782
Langmuir 1990,6, 782-786
by dashed lines in Figures 1-4. The aggregation numbers, some of which correspond to our experimental values and some of which have been interpolated, used in the calculation are given in Tables I and 11. The calculated values for $, and 44.4, the fraction of the neutralized micellar head groups with specifically adsorbed bromide ions (mBrCA)and the fraction of neutralized micellar groups with non specifically adsorbed OH and Br ions (moH and mBrNCA), are also given in Tables I and 11. This treatment only considers an adjustable parameter to explain the kinetic data, the parameter a in eq 26. It is necessary to point out that it cannot explain the kinetic results for this reaction in the mixed micelles with salt added (Figures 3 and 4), contrary to the reaction in simple CTAB micelles for which kinetic results in the presence of KBr are well explained.44 This effect can be related to the change that KBr produces in these mixed micelles structure. According to kinetic results, alcohols have no effect on the micellar phase rate constant, k,, in accordance with their nearly negligible influence on the micellar surface dielectric constant, which is also related to the P value, and the inhibition effect that both alcohols produce is mainly concerned with the substrate displacement from the micellar to the aqueous phase. This electrostatic approach predicts that the micellar surface potential decreases with surfactant concentra-
tion. The incorporation of alcohols into CTAB micelles does not affect (to any great amount) the CTAB surface potential according to our results in a simple CTAB mi~ e l l and e ~ in ~ accordance with the small effect that both alcohols produce in the micellar ionization degree. The substrate binding to micelle increases with the micellar surface drop, according to eq 23. This treatment predicts a variation in the ion-exchange equilibrium constant E(OHB, (Table 111) as well. From all of these results, we can conclude that the concentration of both alcohols, close to saturation of CTAB aqueous solution, does not nearly affect physical p r o p erties of CTAB aggregates such as cmc, micellar aggregation number, micellar effective dielectric constant, and fraction of neutralized micellar head groups. In order to explain kinetic data, the electrostatic treatment is a more realistic approach that takes into account the substrate solubilization into the micelle depending on the micellar surface potential, in accordance with other authors.6eHowever, this treatment is not able to explain all our kinetic results for this reaction in mixed CTAB/l-butanol micelles already reported,44and still new approaches must be developed. Registry No. CTAB, 57-09-0; 1-hexanol, 111-27-3; 1octanol, 111-87-5;crystal violet, 548-62-9.
Photocatalytic Hydrogenation of Acetylene by Mo2 and Mo3 Oxo Species in Colloidal TiOz Solutions Shaeel Al-Thabaiti and Robert R. Kuntz* Department of Chemistry, University of Missouri, Columbia Columbia, Missouri 6521 1 Received October 9, 1989. In Final Form: November 14, 1989
Photoreduction of Mo(V) aquo dimers and Mo(1V) aquo trimers onto colloidal TiO, in the presence of a sacrificial electron donor (PVA) produces catalytic sites for the photoreduction of acetylene to ethane and ethylene during band gap irradiation. The Mo, site assits in production of both ethylene and ethane while the Mo, enhances only the ethane production. In addition, Mo, is capable of reduction of ethylene to ethane. Quantum yields for total reducing equivalenh through the Mo sites are dependent on experimental conditions but maximize at approximately 1.3% for Mo dimeric species and 0.7% for Mo trimeric species based on total incident light at A, > 320 nm. Ethylene is also produced by direct interaction of acetylene with trapped electrons on the colloidal surface. Effects of pH, Mo loading, and intensity on the photocatalytic yields are reported.
Introduction Band gap irradiation of small semiconductor colloidal particles leads to migration of the charge carriers (holes (h+)and electrons (e,)) to surface sites. Those that escape recombination are available to perform redox chemistry a t the particle surface. A wide variety of redox processes induced by gand gap irradiation in colloidal systems has been reported.’-4 Attachment of catalytic cen(1)See, as examples: Kalyanasundaram, K.; Sakata, T.;Kawai, T.; Watanabe, T.; Fumishima, A.; Honda, K. In Energy Resources Through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic: New York, 1983. (2) Kalyanasundaram, K.; Gratzel, M.; Pelizzetti, E. Coord. Chem. Reu. 1986, 69, 57.
ters to the particle surface offers the potential of enhanced charge separation in addition to providing a site for selective reactions. Doping of dry semiconductor powders by metals, especially platinum, is a commonly used process for preparing gas-phase ~atalyst.9.~’~ Platinum on TiO, sols has also been used for trapping of e, in solution, and the effectiveness of such systems in water-splitting reactions has been studied.&’ ~~
(3) Pichat, P.;Tokumaru, K.; Sakuragi, H.; Kanno, T.;Oguchi, T.; Misaya, H.; Shimamura, Y.; Kuriyama, Y.; Fox, M. A.; Chem, C.-C.; Park, K.-H.; Younathan, J. N. In Organic Phototransformutionuin Nonhomogeneous Media; Fox, M. A., Ed.; ACS Sympoeium Series 278, American Chemical Society: Washington, DC, 1985. (4) Anpo, M. Res. Chem. Inter. 1989, 11,67. ( 5 ) Kiwi, J.; Gratzel, M. J . Phys. Chem. 1984, 88, 1302 and references therein.
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Hydrogenation of Acetylene by Mo, and Mo, Oxo Species
Langmuir, Vol. 6, No. 4, 1990 783
Molybdenum is a common doping agent for gas-phase photocatalysis,8 and we recently reported the photoreduction of acetylene in solutions containing colloidal TiO, and M o o t - in the presence of a sacrificial electron donorag Catalytic sites based on transition metals such as Mo which have a large range of oxidation states available could conceivably act as electron sinks through which several electrons could be transferred to a selectively bound substrate, thus effecting multielectron reductions. Molybdenum appeared to be effective as a 4-e reduction catalyst in the reduction of a ~ e t y l e n e . Evidence ~ was also presented which suggested that Mo monomers were more effective at this process than Mo polymers formed at higher Mo concentrations. The current study of stable Mo dimer and trimer species was initiated to further evaluate the effect of clustering on the photocatalytic reduction of acetylene.
The photolysis cell was not thermostated. Temperatures rose from ambient to approximately 40 O C within the first hour of illumination and remained at this temperature throughout the remainder of the photolysis period. The reaction vessel with 26 mL of free gas space above the irradiated solution was equipped with a septum for periodic sample removal for chromatographic analysis using a 6 f t X 0.25 in. Porapak N column and thermal conductivity detection. Authentic samples of product gases were injected for calibration purposes. Duplicate experiments could be reproduced at about the 10-20% level. Blank experiments using PVA alone, PVA with added dimeric and trimeric aquo molybdenum ions, or TiO, with the aquo dimeric or trimeric ions, but without PVA, yielded only traces of the ethylene or ethane products.
Experimental Section Colloidal TiO, with particle sizes of 5-8-nm diameter was prepared by hydrolysis of titanium tetraisopropoxide in HC1 as described earlierSgThe isopropyl alcohol product was not removed since the preparation was to be used in the presence of poly(vinyl alcohol) as a sacrificial electron donor. Stock solutions of 2-4 g of TiO,/L stored at 0 O C were found to be stable for extended periods of time. Aliquots of this stock were used as needed. The MOW)dimeric aquo ion (Mo,02+) was prepared by reaction of Na,MoO, with Hg in 2.5-3 M HCl. The solution was filtered and stored over fresh Hg." Oxidative titration of the stock solution with Ce(1V) using N-phenylanthranilic acid as an indicator" and measurement of the absorption at 384 nm (e = 103 cm-' M-')', confirmed the oxidation state. The Mo(1V) trimeric aquo ion (Mo,O,'+) was prepared under oxygen-free nitrogen from the reaction of stoichiometric amounts of Na,MoO, and K,MoC1, at 80 O C in 2 M HCl.', The resulting solution was diluted and allowed to stand at room temperature overnight. The reddish Mo302+was isolated by cation exchange on a Dowex-50W-X2 column. The concentration of the isolated solution was measured by oxidation with Fe(II1) followed by Ce(1V) titration of the product Fe(I1) using ferroin as an indicator. Concentration was subsequently monitored by measurement of absorption at 505 nm (e = 63 cm-' M-').I4 Poly(viny1alcohol) (PVA, 100% hydrolyzed, MW = 86 OOO) was dissolved in boiling distilled water at a concentration of 2 g/100 mL. Acetylene was purified by bubbling through three water traps to minimize the acetone contaminant. Ethylene (MG Scientific Gases, 99.5%, CP) was used as supplied. Solutions for photolysis were prepared by mixing the appropriate amounts of the TiO, and Mo stocks with 2 mL of the PVA stock and enough H,O to bring the volume to 23.5 mL in a quartz reaction cell with parallel windows and a 2.0-cm light path. The pH was adjusted with NaOH, and the solution was bubbled for 30 min with either acetylene or ethylene to remove air and saturate the solution. Photolysis was performed with an Osram HBO 200-W superpressure Hg lamp filtered through 20 cm of H,O and a 320-nm cutoff fiiter. Actinometry was performed with the uranyl oxalate actinometer using glass cutoff filters to limit the incident radiation to the 320-385-nm region where the TiO, absorbs or scatters nearly all the incident light. Full intensity irradiation into the actinometer gave, typically, 1.0 X lo-, einstein/h of incident light. (6) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J. Phys. Chem. 1985, 89, 626. (7) Furlong, D. N.; Wells, D.; Sasee, W. H. F. J. Phys. Chem. 1985, 89, 1922. ( 8 ) Anpo, M.; Kondo, M.; Kubokawa, Y. Chem. Express 1987,2,65. (9) Cai, 24.;Kuntz, R. R. Langmuir 1988,4,830. (10) Howell,N. F.; Murray, W. M. J . Am. Chem. SOC.1936,58,1689. (11) Stark, J. G. J. Chem. Educ. 1969,46,505. (12) Hahn,M.; Wieghardt, K. Znorg. Chem. 1984,23,3977. (13) Rodgers, K. R.; Murmann, R. K.; Schlemper, E. 0.; Shelton, M. E.Inorg. Chem. 1985,24,1313. (14) Ojo, J. F.; Saeaki, Y.; Sykes, A. G. Inorg. Chem. 1976, 15, 1006.
Results and Discussion Interaction of the MOW) dimer and the M O W ) trimer with trapped electrons on the TiO, surface can be observed directly by mixing degassed (with N,) solutions of the Mo species with preirradiated TiO, solutions which exhibit the characteristic blue color associated with the Ti(II1) centers (or trapped electrons). In each case, rapid bleaching of the blue Ti(II1) color is observed upon mixing while the characteristic colors of the Mo species are also altered. It thus appears that Ti(II1) centers are capable of reducing both MOW)and Mo(1V) to lower oxidation states. During photolysis, the blue color of the Ti(II1) centers redevelops, indicating that production of the trapped electron is more rapid than the rate at which it can be transferred to substrate. The structural fate of the dimeric Mo(V) and the trimeric Mo(1V) while they are undergoing reduction or cycling through oxidation states during the photolysis period cannot be ascertained in this study. It is reasonable that these species, like the Mo(V1) monomer, become attached to the TiO, surface through Ti-O-Mo bonds during the reduction process and remain immobile during the course of the photolysis experiment. I t is also assumed that the exceptional stability of the dimeric and trimeric structures permits them to remain intact even while one or more members of the cluster are undergoing oxidation state changes. Reduction of the MOW)dimer to Mo(1II) without loss of the dimeric structure has been reported,15but reoxidation of the Mo(II1) dimer is not reversible and may involve rate-limiting chemical reactions. Reduction of Mo(1V) to Mo(II1) is quasi-reversible.'&'' The very slow exchange rates of the bridgin oxygens in the MOW) dimerls and the M O W ) trimer' fi,l9also suggests a stable structure for this species. Even if some structural changes occur, however, the binding of the Mo species to the TiO, surface probably results in retention of a spatial relationship similar to that of the parent species. In support of these assumptions, it can be demonstrated that both Mo species are firmly attached to the colloidal surface after photolysis by precipitating the suspension with a heavy ion and observing the color associated with the Mo species remaining with the solid material rather than the supernatant. Also, it is clear from comparison of relative catalytic activity and physical behavior that these species not only differ from each other but also from the monomeric Mo(V1) precursor. For example, while all of the Mo species studied precipitate (with PVA and TiO,), the Mo(V1) solutions typically turned (15) Chalilpoyil, P.; Aneon, F. C. Inorg. Chem. 1978,17,2418. (16) Richene, D. T.; Sykea, A. G.Inorg. Chem. 1982,21,418. (17) Paffett, M. T.;Aneon, F. C. Inorg. Chem. 1983,22,1347. (18) Hinch, G. D.; Wycoff, D. E.; Murmann, R. K. Polyhedron 1986, 5, 487. (19) Richens, D. T.; Helm,L.; Pitted, P.-A.; Merbach, A. E.; Nicolo, F.; Chapuis, G. Inorg. Chem. 1989,28, 1394.
784 Langmuir, Vol. 6, No. 4, 1990
Thabaiti and Kuntz
n
-1
t?*
0
2
4
6
8
0
1
PHOTOLYSIS TIME (HOURS)
Figure 1. Typical product evolution from acetylene reduction as a function of photolysis time. Ethylene from dimeric cluster (A), trimeric cluster (B),and bare TiO, (C).Ethane from dimeric cluster (D) and trimeric cluster (E). No ethane was produced from TiO, in the absence of molybdenum. Loading was 43 dimers/particle and 56 trimers/particle. All data were taken at pH 8-9.
brown-to-black while both the Mo(V) dimer and Mo(1V) trimer attain a color which is dull yellow-brown and show much less precipitate. The presence of partially oxidized PVA in the photolysis mixture precluded determination of the oxidation state of Mo after photolysis. Since the oxidation states are altered during the course of the catalytic action, it is improper to identify the adsorbed species as MOW) or M O W ) . Assuming the structural character is maintained as argued above, these species will be identified only as Mo dimer (Mo,) and Mo trimer (MoJ species hereafter. The primary products from acetylene reduction are ethylene and ethane. Some CO, from complete oxidation of PVA was observed chromatographically but not measured quantitatively. Typical patterns for evolution of these products are shown for bare TiO,, Mo,-doped TiO,, and Mo,-doped TiO, under similar pH conditions in Figure 1. In agreement with an earlier report? bare TiO, produces only the ethylene product, and all ethane produced must be the result of interaction with the Mo cluster site. In addition, the Mo, cluster also enhances the ethylene yield from that observed on the TiO, surface alone. Consequently, the Mo dimer cluster appears to promote the production of both ethylene and ethane. There is no indication of ethylene production from the trimeric cluster above that expected from interaction with the TiO, surface. The clusters differed significantly when ethylene rather than acetylene was used as the substrate. Under otherwise identical conditions, ethylene is not further reduced to ethane on bare TiO, or on the Mo,-doped TiO,. The dimeric cluster does, however, catalyze this transformation. Comparison of the ethane yields from reduction of acetylene and ethylene under similar conditions appears in Figure 2. Preliminary study of this reaction indicated that the ethylene reduction rate was dependent on dimer loading but not significantly on pH. The course of acetylene reduction to ethane on this cluster could involve ethylene as an intermediate, but the absence of an induction period for ethane formation and the significantly larger yield of ethane in the presence of minor amounts of the ethylene product would seem to discount this possibility. I t would appear that ethylene does not bind to the trimeric Mo cluster or a t least is not bound in a way that permits reduction to ethane. We conclude that the ethane is formed by a 4-e transfer through both of the Mo cluster sites. This conclusion is consistent with the
2
3
4
5
6
TIME (HRS)
Figure 2. Ethane production from the reduction of acetylene (A) and ethylene (B)in the presence of the dimeric cluster. Loading was 43 dimers/particle. Data were taken at pH 8-9. Table I. PHEffect on Product Quantum Yields' PH 2.5 2.9 4.8 6.2 9.2 9.4
dimer ethylene ethane 0.0033 0.0029 0.0040
trimer ethylene ethane
O.OOO9 0.0025 0.0020
0.0007 0.0012
0.0029
O.OOO9
0.0017 0.0015
e equivalents 0.0102 0.0078 0.0088 0.0126 0.0120 0.0094
(0.0052) (0.0028) (0.0048) (0.0086) (0.0080) (0.0036)
Dimer loading was 43/particle and trimer loading was 56/particle. The e equivalents column refers to the quantum yield for total reducing equivalents. Values in parentheses refer to reducing equivalents transferred through the cluster (see text).
earlier monomeric Mo study, but it does not clarify the time course of the electron transfer to substrate. pH Effect. The yields of products from bare TiO,, Mo,-doped TiO,, and Mo,-doped TiO, were measured in the pH range 2.5-9.5. These data are shown in Table I. As in the earlier study, ethylene production which reults primarily from the TiO, surface interaction shows a broad minimum in the pH 4 6 region while ethane, which can be attributed entirely to the cluster site, exhibits a maximum in this same region. Correlation of the maximum ethylene production with the pH region in which the colloidal particle is nearly neutral possibly indicates a more favorable binding of acetylene to the colloidal surface or better access to the Ti(II1) centers under neutral conditions. Since ethane apparently results from initial binding of acetylene to the Mo cluster, similar considerations must be applicable here. This study does not provide any evidence about the causes of these results. The total quantum yield of reducing equivalents is calculated as 4aethane+ 2@ethylene. These yields, along with a calculation of reducing equivalents transferred through the Mo cluster, appear in Table I. Reducing equivalents transferred through the Mo cluster site are calculated as In each case, the 4@.,-, + 2[@& ,ens - @'ethylene(TiOz)l. total yield of re&cing equivalents increases with pH, as anticipated from the increase in reducing potential with pHe2O For Mo,, the efficiency of transfer through the catalytic site is enhanced in the neutral region. The decrease at higher pH is not so significant for Mo, in which the cluster site apparently is involved in ethylene production as well. Mo Loading Effect. Loading of Mo, and Mo, species was calculated on the basis of 5-nm-diameter spherical colloidal particles of 4 g/cm3 density. This calcula(20) Duonghong, D.; Ramsden, J.; Gratzel, M. J. Am. Chem. SOC.
1982, 104, 2977.
:=
Langmuir, Vol. 6, No. 4, 1990 705
Hydrogenation of Acetylene by Mo, and Mo, Oxo Species O'Oo5
z
3
's
8
0.002-
5 0.001
I
0
IO
20
.
I
30
.
,
.
40
,
50
LOADING (Mo CLUSTERSIPARTICLE)
Figure 3. Effect of particle loading on product quantum yields: ethylene on dimeric clusters (A) and trimeric clusters (B);ethane on dimeric clusters (C)and trimeric clusters (D). All data at pH 7.
tion gives an average of about 2000 TiO, units/colloidal particle with approximately 700 surface units.' The effect of loadings up to 50 Mo clusters/particle appears in Figure 3. The two clusters show significantly different behavior. The Mo, cluster shows no enhancement of ethylene production over that observed in the bare TiO, sol. In the production of ethane, there is a dramatic increase at loadings below 7 clusters/particle and then a slow increase in efficiency with increased loading. With Mo,, the ethylene increases rapidly a t loadings between 4 and 11 clusters/particle and then maintains this activity unchanged with increased loading. Changes in ethane production with loading of the Mo, cluster are less abrupt but show a steady increase with increased loading. In both cases, loading in excess of 40-50 clusters/ particle led to a rapid decrease in efficiency of production of both ethylene and ethane. Ethylene decreased by 25% and ethane by 33% when the Mo, loading increased from 43 to 47. Yield reductions for a change in Mo, loading from 39 to 64 were 41 % for ethylene and 58% for ethane. A similar decrease was noted for the Mo monomer system' above a loading of approximately 40 Mo/particle. Discoloration and aggregation may be responsible for a rapid decrease in efficiency above a particle loading of approximately 40, but since these phenomena also occur at lower loadings without affecting efficiency, another explanation is required. At the higher loadings, a significant fraction of the particle surface (-2050%) should be covered with the Mo cluster, and interaction of the cluster units is likely. This interaction could lead to greater separation of the reduced Mo sites and a lower probability for multielectron transfer, or it may represent formation of a polymeric layer which is just not effective in catalysis. Ethylene yields, which depend primarily on interaction with the TiO, surface in the Mo, system, decrease much more rapidly than the extent of surface coverage would predict. This observation leads to another possibility-the significant surface coverage decreases the efficiency of the electron-hole separation process, possibly by interfering with hole scavenging by PVA. The efficiency of the catalytic site is represented by the turnover number (molecules/cluster per h). These numbers are represented for the dimer and trimer system in Figure 4. In each case, maximum efficiency occurs at loadings less than 10 clusters/particle for ethane production. The ethylene production catalyzed by the Mo dimer site shows a different behavior with a maximum efficiencyin the 10-15 loading range. It should be noted
- - ,
0
I
10
20
30
.
*
50
40
LOADING (Mo CLUSTERS/PARTICLE)
Figure 4. Turnover number as a function of cluster loading. Excess ethylene from dimeric clusters (B)(see text); ethane from dimeric (A) and trimeric (C)clusters. Data taken at pH 7. Table 11. Intensity Effect on Product Quantum Yields Z = 1.0 X Z = 3.0 X lo4 dimer" trimerb
ethylene ethane ethylene ethane
einstein/ h 0.0029 0.0014 0.0024 0.m9
einateinlh 0.0043 0.0023 0.0033 0.0017
Dimer loading at 43;pH 2.8. Trimer loading at 56, pH 2.9.
that the enhanced efficiency for each cluster site at low loadings does not, in general, result in higher quantum yields. From Figure 3, it is clear that the ethane quantum yields for Mo, clusters continue to increase with loading even though the efficiency/cluster is decreasing. The effect is less important for the Moa cluster since the ethane yields increase only about 20% from a loading of 7/ particle to 39/particle. Only in the case of ethylene from the Mo, cluster does the cluster-catalyzed yield remain constant as loading increases. Longer irradiations (24 h) a t the higher loadings result in only 1-2 ethane molecules/Mo for both the dimeric and trimeric clusters, and the production rate slows considerably. A t lower loadings, however, a turnover of 3-5 ethane molecules/ cluster is observed after only 5 h of irradiation, supporting the idea that the catalytic site does indeed cycle. Accumulation of products from the sacrificial electron donor and aggregation of the sol appear to be responsible for the lower efficiencies at long irradiation times. Intensity Effects. Decreasing light intensity results in increased quantum efficiency for reduction of acetylene. This effect probably occurs because the transfer of electrons to the substrate through the catalytic site or the surface of the particle is a slow process compared to electron-hole recombination or annihilation of Ti(II1) centers by newly created holes. Table I1 shows the effect of to reducing the incident light intensity from 1.0 X 3.0 X low4einstein/h. A similar effect was observed in the Mo monomer system, and higher quantum yields at still lower intensities are to be expected.
Conclusions Dimeric and trimeric Mo species are useful catalysts for transfer of trapped electrons on the colloidal surface to acetylene. The 4-e reduction to ethane occurs only on the Mo cluster sites while 2-e reduction to ethylene can occur on the colloidal surface but, in the case of Moz, is also enhanced by the presence of the cluster. In addition, the Mo, site can also transfer electrons to ethylene while Moa does not. Quantum yields for total reducing equivalents transferred through the Mo cluster sites
786
Langmuir 1990,6, 786-791
Table 111. Comparison of Mo Clusters in Acetylene Reduction. monomer* dimer trimer ethane 0.0010 0.0011 O.oO08 ethylene' 0.0018 reducing equiv 0.0040 0.0080 0.0032 a Comparisons made at a common loading of about 10 clusters/ rarticleand at pH 7. Values in the table are product quantum yields. Data from Zun-Sheng Cai, Ph.D. Thesis, University of Missouri, 1987. Ethylene formed through the Mo cluster.
are dependent on factors such as Mo loading, pH, and light intensity. Under similar conditions of loading, pH, and intensity, the Mo, is more efficient for total electron transfer than either the monomer or trimer (see Table 111). The monomer and dimer are about equally effective for the 4-e process and somewhat better than the trimeric material. In general, the potential enhancement of multielectron reduction efficiency by closely spaced Mo reducing centers as enforced by the dimer and trimer cores is not realized. The relatively low efficiency of these processes even
in the presence of a good electron donor such as PVA indicates that electron transfer to acetylene does not compete effectively with other fates of conduction band electrons. Whether the rate-limiting step is reduction of the Mo clusters by Ti(II1) or substrate binding and subsequent electron transfer cannot be determined from this study. Buildup of the blue Ti(II1) color centers during the photolysis does indicate that the rate of production of charge carriers exceeds the rates of the subsequent steps and suggests that lower intensities should increase the quantum efficiency. Acknowledgment. This work was supported in part by grants from the National Science Foundation (CBT8813146), the University of Missouri Research Council, and the Weldon Spring Endowment administered by the University of Missouri. Assistance from Professor R. Kent Murmann with preparation and purification of the Mo(IV) and MOW)aquo ions is also greatly appreciated. Registry No. TiO,, 13463-67-7; Mo,O:+, 54429-28-6; Mo30d4+,97252-76-1;poly(viny1 alcohol), 9002-89-5;acetylene, 74-86-2;ethane, 74-84-0; ethylene, 74-85-1.
From Cetyltrimethylammonium Bromide Micelles to 2-Butoxyethanol Aggregates Stabilized by Cetyltrimethylammonium Bromide Molecules: A Small-Angle Neutron-Scattering Study Francois Quirion*Pt and Maurice Driffords INRS-Energie, Varennes, Qu6bec, Canada J3X 1S2, and CEN de Saclay, 91911 Gif-sur- Yvette, Cedex France Received June 12, 1989. In Final Form: October 24, 1989
Mixed micelles of cetyltrimethylammonium bromide (CTAB) and 2-butoxyethanol (BE) are investigated through small-angleneutron scattering, and the micellar parameters are compared to those obtained for CTAB micelles in D,O at 26 OC. Although the mole ratio of BE to CTAB is kept constant at 5 or 10, the micellar parameters are not constant with respect to the total concentration (MBECTAB = M TAB + MBE). Our results show that at low concentrations the mixed aggregates resemble micelles of C%AB while at higher concentrations they behave like BE aggregates stabilized by CTAB molecules. This is supported by quasi-elastic light scattering. For the two ratios studied, it is suggested that the micellar parameters depend only on the total concentration. Around MBECTAB = 0.5, there is a transition that is consistent with an increase of the counterion binding. We suggest a dry headgroup shell for CTAB micelles while for the mixed aggregates there might be some hydration associated to BE (ND2,JNBE
-
2).
Introduction Aqueous solutions of surfactant and cosurfactant are able to dissolve large quantities of oil to form stable microemulsions. These systems have been studied extensively over the past 15 years, and reviews'*2 have been published on the subject. INRS-Energie. CEN de Saclav. (1) Prince, L. MI Microemulsions: Theory and Practice; Academic Press: New York, 1977.
0743-7463/90/2406-0786$02.50/0
Because of its name, the cosurfactant is often viewed as a passive ingredient that helps the surfactant to solubilize oil. This is in contradiction with the detergentfree microemulsions, where the role of the surfactant is played by an alcohol such as l - b ~ t a n o l 2-propan01,~~~ ,~ and 2-butoxyethano1.B Recently, Quirion et a17 have studied aqueous solutions of 2-butoxyethanol (BE) through (2) Holt, S. L. J. Dispersion Sci. Technol. 1980, 1,423. (3) Roux-Desgrangp, G.; Grolier,J.-P. E.; Villamanan,M. A.; Casanova, C . Fluid Phase Equilcb. 1986, 25, 209.
0 1990 American Chemical Society
