J. Phys. Chem. 1992, 96,4533-4531 correlate with C-S bond strength. Furthermore, alkyl groups are expected to be strongly bound to Mo( 110); subsequent rapid and irreversible dehydrogenation of the alkyl is anticipated. Future experiments are planned to better address this point, however. The isotopic labeling experiments are also in accord with the proposed reaction scheme. Only a single surface deuterium is incorporated into the isobutane product, consistent with the formation of a single C-H(D) bond. Likewise, no deuterium is incorporated into the isobutene product. The kinetics for hydrocarbon formation are modified by the deposition of sulfur during the course of temperature programmed reaction. We propose that the complex reaction kinetics for hydrocarbon formation reflect the changes in C-S bond activation due to the presence of sulfur. The kinetics for hydrocarbon formation have been previously shown to be dependent upon surface sulfur, carbon, hydrogen, and hydrocarbon fragment^.^,'^ Hydrocarbon formation is accompanied by sulfur deposition on the surface and competing nonselective decomposition of the thiolate intermediate produces surface hydrogen, hydrocarbon fragments, and sulfur. Since these processes occur at low temperatures for 2-methyl-2-propanethio1,the reaction kinetics for hydrocarbon formation are being altered by the presence of surface adsorbates on the surface over a wide range of temperatures. More detailed studies of the effect of sulfur are planned. A possible alternative explanation for the complex kinetics of 2-methyl-2-propanethiol desulfurization could be due to the binding geometry. 2-Methyl-2-propanethiol has been shown to bind in multiple coordination geometries in organometallic comp l e x e ~ . ~ +In~fact, ~ it has been proposed that the reactivity of these different sites would be very different.3s Since the kinetics for isobutane formation seem to increase with 2-methyl-2propanethiol coverage and also seem to consist of a t least two separate pathways, this is a viable alternative. The bonding geometry of 2-methyl-2-propanethiol is not known. It is not likely that the coverage-dependent kinetics are due to repulsive steric interactions between the methyl groups of the (35) Coucouvanis, D.; Hadjikyriacou, A.; Kanatzidis, M. G. J. Chem. Soc., Chem. Commun. 1985, 1224. (36) Christou, G.; Holm, R. H.; Sabat, M.; Ibers, J. A. J . Am. Chem. Soc.
4533
2-methyl-2-propyl thiolate intermediate and the Mo( 110) surface. Previously, interaction between the methyl groups of 2-propanol and the Fe( 100) surface have been proposed to destabilize the 2-propoxide intermediate.38 A similar effect could lead to destabilization of the 2-methyl-2-propyl thiolate intermediate and to desulfurization a t lower temperatures. Electron energy loss spectra also argue against this explanation, as the 2-methyl-2propyl skeleton is relatively unperturbed in the vibrational spectrum, since the loss energies agree well with gas-phase values. Rather, the kinetics of 2-methyl-2-propyl thiolate desulfurization become more complex with increasing thiolate coverage. At low 2-methyl-2-propyl thiolate coverages (Figure 4), desulfurization leads to isobutane and isobutene with relatively simple kinetics. However, as the coverage increases, the kinetics become more complex. This is most readily explained on the basis of a varying amount of sulfur deposition during the course of reaction. Conclusions 2-Methyl-2-propanethiol on Mo( 110) reacts via three competing channels on Mo( 110): thiolate hydrogenolysis yielding isobutane, C-S and C-H bond scission to produce isobutene and nonselective decomposition producing adsorbed carbon and sulfur and gaseous dihydrogen. X-ray photoelectron and high resolution electron energy loss spectra establish that 2-methyl-2-propyl thiolate is formed upon adsorption. At saturation coverage, -80% of the 2-methyl-2-propanethiol produces hydrocarbons, while 20% reacts nonselectively. The rate and selectivity for hydrocarbon production depend on the coverage of sulfur, hydrogen, surface thiolate, and, possibly, hydrocarbon fragments in a complex fashion. The initial rate of hydrogenolysis of 2-methyl-2-propyl thiolate is faster than primary thiolates, such as the ethyl derivative, suggesting that hydrogenolysis kinetics correlate with the homolytic C-S bond strength.
-
Acknowledgment. We gratefully thank the Department of Energy, Basic Energy Sciences, Grant No. DE-FG02-84ER13289 for support of this work. Registry No. Mo, 7439-98-7; Z-methyl-2-propanethio1,75-66-1; isobutane. 75-28-5.
1981. 103. 6269. ~
~~
(37) Coucouvanis, D.; Lester, R. K.; Kanatzidis, M. G.; Kessissoglou, D.
P.J . Am. Chem. SOC.1985, 107, 8279.
(38) Benziger, J. 8.; Madix, R. J. J. Catal. 1980, 65, 36.
Inclusional Complexes of Decyltrimethylammonium Bromide and &Cyclodextrin in Water E. Junquera, E. Aicart, and G. Tardajos* Departamento QuTmica Fhica I , Facultad de Ciencias Quimicas, Universidad Complutense de Madrid, 28040-Madrid, Spain (Received: September 26, 1991) T h e inclusional process of decyltrimethylammonium bromide ( D T A B ) into the cavity of j3-cyclodextrin (j3-CD) has been studied by measuring speed of sound (u) of aqueous solutions of D T A B in the presence of various constant concentrations of j3-cyclodextrin at 298.15 K. The predominant complex formed has a stoichiometry of 1:l. The apparent critical micellar concentration, cmc* (the cmc for the system DTAB + j3-CD + HzO), is found to increase upon the addition of cyclodextrin, while the concentration of free surfactant available for the micellization process in the presence of &CD ([DTAB],) increases slightly. The binding or association constant of the complexation equilibrium is evaluated from u measurements by using a nonlinear regression method. The resulting K value is analyzed and compared with those given by other researchers and t h e discrepancies are discussed.
Introduction cycldextrim (abbreviated as C D , ~hereafter), which are cyclic a(l + 4) linked oligosacharides built UP from six ( a - ~ ~ Seven ), (8-CD), and eight (7-CD) a-Dglucopyranose units, form inclusion Author to whom correspondence should be addressed.
compounds with smaller molecules which fit into their 5-8-A cavity. The most important feature of these compounds is that the 'host" Component can admit 'guestn molecules without any bonds being formed.'-3 (1) Bender, M. L.;Komiyama, M. Cyclodexrrin Chemistry; SpringerVerlag: Berlin, 1978.
0022-3654/92/2096-4533%03.00/0 0 1992 American Chemical Society
4534
The Journal of Physical Chemistry, Vol. 96, No. 1 1 , 1992
Junquera et al.
TABLE I: Values of Apparent Critical Micellar Concentration (cmc* ), [DTAB], Stoichiometry of the Complex [&CD]:[DTAB] (A), Postmicellar Slopes (Sm),and Association Constants (K)Depending on [B-CD] IB-CDl. mM cmc*, mM [DTAB],, m M A S,, m 4 . m M - l K . M-I 0.000 66.5 66.5 0.021 7.501 74.9 67.5 1.02 0.021 387 9.993 76.9 67.5 1.06 0.022 399 13.045 79.9 68.4 1.13 0.021 397 15.959 83.8 69.4 1.11 0.023 263’ A = 1.08 K = 394 f 80 “As has been explained in the text, this value has not been considered in the mean calculation of K.
The increasing number of publications and patents dealing with these inclusion compounds shows that general interest in their physical and chemical properties has grown considerably over the past 20 years. This is due, on the one hand, to the fact that the study of inclusion compounds in fundamental research furnishes information about noncovalent intermolecular forces and that they also serve as models for studying topochemical problems and the mode of action of enzymes. On the other hand, cyclodextrins are becoming widely used in the food, pharmacological, cosmetic, and agricultural industries as encapsulating agents to protect sensitive molecules in hostile environments. Thus, inclusion of organic compounds by CD’s sometimes inhibits an oxidation or a biological digestion of the included compound^;^-^ CD’s also show catalytic activities in many kinds of reactions such as hydrolysis,6 decarboxylati~n,’~~ hydrogenation of olefins? and site-specific substitution reactions of included compounds.I0 Due to a lack of free rotation about the glycosidic bonds, the CD’s are cone-shaped, with all secondary -OH groups located at one end of the toruslike molecule, while all primary -OH groups are a t the other end. As a result of this, the cavity, due to the presence of H atoms and -0- bonds, is slightly apolar having a clear affinity to encapsulate hydrophobic moieties. Actually, it is well-known that the hydrophobic interaction between apolar moieties of host and guest molecules plays an important role in the complexation process of cyclodextrins.’1-13 It is thus interesting to study the interaction of cyclodextrins with surfactant molecules which have an ionic head group as well as a large hydrocarbon chain of varying hydrophobicity. These surfactants molecules are known to form micelles and it is believed that the formation of the inclusion complex may affect the micellization process, though this effect is not well understood yet because, together with the monomer-micelle exchange, an additional equilibrium, due to the association complex process, appears. The characterization of this complexation phenomenon and its influence on micellar properties has been the subject of many different studies in the past years, primarily by using conductometricI4-l9 or conductance stopped-flowz0techniques. However, (2) Saenger, W. Inclusion Compounds; Atwood, J . L., Davies, J. E. D., MacNicol, D. D., Eds.; Academic Press: London, 1984; Vol. 2. (3) Clarke, R. J.; Coates, J. H.; Lincoln, S. F. A d a Carbohydr. Chem. Biochem. 1988, 46, 205. (4) Uekama, K. Pharm. Int. 1985, 6, 61. (5) Szejtli, J. The Cyclodextrins and Their Inclusion Complexes; Academiai Kiado: Budapest, Hungary, 1982. (6) (a) VanEtten, R. L.; Sebastian, J. F.; Clowes, G. A.; Bender, M. L. J. Am. Chem. SOC.1967, 89, 3242. (b) VanEtten, R. L.; Clowes, G . A,; Sebastian, J. F.; Bender, M. L.Ibid. 1967.89, 3253. (c) Kitano, H.; Okubo, T. J. Chem. SOC.,Perkin Trans. 2 1977, 432. (d) Ihara, Y . ;Nakanishi, E.; Nango, M.; Koga, J. Bull. Chem. SOC.Jpn. 1986, 59, 1901. (7) Straub, T. S.;Bender, M. L. J. Am. Chem. Soc. 1972,94,8875, 8881. (8) Cramer, F.; Kampe, W. J. Am. Chem. SOC.1965, 87, 1115. (9) Komiyama, M.; Hirai, H. Bull. Chem. SOC.Jpn. 1983, 56, 2833. (10) Breslow, R.; Campbell, P. J. Am. Chem. SOC.1969, 91, 3085; Biorg. Chem. 1971, 1, 140. ( 1 1) Cramer, F.; Hettler, H. Naturwissenschaften 1967, 54, 625. (12) Saenger, W. Angew. Chem., Int. Ed. Engl. 1980, 19, 344. (1 3) Tanford, C. The Hydrophobic Effect. Formation of Micelles and Biological Membranes, 2nd ed.; John Wiley 8c Sons: New York, 1980. (14) Palepu, R.; Reinsborough, V. C. Can. J . Chem. 1988, 66, 325. (15) Palepu, R.; Reinsborough, V. C. Can. J. Chem. 1989, 67, 1550. (16) Palepu, R.; Richardson, J . E.; Reinsborough, V. C. Longmuir 1989, 5, 218.
the measurement of a property such as the speed of sound has not been often used to study these kind of processes. Only Hoiland et a1.,21-z2 Nomura et al.,23,24 and Verrall et al.25have worked with the speed of sound to study structural changes in micellar systems at different conditions. In this work, speeds of sound of aqueous solutions of DTAB + j3-CD in the pre- and postmicellar region at 298.15 K have been measured, revealing the ability of this technique to detect structural changes in the medium studied. The formation of the 1:1 complex is readily observed, in agreement with previous reports.l6I8 The determination of the corresponding associations constants is surrounded by some controversy, as there exist different values of K for the same system, which depend primarily on the method used to deal with.lkZ0 In the present work, a new method based on speed of sound measurements and a Marquardt algorithm to fit the data is proposed. The validity of this method together with a comparison with those proposed by other authors is rigorously discussed. Experimental Section Materials. Decyltrimethylammonium bromide (DTAB) was purchased from Eastman Kodak Co. and 8-cyclodextrin (0-CD) was obtained from Aldrich Co. and they were used without further purification. Two thermogravimetric analyses (TG) were done in both cases to determine the amount of water. The results of these analyses, which were obviously taken into account to calculate solution concentrations, were the following: 13.5% of H 2 0 in j3-CD and 0% in DTAB. For the preparation of solutions, bidistilled, deionized (taken from a Millipore Super-Q system) and also degasified water was used. Speed of Sound Measurements. Ultrasonic speeds, u, were measured by using a pulse-echo-overlap technique of fixed path type a t a frequency of 10 MHz. This technique operates in a multiple-echo mode with broadband pulses, and the experimental procedure as well as the details of the equipment used are fully described elsewhere.26 The mixtures were formed in a successive dilution cell and the surfactant solution was added by using a 655 Dosimat Metrohm burette with a precision of 2 X m3. The buret cylinder is thermostated by a recirculation water circuit from the bath where the dilution cell is immersed, and whose temperature is maintained at 298.15 K. Calibration of the distance between the transducer and the reflector was made from the speed of sound in pure water (1496.74 m d ) reported by Kroebel and Mahrt.27 (17) Satake, I.; Ikenoue, T.; Takeshita, T.; Hayakawa, K.; Ma&, T. Bull. Chem. SOC.Jon. 1985.58. 2146. (18) Sataie, I.; Yoshida, S.;Hayakawa, K.; Maeda, T.;Kusumoto, Y. Bull. Chem. SOC.Jpn. 1986, 59, 3991. (19) Okubo, T.;Kitano, H.; Ise, N . J . Phys. Chem. 1976, 80, 2661. (20) Okubo, T.; Maeda, Y . ;Kitano, H. J. Phys. Chem. 1989, 93, 3721. (21) Backlund, S.; Hoiland, H.; Kvammen, 0.J.; Ljusland, E. Acra Chem. Scand. 1982, 698. (22) Hoiland, H.; Vikingstad, E. J. Colloid Interface Sci. 1978,64, 126. (23) Zielinski, R.; Ikeda, S.;Nomura, H.; Kato, S. J. Colloid Interface Sci. 1987, 119, 398. (24) Zielinski, R.; Ikeda, S.;Nomura, H.; Kato, S. J. Colloid Inierfoce Sci. 1988, 125, 497. (25) Alauddin, M.; Rao, N . P.; Verrall, R. E. J. Phys. Chem. 1988, 92, 1301. (26) Tardajos, G.; DTaz Pefia, M.; Aicart, E. J. Chem. Thermodyn. 1986, 18, 683.
Inclusional Complexes of DTAB-Cyclodextrin in Water 1524
1520
j
[P-CO]
[fi-CO]
= 0.007 M
= 0013 M
1
6
1
! l
E
\ 2
1 [p-CD]
1
= 0.010 M
\
P
3
a I
I
1496
I
I
0
30
I
60 [DTAB]
I
/
90 mmol I-'
I
120
150
+
Figure 1. Speed of sound u for the systems DTAB + @-CD H20as a function of surfactant concentration at a constant [@-CD]at 298.15 K: Q, [b-CD] = 0.000 M; 0,[@-CD]= 0.007 M; 0,[b-CD] = 0.010 M; A, [b-CD] = 0.013 M; 0 , [@-CD]= 0.016 M. [DTAB]
The u measurements were made for aqueous solutions of 8-CD and DTAB as a function of DTAB concentration for different constant values of [b-CD]. The range of [DTAB] goes from pre- to postmicellar region in order to study how the inclusion complex formed affects to the parameters of the micellization process, Le., critical micellar concentration (cmc), aggregation number, etc. The range of [b-CD] goes up to its solubility limit in water, which is approximately 1.85 g/100 mL (0.016 M). The reproducibility of u data is f0.02 mms-' and temperature stability is f l mK. Results and Discussion The ultrasonic velocities, u, of the H20+ DTAB + 0-CD system as a function of DTAB concentration are given in Figure 1. The characteristic feature of the u-[surfactant] profile is that there can be drawn two straight lines having different slopes, as is clearly shown in the figure. The point of intersection corresponds to the apparent critical micellar concentration, cmc* which represents the cmc for the system in the presence of CD. The cmc* was clearly observed to become larger upon the addition of &CD. These values are shown in Table I. It can be also observed that in the postmicellar region all the curves present the same slopes, S,, independently of [@-CD]. The values of S, have been summarized also in Table I. Since the concentration of the complex is basically constant in this postmicellar zone, all the changes on the speed of sound in this region can be assigned to the micelles. This feature implies that the micelles are the same independently of the @-CDconcentration or which is the same, the cyclodextrin does not participate in the micelle. In that case, it is possible to say that" the aggregation number is constant, whether the surfactant is in the presence or in the absence of the cyclodextrin. In Figure 2, which represents a zoomed view of the marked square in Figure 1, experimental values of Au are plotted against [DTAB] in the premicellar region, being Au = u - uo, where uo is the speed of sound at 8-CD initial solution (before any surfactant has been added). At a particular surfactant concentration, a clear change in Au is observed due to the formation of the DTAB-P-CD inclusion complex. These points can also be fitted by two straight lines of different slopes, whose intersection permits us to obtain the ratio [@-CD]/[DTAB],or which is the same, the stoichiometry of the complex, A . The values of A for the four 8-CD + DTAB systems measured are given also in Table I. As can be seen, the (27) Kroebel, W.; Mahrt, K.H. Acustica 1976, 35, 154.
/
mmol.l-'
Figure 2. Experimental points (symbols from Figure 1) and calculated values (solid line) of Au vs [DTAB] at different constant b-CD initial concentrations.
I:
90
65 60
REGION
4
Figure 3. Apparent critical micellar concentration, cmc*, and [DTABIf in the postmicellar region as a function of [@-CD].
stoichiometry of the complex is approximately 1.1:l &CD:DTAB, indicating that the chief inclusion complex of 8-CD with DTAB in this range is 1:l with p s i b l y a 10% of 2:l complex also present. With the knowledge of cmc* and the stoichiometry of the complex for each 8-CD particular concentration, it is possible to determine the concentration of free monomer on the postmicellar region, [DTABIf, when the CD is present by using the expression [DTABIf = cmc* - [DTAB],,
= cmc* - [@-CD]/A
(1)
where [DTAB],, is the concentration of surfactant forming the complex and [8-CD] the concentration of cyclodextrin kept constant in each case. [DTABIf values are presented in Table I and, together with cmc* values, they are plotted against [B-CD] in Figure 3. As can be clearly seen, the apparent critical micellar concentration, cmc*, increases linearly with 8-CD, while [DTABIf is almost constant, although, rigorously speaking, a slight increase is observed. Extrapolation to zero 8-CD concentration, for both lines, yields the cmc of pure DTAB in perfect agreement with the value measured for us (66.5 X mol-L-I) and with literature value also.**
4536 The Journal of Physical Chemistry, Vol. 96, No. 11, 19‘92
Junquera et al. TABLE II: Statistics of the Fit of Au vs f with Expression 5
W-CDI, mM
I
0
0
20-0
0
0
0
m
E
0
0
0
0
0
I
o
\ 19-0
0
0
0
0
s
0
0
0
0
0
0
0 0 0
0
0
0
0 0 0
10
-Woo
0
0
0
D O 0 E
D
0
0
0 0
0
0
0
0
0
D O 0
DO
0
0
b o o
“
0
0 O
D
0 0
D
0
0
0 0
0
0 0 0 0
0
O
0
0 0
0 00
O
om
0
387 399 397 263
f 80 f 45 f 25 f 20
u2 1.00 f 0.08 0.91 f 0.04 0.86 f 0.03 1.28 f 0.04
a3 1.53 f 1.56 f 1.55 f 1.54 f
0.03 0.01 0.01
0.01
Q
x2
2.52 2.48 2.32 3.47
6.34 9.24 9.41 18.80
As can be seen in Table I, we have obtained a mean value for K of 394 f 80 M-I, without taking into account the last value 263 M-I. If we consider that this value corresponds to the system in which the j3-CD concentration is almost reaching its solubility limit in water (0.016 M), it seems reasonable to wipe it out of the mean calculation. Satake et al.”J8 and Reinsborough et al.I4J6 carried out the determination of binding constants K for surfactant-CD systems from conductivity measurements. They proposed the same 1: 1 association scheme and obtained K by using also a nonlinear method (Gauss-Newton algorithm) to fit their AA experimental values vs f with the expression,
00 0 0
2
a
K = a,, M-’
o m
0
0
0
0-
0
0
;
om
0
s
9
7.501 9.993 13.045 15.959
om
m
AA
(3)
where C,is the total surfactant concentration and C, is the initial concentration of j3-CD; therefore, f is given by
f= (4) where the negative root is chosen to work with a real value off and where everything except K is known. The experimental variables are Au, C,, and c h (which has been kept constant in the present case). On the other hand, substituting experimental C, and C, data in eq 4 and giving aleatory values to K ranging from 1 to 20000, there exists for each experimental point a pair of Au-f values. These curves have been calculated for all the measured systems and are drawn in Figure 4 only for the system with c h = 0.007 M for the sake of clarity. Several analytical functions have been tried in order to find the correct dependence between Au andf. The one which provided the best results, being as well the most reasonable considering the shape of the curves of Figure 4, was the tangent with two fit coefficients to adjust the curvature and the asymptote of the function, respectively, Au = a2 tan ( a d (5) On the basis of eqs 4 and 5, K being another fit coefficient, al, and using a nonlinear least-squares method (Marquard algorithm), Au vs [DTAB] experimental plots ([DTAB] = C,)have been fitted and K values evaluated and summarized in Table I. In Table I1 the statistics of the fit is shown and the resulting calculated curves are plotted (solid line) together with the experimental values in Figure 2. The agreement between experimental and calculated curves is quite satisfactory over the whole range of C,, indicating the validity of a 1:l reaction scheme. (28) Mukerjee, P.;Mysels, K. J. CMC of Aqueous Surfactant Systems; NSRDS-NBS 36; US.Government Printing Office: Washington, DC, 1971.
Af - A = (A, - A,)f
(6)
where Af and A are the equivalent conductivity in the absence and presence of cyclodextrin and Af and A, the equivalent ionic conductivity of the ionic monomer free and included into the CD cavity, respectively. When they use this expression, they are implicitly assuming that the counterion of the surfactant molecule is not complexed by 6-CD to any extent, which means that the counterion equivalent ionic conductivity is the same whether the dissociated monomer is in the absence of cyclodextrin or it is forming a complex with it. However, the same authors, basically from their selective electrode measurement^,'^ affirm that the encapsulated surfactant not only retains its counterions but also immobilizes them to some extent in the entranceway of the j3-CD torus, the mobilities of the complexed counterion being lower than the free one and consequently not cancellable. The latter affirmation is not consistent with expression 6, where an strict linearity between M and f is considered. This assumption causes the K values in ref 15 to be much greater than those given here. Besides, with this method, both researchers groups obtain different values of K depending on the initial concentration of surfactant Cs,which, from a rigorous thermodynamic point of view, is not consistent at all with a proper definition of the thermodynamic equilibrium constant. On the other hand, Okubo et al.,19 also from conductivity measurements, determined association constants for this type of host-guest systems, Le., CTAB (cetyltrimethylammonium bromide) or SDS (sodium dodecyl sulfate) with a-CD or o-CD. They calculate them by using the definition of K , expressed in eq 2. Nevertheless, they implicitly assumed that the increase in the apparent cmc (cmc*) is due only to the increase in concentration of the associated surfactant ion, thus considering that the concentration of the monomeric surfactant ion available for the micellization process remains constant even in the presence of cyclodextrins. As can be obviously observed in Figure 3 this is not rigorously correct because, although the increase of [DTABIf is very low, it is important to realize that the assumptiop is made in the denominator of expression 2. Working with such low quantities, a small error can lead to a mistake in the calculation of K . In spite of that, these authors give only one K for each system, independently of C,,which from our point of view is more correct. The values of K obtained by this group use to be one order of magnitude smaller than the ones reported by Satake et ai. or Reinsborough et al. For example, for SDS + 8-CD a t 298.15 K, Okubo et al. reported K = 356 M-I while Satake et al. obtained K = 3630 M-’ at C, = 2.6 X M and Reinsborough et al. reported values ranging from 7230 to 1380 M-’ depending on the surfactant concentration. Okubo et aLZ0also worked with a conductance stopped flow technique to analyze the relaxation curve corresponding to the inclusion process of sodium dodecyl sulfate or sodium decane-
4531
J . Phys. Chem. 1992,96,4531-4542 sulfonate with CY-, P-, or -/-CD. The results were analyzed following also the scheme I as a singlestep process and, for example, for an inclusion complex of 8-CD with a surfactant of 10 carbon atoms of chain length (like DTAB), i.e., sodium decanesulfonate, the obtained value is K = 180 M-I, which is not much different from the value K = 356 M-l they report for SDS + 8-CD (12 carbon atoms) from conductivity measurements, and with the same order of magnitude. Unfortunately, we have not found values of K for the system DTAB 8-CD in the literature to compare with our values, but in accordance with the foregoing comments we can assume that the binding constant for the complex 8-CD:DTAB should not differ very much from the binding constant for other complexes with surfactants of 10-12 carbon atoms of chain length. It can be noted that our values fall between those of Satake et al. or Reinsborough et al. and Okubo et al., but much closer to those of the last one. From our point of view, Satake et al. and Reinsborough et al. obtain higher values than we do because in their calculations they are erroneously considering the same mobilities for the free and associated counterion. However, we agree with Okubo et al. in giving only one K for each system, though, since he does not consider that in the presence of cyclodextrin the cmc for the pure surfactant is not strictly constant, his constants are lower than ours, but in any case of the same order.
postmicellar region and at various initial concentrations of 8-CD. The results of this study reveal the following: (i) The inclusion complex 8-CD:DTAB is believed to have a stoichiometry of 1:1, with a possible contribution (around 10%) of 2:l complex. (ii) The apparent critical micellar concentration, cmc+,increases linearly with the addition of F C D up to its solubility limit in water. (iii) Neither the 8-CD nor the complex form part of the micelle. Consequently, the aggregation number is the same whether the micelle is in the presence or in the absence of cyclodextrin. (iv) The concentration of surfactant available for micellization process in the postmicellar region, [DTABIf,show a slight increase with 8-CD concentration. (v) The binding constant for the complex 8-CDDTAB has been obtained by fitting Au vs [DTAB] data with a Marquardt algorithm. The resulting value is 394 M-l, independent of the concentration of the involved species, as might be expected.
Conclusions Speed of sound measurements for solutions of DTAB + 8-CD are reported as a function of DTAB concentration from pre- to
Supplementary Material Available: Tables of speed of sound data (6 pages). Ordering information is given on any current masthead page.
+
Acknowledgment. We are grateful to MEC of Spain for financial support through a DGICYT grant no. PB86-0568 and to V.G. Baonza for computer assistance. E.J.also thanks MEC for a scholarship from the FPI program. Registry NO. DTAB, 2082-84-0; &CD, 7585-39-9;&CD.DTAB, 138558-38-0.
Role of Medium-Chain Alcohols In I nterfaciai Films of Nonionic Mlcroemulsions R. Strey* and M. Jonstromer Max PIanck Institut fiir Biophysikalische Chemie, Postfach 2841, 3400 Gottingen, Germany (Received: October 4, 1991)
In this paper we report on experiments aimed at elucidating the role of medium-chain alcohols in bicontinuous micrcemulsions. We have determined isothermal sections through the phase tetrahedra of quaternary systems HzO-n-octane-C,&-C8E5 at equal amounts of water and oil. From these measurements we obtained the compositions of the middle-phase microemulsions as well as estimates for the alcohol concentrations in the bulk excess phases. The efficiency of the amphiphile combination (C,& + CsE5)increases with the alcohol chain length (n = 4,5,6, 8, 10). Furthermore, we obtained from the phase diagrams estimates for the ratio of alcohol to surfactant molecules in the interfacial films. In order to independently determine the fraction of alcohol effectively present in the film, we used perdeuterated C,Eo in a small-angle neutron scattering (SANS) experiment utilizing the technique of contrast-matching. From absolutely scaled intensities we found that the film contained 2.1 C,& molecules per surfactant molecule in sufficient agreement with the estimate (2.6) from the phase diagram. Apparently, the surfactant monolayer acts as a two-dimensional third phase (Schulman’s “interphase”) into which the medium-chain alcohols preferentially dissolve. They do so because of the intermediate polarity of the films rather than by true surface action. Accordingly, one has to discuss to what extent the action of the alcohols is that of “cosolvents” as opposed to “cosurfactants”. Finally, the influence of alcohol addition on the bending properties of the film is briefly discussed.
Introduction The use of alcohols (referred to as “cosurfactants*) in making microemulsions has a long tradition. Starting with Schulman’ and Winsor,2 the addition of cosurfactants was considered an indispensable ingredient to microemulsions. However, already from Shinoda’s early work on nonionic surfactants demonstrating that microemulsions can be made with only three components, water, oil and a surfactant, it was evident’ that cosurfactants were not a necessity. What, then, is the purpose of adding alcohols to microemulsions? One reason is that most commercial surfactants are not balanced with respect to their affinity to water and oil, but can be made so by alcohol addition, therefore, the To whom correspondence should be addressed.
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name cosurfactants. Furthermore, if one uses ionic surfactants electrostatic interactions have to be screened by salt as a fifth component. The unwanted effects of salt (salting out) can again be compensated by adjusting the alcohol concentration. Fivecomponent systems are difficult to handle, and the phase behavior cannot be represented exactly in three dimensions. Kahlweit et a].? therefore, studied the effect of medium-chain alcohols on the phase behavior of microemulsions in ternary systems. They concluded that referring to alcohols as cosurfactants may obscure ~
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Bowcott, J. E. L.; Schulman. J. H. Z . Electrochem. 1955, 59, 283. Winsor, P. A. Chem. Rev. 1968, 68, 1. Shinoda, K. J . Colloid Interface Sci. 1967, 24, 4. Kahlweit, M.; Strey, R.;Busse, G . J . Phys. Chem. 1991, 95, 5344.
0 1992 American Chemical Society
