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Langmuir 1999, 15, 4380-4387
Electromotive Force Studies Associated with the Binding of Sodium Dodecyl Sulfate to a Range of Nonionic Polymers S. M. Ghoreishi, Y. Li, D. M. Bloor, J. Warr,† and E. Wyn-Jones* Division of Chemical Sciences, University of Salford, Salford M5 4WT, U.K. Received September 23, 1998. In Final Form: March 30, 1999 The interaction between sodium dodecyl sulfate (SDS) and a variety of nonionic polymers has been studied using an SDS membrane selective electrode. From the experimental data critical concentrations associated with the binding process have been evaluated. These are (I) the onset of binding T1, (II) the SDS concentration T2 corresponding to the polymer becoming “saturated” with bound SDS, and (III) the SDS concentration (Tf) when free micelles occur in solution. The binding isotherms have also been measured at different added salt concentrations and in some cases different molecular weight polymers. At low salt concentration the different polymers show some selectivity toward SDS in the sense that the maximum amount of SDS they can bind per gram varies from polymer to polymer. However, in the presence of added salt, this selectivity is almost removed and all polymers seem to behave in a similar fashion. The presence of free sodium counterions in solution whether generated as a result of low counterion binding by the bound surfactant or by simply adding salt also governs the concentrations at which free micelles occur in solution.
Introduction Mixtures of polymers and surfactants have found practical applications in diverse areas of commercial importance. As a result their interactions have been extensively studied and have been the subject of review papers.1-3 One of the prerequisites in any fundamental studies is a reliable binding isotherm. Up to a few years ago the number of polymer/surfactant systems studied far exceeded the number of available binding isotherms. This situation has now been remedied by the use of surfactant selective electrodes which provide a quick, efficient, and reliable method to monitor the monomer concentration of an ionic surfactant in the presence of various additives.4-10 The interactions between sodium dodecyl sulfate (SDS) and a number of water-soluble nonionic (neutral) polymers have been studied using an SDS electrode. From the measured electromotive force (EMF) data and also the variation of monomer (m1) SDS concentration with total added SDS (C1), various critical concentrations associated with the binding process have been determined. These include the value of the total SDS concentration corresponding to (I) the onset of binding (T1), (II) the saturation of the polymer with bound SDS (T2), and (III) the formation of free micelles (Tf). In addition, † Unilever Research Port Sunlight, Quarry Road, East Bebington, Wirral L63 3JW, U.K.
(1) Goddard, E. O. In Interactions of Surfactants with Polymers and Proteins Goddard, E. D., Ananthapadamanabham, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 22, 85. (3) Hayawaka, K.; Kwak, J. C. T. Cationic Surfactants. Surf. Sci. Ser. 1991, 37, 189. (4) Davidson, C. J. Ph.D. Thesis, University of Aberdeen, Aberdeen, U.K., 1983. (5) Thuresson, K.; Karlstrom, G.; Lindman, B. J. Phys. Chem. 1995, 99, 3823. (6) Valasques, D. L.; Galin, J. C. Macromolecules, 1986, 19, 1096. (7) Painter, D. M.; Bloor, D. M.; Takisawa, N.; Hall, D. G.; WynJones, E. J. Chem. Soc. Faraday Trans. 1 1988, 84, 2087. (8) Takisawa, N.; Brown, P.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2099. (9) Wan-Badhi, W. A.; Wan-Yunus, W. M. Z.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans 1993, 89, 2737. (10) Li, Y. Ph.D. Thesis, University of Salford, Salford, U.K., 1997.
binding isotherms and a parameter which is a measure of the relative extent of binding have been determined. The variation of these binding data with salt concentration, molecular weight, and structure of the monomer units of the polymers has been examined. A review describing other techniques which can be used to determine some of the above binding parameters is given in refs 2 and 3. Experimental Section Materials. The SDS used in this work was synthesized according to the method described by Davidson.4 The polymers together with their sources and molecular weights (when available) are listed in Table 1. EHEC and HM-EHEC were purified by dialysis and freeze drying,5 and the remaining polymers were used as received. The information concerning the molecular weights of the polymers was given by the suppliers. We do not have any data on polydispersity. EMF Measurements. Surfactant membrane electrodes selective to SDS were constructed in the laboratory4,7-9 and used to determine the concentrations of monomer SDS and Na+ counterions by measuring their EMF relative to a bromide ion selective electrode (Corning solid-state ISE 30-35-00), a chloride ion selective electrode (Orion 91-CIDN), and also a commercial sodium (Corning 476211) reference electrode. The cells used for these measurements and the procedures to calculate the respective monomer concentrations have been described elsewhere.4-14 In the EMF experiment a concentrated surfactant solution is titrated into an aqueous solution containing a constant amount of polymer. After each injection, the EMF of the solution is measured. The EMF data are then plotted as a function of SDS concentration for the solutions with and without the polymer, the latter being the “control” experiment. Typical EMF data are displayed in Figure 1 for the SDS/PVI system at 1 × 10-4 mol dm-3 NaBr. Normally, when binding is taking place the EMFs are different for each corresponding titration for the solutions with and without polymer. The EMF data were carried out in 1 × 10-1 NaCl and 1 × 10-4 mol dm-3 NaBr solutions. (11) Wan Badhi, W. B. Ph.D. Thesis, University of Salford, 1993. (12) Bloor, D. M.; Wan-Yunus, W. M. Z.; Wan-Badhi, W. A.; Li, Y.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 3395. (13) Bloor, D. M.; Li, Y.; Wyn-Jones, E. Langmuir 1995, 11, 3778. (14) Bloor, D. M.; Mwakibete, B. O.; Wyn-Jones, E. J. Colloid Interface Sci. 1996, 178, 374.
10.1021/la981313z CCC: $18.00 © 1999 American Chemical Society Published on Web 05/26/1999
EMF Studies Associated with the Binding of SDS
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Table 1. Sources and Molecular Weights of Polymers Used in This Work polymer
abbreviation
methyl cellulose hydroxypropyl cellulose ethylhydroxy ethyl cellulose hydrophobically modified EHEC hydroxybutylmethyl cellulose hydroxypropylmethyl cellulose hydroxyethyl cellulose poly(vinylpyrrolidone) poly(vinylpyrrolidone) poly(vinylpyrrolidone) poly(vinylpyrrolidone) poly(vinylpyrrolidone) poly(vinylpyrridine) nitrogen oxide poly(vinylpyrrolidone)/poly(vinylimidazole) copolymer poly(propylallylamine) poly(vinylimidazole) poly(ethylene oxide) poly(ethylene oxide) poly(propylene oxide) poly(vinyl methyl ether) methylvinylimidazole/vinylpyrrolidone copolymer N-vinylacryloylpyrrolidine and vinylpyridine dicyanomethylide copolymer
MC HPC EHEC HM-EHEC HBMC HPMC HEC PVP PVP PVP PVP PVP PVPy-N-O PVP/PVI PPAA PVI PEO PEO PPO PVME MVI/VP PAPR*
molecular weight >50 000 100 000 120 000 120 000 120 000 >50 000 250 000 360 000 40 000 24 000 15 000 10 000 200 000 40 000 4 000 6 000 1 000 27 000 40 000 10 000
supplier Aqualon Aldrich Bernol Noble AB, Sweden Bernol Noble AB, Sweden Aldrich Aqualon Aldrich Sigma Unilever Unilever Unilever Sigma Unilever Unilever Unilever Unilever BDH BDH Aldrich Aldrich Unilever synthesized by Dr. Bloor6
Figure 1. Plot of the EMF of the SDS electrode (reference Br-) as a function of the total SDS concentration for the SDS/PVI system in 1 × 10-4 mol dm-3 NaBr: ([) pure SDS; (9) SDS + PVI (0.1% w/v).
Results In the present study of SDS/polymer systems, the methods used to determine the critical concentrations associated with the binding of SDS to the polymers using the SDS selective electrode are described below. We will report the experimental results for the system whose experimental data behave ideally, namely, SDS/PVI in Figure 1, and then show how EMF data from some of the other systems have been interpreted. Figure 1 shows three distinct regions of EMF behavior. Initially, in region I, at low surfactant concentration, the EMF data coincide and display Nernstian behavior. In this region no detectable interaction occurs between the SDS and polymer. This region finishes at T1 when the EMF data start diverging in such a way that the EMF of the polymer/surfactant system is always greater than the corresponding polymerfree SDS solution. The “break point” in the EMF at T1 is
almost as sharp as some of those found when determining the critical micelle concentrations (CMCs) of pure surfactants using different physicochemical methods. T1 represents the onset of surfactant binding to the polymer, and at total SDS concentrations exceeding T1, the monomer SDS concentration is less than the total added SDS, the difference being equal to the amount of bound SDS. As further SDS is added in excess of T1, the EMF data continue to diverge before finally merging again at T2. This is referred to as region II. The merging of the EMF data at T2 is an observation which we have found previously in connection with the binding of SDS to neutral polymers,9-13 cyclodextrins,15 and also dendrimers16 and (15) Wan-Yunus, W. M. Z.; Taylor, J.; Bloor, D. M.; Hall, D. G.; WynJones, E. J. Phys. Chem. 1992, 96, 8979. (16) Ghoreishi, S. M.; Li, Y.; Holzwarth, J. F.; Khoshdel, E.; Warr, J.; Bloor, D. M.; Wyn-Jones, E. Langmuir, to be submitted.
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Table 2. Critical Concentration Associated with Binding in 1 × 104 mol dm-3 NaBr (N/A Means Not Applicable)a
polymer
Tl × 103 (mol dm-3)
T2 × 103 (mol dm-3)
Tf × 103 (mol dm-3)
m1 at T2 × 103 (mol dm-3)
T2 - m1 × 103 (mol dm-3
(T2 - m1)/Cp × 103 (mol of bound SDS/g dm-3 polymer)
a
MC (0.5%) HPC (0.5%) HPC10 (0.5%) EHEC13 (0.5%) (120 000) HMEHEC (0.5%) HBMC (0.5%) HPMC(0.5%) HEC (0.5%) PVP (1%) (3 600 000) PVPII (1%) (360 000) PVP (0.5%) (40 000) PVP (1%) (24 000) PVP (1%) (15 000) PVP (1%) (10 000) PVI (0.1%) PVPy-NO (0.1%) PPAA (0.1%) PVP/PVI (0.1%) PVME10 (0.5%) PEO (1%) (4000) PEO (1%) (6000) PAPR*13 (0.5%) (10 000) PPO12 (0.5%) MVI/VP (0.5%)
0.003 0.0015 0.001 0.002 0.002 0.0035 0.004 0.0024 0.0015 0.002 0.0009 0.0012 0.0013 0.002 0.0004
