Solubilization of n-Alkylbenzenes in Aggregates of Sodium Dodecyl

below the cac and then increased with increasing SDS concentration above the cac and with decreasing alkyl chain length of the solubilizates. Also, th...
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Langmuir 2004, 20, 6116-6119

Solubilization of n-Alkylbenzenes in Aggregates of Sodium Dodecyl Sulfate and a Cationic Polymer of High Charge Density (II) Jungno Lee and Yoshikiyo Moroi* Chemistry and Physics of Condensed Matter, Graduate School of Sciences, Kyushu University, Chuo-ku, Ropponmatsu 4-2-1, Fukuoka 810-8560, Japan Received December 30, 2003. In Final Form: March 24, 2004 The solubilization property of the aggregate composed of sodium dodecyl sulfate (SDS) and a cationic polymer (polydiallyldimethylammonium chloride, PDADMAC) was investigated. From the binding isotherm, the increasing free SDS concentration (Cf) above the critical aggregation concentration (cac) was clearly confirmed and used to calculate the Gibbs free energy change of solubilization. The maximum additive concentration of the alkylbenzene solubilizates remained almost constant around their aqueous solubilities below the cac and then increased with increasing SDS concentration above the cac and with decreasing alkyl chain length of the solubilizates. Also, their solubility increased with increasing temperature over the concentration range of the surfactant examined. Because the monomeric DS- concentration in the aqueous phase (Cf) increased with the SDS concentration above the cac in the SDS/PDADMAC system, Cf was evaluated from the binding isotherm to calculate the change in the Gibbs energies of transfer of the solubilizates using the phase separation model. The Gibbs energy change for the solubilizates decreased with increasing temperature and increasing alkyl chain length. The decrease in the Gibbs energy per CH2 group (∆GCH2°) was favored by an increase of temperature, and it was larger in magnitude than that for micelles of single-surfactant systems. From the values of ∆H° and T∆S°, the solubilization of alkylbenzenes into SDS/PDADMAC was found to be entropy-driven.

Introduction Interaction between surfactants and polymers has been of interest to researchers and field engineers. The aqueous surfactant systems formulated together with polymers are extensively used in modern cosmetics, household products, paints, and so forth. Currently, as a result of their capability to form hydrophobic aggregates, the systems are attracting great interest as a drug delivery system too.1 Binding isotherms of ionic surfactants to ionic polymers have been studied2,3 thoroughly by many researchers using a potentiometric titration method with a surfactant ionselective electrode in the solid state.4-6 Through this method, one can evaluate the binding parameters and, at the same time, measure a clear onset for the critical aggregation concentration (cac) and the free surfactant ion concentration (Cf) in the aqueous phase above the cac. Evaluation of the Cf value in the surfactant/polymer system above the cac is very important, because it does not maintain a constant value but increases with increasing surfactant concentration, especially for the case of cross-linked polymers7 and polysoaps.8,9 So, one should * To whom correspondence should be addressed. Telephone: +81-92-726-4742. Facsimile: +81-92-726-4842. E-mail: moroiscc@ mbox.nc.kyushu-u.ac.jp. (1) Barreiro-Iglesias, R.; Alvarez-Lorenzo, C.; Concheiro, A. J. Controlled Release 2003, 93, 319. (2) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642. (3) Shirahama, K.; Sato, S.; Niino, N.; Takisawa, N. Colloids Surf., A 1996, 112, 233. (4) Davidson, C. J.; Meares, P.; Hall, D. G. J. Membr. Sci. 1988, 36, 511. (5) Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1990; Chapter 5. (6) Liu, J.; Nakama, M.; Takisawa, N.; Shirahama, K. Colloids Surf., A 1999, 150, 275. (7) Gong, J. P.; Osada, Y. J. Phys. Chem. 1995, 99, 10971.

not regard [surfactant] - cac as the concentration of bound surfactant for evaluation of both the aggregation number and the Gibbs free energy change for solubilization into the surfactant/polymer system above the cac. Compared to the interaction between the polymer and the surfactant, the solubilization into the surfactant/ polymer aggregate has not been studied much.10 Only a few studies on dye solubilization have been reported.11-13 Hayakawa et al. have studied dye solubilization into cationic surfactant/anionic polymer systems using a potentiometric titration method with an electrode selective to a cationic surfactant ion.14,15 Recently, the solubilization of phenols and estradiol into the surfactant/polymer system has been reported.1,16 In this study, we evaluated the Cf value through the binding isotherm using a surfactant ion-selective electrode. The maximum additive concentrations of n-alkylbenzenes into the mixed system were determined with absorption spectra of the solubilizates. Then, we evaluated the Gibbs energy change using the Cf data from the binding isotherm. Finally, the corresponding enthalpy and entropy changes were also obtained from the temperature dependence of the Gibbs energy change. Experimental Section Materials. The cationic polymer examined in this study is a homopolymer of polydiallyldimethylammonium chloride (PDAD(8) Benrraou, M.; Zana, R.; Varoqui, R.; Pefferkorn, E. J. Phys. Chem. 1992, 96, 1469. (9) Anthony, O.; Zana, R. Langmuir 1996, 12, 3590. (10) Saito, S. J. Colloid Interface Sci. 1967, 24, 227. (11) Jones, M. N. J. Colloid Interface Sci. 1968, 26, 532. (12) Goddard, E. D.; Hannan, R. B.; Matteson, G. H. J. Colloid Interface Sci. 1977, 60, 214. (13) Thalberg, K.; Lindman, B. J. Phys. Chem. 1989, 93, 1478. (14) Hayakawa, K.; Shinohara, S.; Sasawaki, S.; Satake, I.; Kwak, J. C. T. Bull. Chem. Soc. Jpn. 1995, 68, 2179. (15) Hayakawa, K.; Tanaka, R.; Kurawaki, J.; Kusumoto, Y.; Satake, I. Langmuir 1999, 15, 4213. (16) Olea, A. F.; Gamboa, C. J. Colloid Interface Sci. 2003, 268, 63.

10.1021/la030443r CCC: $27.50 © 2004 American Chemical Society Published on Web 06/17/2004

Solubilization into SDS/Polymer Aggregates MAC) (McIntire), whose average molecular weight is about 1.5 × 105. It has a relatively high cationic charge density (5.2 mequiv g-1). The amount of cationic charge of this linear polymer was determined by a colloidal titration method using a toluidine blue solution (0.1 w/v %)17 as an indicator and an N/400 potassium polyvinyl sulfate solution as a titrant. Sodium dodecyl sulfate (SDS; Nacalai Tesque, 99%) was purified by ether extraction followed by two successive recrystallizations from water. Benzene (Kishida, 99.5%), toluene (Kishida, 99.5%), ethylbenzene (Aldrich, 99.8%), n-propylbenzene (Tokyo Kasei Kogyo, 99%), n-butylbenzene (Aldrich, 99+%), and n-pentylbenzene (Tokyo Kasei Kogyo, 99%) were purified by extraction with concentrated sulfuric acid three times and then by washing once with dilute NaOH solution and finally three times with a large amount of water. The water used was distilled twice from alkaline permanganate solution. All solutions were prepared using a 1 mmol dm-3 NaCl solution. Electrode Potential Measurements. A surfactant anionselective cationic poly(vinyl chloride) (PVC) membrane similar to that of the previous study18 was used to determine the free anion concentration of DS- (Cf). The potentiometric measurements were conducted using a digital voltmeter (Advantest R6441B) with an accuracy of (0.5 mV. The potentiometric titration apparatus is also similar to that of described previously.18 In these measurements, the temperature of the apparatus was controlled in a thermostat (Tamson TMV 40) within (0.003 K, and the sample solution was continuously stirred using a magnetic stirrer. The cell system used was as follows: Ag | AgCl || reference solution (1 mmol dm-3 SDS solution) | cationic PVC membrane | sample solution || AgCl | Ag. The concentrations of the sample solutions were changed stepwise by adding a certain volume of the stock SDS solution to the sample solution using a microsyringe. After each addition, about 10 min were allowed to pass until the potential ∆E became stabilized. The experiments were carried out at 288.2, 298.2, and 308.2 K in the presence of 200 ppm of PDADMAC. The solutions became turbid in the course of titration as a result of the neutralization of the cationic polymer charges by DS- ions and their subsequent aggregation by hydrophobic interaction. Therefore, the experiments were usually stopped before the system became turbid because the data thereafter became less reproducible and the precipitation was unfavorable for the membrane electrode. Solubilization of n-Alkylbenzenes into SDS/PDADMAC Solution. The stock solution of SDS was passed through a membrane filter of pore size 0.22 µm (Millipore Millex VV) to remove dust and was then introduced in different amounts into the eight solutions of PDADMAC (200 ppm). Then, the mixed solutions were stirred for 24 h for complete equilibrium binding of surfactant to polymer. They were poured separately into eight photocells, which were then set into a solubilization apparatus. The solubilization apparatus was similar to the one used in the previous work.19 A sufficient amount of n-alkylbenzene was placed in a hollow place in the middle of the glass apparatus. The whole glass vessel with its cover was kept in a thermostat (Advantec LCH-19D) at 288.2, 298.2, and 308.2 ( 0.1 K for more than 76 h for the complete solubilization equilibration, while the surfactant/polymer solutions were agitated with rotors in the photocells. Inside the apparatus, the volatile solubilizates (nalkylbenzenes) easily evaporated because of their high volatility, and then, the chemical potential of the gaseous solubilizate molecule becomes the same throughout the phases or the concentration of monomeric solubilizate can be set identically in the eight solutions. After the equilibration, each photocell was capped immediately except for benzene and toluene, for which the solution was diluted 60 times by 11.57 mmol dm-3 SDS solution because of the large optical density. The solubilizate concentrations in the surfactant/polymer solutions were determined spectrophotometrically by the optical density of the (17) Ueno, K.; Kina, K. J. Chem. Educ. 1985, 62, 627. (18) Lee, J.; Moroi, Y. Bull. Chem. Soc. Jpn. 2003, 76, 2099. (19) Lee, J.; Moroi, Y. J. Colloid Interface Sci. 2004, 273, 645.

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Figure 1. Binding isotherms of DS- to PDADMAC in the presence of PDADMAC (200 ppm) in 1 mmol dm-3 NaCl solution.

Figure 2. Changes of free DS- concentration (Cf) against the SDS concentration in the presence of PDADMAC (200 ppm) in 1 mmol dm-3 NaCl solution. solutions and the molar extinction coefficients. The values of the molar extinction coefficients () used were reported in a previous study.20

Results and Discussion Figure 1 shows the binding isotherm of the DS- ion for the SDS/PDADMAC system. From the isotherm, the Cf value can be easily evaluated. The change of Cf with the SDS concentration is illustrated in Figure 2. As can be seen in Figure 2, the Cf values increased slowly and then started to increase steeply near the electrically neutral concentration of polymer. So, this increasing value of Cf should be used instead of the cac to calculate the aggregation number of the aggregates21 and the Gibbs energy of transfer of the solubilizate from the bulk to the surfactant/polymer aggregate at surfactant concentrations above the cac. Figure 3 shows the changes of the maximum additive concentration of n-alkylbenzene with increasing SDS concentration at 308.2 K. Solubilizate concentrations below the cac remained almost constant for all the solubilizates, which indicates the constancy of the chemical (20) Take’uchi, M.; Moroi, Y. Langmuir 1995, 11, 4719. (21) Lee, J.; Moroi, Y. Langmuir 2004, 20, 4376.

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the maximum additive concentration at the highest SDS concentration above the cac to that around the cac, n-pentylbenzene showed the largest increase (ca. 9-11 times), while benzene showed the least increase (ca. 1.141.16 times) for the temperature change of 288.2-308.2 K. This can be related to the difference in the Gibbs energy change for solubilization. The maximum additive concentration increased with increasing temperature, which results from a larger value of [R] at a higher temperature. The solubilized amount of n-alkylbenzene for the present surfactant/polymer system was smaller than that for single surfactant aggregates22 because of the much smaller aggregation number of SDS in the polymer/SDS aggregates. However, the average number of solubilizate molecule per surfactant molecule in these aggregates was larger than that of a single surfactant micellar system. For example, the average number of n-butyl benzene per surfactant in this system was 0.58 at 298.2 K. This value is much larger than those of anionic surfactant micelles: 0.02 for lithium 1-perfluoroundecanoate,23 0.04 for sodium cholate,24 and 0.24 for 1-dodecanesulfonic acid.20 However, it is comparable to that of a cationic surfactant: 0.56 for n-tetradecyltrimethylammonium bromide.22 The number of solubilized molecules per surfactant/ polymer aggregate was somewhat large. Hence, we evaluated the Gibbs energy of transfer of the solubilizate molecules from the aqueous bulk to the molecular aggregates by the partition equilibrium, regarding the aggregates as a separate phase. The following relationships based on the phase separation model of polymer with the aggregates give the Gibbs energy of the transfer:

XAR ) ([Rt] - [R])/{([SDS] - Cf) + ([Rt] - [R])}

Figure 3. (a) Changes of the maximum additive concentration of n-alkylbenzene with SDS concentration in the presence of PDADMAC (200 ppm) in 1 mmol dm-3 NaCl solution at 308.2 K. (b) Exaggerated ordinate for longer n-alkylbenzenes.

potential of the solubilizate molecule throughout the phases inside the glass apparatus. The mean value of the solubilizate concentrations below the cac gives the monomer concentration [R] of the solubilizates in the SDS/ polymer solution. This value, within the experimental error, is equal to the solubility of the solubilizate in water. The increase in the solubility of solubilizate or the maximum additive concentration above the cac is brought about by the incorporation of solubilizates into SDS aggregates formed with PDADMAC. However, the determination of the solubility was limited to SDS concentrations below the electroneutral point of the charged aggregates because above these SDS concentrations it became impossible to measure the absorbance due to the appearance of turbidity in the system. At a given temperature, the maximum additive concentrations decreased in the order of benzene > toluene > ethylbenzene > n-propylbenzene > n-butylbenzene > n-pentylbenzene, which is the order observed in the aqueous solution [R]. This indicates that, as the alkyl chain of the solubilizate becomes shorter, the maximum additive concentration becomes higher. The present results agree well with the fact that the solubilized amount in the aggregates becomes larger for more soluble solubilizate in an aqueous bulk. However, from the viewpoint of the change in the ratio of

(1)

XW R ) [R]/(55.5 + Cf + [R])

(2)

0 0,A θ,W RT ln(XAR/XW R ) ) - (µR - µR ) ) -∆G

(3)

where XAR and XW R are the mole fractions of the solubilizate in the aggregate phase (superscript A) and in aqueous phase (superscript W), respectively, [Rt] is the total equivalent concentration of solubilizates, [SDS] is the total surfactant concentration, and µθ is the standard chemical potential at infinite dilution. The mole fractions are easily available from Figure 3. In evaluating XRA, the analytical DS- concentration in the aggregate phase should be obtained from the concentration of free DS- (Cf) derived from the binding isotherm and is not [SDS] - cac, because the Cf value does not remain constant above the cac. In this experiment, Cf was obtained from the binding isotherm based on the potentiometric titration method. The Gibbs energy change at 288.2, 298.2, and 308.2 K decreased, or the solubilization became more stabilized with increasing alkyl chain length of the solubilizates, while remaining almost the same regardless of the SDS concentration. The Gibbs energy changes were plotted against the number of carbon atoms in the alkyl chain of the solubilizates at different temperatures (Figure 4). The contribution per methyl group in the alkyl chain to the Gibbs energy change ∆GCH2° decreased with the increase in temperature, that is, -2.55 kJ mol-1 at 288.2 K, -2.66 kJ mol-1 at 298.2 K, and -2.82 kJ mol-1 at 308.2 K. Although the SDS aggregates on PDADMAC have a (22) Doi, Y.; Kawashima, Y.; Matsuoka, K.; Moroi, Y. J. Phys. Chem. B 2004, 108, 2594. (23) Take’uchi, M.; Moroi, Y. J. Colloid Interface Sci. 1998, 197, 230. (24) Moroi, Y.; Sugioka, H. J. Colloid Interface Sci. 2003, 259, 148.

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Figure 4. Gibbs energy change for solubilization against the number of carbon atoms in the alkyl chain of the solubilizates at different temperatures.

Figure 5. Enthalpy and entropy changes of solubilization, ∆H° and -T∆S°, against the number of carbon atoms in the alkyl chain of the solubilizates.

smaller aggregation number21 than the micelle of the single SDS system, the present values are larger in magnitude than expected (see the values of -2.35 to -2.81 kJ mol-1 for 298.2 K for the usual single surfactant systems,20,22 where the transfer from the aqueous bulk into the single micelle was investigated). This difference suggests that the cationic polyelectrolyte binds to the anionic micelle more strongly than the single electrolyte monovalent or divalent cations. This binding results in the formation of a more hydrophobic environment in aggregates for the solubilizates and gives rise to the larger Gibbs energy decrease for the transfer than that for the micelle formed by single-surfactant systems. From the variation of ∆G° with temperature, the enthalpy and entropy changes, ∆H° and ∆S°, of solubilization can be obtained by the following relations:

showed being entropy-driven even from the benzene. The negative entropy contribution to the negative Gibbs energy of transfer increased with the alkyl chain length of alkylbenzene as a result of the breaking of structured water molecules around the alkyl chain. In addition, it is highly possible that the molecular aggregates of a smaller aggregation number in the present study were destructured more for their mother structure by the intrusion of larger numbers of solubilizate molecules than the SDSonly aggregates of a larger aggregation number incorporated with less solubilizate molecules.

∆H0 )

[

]

∂(∆G0/T) ∂(1/T)

(4)

P

∆S° ) - (∆G° - ∆H°)/T

(5)

The enthalpy change can be estimated from the slope of linear plots of ∆G°/T versus 1/T. The ∆H° and T∆S° values are illustrated in Figure 5, and they imply that the solubilization of n-alkylbenzenes into SDS/PDADMAC aggregates is driven by a positive entropy change. In contrast, solubilization studies of the n-alkylbenzenes in other surfactant micelles showed that the processes were enthalpy-driven for the shorter alkyl chain solubilizates and became entropy-driven for the longer alkyl chain lengths.20,24 On the contrary, the SDS/PDADMAC system

Summary The free surfactant concentration Cf increased with increasing total surfactant concentration above the cac from the binding isotherm at 288.2, 298.2, and 308.2 K. The maximum additive concentration for the n-alkylbenzenes increased gradually with the SDS concentration, with temperature, and with the decreasing alkyl chain length of the solubilizates. The Gibbs energy of solubilization calculated on the basis of the Cf value from the binding isotherm decreased with temperature and the alkyl chain length of the solubilizates, while it remained almost constant with SDS concentration. The increase in temperature decreased the Gibbs energy of transfer per methyl group ∆GCH2° quite steeply, and the decreasing rate of the Gibbs energy change with temperature became more pronounced with increasing alkyl chain length of the solubilizates. ∆H° and T∆S° values indicate that the solubilization of alkylbenzenes into SDS/PDADMAC was entropy-driven. LA030443R