Langmuir
1994,10,
423-431
423
Volumes, Heat Capacities, and Conductivities of Water-Surfaotant-18-Crown-6Ether Systems M. S. Bakshi,? R. Crisantino, R. De Lisi,* and S. Milioto Dipartimento di Chimica Fisica, Uniuersith di Palermo, via Archirafi 26, 90123 Palermo, Italy Received July 19, 1993. I n Final Form: Nouember 9, 1993" Volumes, heat capacities,and conductivitiesof water-18-crown-6 ether (CR)-surfactant ternary systems were measured at 25 OC as functions of the surfactant (ms) and the CR (mcR) concentrations and at fixed CR/surfactant (R) ratios. The surfactants studied are sodium dodecyl sulfate (NaDS) and dodecyltrimethylammonium bromide (DTAB). From conductivity data the cmc and the degree of counterion dissociation (8) were evaluated. The increase of 8 with mcR is essentially the same for the two surfactants while it is not so for the cmc. In fact, the cmc always increase with mCR for DTAB while it is a concave curve for NaDS. The apparent molar volume of transfer (AV*,s(W--W+CR)) of DTAB from water to water-CR mixtures indicates the presence of CR-DTAB interactions in the aqueous phase and the lack of specific interactions in the micellar phase. A plot of AV*,s(W-W+CR) of NaDS shows a maximum just beyond the cmc and then decreases with the increase in ms. The C*,s us ms curves are similar to those in water and slightly affected by the presence of CR. The apparent molar volumes and heat capacities of the composite mixtures, at fixed R, are higher and smaller, respectively, than those calculated on the basis of the additivity of both binary mixtures. At a given total concentration, the excess volumes (Vexc) and heat capacities (obtained as a difference between the experimental and calculated properties) present a maximum and minimum, respectively, us the mole fraction. Vexcis slightly negative for CR-DTAB and positive for CR-NaDS. Since positive Velrcvalues have been observed for CR-NaC1 and pentanol-NaDS mixtures, therefore, we cannot ascertain if hydrophilic and hydrophobic interactions are involved between NaDS micelles and CR. The apparent molar volumes (VQ,CR)and heat capacities (C*,CR)of CR at 0.04 m in micellar solutions of the two surfactants were also determined. For NaDS,the profiles of the two properties are similar to those observed for other additives which distribute between the aqueous and the micellar phases; for DTAB they essentially do not depend on ms as it was observed for those additives which do not solubilize in the micelles. V*,CRwas rationalized as a function of ms by using a previously reported approach for the distribution betweenthe aqueous and micellar phases where the CR complexation with Na+ ions in the aqueous phase was taken into account. From the resulting equation the distribution constant of the complexed CR between the two phases and its partial molar volume in the micellar phase were derived. These properties indicate that strong interactions between NaDS micelles and CR are present. The site of solubilization of CR is the micellar surface where its complex is the counterion of the micelle. Introduction Recently, the thermodynamic investigations of aqueous surfactant solutions have been reported extensively. Especially the effect of the alkyl chain length, the nature of polar head group, the counterion binding to the surfactant molecules, and the effect of additives like alcohols and ureaon the process of micellization in aqueous solutions have been explored.1-6 Due to the strong ability of macrocyclic compounds to form inclusion complexes,6J they have been widely used in the fields of environmental chemistry, food industries, and pharmacology. However,to the best of our knowledge, the effect of macrocyclic compounds on solutions has not yet been widely studied. Accordingly, the partial molar volume and compressibility of sodium decanoate in aqueous crown ethers were reported by Vikingstad and t Visiting fellowfrom Department of Chemistry, Panjab University, Chandigarh-160014, India. Abstract published in Advance ACS Abstracts, January 1,1994. (1) De Liai, R.; Milioto, S.; Castagnolo, M.; Inglese, A. J. Solution Chem. 1990,19, 767. (2) Causi, S.; De Lisi, R.; Milioto, S.; Tirone, N. J.Phys. Chem. 1991, 95,5664. ( 3 ) De Lisi, R.; Lizzio, A.; Milioto, S.; Turco Liveri, V. J. Solution Chem. 1986,15,623. (4) Milioto, S.; Causi, S.; De Lisi, R. J. Solution Chem. 1993, 1, 22. (5) De Lisi, R.; Fisicaro, E.; Milioto, S. J. Solution Chem. 1988, 17,
Bakken8 and the free energies of transfer of crown ethers from water to NaDS and DTAB micellar phases were determinedSBAlso, data for the thermodynamics of inclusion complexes of macrocyclic compounds with dispersed surfactantsin aqueous solutions are available.'O Macrocyclic polyether selected for the present study is 18-crown-6 ether (CR), which is accepted to form stable complexes with cations and especially with Na+ ions.7 These complexes are mostly of 1:l stoichiometry and are the result of strong ion-dipole interactions. Herein, also, it is expected that the effect of complex formation by the counterions in case of sodium dodecyl sulfate ((NaDS) with polyether cavity subsequently leads to the significant alterations in various thermodynamical properties. Therefore, volumes and heat capacities for aqueous NaDS + CR systems at fixed NaDS/CR ratios as well as at fixed CR molalities were performed at 25 "C. Since it is also possible that the hydrophobic surface of CR interacts with the hydrophobic alkyl chain of the surfactant, therefore, in order to discriminate the twoeffects, the study was extended to dodecyltrimethylammonium bromide (DTAB) whose counterion cannot be complexed by the CR cavity. In addition, the specific conductivities of NaDS and DTAB in water + CR mixtures were measured in order to evaluate the critical micelle concentration (cmc)
(6) Gokel, G., Crown Ethers & Cryptands; The Royal Society of Chemistry: Cambridge, 1991. (7) Hoiland, H.; Ringseth, J. A.; Brun, T.S. J. Solution Chem. 1979, 8,779.
(8)Vikingstad, E.; Bakken, J. J. Colloid Interface Sci. 1980, 74, 8. (9) Stilbs, P. J. Colloid Interface Sei. 1982,87, 385. (10) Evans, D. F.; Sen, R.; Warr, G. G. J. Phys. Chem. 1986,90,5500.
@
1015.
0743-7463/94/2410-0423$04.50/0
0 1994 American Chemical Society
Bakehi et al.
424 Langmuir, Vol. 10,No. 2, 1994
Table 1. Critical Micelle Concentration (cmc) and Degree of Counterion Dirlociation 69) of Micelle of Sodium Dodecyl Sulfate (NaDS) and Dodecyltrimethylammonium Bromide (DTAB)in Aqueous lWrown-6 Ether (CR) Solutions.
NaDS
0
5
10
15
lo-' m,
Figure 1. Specific conductivities corrected for the solvent of sodium dodecyl sulfate in water-18-crown-6 ether (CR) mixtures as a function of the surfactant molality. The CR molalities are (a) m C R = 0, (b) mCR = 0.007, (c) mcR = 0.02, (d) m a = 0.06, (e) mcR = 0.13, and (0 mCR = 0.29.
and the degree of the counterion dissociation as functions of CR concentration. Experimental Section Materials. 18-Crown-6 ether (CR), Sigma, was dried in a vacuum oven at 35 OC for at least 4 days before use. Sodium dodecyl sulfate (NaDS), Fluka, and dodecyltrimethylammonium bromide (DTAB), Sigma, were crystallized from ethanol and ethanol-ethyl acetate mixture, reapectively, and dried in a vacuum oven at 60 OC for 2 days. All solutions were prepared by mass using degassed conductivity water and their concentrations were expressed as number of solute moles per kilogram of the solvent. Equipment. The solution densities were measured at 25 OC using a vibrating tube flow densimeter (Model 03D, Sodev, Inc.) sensitive to 3 ppm or better. The temperature was maintained constant within 0.001 "C by using a closed loop temperature controller (Model CT-L, Sodev, Inc.). The calibration of the densimeter was made with water (d = 0.997 047 g cma)11 and sucrose solutions whose densities as a function of concentration are reported in the literature.12 The relative differences in heat capacities per unit volume Aula0 = (a - UO)/UO were determined with a Picker flow microcalorimeter (Setaram) at 25 OC. Using a flow rate of about 0.01 cma 8-1 and a basic power of 21.2 mW, the temperature increment was approximately 0.5 OC. The specific heat capacities (c,) of solutions of density d are related to Aula0 through the equation
DTAB
fiied CR
fiied CR/
fiied CR
concentration
surfactant ratio
concentration
0 0.00718 0.02240 0.06182 0.1374 0.2474 0.2988
0.0084 0.0067 0.0066 0.0062 0.0049 0.0060 0.0051
a Units
0.40 0.41 0.44 0.46 0.60 0.49
0 0.0056 0.36 0.1OOO 0.0069 0.36 0.2026 O.Oo60 0.39 0.2890 0.0065 0.36 0.4002 0.0067 0.38 0.50 0.485 0.0069 0.37 0.200 0.0073 0.38 5.02 3.36 1.67 1.00 0.664
0.0156 0.0160 0.0164 0.0181 0.0185
0.24
0.28 0.29
0.30 0.31
are mol kgl for cmc and ~ C R .
ciation, 8. It is known that the specific conductivity is linearly correlated to the surfactant concentration in both the premicellar and in the postmicellar regions, being the slope in the premicellar region greater than that in the postmicellar region. The intersection point between the two straight lines gives the cmc while the ratio between the slopes of the postmicellar region to that in the premicellar region gives 8. Examples of the specific conductivities corrected for the solvent (1/R - 1/Ro) as functions of the surfactant molality are shown in Figure 1 while the /3 and cmc values are reported in Table 1. The apparent molar volumes (VO)and heat capacities (CO)of CR in water and of the composite mixture of CR + surfactant at fiied ratios in water were calculated by means of the following equations
M 103(d-d0) V,=-d mdd,
Results Conductivity measurements were performed in order to evaluate the cmc and the degree of counterion disso-
1o3(Cp - cp,o) (3) m where m and M are the molality and the molecular weight of the solute, d and cp are the densities and the specific heat capacities of solutions, respectively, and do and cP,o are the corresponding properties for water. For the composite mixture at a fixed ratio, m indicates the molality of the composite mixture and M the averaged molecular weight given by M = MC&CR + M&s, where XCRand Xs are the mole fractions of CR and surfactant, respectively. VO and COof surfactants in water CR mixtures at fixed CR concentrations as well as those of CR at 0.04 m in water + surfactant systems were also calculated by means of eq 2 and 3, where d and cp are, once again, the densities and the specific heat capacities of the ternary systems, respectively, and do and C,O the corresponding properties for the binary mixtures. The VOand COvalues of CR and the composite mixtures at fixed ratios in water, CR in water + surfactant systems and of surfactants in water + CR systems together with the excess densities (Ad = d - do) and the relative differences in heat capacities per unit volume are collected in Tables 2-4. In addition some density measurements of the CRNaC1-water system were performed in order to calculate
(11) Kell, G. S. J. Chem. Eng. Data 1967,12,66. (12) Garrod, J. E.; Herrington, T. M. J. Phys. Chem. 1970, 74, 363. (13) Stimaon, M. F. Am. J. Phys. 1955,23,614.
(14) Daggett, H. N.; Bair, E. S.;Kraue, C. A. J. Am. Chem. SOC.1951, 73,799. (15) Janz,G. J.; McIntyre, J. D. E. J. Electrochem. SOC.1961,108,72.
+
c, = c,,~ (1 Aa/ao]d,jd
(1)
where cP,o and do correspond to the specific heat capacity and density of water for measurements of water CR mixtures and for ternary systems at fixed crown/surfactant ratios, to those of the water CR mixtures for measurements of surfactant in water CR systems and to those of the water surfactant mixtures for measurements of CR in water surfactant systems. The value of 4.1792 J K-1 g-1 for specific heat capacity of water was taken.I3 For the conductance measurements a cell similar to that described by Daggett et al.," with unplatinized electrodes, was used. All measurements were performed with the cell in a constant temperature oil-bath controlled within 0.005 "C by a Hewlett-Packard 2804 A quartz thermometer. The electrical resistance measurements were made with a calibrated ac bridge's at a frequency of 2 kHz.
+
+
+
+
+
C, = Mc, +
+
WatelcCrown Ether-Surfactant Systems
Langmuir, Vol. 10, No. 2, 1994 425
Table 2. Volumes and Heat Capacities of 18-Crown-6Ether (CR) in Sodium Dodecyl Sulfate (NaDS) and Dodecyltrimethylammonium Bromide (DTAB) Aqueous Solutions at 298 Ka
ma
ms
d(W + S)
d(W + S
+ CR)
V*
cD(w + s)
CD(W+ s + CR)
C*
223.84 223.25 222.99 223.25 223.47 223.12
4.15131
4.14114
823.1
4.12072 4.08417 4.01205 3.96595
4.11002 4.07414 4.00223 3.95701
818.5 826.6 813.5 821.7
223.66 223.47 225.05 226.18 227.39 228.41 229.74 230.10 230.57 231.41 231.34 231.72 231.71 232.16
4.17863
4.16722
816.7
4.17290 4.16056 4.15640 4.13475 4.11139 4.10145 4.07875 4.05048 4.03980 4.01057 3.95655
4.15768 4.14607 4.14225 4.11791 4.09331 4.08480 4.06105 4.03816 4.02545 3.99420 3.93836
760.6 733.5 732.0 713.2 679.9 658.7 678.1 759.4 743.5 691.6 590.2
DTAB 0.04957 0.06818 0.1032 0.1524 0.2684 0.3602
0.037470 0.039960
0.002668 0.004060 0.01493 0.o3001 0.03855 0.06918 0.1072 0.1191 0.1504 0.1907 0.2095 0.2551 0.3614 0.4872
0.040080 0.044990 0.044990 0.039980 0.039020 0.044900 0.044980 0.039550 0.044780
0.039960 0.O40080 0.040190 0.039890
0.997832 0.997922 0.998529 0.999159 1.000593 1.001674
0.999350 0.999563 1.000176 1.000796 1.002215 1.003290
0.997183 0.997263 0.997722 0.998293 0.998627 0.999780 1.001201 1.001636 1.002787 1.004208 1.004880 1.006470 1.009944 1.012641
0.998818 0.999104 0.999489 0.999816 1.000064 1.001377 1.002734 1.002964 1.004258 1.005479 1.006315 1.007862 1.011167 1.013961
NaDS
a Unite
0.040030 0.044800 0.044970 0.040360 0.045110
are mol kg-1 for m,g cm9 for densities,
03113 mol-'
for volumes, and J
both the apparent molar volume of CR in the complexed form and the excess volume of CR-NaC1 composite mixture. Discussion Critical Micelle Concentration and Counterion Dissociation. Conductivity measurements were carried out at fixed mcR. In the case of NaDS they were also measured at CR/NaDS fixed ratios. As said above, from these data cmc and j3 were calculated. They are plotted in Figures 2 and 3, respectively, as a function of mcR. The ordinate scale was compacted by choosing as reference state the corresponding properties in pure water. To compare the two sets of data for NaDS and to account for the difference in the cmc between NaDS and DTAB, the R value at the cmc (Rcmc)was used for the abscissa scale. The plots of cmc us R,, (Figure 2) show that within the uncertainties the two sets of data for NaDS agree themselves. Figure 2 also shows that by increasing the amount of CR the cmc of DTAB increases while that of NaDS abruptly decreases tending to a constant value and, then, increases at high concentration. This concave trend was observed for amphiphilicadditives whose hydrophobic moiety is predominating on the hydrophilic one and leads to the additive distribution between the aqueous and the micellar phases.I6 Crown ethers are peculiar amphiphilic moleculesbecause they are formed by an hydrophobicmoiety,Le. the outward part, and an hydrophilic one, Le. the cavity. Consequently, the decrease in the cmc can be due to the penetration in the micelles and to the reduction in the electrostatic interactions at the micellar surface. This explanation cannot be valid for DTAB. The different behaviors of DTAB and NaDS cannot be ascribed to a different partitioning of CR. In fact, if on one side, the distribution constant of hydrophobic additives between the aqueous and the micellar phases depends on the nature of the head group of the surfactant,I160nthe other side, ita dependence is not so large to justify the absence of the distribution in DTAB predicted by the cmc us mCR trend. So, according to the literature? these results can be explained by (16) DeLi&R.;Turm,Liveri,V.;Caetagnolo,M.; Ingleae,A.J.Solution Chem. 1986,16,23.
K-1
mol-' for heat capacities.
considering that CR strongly interacts with sodium ions forming complexes of high stability constant and that the CR.Na+ complex solubilizes in the NaDS micellar phase. Looking at experimental data at fixed ~ ~ L Cshown R in Figure 3, for both surfactants j3/j3" increases with R , tending to a constant value at high mCR. In spite of the extensive literature on the effect of a d d i t i ~ e s ~ on J ~j3- ~ values, as far as we know, the only data available for surfactants in aqueous CR solutions deal with sodium decanoate (NaDec).8 From the dependence of j3 on surfactant and CR concentrations, it was concluded that the CR-Na+complex is partially associatedto the micelle? The present data do not exclude this possibility even if it is quite difficult to explain the dependence of j3 on CR concentration for DTAB whose profile is similar to that for NaDS. At present we are unable to explain the decreasing trend of j3 with the added CR observed for NaDS at fixed R. The difficulty deals with the reliability of j3 using this unusual (from the conductivity point of view) approach. This is shown in Figure 4 where the slopes in the pre- and post-micellar regions are plotted against the R,,. As can be seen, in the premicellar region the slope values at fixed ratios are higher than those at fixed molalities while the opposite behavior is observed in the postmicellar region. It is to be noted that the unreliability of the j3 value obtained from the study at fixed ratios does not imply that the cmc values obtained by using the same experimental conditions are also unreliable. Apparent Molar Volume and Heat Capacity of Crown in Water and NaCl Solutions. Starting with this study, we observed a lack of reproducibility in the apparent molar volume of CR in water which was ascribed to the different content of water in the product. Therefore, density measurements were carried out using the products as received and by keeping it in a vacuum oven at 35 OC at different time intervals before use. As Figure 5 shows, (17) Vikingstad,E.; Kvammen, 0. J. Colloid Interface Sci. 1980, 74, 16.
(18) BostrBm,G.; Backlund, 5.;Blokhue, A. M.; HoiImd, H. J. Colloid Interface Sci. 1989,128, 179. (19) Zana, R.;Yiv, S.;Strazielle,C.; Limos, P. J. Colloid Interface Sci. 1981, 80, 208. (20) Abu-Hamdiyyah, M.; Kumari, K. J.Phys. Chem. 1990,94,6446.
Bakshi et al.
426 Langmuir, Vol. 10, No. 2, 1994 Table 3. Voluma and Heat Capacities of Sodium Dodecyl Sulfate (NaDS) and Dodecyltrimethylammonim Bromide (DTAB) in Water 18-Crown-6 Ether at Fixed Crown Molality at 298 K*
+
~~
~~
$Ad NaDS mCR = 0.2014 0.01160 0.424 0.02020 0.679 0.04170 1.328 0.07290 2.251 0.1074 3.307 0.1363 4.171 0.1743 5.315 0.2256 6.881 0.2795 8.490 0.3261 9.903 0.3823 11.567 0.4517 13.595 0.5231 15.659 m5
mCR
0.00195 0.00384 0.00539 0.00711 0.01156 0.02805 0.05229 0.08781 0.1226 0.1899 0.2280 0.2820 0.3565 0.4079 0.5036 0.5825
= 0.4771 (do = 1.015410; cpo= 4.04255) 0 0.090 239.36 0.168 241.41 0 0.225 243.47 0.253 249.43 0.23 0.365 253.29 0.77 0.746 258.02 3.12 1.345 258.71 6.47 2.231 258.79 11.35 3.088 258.79 16.68 4.733 258.63 26.00 5.670 258.44 31.23 6.990 258.18 38.41 8.809 257.80 48.50 10.073 257.50 54.90 12.399 257.00 66.54 14.323 256.54 75.85
-
Cr.5
737.9 620.4 556.6 526.9 470.4 467.4 459.6 457.1 455.0 454.0 452.8 455.5 456.9 1004 991
891.0 770.9 606.9 555.3 527.7 494.5 480.8 473.7 468.4 457.0 454.8 450.5 446.6
0.4003 (do = 1.012599; cpo= 4.06486) 0.092 284.98 0.17 1024 0.171 285.38 0.35 1012 0.230 285.91 0.44 1028 0.266 286.18 0.63 998 0.355 287.06 0.77 1024 1.82 941.8 0.449 289.65 0.651 292.44 5.08 812.6 0.785 293.29 7.34 771.5 1.024 293.96 11.12 734.9 1.489 294.51 18.64 692.8 1.813 294.65 23.29 687.3 2.406 294.85 32.54 671.4 2.890 294.90 40.50 655.5 3.623 294.95 52.20 642.7
DTAB mCR 0.004634 0.008781 0.01211 0.01427 0.01999 0.02979 0.05366 0.06991 0.09791 0.1525 0.1904 0.2631 0.3233 0.4178
Vrs -1O*Au/ a0 (do = 1.005136; cpo= 4.12147) 250.62 0.84 253.46 2.09 255.05 5.00 255.77 9.25 255.59 14.92 255.56 18.89 255.37 24.22 254.97 31.00 254.69 37.98 254.34 43.79 254.04 50.67 253.68 58.52 253.33 66.37
a Units are mol k g 1 for m, g cma for densities, cm3 mol-' for volumes, and J K-1 mol-' for heat capacities.
the reproducibility was attained when the product was dried at least 4 days. The standard (infinite dilution) partial molar volume and heat capacity were obtained from the dependence of the corresponding apparent molar properties as functions of concentration by means of the following equation
Y*= Y 2 0 + B y m + C y m 2 (4) where By and Cy are parameters which account for the solute-solute interactions. The best fits of experimental data up to CR 2 m to eq 4 gave the following values: Vzo = 223.35 f 0.03 cm3 mol-', Bv = -2.6 f 0.1 cm3 kg mol-2, CV= 0.69 f 0.05 cm3 kg2 mol" and Cp2O = 814.8 f 0.8 J K-' mol-', Bc = -64 f 3 J K-1 kg mol-2, Cc = 13 f 1J K-l kg2mol" from volume and heat capacity data, respectively. The standard partial molar volume is quite good in agreement with that (223.2 cm3 mol-') reported by Vikingstad and Bakkens while slightly higher than that (222.5cm3 mol-') reported by Letcher et a1.2' Within the experimental uncertainties, also the standard partial molar
heat capacity is in good agreement with that (808 f 7 J K-1 mol-') obtained by Briggner and Wads622from the study of the standard enthalpy of solution at 20 and 25 "C. VzOand the Bv parameter for CR in the complexed form were obtained by additivity from the apparent molar volume of CR in NaCl aqueous solutions. By indicating the complexed and uncomplexed form of CR with the subscripts i and j, respectively, we can write
+
(5) v*,c,= xi v*j xjv*j By neglecting the complexed CR concentration with respect to that of NaCl and by indicating with K c the equilibrium constant for the complex formation, Xi and Xj are given by
By taking for K c the value of 6.6 reported the literature,' a t a given mNaCl,Xi and X j can be calculated by means of eq 6. Then, at the given stoichiometric CR concentration (mCR),mi and mj were derived according to mi = XimcR and mj = XjmcR and the V*j value at mj by means of eq 4 and the VZ",Bv, and CVvalues reported above for CR in water. Thus, the apparent molar volume of CR in the complexed form V*,i was calculated by using eq 5. As Figure 6 shows, in the narrow range of concentration analyzed the plot of V*,i us mi gives a straight line whose intercept and slope are 235.2 f 0.2 cm3 mol-' and -95 f 6 cm3 kg mol-2, respectively. In conclusion equations correlating the apparent molar volume of CR to its concentration for complexed (i) and uncomplexed 0') CR are V a j = 223.35 - 2.6mj
+ 0.69mt
(7)
V,,i = 235.2 - 95mi (8) The difference between the standard partial molar volumes of CR in the two forms (11.8 cm3 mol-') agrees with that reported for the sodium ion in the presence and absence of CR (12.1 f 0.5 cm3 mol-') reported in the l i t e r a t ~ r e .Although ~~ infrared" and kinetic2Sinvestigations evidenced a change in the structural conformation between complexed and uncomplexed CR which should involve a volume change, the large and positive difference in the intercepts and negative in the slopes should be essentially ascribed to the loss of the hydrophilic hydration of the sodium ion screened by CR in the complex formation. Apparent Molar Volume and Heat Capacity of Surfactants in Water-Crown Systems. Figures 7 and 8 show the plots of the apparent molar volume ( V*,S) and heat capacity (C*,s) of NaDS and DTAB in water CR mixtures as functions of ms. In the case of volume, the property of transfer from water to water CR mixtures
+
+
AV*,s(W-W+CR) = V*s(W) - V*,s(W+CR) (9) is plotted. For the present surfactants it is quite difficult to obtain unquestionable results in the premicellar region because of their low cmc values. This means not only that a very (21)Letcher, T.M.;Paul, J. J.; Kay, R. L. J. Solution Chem. 1991,20, 1001. (22)Briggner, L. E.;and Wadsb, I. J. Chem. Thermodyn. 1990, 22, 143. (23)Hoiland, H.; Ringseth, J. A.; and Vikingstad,E. J. Solution Chem. 1978, 7,515. (24)Dale, J.; Kristiansen, P. 0. Acta Chem. Scand. 1972,26, 148. (25)Rodriguez,L. J.; Liesegang,G. W.; White, R. D.; Farrow,M.M.; Purdie, N.; Eying, E. M.J. Phys. Chem. 1977,81, 2118.
Langmuir, Vol. 10, No. 2,1994 421
Watel-Crown Ethel-Surfactant Systems
Table 4. Volumes and Heat Capacities of Sodium Dodecyl Sulfate + 18-Crown-6Ether Composite Mixture at Fixed Crown/ Surfactant Ratios (R)in Water at 298 Ka
m
108Ad
V*
0.06392 0.09410 0.1305 0.1576 0.1987 0.2649 0.3202 0.3933 0.5396 0.7196 0.8950 1.0172 1.2844 1.4807
2.547 3.714 5.022 6.022 7.504 9.753 11.635 14.012 18.575 23.788 28.550 31.661 38.018 42.313
R=2 232.47 232.58 233.27 233.31 233.41 233.83 233.87 234.02 234.16 234.30 234.35 234.39 234.43 234.45
-3.6 -5.6 -8.1 -10.0 -12.5 -16.8 -20.9 -24.6 -33.7 -43.8 -54.4 -59.6 -74.1 -83.3
729.8 713.6 706.5 697.3 697.6 692.9 681.4 689.8 681.8 679.1 669.3 673.6 663.4 660.4
0.02187 0.04567 0.06887 0.09094 0.1162 0.1600 0.2103 0.2603 0.3112 0.3648 0.4183 0.4759 0.5281 0.6242 0.7024 0.8146
0.920 1.820 2.633 3.436 4.332 5.869 7.571 9.232 10.865 12.562 14.226 15.925 17.485 20.160 22.298 25.196
0.02007 0.03968 0.06231 0.08359 0.1034 0.1484 0.1950 0.2382 0.2847 0.3326 0.3819 0.4284 0.4819 0.5623 0.6458
0.873 1.599 2.420 3.224 3.958 5.520 7.160 8.617 10.157 11.701 13.275 14.740 16.367 18.689 21.038
R '15 241.33 244.36 245.63 245.70 245.81 246.52 246.58 246.78 246.89 247.00 247.04 247.03 247.07 247.23 247.30
-1.2 -3.7 -6.4 -8.7 -11.3 -16.7 -21.5 -27.0 -31.9 -37.0 -42.5 -47.0 -52.6 -60.3 -68.6
752.8 624.7 587.7 579.9 556.0 539.8 544.6 526.9 527.6 526.2 520.6 522.4 519.2 519.9 515.8
0.01974 0.03775 0.05514 0.07663 0.09624 0.1363 0.1786 0.2197 0.2629 0.3065 0.3514 0.3978 0.4453 0.5079 0.5927 0.6728
0.834 1.530 2.187 2.974 3.714 5.155 6.654 8.079 9.538 10.977 12.456 13.939 15.400 17.306 19.761 22.001
0.01796 0.03631 0.05355 0.07347 0.09226 0.1307 0.1675 0.2069 0.2491 0.2867 0.3320 0.3705 0.4186 0.4852 0.5443 0.6318
0.801 1.503 2.122 2.850 3.575 4.999 6.295 7.659 9.172 10.444 11.852 13.130 14.652 16.748 18.464 20.980
R = '1% 243.20 246.25 247.87 248.53 248.39 248.55 248.89 249.12 248.93 249.01 249.38 249.33 249.39 249.35 249.52 249.60
-1.2 -3.6 -5.9 -8.1 -10.5 -15.4 -20.2 -24.4 -29.6 -33.4 -38.7 -42.6 -49.7 -54.7 -60.1 -69.3
732.9 608.0 566.3 566.5 548.5 527.4 512.2 519.9 509.9 516.0 511.7 514.1 491.3 511.0 515.8 509.6
a
-l@Aa/oo
Ca
1@Ad
m
-l@Aa/un
Ca
240.68 242.69 244.12 244.37 244.65 244.88 245.15 245.27 245.44 245.49 245.51 245.64 245.62 245.77 245.79 245.90
-1.3 -4.1 -6.6 -9.2 -12.1 -16.8 -21.8 -26.6 -32.9 -38.3 -44.1 -49.9 -54.3 63.0 -70.3 -80.0
753.1 631.9 610.0 586.1 571.8 564.3 566.0 567.7 547.3 545.0 537.3 534.2 538.2 537.8 533.9 532.2
R = '/io 244.33 245.89 246.59 247.26 247.30 247.71 247.90 248.04 248.17 248.27 248.27 248.32 248.41 248.45 248.57 248.65
-1.3 -3.8 -5.9 -8.3 -10.7 -15.7 -20.7 -24.6 -31.0 -35.9 -40.3 -45.7 -50.7 -57.5 -65.9 -73.3
744.5 602.9 577.2 572.0 557.7 537.5 530.0 543.1 512.1 510.6 516.3 510.1 509.5 505.4 505.6 507.5
Va
R = '13
Unite are mol kg-' for m, g cm-9 for densities, cms mol-' for volumes, and J
few data points can be collected but also that they are affected by a large uncertainty which, in the worst conditions, is of the order of 1cm3mol-' for volumes and 30 J K-1 mol-' for heat capacity. However, according to the experimental data the presence of CR slightly affects C*,s and the C*,s us ms slopes; in addition, the values for NaDS are very close to those for DTAB. According to literature data8for NaDec in CR solutions, positive slopes for V*,s were observed for both surfactants. The addition of CR leads to a decrease of the standard partial molar volumes which is about 3.5 and 1cms mol-l for DTAB and NaDS, respectively. This difference accounts for hydrophobic CR-DTAB and CR-NaDS interactions and, in the case of NaDS, for the decrease in the hydrophilic interactions due to the screened sodium ion when it is complexed in the CR cavity. In the postmicellar region, just beyond the cmc for both surfactants, V*,s increases and C0,s decreases as in pure water tending to a constant value at high ms. Looking at
K-1
mol-' for heat capacities.
Figure 8, we can observe that C*,s is larger and smaller by about 30 J K-' mol-' than that in pure water for DTAB and NaDS, respectively. In addition, by changing the CR concentration the C*,Svs ms profiles for NaDS are slightly affected. More interesting are the experimental data plotted in Figure 7. In fact, the DTAB AV*s(W-.W+CR) is negative at the cmc indicating CR-DTAB interactions in the aqueous phase and nil for the micellized surfactant according to a lack of specific interactions between CR and DTAB micelles. For NaDS,with increasing ms AVe$(W-W+CR) steeply increases just beyond the cmc, reaches a maximum value which is 8 and 12 cm3 mol-' at mCR = 0.2 and 0.5 mol kgl, respectively, and then decreases. This behavior is similar to that observed for surfactants in water + alcohol mixtures and was ascribed to a progressive extraction of the additive from the aqueous phase and its solubilization in the micellar phase. Apparent Molar Volume and Heat Capacity of Crown-NaDS Composite Mixtures. Figures 9 and 10
Bakshi et al.
428 Langmuir, Vol. 10, No. 2, 1994
. . . . . . . .' L . .. I
A
' . . . . l . . . . i . . . .
DTAB
A
I.'
0
L
t
s
5 230 -. 0
235
1
-0.5
i
h
h 4
8
O
220 0
10
20
30
40
50
0
60
Rnnc
Figure 2. Dependence of the relative cmc with respect to that in water on the 18-crown-6ether (CR)/surfactantratio at the cmc: circles, sodium dodecyl sulfate, filled symbols, at fixed R; open symbols, at fixed CR molality; triangles, dodecyltrimethylammonium bromide.
0.1
* o
0.2
*
0.3
O
0.4
0.5
mcR
Figure 5. Apparent molar volume of 18-crown-6ether in water as a function of its molality: A, 1day dried; A,2 days dried; , 3 days dried; 0 , 4 days dried; .,5 days dried. 236
t 235 234
A A
0
0 O
I
+. s
2: 233
> 232
L
23 1 230 0
0.01
0.02
0.03
0.04
0.05
"."+
0.8 0
10
20
30
40
M
60
Rcmc
Figure 6. Plot of the apparent molar volume vs molality for CR in the complexed form.
Figure 3. Dependence of the relative degree of counterion dissociationwiih respect to that in water on the 18-crown-6ether (CR)/surfactantratio at the cmc: circles,sodium dodecylsulfate, filled symbols,at fiiedR; open symbols,at fied CR concentration; triangles, dodecyltrimethylammoniumbromide.
I.
* *
NnDS in CR 0.5m
e
*
*
NaDS in CR O.2m
4
-c .
z
i 6 5
O
0
0
DTAB in CR 0.4m
1
-2 4 O0
0.1
0.2
03
OA
0.5 n
x
B 4
m,
0
O.'
mcR(cmc)
0.2
0.3
Figure 4. Slopes of the plots of conductivity us surfactant concentration in the pre-(opensymbols) and postmicellar (filled symbols) regions at fixed R (triangles)and at fixed CR concentration (circles) as functions of the CR molality at the cmc. show the plots of V*,c.m.and C O , ~ for . ~ . the composite mixture (SDS+ CR) at different mole ratios us its molality in water It can be seen that the curves are regularly shifted with the increase in the mole ratio being comprised between those of pure N O S and CR in water. As a general feature, all the curves are tending to a constant value with the increase in mc.m.. In the case of heat capacity, points around 0.2-0.3 m are scattered probably due to the NaDS postmicellar transition.
Figure 7. Apparent molar volume of transfer of sodium dodecyl sulfate (NaDS) and dodecylbimethylammonium bromide @TAB) from water to water-18-crown-6 ether mixtures as a function of surfactant molality. The propertiea of the composite mixture can be calculated on the basis of the additivity of both binary mixtures. This was done for all the mole ratios experimentally investigated but, for simplicity, those at R = are shown in Figures 9 and 10. As can be seen, the profiles are similar to the experimental ones but they are shifted toward lower and higher values for volumes and heat capacities, i.e. respectively. In addition, the excess properties (Yexc), the difference between the experimental and calculated properties, depend on the R value. As a general feature, at a given me.m.(where the surfactant is always in the micellized form) VmC and Cp"" present a maximum and a minimum, respectively, as a function of the mole fraction
WaterCrown Ethel-Surfactant Systems
Langmuir, Vol. 10, No. 2, 1994 429
, , , 1 , , ' , 1 , , , , 1 , , , , 1 , , , ,
3 1
-
'.
t
J.
P
800
2
700
. 3
DTAB in CR 0.4m
c,
600
U
500
400
300
0
0
a
m o a 0
N O S in CR 0.2 and 0.5m
x 0.2
0.1
0
0.3
m,
-2
0.5
0.4
0
0.2
0.4
0.6
0.8
1
x2
Figure 8. Apparent molar heat capacity of sodium dodecyl sulfate (NaDS) and dodecyltrimethylammoniumbromide (DTAB) in water 18-crown-6 ether mixtures as a function of surfactant molality; broken line, DTAB in pure water; solid line, NaDS in pure water.
Figure 11. Excess volumes of composite mixtures as fuctions of the mole fraction of the second component: 0, CR-NaDS; A, pentanol-DTAB (from ref 16);0,pentanol-NaDS (from ref 26); A, urea-DTAB (from ref 2); e, CR-DTAB; 0, CR-NaCl.
+
250
-%
245
6
.-
!240
1 235
0
0
.
650
c,
1600
P
550
500
a
L ~
~
1
0
1
1
0.1
1
1
"
1
0.2
'
1
1
1
0.3
1
'
1
1
1
0.4
4 '
1
1
1
0.5
1
1
1
1
~
~
0.6
"n.
Figure 10. Apparent molar heat capacity us molality for sodium
+
dodecyl sulfate 18crown-6ether c o m p i t e mixture at different 0, R = V10; A, R = lis; A, crown/surfactant ratios: 0, R = R =. :I?;0, R * 2; broken line, calculated according to the additiwty of both binary mixtures at R = Vg.
of the mixture. However the correlation for CpXcis not good as for VaC because the NaDS postmicellar transition affects heat capacity data. The plots of Velcus the mole fraction of the composite mixture are shown in Figure 11,where V-0 evaluated from literature data for pentanol-NaDS,M pentanol-DTAB,' and urea-DTAB2 are also reported. In the case of CR-DTAB and urea-DTAB, the data did not permit calculation of the VeXcat a constant 80
I
0.1
0.2
0.3
0.4
0.5
m, Figure 12. Dependence of the apparent molar volume on the surfactant molality for ( 0 ) 18-crown-6 ether (CR) in sodium dodecyl sulfate (NaDS), (0) CR in dodecyltrimethylammonium bromide (DTAB),(A)hexanol in NaDS (from ref 26), (A)hexanol in DTAB (from ref 16), and (e)urea in DTAB (from ref 2). Line indicates beet fit according to eq 10. Vw" indicates the standard property of the additive in pure water.
Figure9. Apparent molar volume us molality for sodium dodecyl sulfte-18-crown-6 ether composite mixture at different crown/ surfactant ratios: 0,R = VM; 0, R = illo; A,R = Vs; A! R = lis; 0, R = 2;broken line, calculated according to the additivity of both binary mixtures at R = VS.
450
, , , * ~ , , ,;
-2
230
-2
r;=;;,; 1 ,I *,;
2
>P
~
~
mc.,,,.. As can be seen, Vexcvalues of CR-NaDS mixture are positive in the whole range of composition and comparable to those for pentanol in the two surfactants. Within the experimental uncertainties, Vexc= 0 for ureaDTAB mixture and Vexc < 0 for CR-DTAB. These results agree with the CR solubilization in the complexed form in the NaDS micelles. However, positive Vexcvalues are also obtained for the CR.Na+ complex formation from density measurements of sodium chloride-CR mixtures in water. Since both hydrophilic and hydrophobic interactions involve positive Vwcvalues,we cannot discriminate if the solubilization process of CR in NaDS micelles is driven by one or other interaction. Apparent Molar Volume and Heat Capacity of Crown i n Water-Surfactant Systems. Figures 12 and 13show V*,~Rand C*,cRfOrCR 0.04 m in NaDS and DTAB, respectively, against ms. To compact the ordinate scale, in the case of volume, the standard partial molar volume of CR in water (V2")was subtracted. In the same figures the literature data for hexanol (HexOH) in both surfact a n t ~ . and ~ ~for~urea ~ ~in~DTAB2 . ~ ~ were also plotted. As can be seen, (V*,CR- V2")for CR in NaDS sharply increases (26) De Lisi, R.; Genova, C.; Testa,R.; Turco Liveri, V. J. Solution
Chem. 1984,13,121.
(27) Rous Desgrangea, G.;R o u , A. H.; Viallard, A. J. Chim. Phyu. 1981, 82, 441. (28) De Lisi, R.; Milioto, S.;Inglese, A. J.Phys. Chem. 1991,95,3322.
Bakshi et al.
430 Langmuir, Vol. 10, No. 2, 1994 900
800
-%
--
....-
-'ii E.
700
,
,E600
con
/ -
L
E
.I
L
i
je
2
500
U
400
L !'
'
300 -2
200 0
0.1
0.2
0.3
m,
0.4
0
0.5
Figure 13. Dependence of the apparent molar heat capacity on the surfactant molality for (0) 18-crown-6ether (CR) in sodium ( 0 )CR in dodecyltrimethylammonium dodecyl sulfate (NaDS), bromide (DTAB), (A)hexanol in NaDS (from ref 27), and (A) hexanol in DTAB (from ref 28). with the increase in ms tending to a constant value at higher concentration as it occurs for HexOH in both surfactants. For CR in DTAB, (V*,CR-VZ")is essentially independent of ms as observed for urea in the same surfactant. As expected, the heat capacity has similar behavior and opposite trend with respect to that of volume. In addition, in the case of NaDS a maximum, whose amplitude and location are comparable to that of HexOH, is present. This maximum was ascribed to the NaDS postmicellar transition.27 Since it is known that HexOH distributes between the aqueous and the micellar phases while urea does not, the above trends can be explained and rationalized on the basis of the CR solubilization in the micellar phase of NaDS but not in that of DTAB. We have reported elsewhere3 an approach following which a given bulk thermodynamic property for additives in micellar solutions can be rationalized in terms of the distribution constant of the additive between the aqueous and the micellar phases ( K ) and the corresponding property of the additive in both phases. This approach cannot be applied to C*,CRdata in NaDS. In fact, the presence of a maximum makes the simulation difficult because of the narrow range of ms analyzed. V*,CRin NaDS as a function of ms was fitted by3
Nf =
1
1+ K(ms - cmc)
Nb
K(ms - cmc) (11) 1 K(ms - cmc)
+
Acdc=c{2.3Ks+( I+@)@ (12) 2 where Ks is the Setchenov constant. The three parameters ( v b , Vf, and K) in eq 10 were solved by means of a nonlinear regression. In order to recover the Vf value, the Acd,AVmterm was calculated by taking the cmc and p values in pure water (Table 1)and the value of 11.8 cm3 mol-l for AVm.26 As far as the KS value is concerned, we have assumed it to be -0.5 as that
0.3
0.4
0.5
%
Figure 14. Plot of the best fit of eq 17 to weighted data and of the CR-Na+ formation in the aqueous phase and CR-Na+ distributionbetween the aqueous and the NaDS micellar phases. for HexOH in NaDS.29 It is to be noted that Ks affects only the Vf value and ita contribution to the Adc term is negligible with respect to that of distribution. The minimizing procedure gives vb = 233.4 f 0.3 cm3 mol-', Vf = 223.0 f 0.6 cm3mol-', and K = 17 f 3. The v b value is reasonably in agreement with that (232.4 cm3mol-') in NaDec 1.0 me while our K value is half to that we have calculated (see later) from the value reported by Stilbsg at ms = 0.25 m and mCR = 0.02 m. It is to be stressed that the above equations (and, then, the v b and K values) were obtained on the basis of a simple distribution model which does not account for the concomitant complexation equilibrium present in our system. In fact, according to eq 10 Vf is a constant quantity while it is not if the complexation process is considered. We have shown that the standard partial molar volume of CR in the complexed form is 11.8 cm3 mol-' larger than that in the uncomplexed form. This implies that, at fixed ~ C R the mole fraction of CR in the complexed form and, then, the total volume increase with mg. By assuming that the presence of the surfactant in the aqueous phase does not affect the apparent molar volume of CR and that the complexation constant is 6.1 as that in NaDec? Vf at a given mCR can be calculated on the basis of eqs 5 through 8, where ms is introduced instead of mNecl Vf = X,V*,
where Vf and vb are the partial molar volumes of CR in the aqueous and the micellar phases, respectively, AVm is the volume change for the NaDS micellization process, Nf is the fraction of CR solubilized in the aqueous phase, and Adc is the displacement of micellization equilibrium due to the presence of the additive. Nfand, then, the fraction of the additive in the micellar phase (Nb = 1- Nf)and the Acdc terms are given by3
0.2
0.1
+ x,v*,=
+
1 Kcms 1
Kcms K c s mCRf 1 235*1 1 + K c s mCR,f
223.3- 2.6
+
+ Kcms (
where mcRp indicates the total CR concentration in the aqueous phase. According to eq 11 it is given by mCR
"f,
= 1+ K(ms - cmc)
(14)
The plot of Vf,Le. the complexation processcontribution to the apparent molar volume, us ms at mCR = 0,04 m is shown in Figure 14. As can be seen, this effect cannot be neglected when experimental data are fitted by means of a model. Another equilibrium considered deals with the uncomplexed CR distribution between the aqueous and the micellar phase. Actually, we have mentioned above that CR scarcely solubilizes in the micellar phase of DTAB. Accordingly, it is reportedg that the K value for the ~
(29) Trainer, C. J. Colloid Interface Sci. 1982, 90,444.
,
Langmuir, Vol. 10, No. 2, 1994 431
Water-Crown Ether-Surfactant Systems solubilization process of different CR is =l. Since a K value of this order of magnitude cannot be detected by our approach, we assume that the uncomplexed CR does not solubilize in the micellar phase as also reported in nonionicsurfactants micelles of hexaethyleneglycol monon-dodecyl ether.30 In addition, we assume that the KS value for complexed and uncomplexed CR is the same. Therefore, eq 10 can be rewritten in the following form V*,CR= Yf,iVf,i + Yfjvfj+ (A,cIYf,i + AcdejYfj)AVm +
where Y indicates the mole fraction of the complexed CR (subscript i) or uncomplexed CR (subscript j ) component in the aqueous (subscript f) or micellar (subscript b) phase with respect to the stoichiometricCR concentration (mcR). In eq 15 Ki indicates the distribution constant of the complexed CR between the two pseudophases. Since Yf,i and Yfj can be expressed in terms of the mole fraction of each component in the aqueous phase Xi = 1 - Xj = mf,J(mf,i+ mfj) and Nf and since Xb,i = 1 it follows Yf,i = XiNf Yfj = X?Vf Yb,i = Nb (16) Therefore, eq 9 takes the form cmc AV, V*,c, = [XiVf,,+ XjVfj" + 2 [2.3Ks
+
vb,i
+
cmc AV, [2.3Ks + (1 + 0)KiXiI 2
In order to solve eq 17 Nf should be expressed in terms of Ki. Since mbj = 0, according to eq 11 we can write
is replaced by ms, with By combining eqs 6, where eqs 11 and 18, the following ones are obtained (19)
Equation 17 through eqs 6 and 20 is a four-parameter (Vf,i, Vfj, vb,i, and Ki) equation. By using eqs 7 and 8, it
can be reduced to a two-parameter (Ki and vb,i) equation which was solved by a nonlinear regression. Figure 14 showsthe best fit of eq 17 to the VQ,CR-Vw" data together with the complex formation and the distribution contributions; the micellization shift contribution is not shown since it is very small. It is interesting to note the relevant contribution of the complexation equilibrium in the aqueous phase. The best fit to the experimental weighted data gave the values of 231.87 f 0.04 cm3 mol-' for vb,j and 33 f 1for Ki. These results are quite interesting. In fact, the volume of the complexed CR solubilized in the micellar phase is quite smaller than that in water (235.2 cm3 mol-') while for polar additives penetrating the micelles, opposite results have generally been found. This strongly suggests the substitution at the micellar surface of the uncomplexed sodium ion with the complexed one. This process involves the formation of free sodium ions resulting in an increase in the hydrophilic interactions and, then, a decrease in the volume. As far as Ki is concerned, our value is very close to that of 37 f 3 obtained by converting the , equilibrium constant in the molarity scale ( K M )reported by Stilbs? into our scale (K = KMVS,where Vs is the partial molar volume of the micellized surfactant). Although the latter value seems to be related to the bulk distribution constant as that obtained by using eq 10, ita closness to Ki could be ascribed to a low sensitivity of the technique used by Stilbs to the CR complexation process in the aqueous phase. Consequently, while the complexation equilibrium strongly affects the volume data and, therefore, different results are obtained from the fit if it is taken into account or not, it is probably not so for NMR experiments.9*30 In addition, from the above results was calculated the fraction of sodium in the complexed form bonded to the micelle as a function of both the CR and the surfactant concentrations; at a given mcR value, it decreases by increasing ms and, at a given ms value, it increases with mcR. These results agree with literature findings for C211 cryptand in lithium dodecyl sulfate micellar solutions.31
Acknowledgment. The authors are grateful to the National Research Council of Italy (CNR, Progetto Finalizzato Chmica Fine) and to the Ministry of University and of Scientific and Technological Research (MURST) for financial support. M.S.B. is also indebted to Foreign Ministry of Italy for a fellowship which allowed him to come to Italy. (31) Cinley, M.; Henriksson, U. J. Colloid Interface Sci. 1992, 150,
(30)Stilbs, P.J . Colloid Interface Sci. 1983,94,463.
281.
