-pentanol to mole of potassium stearate, comparable to the ratios tabulated in Table I. Entropy effects in microemulsion systems have been discussed f i s t by Ruckenstein and Chi2’ and Talman and PragerZ2and lately by De Gennes and Tauphzo From these studies a microemulsion system would result from the melting of a macrocrystalline structure. It is appropriate to mention that back in 1943 Schulman and M c R o b e r t ~pictured ~~ the role of the surfactant as increasing the disorder of the interfacial film necessary to produce high radii of curvatures of the microemulsion particles. Our results lead to the conclusion that spontaneous formation of n-hexadecane/potassium stearate/HzO/ 1-pentanol very fine dispersions depends on the way the various components are added. These facts account for the small AG values found. Moreover, these processes are entropy driven. Acknowledgment. W e gratefully acknowledge the financial ~MA in~supporting t this ~ assistance of the Gillette carp., ~ work. Registry No. Hexadecane, 544-76-3;potassium stearate, 593-29-3; I-pentanoly 71-41-0. (20) de Gennes, P. G.; Taupin, C. J . Phys. Chem. 1982,86,2294-2304. (21) Ruckenstein, E.; Chi, J. C. J . Chem. Soc., Faraday Trans. 2 1975, 1690-1707. (22) Talmon, Y.;Prager, J. J . Chem. Phys. 1978,69,2984. McRoberts, M. Trans. Faraday Soc. 1946,42,165. (23) Schulman, J. H.; (24) Hill, A. E.; Malisoff, W. M. J . Am. Chem. SOC.1926,48,918-927. (25) Tadros, Th. F. In “Structure/Performance Relationships in Surfactants”; Rosen, M. J., Ed.; American Chemical Society: Washington, DC, 1984;ACS Symp. Ser. No. 253. (26) Scriven, L.E. In ‘Micellization, Solubilization, and Microemulsion“; Mittal, K. L., Ed.; Plenum Press: New York, 1977;p 277.
Voltammetric Study of CO and CO, Adsorption on Smooth and Platinized Platinum Electrodes Jerzy Sobkowski and Andrzej Czerwinski* Chemistry Department, The Warsaw University, 02-089 Warsaw, Poland (Received: April 13, 1984; In Final Form: July 9, 1984)
The adsorption of carbon monoxide and carbon dioxide on platinized and smooth platinum electrodes has been studied by the potentiodynamic technique. It is shown that the main product of CO and COz adsorption in the presence Hadon the Pt surface is similar, probably the COOHadradical. It has been found that there are some differences between CO and C 0 2 adsorption products on Pt/Pt and smooth platinum electrodes.
Introduction The phenomena of CO and C 0 2 adsorption on Pt electrodes from acid solutions have been known for more than 20 years.’“ About 60 papers*”4 on these problems have appeared, but the (1) S. Gilman, J . Phys. Chem., 66,2657 (1962). (2) S.Gilman, J . Phys. Chem., 67,78 (1963). (3) S.Gilman, J. Phys. Chem., 67, 1898 (1963). (4) S.Gilman, J . Phys. Chem., 68, 70 (1964). (5) J. Giner, Electrochim. Acta, 8,857 (1963). (6)J. Giner, Electrochim. Acta, 9,63 (1964). (7) W.Vielstich and V. Vogel, 2.Elektrochem., 68, 688 (1964). (8) P. R. Johnson and A. T. Kuhn, J . Electrochem. Soc., 122,599(1965). (9)T. B. Warner and S. Schuldiner, J. Electrochem. Soc., 111,992 (1964). (IO) B. J. Rersma, T. B. Warner, and S . Schuldiner, J. Electrochem. Soc., 113, 84 (1966). (11) S. B. Brummer and J. I. Ford, J . Phys. Chem., 69, 1355 (1965). (12) S.B. Brummer and M. J. Turner, J . Phys. Chem., 71,3494 (1967). (13) S.B. Brummer and K. Cahill, Discuss. Faraday Soc., 45,67 (1968). (14) S . B. Brummer and K. Cahill, J. Electroanal. Chem., 21,463(1969).
opinions of the authors are not consistent. They mostly agreed that the final product of COzadsorption (which is called “reduced (15) M. W.Breiter, J . Phys. Chem., 72, 1305 (1968). (16) M. W. Breiter, Electrochim. Acta, 12, 1213 (1967). (17) M. W.Breiter, J. Electroanal. Chem., 19, 131 (1968). (1 8) M. W. Breiter in “Proceedings of the Symposium on Electrocatalysis”, The Electrochemical Society, Princeton, NJ, 1974,p 115. (19) M. W. Breiter, J . Electroanal. Chem., 65,623,(1975). (20) M. W. Breiter in “Modern Aspects of Electrochemistry”, Vol. 10, Plenum Press, New York, 1975. (21) T. Biegler, J . Phys. Chem., 72, 1571 (1968). (22) B. I. Podlovchenko, W. F. Stenin, and A. A. Ekibaeva, Elektrokhimiya, 4, 1374 (1968). (23) M.W. Brieter, Z . Phys. Chem. (Frankfurt am Main), 98.23 (1975). (24) G. L. Padyukova, A. B. Fasman, and D. V. Sokolsky, Elektrokhimiya, 2, 885 (1966). (25) P. Stonehart, Electrochim. Acta, 18,63 (1973). (26) J. Bett, K. Kinoshita, K. Rontsis, and P. Stonehart, J. Catal., 29, 160 (1973). (27) P. Stonehart and G. Kohlmayer, Electrochim. Acta, 17, 369,(1972).
0022-3654/85/2089-0365$01.50/00 1985 American Chemical Society
~
,
