J . Phys. Chem. 1988, 92, 1301-1307 schematically indicated as -OH.-C1CH3. (3) The reaction of CH3Cl with surface OH groups preferentially consumes high-frequency OH species, suggesting that OH groups exhibiting the lowest degree of OH-OH association are preferentially reactive. (4)At temperatures between 350 and 400 K, an intermediate species is irreversibly formed, whose structure we are not able to
1301
determine. This species spectroscopically resembles a strongly bound CH3C1 species more than an -OCH3 species. Acknowledgment. We acknowledge with thanks the support of this work by ALCOA. We also thank Dr. Lawrence Dubois of AT&T Bell Laboratories for supplying the CH3CI and CD3Cl. Registry No. CH,Cl, 67-66-3; CD3CI, 11 11-89-3.
Apparent Molar Volume, Apparent Molar Adiabatic Compressiblllty, and Solubilization Studies of Aqueous Solutlons of Sodium p-(n-Dodecyl) benzenesulfonate as a Function of Surfactant and Solubillzate Concentrations and Temperature M. Alauddin, N. P. Rao, and R. E. Verrall* Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S 7 N 0 WO (Received: April 21, 1987;In Final Form: September 30, 1987)
Ultrasonic velocities and densities of aqueous solutions of sodium p-(n-dodecy1)benzenesulfonate have been measured at concentrationsbelow 0.30 mol kg-' at 15,25, and 40 OC. Apparent molar adiabatic compressibilities,apparent molar volumes, and adiabatic compressibilitycoefficients of the surfactant solutions were derived from these data. Apparent molar volumes and apparent molar compressibilitiesof 2,6-di-tert-butyl-4-methylphenol and 2-tert-butyl-4-methoxyphenoldissolved in aqueous micellar solutions of sodium p-(n-dodecy1)benzenesulfonate were determined as a function of surfactant concentration and temperature. The partial molar volumes and partial molar compressibilities of solubilizates at infinite dilution in the aqueous micelle solutions were obtained from these apparent molar properties. Thermodynamic transfer function parameters AG,,O, AZf,,', and M,Owere calculated from solubility measurements. The sign and magnitude of these parameters were used to analyze the location of the solubiiizates in the micelle. The results show that sodium p-(n-dodecy1)benzenesulfonatemicelles undergo a second transition, sphere-to-rod, in the postmicellar region at ca. 0.15 mol kg.-'. The solubilization behavior of the micelles below and above the transition concentration is found to be significantly different. The results show that 2,6-di-tert-butyl-4-methylphenol is primarily solubilized in the interior of the micelle while 2-tert-butyl-4-methoxyphenol can be solubilized at the micelle surface and in the palisade layer, depending upon the micelle concentration. Also, it appears that 2,6-di-tert-butyl-4-methylphenol is less accessible to the inner part of the micelle at higher surfactant concentration. Proton NMR chemical shift measurements in micelle solutions containing 2,6-di-tert-butyl-4-methylphenol support the conclusions drawn from the thermodynamic results.
Introduction Micellar flooding is a recent development in enhanced recovery of It consists of injecting into the oil reservoir a slug of micellar fluid composed of surfactant, cosurfactant, light oil, and brine. The interfacial tension is reduced and the oil in the form of a microemulsion is displaced toward the producing well. More recently, it has been shown that microemulsions in the form of light oils are effective in the aqueous extraction of bitumen from tar sand^.^,^ A number of double-tailed sodium alkylbenzenesulfonates have been investigated as model compounds5S6for the commercial petroleum sulfonates used in enhanced oil recovery. (1) Shah, D. O., Schechter, R. S., Eds. Improved Oil Recovery by Surfactant and Polymer Flooding Academic: New York, 1977. (2) Johansen, R. T., Berg, R. L., Eds. Chemistry of Oil Recovery; ACS Symposium Series 92; American Chemical Society: Washington, DC, 1979. (3) Sarbar, M.; Brochu, C.; Boisvert, M.; Desnoyers, J. E. Can. J . Chem. Eng. 1984.62, 267. (4) Desnoyers, J. E.; Sarbar, M.; Lemieux, A. Can. J. Chem. Eng. 1983, 61, 680. (5) (a) Doe, P.; El-Emary, M.; Wade, W. H.; Schechter, R. S. J. Am. Oil Chem. Soc. 1977,54, 570. (b) J. Am. Oil Chem. SOC.1978, 55, 505, 513. (c) Wade, W. H.; Morgan, J. C.; Schechter, R. S.;Jacobson, J. K.;Salagar, J.-L. SOC.Per. Eng. J . 1978, 18, 242. (6) (a) Franses, E. I.; Puig, J. E.; Talmon, Y.; Miller, W. G.; Scriven, L. E.; Davis, H. T.J. Phys. Chem. 1980, 84, 1547. (b) Franses, E. I.; Davis, H. T.; Miller, W.G.; Scriven, L. E. A C S S y m p . Ser. 1979, No. 9, 35. (c) Puig, J. E.; Franses, E. I.; Davis, H. T.; Miller, W. G.; Scriven, L. E. SOC. Pet. Eng. J . 1979, 19, 71.
0022-3654/88/2092-1301$01 SO/O
Some of these investigations6have focused on the role of surfactant aggregates in the production of ultralow surface tensions between oil and water. Small-angle neutron scattering measurements of micellar solutions of sodium alkylbenzenesulfonates having branched alkyl groups also have been reported.' Porte and Poggi8 studied the magnetic birefringence of aqueous solutions of sodium octylbenzenesulfonate and reported a second transition at high surfactant concentration. A more recent thermodynamic study9 of a homologous series of sodium alkylbenzenesulfonates up to n-octyl has confirmed this postmicellar transition. Another important feature of micellar systems is their ability to solubilize various organic compounds which are sparingly soluble or insoluble in water.1° This is a primary reason why surfactants find useful application in many biological and pharmaceutical applications. As well, since lipid bilayers are somewhat similar to micelles in terms of their physical chemical properties, micellar solubilization can be used as a model system to provide useful insight into the understanding of such processes in these biological systems. (7) (a) Magid, L. J.; Triolo, R.; Johnson, Jr., J. S.;Koehler, W. C. J. Phys. Chem. 1982, 86, 164. (b) Triolo, R.; Hayter, J. B.; Magid, L. J.; Johnson, Jr., J. S.J. Chem. Phys. 1983, 79, 1977. (8) Porte, G.; Poggi, Y. Phys. Rev. Lett. 1978, 41, 1481. (9) Caron, G.; Perron, G.; Lindheimer, M.; Desnoyers, J. E. J . Colloid Interface Sci. 1985, 106, 324. (10) Elworthy, P. H.; Florence, A. T.; McFarlanc, C. B. Solubilization by Surface Active Agents; Chapman and Hall: London, 1968.
0 1988 American Chemical Society
1302 The Journal of Physical Chemistry, Vol. 92, No. 5, 1988
This paper also describes the results of our continuing studof micellar solutions that contain chemical compounds which have been reported to be either a potent inactivator of mammalian and bacterial viruses containing lipid structuresI4or inhibitors of processes leading to cancer growth.15 We report volumetric, adiabatic compressibility, solubility, and NMR studies (BHT) and 2- or 3-tert-b~of 2,6-di-tert-butyl-4-methylphenol tyl-4-methoxyphenol (BHA) in aqueous micelle solutions of sodium p-(n-dodecy1)benzenesulfonate (NaDDBS) above the critical micelle concentration (cmc). As well, we have examined the variation of ultrasonic velocity in these micellar systems in the presence of n-butoxyethanol (BE), a cosurfactant that we have been studying in other micellar systems. The results are interpreted in terms of the approximate location of the solubilizates in the micellar aggregates, possible changes in the shape and size of the micelles, interactions within the micelle between the solubilizate and surfactant species, and the sphere-to-rod transition that has been reportedss9 for this micellar system. iesl 1-1 3
Experimental Section Materials. Sodium p-(n-ddecy1)benzenesulfonate (Pfaltz and Bauer Inc), 85% pure, was purified by extracting the impure salt with a large volume of an acetone-absolute ethanol mixture (50/50, v/v) with constant stirring for a period of at least 24 h. The clear supernatant liquid was then separated from the undissolved materials and the amount of solvent was reduced to induce the precipitation of the dissolved material. It was further purified by twice recrystallizing from an acetoneethanol mixture and cooling to -70 "C prior to filtering. The product was dried under vacuum at 100 OC. The purity of the recrystallized NaDDBS was analyzed by using reverse phase high-performance liquid chromatography (HLPC) and N M R techniques. A mixture of methanol-water (85/15, v/v) was used as the mobile phase in the HLPC analysis. Detection was made by means of ultraviolet spectroscopy (254 nm). Chromatograms were obtained both in the acidic mobile phase and, also, by using ion pairing with the counterion Bu4Nf dissolved in the mobile phase. The spectra showed good agreement with those obtained by Desnoyers et and Thomas and Rocca." Proton N M R analysis of an aliquot of each HLPC peak revealed absorption peaks characteristic of an aromatic proton ortho to a sulfonate group. These results suggest that the surfactant is a possible mixture of compounds having different alkyl chain lengths with C I 2being the major component. 2-Butoxyethanol (Aldrich) (BE) was purified by fractional distillation under high vacuum and kept over molecular sieves. Deuterium oxide (Merck, 99.7%isotopic punty) was used without further purification as solvent in IH N M R studies. Deionized water used in these experiments was obtained from a Millipore Super-Q-System. All other chemicals were purified as described previously.11J2 Apparatus and Procedure. Ultrasonic velocities were measured at a frequency of 4 MHz with a Nusonics (Model 6080, Mapco) single transducer velocimeter using the sing-around technique.'* The sound speed, u, in m s-I, was calculated from the average round-trip period of the ultrasonic wave in the fixed path length between the piezoelectric transducer and reflector. The period of the sound wave was measured with a frequency meter (Fluke 7261 Universal Counter). The following equation was used to calculate t i (11) Alauddin, M.; Verrall, R. E. J. Phys. Chem. 1984, 88, 5725. (12) Alauddin, M.; Verrall, R. E. J . Phys. Chem. 1986, 90, 1647. (13) Alauddin, M.; Verrall, R. E. J . Phys. Chem. 1987, 91, 1802. (14) Cupp, J.; Wanda, P.; Keith, A,; Snippes, W. Antimicrab. Agents Chemother. 1975, 8, 698. (15) Anderson, M. W.; Boroujendi, M.; Wilson, A. G. E. Cancer Res. 1981, 41, 4309. (16) Desnoyers, J. E.; Sarbar, M.; Lemieux, A. Can. J. Chem. Eng. 1983, 61, 680. (17) Thomas, D.; Rocca, J.-L. Analusis 1979, 7, 386. (18) Garnsey, R.; Mahoney, R.; Litovitz, T. A . J . Chem. Phys. 1964, 64, 2043.
Alauddin et al. u =
fA(1 + cut) 7 -fB
(1)
where A is the sonic path length in meters, B is the electrical delay time in seconds, f is the frequency in hertz, (Y is the coefficient of thermal expansion of the transducer holder material, and t is the temperature of the solution in degrees Celsius. The constants A and B were obtained at different temperatures by calibrating the transducer probe using the velocity data for waterI9 and aqueous solutions of NaCL20 The sample container was an insulated double-walled glass cell which has a liquid volume capacity of ca. 80 mL. Accurate and rapid measurements of sound velocity as a function of solute concentration were made by successiveadditions of known weights of the solute to the container. The ultrasonic velocity measurements were estimated to be accurate to fO.005 m SKI.Adiabatic compressibility coefficients, &, in units of Pa-' were derived from the relation
0,=
l/u2d
(2)
where d is the solution density in kg m-3. The apparent molar volume (&) and apparent molar adiabatic compressibility (&) of liquid solutions were calculated from the following relations (3)
(4) where m is the molality of the solution, M is the relative molar mass of the solute, d is the density of the solution, do is the density of the solvent, and Po and p are the compressibility coefficients of the solvent and solution, respectively. Density data were obtained with a high-precision flow digital densimeter (Model 02D, Sodev Inc). The operation and calibration of the instrument has been previously described." 21 The precision in the density data was found to be better than f 1.5 ppm. The solution temperatures in both the velocimeter and densimeter were maintained to fO.OO1 "C by using a closed loop temperature controller (Model CT-L, Sodev Inc.). Data for all systems are found in the supplementary material (see paragraph at the end of the text regarding supplementary material). Ultraviolet spectra of solutions were measured at 25 "C with a Cary 118C spectrophotometer. The ultraviolet spectra of solubilizates in aqueous surfactant solutions were obtained with respect to the aqueous surfactant solution as reference. Proton N M R measurements were performed on a Bruker (Model AM-300) N M R spectrophotometer at 300 MHz in order to investigate possible solubilization sites of additives in the micellar phase. The method depends on the ability of the solubilizates to affect the chemical shift of different proton signals of the surfactant molecule. These chemical shifts were measured in the presence of BHT as a function of both solubilizate and surfactant concentration. All spectra were measured relative to sodium 2,2-dimethyl-2-silapntane-5-sulfonate (2% in D20) as an external standard at 25 "C. Chemical shift measurements of various resonance peaks of NaDDBS are given on the 6 scale in parts per million (ppm) of the applied radio frequency. The precision of the data are estimated to be 0.002 ppm from repeated determinations. Results The concentration dependence of the ultrasonic velocities and the adiabatic compressibility coefficients of aqueous solutions of NaDDBS are shown in Figures 1 and 2, respectively. A sharp break in sound velocity occurs at very low concentrations. As well, at higher concentrations the sound velocity and adiabatic com(19) Del Grosso, A,; Mader, C. W. J. Acoust. Sac. Am. 1972, 52, 1442. (20) Sakurai, M.; Nakajima, T.; Komatsu, T.; Nakagawa, T. Chem. Lett. (Chem. Sac. Jpn.) 1975, 971. (21) Picker, P.; Tremblay, E.; Jolicoeur, C. J . Solution Chem. 1974, 3, 377.
Properties of Sodium p-(n-Dodecy1)benzenesulfonate
1T
The Journal of Physical Chemistry, Vol. 92, No. 5, 1988 1303
8 -5 / I
0
I
1
0.20
I
I
0.60
0.40
Figure 4. Apparent molar compressibility of aqueous solutions of NaDDBS as a function of the square root of molality at 15, 25, and 40 OC. I
0
I
0.10
I
0.20 0.30 m ( m o l kg-')
,
1560
040
- EEtH20 A-BEtH20tO06mNoDDES o-BEtH20+020mNaDDES - EE+HzOtO 40 m NaDDBS
Figure 1. Ultrasonic velocities in aqueous solutions of NaDDBS as a function of molality at 15, 25, and 40 OC.
1500-
XBE
+
Figure 5. Ultrasonic velocities in BE NaDDBS function of BE concentration at 25 OC.
+ H20 systems as a
TABLE I: Solubilities of BHT and BHA in Aqueous Solutions of NaDDBS at 25,35, and 45 O C 1O3(concn),mol kg-' NaDDBS
'
L -I
mp (mol' kg
'I
Figure 2. Adiabatic compressibility coefficient of aqueous solutions of NaDDBS as a function of the square root of molality at 15, 25, and 40
OC. 40'C
-
m; (mol+ k g - i )
Figure 3. Apparent molar volume of aqueous solutions of NaDDBS as a function of the square root of molality at 15, 25, and 40 OC.
pressibility coefficient show another change in slope, with the change in the case of sound velocity being more pronounced at higher temperatures. Both the apparent molar volume, 4" (Figure 3), and apparent molar compressibility, & (Figure 4), of NaDDBS solutions increase with increasing concentration and temperature. A large increase in these properties occurs at low concentrations. At higher concentrations, a small but well-defined break is observed in the vs m1I2at all temperatures. The change is readily slope of apparent in the case of the & property.
concn, mol ka-'
temp, OC
BHT
BHA
0.10
25 35 45 25 35 45 25 35 45
8.7 12.5 16.4 21.9 27.1 33.4 34.6 42.10 49.9
24.9 34.8 44.4 29.1 37.2 45.6 32.9 39.9 46.6
0.20 0.30
The variation of ultrasonic velocity with BE concentration in aqueous NaDDBS solutions of fixed concentration was studied at 25 OC. The results (Figure 5) show that a maximum in u occurs at low concentration of BE for all compositions of surfactants and that the magnitude of this velocity maxima decreases with increasing surfactant concentration. The data for BE-H20 system is included in Figure 5 for reference. The solubilities of BHT and BHA in aqueous micelle solutions of NaDDBS in the concentration range 0.10-0.30 mol kg-' at 25, 35, and 45 O C are shown in Table I. The solubilities of these additives in water at various temperatures have been reported previously.I2 The solubilities in NaDDBS micelle solutions increase with increasing surfactant concentration and temperature. Molar concentrations derived from spectroscopic measurements were converted to mol kg-' by using the density data obtained for these solutions. The maximum estimated error in the solubilities of these systems, based on the standard deviation of repeated determinations, is f 2 % . The equilibrium constants for the distribution of the solubilizate between the aqueous and micellar phases were determined from solubility measurements in water and aqueous micellar solution at saturation conditions. The behavior of the solubilizates was assumed to be ideal at the concentration conditions reported. However, for the relatively high solubilities obtained, particularly
1304
Alauddin et al.
The Journal of Physical Chemistry, Vol. 92, No. 5, 1988
TABLE II: Values of K , A c e o , AH,', and AS,O for Transfer of BHT and BHA from Water to Micellar Solutions of Different NaDDBS Concentrations 1O-'K -AG,,O f 0.05: kJ mol-' NaDDBS concn, mol kg-I 25 OC 35 o c 45 o c 25 O C 35 o c 45 OC AH,,', kJ mol-' h S , , O , J K-' 0.10 0.20 0.30
54.9 65.1 65.8
72.1 16.4 14.6
86.0 83.8 80.5
0.10
5.1 2.0 1.5
2.9 1. I 1.2
2.6 1.5 1.1
_RHT ___ 21.04 21.48 27.48
28.66 28.19 28.13
30.04 29.91 29.86
11.12 f 2.23 8.61 f 1.54 1.91 f 0.91
20.41 19.05 18.15
20.79 19.33 18.51
-26.12 f 9.86 -1 1.35 f 0.65 -12.26 f 2.86
*
150 8 121 f 5 119 f 3
BHA 0.20 0.30
21.15 18.80 18.12
-19 h 30 25 f 2 20 f 9
Maximum error. 2
4
4
7
..
-
--.__ .-e.-
ms
'
'--.--
1
7
8
8
40'C
- - - --o-Q-+,
243-
I
'
I
0-c
0 ' -Q-0,.
400~
2411 >
7 . _ ,_ o ,_-_
238
0
236
171
25°C
Oo\--eo-
0
I
4
8 12 m x IO3 ( m o l kg-ll
aqueous solutions of NaDDBS of different concentrations at 25 and 40 OC: 0,0.10 m;0 , 0 . 2 0 m.
I
25
10
16
Figure 6. Apparent molar volume of BHT as a function of molality in
I
I
15 20 m x IO3 (mol kg-ll
5
0
,
Figure 7. Apparent molar volume of BHA as a function of molality in aqueous solutions of NaDDBS of different concentrationsat 25 and 40 "C: 0,0.10 m ;0 , 0.20 m.
at higher temperatures, this will contribute to some error in the absolute values of the thermodynamic parameters calculated. Since we are interested in discussing trends in these properties, this assumption is not expected to change the qualitative arguments presented. The following equation is used to calculate the distribution coefficient12 (CA - Cr)(55.5 - 4 t s c d / 1 8 . 0 )
K=
cr(c+ CA - cy)
(5)
where the value of K is based on the concentration standard state of mol kg-', C, is the total concentration of the solute in solution, CF is the concentration of the solute in water, is the concentration of micellized surfactant defined by [surfactant] - cmc, is the apparent molar volume of the micellized surfactant, and d IS the density of the surfactant solution. The cmc values were taken from the literature.22 It also was assumed that the solubilities of these solubilizates in water would not cause a significant change in the cmc of NaDDBS in water. However, the partition coefficient data may be subject to some error from this source. Further, for solutions a t the solubility limit, it is assumed that the solubilities of these compounds in water are constant and equal to cr.23 The standard molar Gibbs energy of transfer, AGtro, for the process, solubilizate(aq) solubilizate(mic) was calculated from the distribution coefficients for each system at specified conditions by using the relation ACIro= -RT In K. The standard enthalpies of transfer, AHtr0were estimated from the temperature dependence of the partition coefficient by using the van't Hoff equation. Values of AH,,' were assumed to be. independent of temperature over the narrow temperature range studied. The entropies of transfer, AStr', were calculated from AGIr0 and AHtro.Values
l
0
L
2
l
4
