Thermodynamics of the Solubilization of Water-Insoluble Dyes by

Langmuir 1999, 15, 4213-4216

4213

Thermodynamics of the Solubilization of Water-Insoluble Dyes by Complexes of Cationic Surfactants with Poly(vinyl sulfate) of Different Charge Densities† Katumitu Hayakawa,* Ryoya Tanaka, Junichi Kurawaki, Yoshifumi Kusumoto, and Iwao Satake Department of Chemistry and BioScience, Faculty of Science, Kagoshima University, 1-21-36 Korimoto, Kagoshima 890, Japan Received August 20, 1998. In Final Form: February 25, 1999 The solubilization of two water-insoluble dyes, o-(2-amino-1-naphthylazo)toluene (OY) and 1-pyrenecalbaldehyde (PyA), by complexes of anionic polyelectrolyte and alkyltrimethylammonium bromide (C12TAB, C14TAB, C16TAB) was examined using sodium poly(vinyl sulfate) (PVS) with different charge densities at 298.2 K. The change in the Gibbs function was estimated for this solubilization equilibrium. These complexes showed a larger solubilizing capacity for OY than for PyA. Their capacity for OY was larger than that of the corresponding surfactant micelles. The charge density of PVS influenced the solubilizing capacity in C12TAB and C14TAB solutions but had little effect in C16TAB solutions. PVS with a 40% charge density had the lowest solubilizing capacity. Bound short-chain surfactants are expected to have a smaller lateral interaction with neighboring surfactants bound to PVS with a lower charge density, thus forming a less hydrophobic complex with a lower solubilization capacity. The observed weak solubilization by PVS with a low charge density was ascribed to this less hydrophobic complex.

Introduction Solubilization by surfactant micelles has been studied extensively, and both the physicochemical aspects and applications of solubilization in a wide variety of fields have been revealed to be a function of the chemical structures of the surfactants and solubilizates, added salts, and organic modifiers.1-4 Although solubilization enhancement by polymer-surfactant complexes was observed at concentrations far below the surfactant critical micelle concentration in the early 1970s,5-8 little systematic research has been conducted in this area and the physicochemical aspects of the process still remain unclear. The characteristics of surfactant binding by a polyelectrolyte with an opposite charge include a strong affinity through the electrostatic interaction and a highly cooperative property through the hydrophobic interaction between bound surfactants.9-29 We found a correlation between the solubilization capacity and cooperative bind†

Presented at Polyelectrolytes ‘98, Inuyama, Japan, May 31June 3, 1998. (1) Nakagawa, T. In Nonionic surfactants; Schick, M. J., Ed.; Dekker: New York, 1966; Chapter 17. (2) Mukerjee, P. In Solution chemistry of surfactants; Mittal, K. L., Ed.; Plenum: New York and London, 1979; Vol. 1, p 153. (3) Mackay, R. A. In Nonionic Surfactants: Physical Chemistry; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; Chapter 6. (4) Christian, S. D.; Scamehorn, J. F. Solubilization in surfactant aggregates; Dekker: New York, 1995. (5) Arai, H.; Murata, M.; Shinoda, K. J. Colloid Interface Sci. 1971, 37, 223. (6) Saito, S.; Taniguchi, T. J. Colloid Interface Sci. 1973, 44, 114. (7) Tokiwa, F.; Tsujii, K. Bull. Chem. Soc. Jpn. 1973, 46, 2684. (8) Goddard, E. D.; Hannan, R. B. In Micellization, Solubilization, and Microemulsions; Mittal, K. L., Ed.; Plenum: New York, 1977; p 835. (9) Hall, D. G. J. Chem. Soc., Faraday Trans. 1 1981, 77, 1121. (10) Shirahama, K.; Yuasa, H.; Sugimoto, S. Bull. Chem. Soc. Jpn. 1981, 54, 375. (11) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (12) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1983, 87, 506. (13) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642. (14) Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1984, 88, 1930.

ing in some polyion-surfactant systems.30-32 The complex of poly(styrene sulfonic acid) with cationic surfactants showed a low solubilization capacity for o-(2-amino-1naphthylazo)toluene (OY) and 1-pyrenecalbaldehyde (PyA). On the other hand, the complex of a hydrophilic polyion, dextran sulfate, with cationic surfactants had an even larger solubilization capacity for both dyes than did surfactant micelles.32 However, only limited quantitative data on solubilization by polymer-surfactant complexes are available. More experimental data are necessary to make generalizations on the correlation between the solubilizing capacity and the cooperative binding of surfactant by a polyion. In this paper, we report the solubilization of two water-insoluble dyes, OY and PyA, by complexes of cationic surfactant and anionic polymer, sodium poly(vinyl sulfate) (PVS), with different charge densities. (15) Leung, P. S.; Goddard, E. D.; Han, C.; Glinka, C. J. Colloids Surf. 1985, 13, 47. (16) Santerre, J. P.; Hayakawa, K.; Kwak, J. C. T. Colloids Surf. 1985, 13, 35. (17) Binana-Limbele, W.; Zana, R. Colloids Surf. 1986, 21, 483. (18) Chu, D.-Y.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270. (19) Goddard, E. D. Colloids Surf. 1986, 19, 301. (20) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 99, 16684. (21) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 99, 16694. (22) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, 1993; Chapter 4. (23) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. (24) Thalberg, K.; Lindman, B.; Bergfeldt, K. Langmuir 1991, 7, 2893. (25) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1991, 95, 6004. (26) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1991, 95, 3370. (27) Thalberg, K.; van Stam, J.; Lindblad, C.; Almgren, M.; Lindman, B. J. Phys. Chem. 1991, 95, 8975. (28) Thalberg, K.; Lindman, B. Langmuir 1991, 7, 277. (29) Chu, D. Y.; Thomas, J. K. Macromolecules 1991, 24, 2212. (30) Hayakawa, K.; Fukutome, T.; Satake, I. Langmuir 1990, 6, 1495. (31) Sudbeck, E. A.; Dubin, P. L.; Curran, M. E.; Skelton, J. J. Colloid Interface Sci. 1991, 142, 512. (32) Hayakawa, K.; Shinohara, S.; Sasawaki, S.; Satake, I.; Kwak, J. C. T. Bull. Chem. Soc. Jpn. 1995, 68, 2179.

10.1021/la9810657 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/24/1999

4214 Langmuir, Vol. 15, No. 12, 1999

Hayakawa et al.

Figure 1. Dependence of absorbance on surfactant concentration. (A) OY in PVS/C12TAB mixed solutions. (B) PyA in PVS/ C12TAB mixed solutions. Charged density of PVS: (O) 100%; (b) 93%; (0) 88%; (9) 62%; (3) 40%.

Experimental Section The potassium salt of PVS (Wako, Osaka) was hydrolyzed in 0.2 mol dm-3 hydrochloric acid, and the degree of hydrolysis was calculated from the increase in the acid concentration. The relative charge density of PVS was estimated from the degree of hydrolysis (the charge density of the original PVS was 100%) because a greater degree of hydrolysis leads to a lower charge density. After the counterions (K+ and H+) were exchanged with sodium ions, the ionic concentrations of PVS stock solutions were determined by colloid titration. The surfactants used were dodecyl- (C12TAB), tetradecyl- (C14TAB) and hexadecyltrimethylammonium bromides (C16TAB). They were all purchased from TCI, Tokyo (all GR) and were further purified by repeated recrystallization. OY (TCI, Tokyo) and PyA (Aldrich, Tokyo) were used without further purification. A few milligrams of OY or PyA powder was added to 20 mL mixed solutions of 0.5 mmol dm-3 PVS and surfactant at the required concentration, and the mixture was shaken at 25 °C for 3 days. The amount of unbound surfactant was independently determined potentiometrically using electrodes responsive to cationic surfactant ions. The amount of OY solubilized in the PVS-surfactant solution mixture was calculated from the molar absorbance of OY (13 300 mol-1 dm3 cm-1) at 454 nm in the supernatant of the mixture. For PyA, the molar absorbance at 397 nm (4350 mol-1 dm3 cm-1) was used. OY and PyA were completely insoluble in the absence of the mixture of PVS and surfactant. For turbid samples, the absorbance at 600 nm was subtracted as a baseline correction. A similar procedure was used to measure the micellar solubilization, in which the amount of surfactant varied from concentrations below the critical micelle concentration (cmc) to concentrations greatly exceeding the cmc.

Results and Discussion Solubilizing Capacity of PVS-Surfactant Complexes. The absorbance at 454 nm for OY and at 397 nm for PyA depended on the C12TAB concentration. Figure 1 shows an example for absorbance in the presence of 0.5 mmol dm-3 PVS with different charge densities. No absorbance is seen in PVS solutions without C12TAB, but the absorbance increases with the C12TAB concentration at concentrations far below the cmc (16 mmol dm-3),

Figure 2. Dependence of solubilized amounts of OY on the binding degree of surfactant: (A) PVS/C12TAB mixed system; (B) PVS/C14TAB mixed system; (C) PVS/C16TAB mixed system. Symbols are the same as those in Figure 1.

indicating solubilization of the water-insoluble dyes, OY and PyA, into PVS-C12TAB complexes. A rapid increase in absorbance is observed when the PVS charge density is high. This observation suggests that PVS with a higher charge density has a greater solubilization capacity at a given C12TAB concentration. The solubilized amounts of OY and PyA are plotted against the amounts of surfactant bound by PVS in Figures 2 and 3, respectively. The solubilizing capacity (PS), defined as the mole ratio of solubilized dye to bound surfactant, was calculated from the slopes of the linear parts of the curves in each figure. The values for OY and PyA are listed in Tables 1 and 2. These tables also include the solubilizing capacity of micellar surfactant without polyelectrolyte. From Tables 1 and 2 we observe the following. (1) The solubilizing capacity for OY and PyA increases with the PVS charge density in the PVS-C12TAB mixed system but is independent of the charge density in the PVSC16TAB mixed system. (2) PS only increased with the

Solubilization of Water-Insoluble Dyes

Langmuir, Vol. 15, No. 12, 1999 4215 Table 1. Thermodynamic Parameters for the Solubilization of OY by PVS-CnTAB Complexes charge density (PVS) 100% 93% 88% 62% 40%

PS

∆G°/kJ mol-1

KS

OY in PVS-C12TAB Complex 0.320 0.242 0.323 0.244 0.284 0.221 0.197 0.165 0.127 0.113

3.51 3.49 3.74 4.47 5.41

100% 93% 88% 62% 40%


3.02 3.15 3.50 3.31 4.07

100% 93% 88% 62% 40%


3.29 3.33 3.74 3.65 3.96

C12TAB micelle C14TAB micelle C16TAB micelle

0.075 0.142 0.218

0.070 0.124 0.179

6.60 5.17 4.26

Table 2. Thermodynamic Parameters for the Solubilization of PyA by PVS-CnTAB Complexes charge density (PVS) 100% 93% 88% 62% 40%

Figure 3. Dependence of solubilized amounts of PyA on the binding degree of surfactant: (A) PVS/C12TAB mixed system; (B) PVS/C14TAB mixed system; (C) PVS/C16TAB mixed system. Symbols are the same as those in Figure 1.

surfactant chain length when the PVS charge density was 40%; for higher charge density PVS there is no clear trend with surfactant chain length. (3) PS is larger for OY than for PyA. (4) The PVS-surfactant complexes solubilize more OY and less PyA than the corresponding surfactant micelles. From the PS values we estimate that PVS(100%)surfactant complexes solubilize 3 or 4 OY molecules for every 10 bound surfactant ions but only 1-2 PyA molecules for every 10 bound surfactant ions. A lower solubilizing capacity for both OY and PyA is observed with PVS(40%)surfactant complexes, and PVS(40%)-C12TAB complex solubilizes only 1 OY molecule for every 8 bound C12TA ions and 1 PyA molecule for every 25 bound C12TA ions. This observation suggests that much of the bound surfactant does not contribute to the solubilization of OY and PyA. Gibbs Functions for the Solubilization. From the solubilizing capacity we estimate the solubilization constant (KS) and the change of the standard Gibbs function (∆G°) for solubilization of the dyes. When we represent

PS

∆G°/kJ mol-1

KS

PyA in PVS-C12TAB Complex 0.119 0.106 0.112 0.101 0.108 0.097 0.054 0.051 0.040 0.038

5.55 5.69 5.77 7.36 8.07

100% 93% 88% 62% 40%


4.25 4.13 4.33 4.69 6.79

100% 93% 88% 62% 40%


4.02 4.41 4.23 4.33 4.66

C12TAB micelle C14TAB micelle C16TAB micelle

0.125 0.217 0.342

0.111 0.178 0.255

5.44 4.27 3.39

the solubilization equilibrium by KS

D (solid) {\} D (P/S complex)

(1)

the following equation for the solubilization constant KS (mole fraction units) is derived:

KS )

PS ∆G° ) exp 1 + PS RT

(

)

(2)

Here, ∆G° corresponds to the standard Gibbs function for transfer of a dye from the solid phase to a polyionsurfactant complex. The values of KS and ∆G° for OY and PyA solubilization are also presented in Tables 1 and 2. The changes in the Gibbs function are positive because the solid state of the dye was selected as the standard for this comparison (eq 3).

∆G° ) µD° (in P/S complex) - µD° (solid)

(3)

The change in the Gibbs function for solubilization by

4216 Langmuir, Vol. 15, No. 12, 1999

Hayakawa et al.

Figure 4. Comparison of Gibbs function for solubilization of OY by PVS/CnTAB mixed system. The micellar solubilization for OY is selected as the standard.

complexes of PVS of different charge densities with a surfactant cation is compared in Figure 4 for OY and in Figure 5 for PyA, where ∆G° for the solubilization by the corresponding surfactant micelles is used as the standard. As above, solubilization of OY by PVS-surfactant complexes is greater than that of surfactant micelles; accordingly, the change in Gibbs function is lower (less positive) than in surfactant micelles. In the case of the solubilization of PyA, the PVS-surfactant complexes with higher PVS charge density have a change in Gibbs function comparable to that of the corresponding micelles. These complexes exhibit a larger (more positive) change in the Gibbs function at a PVS charge density of 40% than do the corresponding surfactant micelles, indicating a lower solubilizing capacity for PyA. The cmc of these surfactants was determined as 13.4 mM for C12TAB, 2.83 mM for C14TAB, and 0.69 mM for C16TAB, from the solubilization of OY, and 15.5 mM for C12TAB, 3.65 mM for C14TAB, and 0.99 mM for C16TAB, from the solubilization of PyA. The reference values are 16 mM for C12TAB, 3.6 mM for C14TAB, and 0.92 for C16TAB at 25 °C.33 The low cmc for OY suggests a specific interaction of OY with these cationic surfactants (for example, dye-induced premicelle34) and may lead to a large solubilizing capacity. The solubilization of both dyes is independent of the PVS charge density in PVS- C16TAB complexes, whereas the PVS-C12TAB and PVS-C14TAB complexes at 40% PVS charge density show a large change in the Gibbs function. Since, on average, there is a longer distance (33) Kagaku Binran, 4th ed.; Nippon Kagakukai, N., Ed.; Maruzene: Tokyo, 1993; Vol. 1, p 613. (34) Sato, H.; Kawasaki, K.; Kasatani, K. J. Phys. Chem. 1983, 87, 3759.

Figure 5. Comparison of Gibbs function for solubilization of PyA by PVS/CnTAB mixed system. The micellar solubilization for PyA is selected as the standard.

between neighboring ionic sites in PVS with a lower charge density, the weak hydrophobic interaction between bound surfactant chains is expected to induce small surfactant clusters in polyion domains. We estimate the average distance between the neighboring ionic sites as 2.5 Å for 100%, 2.7 Å for 93%, 2.85 Å for 88%, 4.0 Å for 62%, and 6.3 Å for 40% PVS. The surfactant chain length of C12TAB is still much longer than these values, but the lateral interaction between bound surfactant may be weakened considerably. Therefore, we expect that the weak hydrophobic interaction between bound surfactant chains leads to less cooperative binding of the surfactant ions and small aggregates in polyion domains. This is more effective for a surfactant with a short chain. Conclusion Surfactant complexes with PVS of different charge densities have a greater solubilizing capacity for OY than for PyA. The capacity for OY is larger than that of a surfactant micelle. This observation is ascribed to a specific interaction of OY with CnTAB surfactants. PVS with 40% charge density increases the solubilization capacity in C12TAB and C14TAB solutions but not in C16TAB solutions. This is ascribed to the weak lateral hydrophobic interaction between bound surfactant chains in the case of shortchain surfactants. LA9810657

Thermodynamics of the Solubilization of Water-Insoluble Dyes by

Recommend Documents