Langmuir 1994,10, 2219-2222
22 19
The Partitioning of n-Hexanol into Micelles of Cetyltrimethylammonium Bromide in Aqueous Solution. Ultrasonic Relaxation and Head-SpaceAnalysis Measurements W. Wan-Badhi, D. M. Bloor, and E. Wyn-Jones* Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4 WT, England Received February 1, 1994. I n Final Form: April 18, 1994@ The partitioning of n-hexanol between aqueous solution and cetyltrimethylammoniumbromide micelles has been studied using head-space analysis involving gas chromatographyand ultrasonic relaxation. The study has been carried out over varying concentrations of hexanol up to 0.18 mol dm-3 in both 0.05 and 0.075mol dm-3 surfactant solutions. Over the concentration range in question the parameters that monitor the equilibrium aspects of the partitioning process can be adequately described on the basis of a simple two-state process involving “free”and “solubilized”hexanol. On the other hand, the kinetic parameters show marked deviations at the higher hexanol concentrations which are close to the phase boundary.
Introduction Microemulsions are thermodynamically stable isotropic fluid phases containing substantial amounts of surfactants, cosurfactants (usually an alcohol), oil, and water and have found many industrial and pharmaceutical appli~ations.l-~ The key ingredient which is thought to control the properties of microemulsions is the composition of the surfactant/cosurfactant mixture. As a result much attention has been paid to the properties of surfactant/ alcohol mixt~res.~-ll In dilute micellar solutions, alcohols such as n-butanol, n-pentanol, and n-hexanol usually form mixed micelles with the surfactant. In these mixed micelles the alcohol is thought to reside in the palisade layer of the micelle, and in dilute systems it is usually the case that the amount of surfactant in the micelle exceeds that of the solubilized alcohol. Recently, in certain mixed micelles involving hexanol and cetyltrimethylammonium bromide (CTAB) micelles and also hexanoVsodium dodecyl sulfate (SDS)it has been shown that a t certain surfactant concentrations it is possible to get mixed hexanollsurfactant micelles even when the overall concentration of hexanol is about 2-5 times that of the surfactant.12J3 In these studies, measurements on
* Author to whom correspondence should be addressed: Professor E. Wyn-Jones, Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4W”, England. Abstract published in Advance ACS Abstracts, June 15,1994. @
(1)Microemulsions: Robb. I. D.. Ed.: Plenum Press: New York. 1982. (2)Solution Behadour o j Surfactunts; Mittal, K. L., Ed.; Plenum Press: New York, 1982. (3)Friberg, S. E.; Qamheye, K. In The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Bloor, D. M., Wyn-Jones, E.. Eds.: Kluwer: Dordrecht. 1990. ’(4) H d l , D. G.; Jobling, P. L.; Rassing, J. E.; Wyn-Jones, E. J.Chem. SOC.,Faraday Trans. 2,1977,73, 1502. (5)Gettins, W. J.; Hall, D. G.; Jobling, P. L.; Rassing, J. E.; WynJones, E. J. Chem. SOC.,Faraday Trans. 2 1978,1957. (6)Stilbs, P.J. Colloid Interface Sci. 1982,89, 547. (7)Almgren, M.; Lofroth, J. E. J.Phys. Chem. 1982,76,2734. (8) Hayter, B.; Mayoun, M.; Zemb, T. Colloid Polym. Sci. 1984,262, 798. (9)Lang, J.; Zana, R. J.Phys. Chem. 1986,90,5258. (lO)Ljosland, E.; Blockhus, A. B.; Veggeland, K.; Backlund, S.; Hoiland, H. Prog. Colloid Polym. Sci. 1986,70, 34. (11)Zana, R.; Yir, S.; Strazielli, C.; Limos, P. J. Colloid Interface Sci. 1981,80,208. (12)Backlund, S.;Bakken, J.; Blokhus, A. M.; Hoiland, H.;Vikholm, I. Acta Chem. Scand. 1986,A40,241. (13)Vikholm, I.; Doukinet, G.;Backlund, S.; Hoiland, H. J . Colloid Interface Sci. 1987,116,582.
the viscosity, conductivity, density, and ultrasonic velocity have been carried out to high hexanol concentrations. In some cases these measurements have shown a distinct change in behavior which is sometimes characterized by a well-defined break when the experimental parameter is plotted against hexanol concentration. The observations have been interpreted as a change in the hexanol solubilization pattern in which two solubilization sites are involved in the mixed micelle.12J3 This interpretation has been challenged in recent years14and has become the subject of some controversy. It therefore it seems desirable to carry out more systematic and independent measurements on these systems. The strategy used in the present project was to combine equilibrium data from head-space analysis involving gas chromatography with ultrasonic relaxation measurements so that new thermodynamic and kinetic parameters for the partitioning process could be evaluated. The strategy therefore takes the following format: (i) to carry out measurements on the partition coefficient of the hexanol between the aqueous and micellar phase using head-space analysis involving gas chromatography; (ii) to carry out ultrasonic relaxation studies associated with the pertur-. bation of the equilibrium between “free” and “solubilized” hexanol; and (iii) to combinethe data from the experiments described in i and ii so that kinetic data associated with the partitioning ofhexanol can be evaluated. This will be achieved by use of a phenomenological treatment which This paper describes has been described previ~usly.’~-’~ such measurements on the system hexanolketyltrimethylammonium bromide (CTAB).
Experimental Section The samples of CTAB and n-hexanol used in this work were purified samples obtained from BDH and Fluka AG, respectively. All measurements were carried out at a constant temperature of 25 f 0.1 “C. (14) Stilbs, P. J. Colloid Interface Sci. 1988,116,593. (15)Gormally, J.; Sztuba, B.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 2 1985,81,395. (16)Smith, P.; Gould, C.; Kelly, G.; Bloor, D. M.; Wyn-Jones, E. In Reactions in Compartmentalised Liquids; Knoche, W., Schumaker, R., Eds. 1989,p 83. (17) Kelly, G.;Takisawa, N.; Bloor, D. M.; Hall, D. G.;Wyn-Jones, E. J. Chem. SOC.,Faraday Trans. 1 1989,85,4321.
0743-746319412410-2219$04.50/0 0 1994 American Chemical Society
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2220 Langmuir, Vol. 10, No. 7, 1994 - - I
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Figure 1. Plot of the concentration of solubilized alcohol (Cp) versus amount of alcohol monomer (md:(W) 0.05, (0) 0.075 mol dm-3 CTAB.
Equilibrium Measurements Quantitative information containing the partitioning of the hexanol between the bulk phase and the CTAB micelles has been obtained using the head-space analysis method involving gas chromatography. In this method, solutions of alcohol and alcohoVCTAB are placed in a conical flask with fitted septum and placed in a thermostatic bath. After appropriate equilibration,vapor samples are withdrawn from the head space using a gas-tight syringe and are analyzed by gas chromatography to determine the amount of alcohol in the vapor from each solution. The underlying principle of this method is that the amount of alcohol per unit volume of head space from solution in equilibrium with vapor is the same for any two solutions in which the alcohol chemical potential is the same. If we assume therefore that a given activity corresponds to a given concentration in the continuous phase we may then from the total hexanol concentration estimate the amount solubilized in the micelle. The measurements were carried out on a Shimadzu model GC8A fitted with an integrator (type CR18) and a flame ionization detector. The sensitivities were adjusted so that the samples in the given set could be run under the same conditions. The handling procedure was similar to that described by Spink and Colganl*and has successfully been used by the present gr0up.15-17 From the data it is then possible to evaluate the amount of solubilizedalcohol at each total alcohol concentration. These measurements were carried out for two constant concentrations of CTAB micelles, 0.05 and 0.075 mol dm-3 and were repeated several times. In the present work the critical micellar concentration of CTAB is extremely low, of the order mol dm-3 l9 and in these circumstances it is assumed that the micellar concentration equals the total weighed in surfactant concentration 4. The experimental data are displayed graphically in Figure 1in which the amount of solubilized : C has been plotted against the amount of alcohol, , monomer alcohol denoted m2. For the purpose of evaluating the present work the partitioning coefficient K of n-hexanol between bulk and micellar phases is defined as follows:
The plots of K as a function of solubilized alcohol Cz are also shown in Figure 2 for 0.05 and 0.075 mol dm-3 CTAB concentrations giving an average value of K for the two (18) Spink,C. H.; Colgan, S. J.Phys. Chem. 1983,87,888. (19) Mukerjee, P.;Myeels, K. J. Natl. Stand. Ref. Data Ser. (U.S. Natl. Bur. Stand.) 36.
Ultrasonic Relaxation Ultrasonic measurements were carried out using the Eggers resonance technique whose frequency range has recently been extended to cover the range 0.2-20 In the present work no relaxation was observed in pure CTAB micelles nor in dilute aqueous hexanol solutions. However, when hexanol was added to CTAB micelles a single well-definedrelaxation was always observed in the above frequency range. The absorption data were analyzed using the equation
where a is the sound absorption coemcient at frequency f,Ais the relaxation amplitude parameter, f c the relaxation frequency, and B represents contributions to alp which are independent of frequency. The relaxation time, z and the maximum absorption per wavelength, ,umare given by
llz = 2nfC
(3)
and (4)
where v is the velocity of sound through the sample. As we have stated previously the relaxation observed in this work only occurs when alcohol and CTAB micells are mixed in aqueous solutions. We therefore conclude that the origin of the relaxation is associated with the exchange process between free alcohol in the bulk phase and solubilized alcohol. Typical plots of alp versus frequency (20) Chaudhuri,A.;Romsted, L. S.;Yao, J. J.Am. Chem. SOC.1993, 115,8362. (21) Eggers, F.Acowtica 1968,19, 323. (22) Eggers, F.;Funk,T.; R i c h " , K. H.Rev. Sci. Znst. 1976,47, 361.
The Partitioning of n-Hexanol
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Langmuir, Vol. 10,No.7,1994 2221
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Figure 3. Plot of a/f2 versus frequency for CTABhexanol mixtures. CTAB concentration = 0.05 mol dm-3; hexanol concentration = (m) 0.02, (A)0.06,(*) 0.12, (v)0.17 mol dm-3.
0.08
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Figure 5. Plot of hdt)against (CF) according to eq 5 : (m) 0.05 and (0)0.075 mol dm-3 CTAB.
dodecyl sulfate where AV = 2.8 cm3 mol-' from dilatometric measurements. Kinetic Analysis. The kinetic behavior of the alcohol partitioning with the micelle can be investigated using the phenomenological equation
160
CC;
m2I/C*[mol dm.31
Figure 4. Plot of the maximum sound absorption per wavelength hm) against (m,CF/C,) according to eq 5:).( 0.05 and (0)0.075 mol dm-3 CTAB.
are shown in Figure 3 with an estimated experimental error of f3%. Analysis of Data using Phenomenological Approach. We have already established a method by which the ultrasonic relaxation associated with alcohol partitioning of this kind may be analyzed.15-17 AmplitudeAnalysis. One ofthe prerequisites in using this treatment is that equilibrium partitioning data, as displayed in Figure 1, is available. For the relaxation process in question which essentially occurs at constant micelle concentration, Cm, and constant micellar surfactant concentration C1, the maximum sound absorption per wavelength p m is expected to have the following concentration dependence:
where AVis the reaction volume change, K~ the adiabatic compressibility, CZ the total n-hexanol concentration (CZ = m2 and R and T have their usual meaning. According to the above equation, a plot of p, against Cm,CzplC2)should be a straight line passing through the origm with a gradient equal to the thermodynamic term in parentheses, from which AVcan be evaluated. For the two CTAB concentrations in question these plots are shown in Figure 4 and display the linearity predicted by the above equation. From the slopes of these plots the following AV values have been obtained: 0.05 mol dm-3 CTAB, AV = 5.7 cm3mol-'; 0.075mol dm-3 CTAB, AV = 5.5 cm3mol-'. These values are very close in magnitude as expected and may be compared with the hexanolhodium
+ e),
0.06
where R is the equilibrium forward (= backward) rate of the equilibrium being perturbed by the sound wave, i.e. alcohol associating to and dissociating from the CTAB micellar aggregate. In dilute hexanoVCTAB solutions it is expected that the solubilized hexanol resides in the palisade layer of the micelle. In these circumstances the kinetics of the dissociation of the hexanol molecule from the mixed micelle is expected to be a first-order process whose rate is directly proportional to the amount of solubilized hexanol as indicated in the above equations. Thus
where k- is backward rate constant. Combining eqs 6 and 7 we obtain
t -
(~Z ATP )k-c; K ,
(8)
In the above equation pm and t are known from the relaxation experiments, the thermodynamic term in parentheses is given by the gradient of Figure 4 and C y follows from the head-space data. Thus for a firstorder dissociation, a plot ofthe left-hand side ofthe above equation against CF should be a straight line passing through the origin. These plots are shown in Figure 5 for the two concentrations of CTAB studied in this work. The plots show the following features; in the dilute n-hexanol regime the left-hand side of the above equation is directly proportional to Cp, giving good straight lines passing through the origin. In this region it is clear that the backward rate is a first-order rate process whose rate is directly proportional to the amount of solubilized alcohol. However, at a certain alcohol concentration the reaction rate then appears to level off to a constant value. This is clearly seen in the two plots and a relatively sharp break point in the rate is observed. The leveling off of the reaction rate only takes place in the concentrated alcohol solutions which are also close to the phase boundary. (23) Manabe, M.; Shirahama, K.; Koda, M. Bull. Chem. SOC.Jpn. 1976,49, 2904.
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2222 Langmuir, Vol. 10, No. 7, 1994
We now discuss the experimental data described above in relation to the dilute solution regime in which the dissociation of the hexanol monomer from the mixed micelle is a first-order rate process governed by eq 7. The k- values estimated from the slope of the plots in Figure 5 are as follows: k- = 4.2 x lo6 s-' for the 0.05 mol dm-3 CTAB; k- = 3.8 x lo6 s-l for the 0.075 mol dm-3 CTAB. If we now extend the kinetic considerations to the forward rate of the equilibrium in question, that is, the solubilization of n-hexanol then this is expected to be a bimolecular step whose rate R+ is therefore expected to be given by
where k+ is the forward rate constant and n the micellar aggregation number. At equilibrium we can equate the right sides of eqs 7 and 9 thus (10) or k+ = nk-K
(11)
If we use the above equation in the limit when approaches zero, n becomes the aggregation number of CTAB micelles (n = 85hZ7 On this basis the diffision controlled value ofk+ can be estimated from eq 11to give k+ = 1.5 x 1O'O mol-' dm3s-l for 0.05 mol dm-3 CTAB and k+ = 1.6 x 10'O mol-' dm3 s-l for 0.075 mol dm-3 CTAB. On the basis of the above treatment the data in the dilute solution regime are consistent with a mechanism in which n-hexanol monomers exchange between the bulk solution and the palisade layer in the mixed micelle. At this stage, however, it is worth pointing out that the behavior of the association rate measured in the present work is different to similar data that we have previously reported15-17in connectionwith the exchange ofn-pentanol with various mixed micelles. In these latter studies it was found that k- is not a rate constant but rather a rate coefficient which increases linearly with the amount of solubilized pentanol. In contrast, in the present work kis constant. The values ofk+ also compare well with data on the exchange rate involving small molecules and
micelles and the numerical values indicate that k+ is almost diffision controlled. Overall the numbers that emerge from the analyses of eqs 5,8, and 11are reasonable in relation to the mechanism involving a two-state exchange rate between "freenand "solubilized" hexanol. At the higher hexanol concentrations our kinetic data as described by eq 8 deviates from the first-order behavior described above as shown by the distinct breaks in the plots of Figure 5. We only observed this break in the kinetic data obtained from the relaxation time in the ultrasonic experiment. No such breaks were observed in the thermodynamic data involving head-space analysis (Figures 1 and 2) nor in the thermodynamic treatment using the maximum absorption per wavelength. At the higher hexanol concentration the anomalous behavior of these systems as described by Hoiland et al. and also observed in our kinetic measurements is observed when the system is very close to a phase boundary. In these circumstances it is well known (for example, at phase boundaries in lyotropic liquid crystals) that an additional contribution to ultrasonic relaxation occurs as the phase boundary is a p p r ~ a c h e d . ~ In~ an analogous situation, when liquids or liquid mixtures approach a critical point it is well known that ultrasonic relaxation occurs due to concentration fluctuations near the phase b ~ u n d a r y . ~ ~ , ~ ~ It is very likely that at the higher alcohol concentrations additionalcontributionsto the ultrasonic relaxation occurs due to dynamic processes associated with a phase change. We tested our solutions using polarizing optical microscopy and found no evidence of lyotropic liquid crystal phase formation. Some of the solutions,however, became cloudy over a period of time-the time scale ofwhich was arrested from approximately 30 min to several days by ultrasonification. At this stage there is no doubt that solutions of hexanol in CTAB show anomalous behavior at high hexanol concentrations, the question still remains as to the origin of the molecular process giving rise to this behavior. We believe that the only way that this can be resolved is via structural investigation possibly using small angle neutron scattering. Acknowledgment. W. A. Wan Badhi thanks the Malaysian government for a scholarship. We also thank Mr. Raffiqfor carrying out the head-space measurements. (24) Walsh, M. F. Ph.D. Thesis, University of Salford, 1980. (25) Fixman, M. M u . Phys. Chem. 1964,6, 175. (26) Fisher, M. E. J. Math. Phys. l864,5, 944. (27) Tartar, H.V. J.Colloid Sci. 1989, 14, 115.
