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Langmuir 1999, 15, 6326-6332
Interactions between a Nonionic Copolymer Containing Different Amounts of Covalently Bonded Vinyl Acrylic Acid and Surfactants: EMF and Microcalorimetry Studies Y. Li,† S. M. Ghoreishi,‡ J. Warr,§ D. M. Bloor,‡ J. F. Holzwarth,*,† and E. Wyn-Jones*,†,‡ Division of Chemical Sciences, Science Research Institute, University of Salford, Salford M5 4WT, U.K., Fritz-Haber Institut der Max-Planck Gesellschaft, Faradayweg 4-6, D-14195 Berlin-Dahlem, Germany, and Port Sunlight Laboratory, Unilever Research, Quarry Road East, Bebington, Wirral L63 3JW, U.K. Received March 22, 1999. In Final Form: May 12, 1999 The binding of both an anionic surfactant, sodium dodecyl sulfate (SDS), and a cationic surfactant, tetradecyltrimethylammonium bromide (TTAB), to various water soluble copolymers containing equal amounts of the monomers of methyl vinyl imidazole (MVI) and vinyl pyrrolidone (VP) and various amounts of a third monomer, vinyl acrylic acid(AA), have been studied using surfactant selective electrodes and isothermal titration microcalorimetry (ITC). The change in the binding behavior of both surfactants has been investigated as the charge and content of the acrylic acid monomer has been systematically altered. The data clearly show how surfactant binding can be both moderated and enhanced. In the presence of salt there is almost no binding of the MVI/VP/AA polymer in its anionic form with TTAB, whereas the binding with SDS is somewhat reduced in comparison with the data for no salt. The ITC measurements have also been used to investigate the binding of various forms of the polymer with a commercial sample of sodium dodecylbenzenesulfonate. The binding data are very well defined and clearly show similar trends to those observed for SDS.
Introduction Polymers and surfactants are used widely both in industry and everyday life and are frequently employed together, particularly in complex colloidal systems, to achieve emulsification, colloidal stability of flocculation, structuring and suspending properties, and rheology control.1-3 The development of polymer/surfactant formulations of acceptable blends requires methods for optimizing the physicochemical properties of specific polymer/surfactant systems. This can only be achieved through controlled manipulation of the polymer/surfactant interactions. As far as we are aware, there is a paucity of fundamental studies on this topic. Recently we have developed an experimental strategy for monitoring the controlled adsorption and desorption of sodium dodecyl sulfate (SDS) to and from a neutral polymer. In this work a nonionic surfactant was used to modify the SDS/polymer interactions, and the changes were monitored using a dye labeled polymer,4 isothermal titration microcalorimetry (ITC),5 and electromotive force (EMF) methods6 with an SDS selective membrane electrode. An alternative strategy * Corresponding authors. † Fritz-Haber Institut der Max-Planck Gesellschaft. ‡ University of Salford. § Unilever Research. (1) Goddard, E. O. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. O., Ananthapadamanabham, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) (a) Robb, I. P. Anionic Surfactants. Surfactant Sci. Ser. 1981, 11, 109. (b) Brackman, J. C.; Engberts, J. B. F. E. Chem. Soc. Rev. 1993, 22, 85. (3) (a) Hayawaka, K.; Kwak, J. C. T. Cationic Surfactants. Surfactant Sci. Ser. 1991, 31, 189. (b) Hansson, P.; Lindman, B. Curr. Opin. Colloid In 1996, 1, 604. (4) (a) Kwak, J. C. T.; Linse, P. Surfactant Sci. Ser. 1998, 77. (b) Li, Y.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1996, 12, 4476. (5) Fox, G. J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1998, 14, 1026. (6) Li, Y. Ph.D. Thesis, University of Salford, 1998.
Table 1. Composition of the Different Monomers in the Various Copolymers (mol %)a MVI
VP
AA
50 48.5 47.5 46.5 45
50 48.5 47.5 46.5 45
3 5 7 10
a MVI ) methylvinylimidazole; VP ) vinylpyrrollidone; AA ) vinyl acrylic acid.
which has been used successfully involves the synthesis of hydrophobically modified polymers in which systematic modification of a polymer is achieved by introducing alkyl groups into the chain.7,8 These hydrophobic polymers undergo specific interactions with surfactants which can drastically change the rheological behavior of the solutions. In this report we have further developed this theme of modifying polymer/surfactant interactions using a copolymer which contains various amounts of a covalently bonded monomer unit which can electrostatically repel or attract specific surfactants. In the present work the base macromolecule is a 50/50 mol % copolymer of methylvinylimidazole (MVI) and vinyl pyrrolidone (VP). The other copolymers contain equal amounts (in moles) of MVI and VP and various amounts of a third monomer, vinyl acrylic acid (AA), in such a way that the three monomers in all copolymers are randomly distributed. The molar composition of the variations of the copolymer denoted MVI/VP/ AA and used in this work are listed in Table 1. In the present work the experiments have been carried out at pH’s 5-6 and 9-10 for both surfactants. At pH 5-6 the carboxylic acid groups in the vinyl acrylic acid monomer (7) Carlsson, A.; Karlstrom, G.; Lindman, B.; Stenberg, O. Colloid Polym. Sci. 1988, 266, 1031. (8) Thureson, K.; Lindman, B.; Nystrom, B. J. Chem. Phys. B 1997, 101, 6450.
10.1021/la990342m CCC: $18.00 © 1999 American Chemical Society Published on Web 07/02/1999
Nonionic Copolymer and Surfactant Interactions
units of the polymer contain a residual amount of negative charge, and at pH 9-10 these acid groups are completely dissociated. We have used EMF and ITC methods to study how the binding of the anionic surfactant SDS and the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) to this polymer is affected by the charge and amount of the vinyl acrylic acid (AA) monomer. ITC measurements have also been carried out on both surfactants in the presence of 0.1 mol dm-3 salt and also with various forms of the polymer in the presence of a commercial sample of the surfactant sodium dodecylbenzenesulfonate. In fundamental studies carried out on the binding of surfactant to a polymer the following critical concentrations referring to the total amounts of added surfactant to a known amount of polymer are of prime importance: (i) T1, signaling the onset of binding; (ii) T2, signaling the saturation of the polymer with bound surfactant; (iii) Tf, signaling the formation of free micelles. In addition binding isotherms representing (i) the amount of bound surfactant as a function of monomer surfactant concentration and (ii) the amount of bound counterions are also important in order to understand the behavior of the systems. Although the importance of the above parameters in relation to binding studies has long been recognized,1 it is only recently that quick, reliable, and effective experimental methods involving surfactant selective electrodes (EMF) and isothermal titration calorimetry have been developed by us to determine these parameters.9-15 Experimental Section SDS was synthesized according to the procedure described by Davidson,17 and TTAB was a Sigma product purified by repeated recrystallization. The polymers were specially prepared commercial samples obtained from BASF, all with molecular weights in the region 40 000.18 These polymers only contain a small amount of impurities ( q). j, k, and m are the numbers of these polymer bound aggregates which are formed during various stages of binding. m and n represent these numbers at the binding limit when the polymer is “saturated” with bound aggregates; SN represent free micelles. It is clear that the introduction of negative charges on the polymer makes it more difficult for the surfactant monomers to accumulate in the vicinity of the polymer, thus making it energetically more difficult for binding to take place as shown by the increase in the T1 values. In comparison to SDS the binding of TTAB to the parent copolymer MVI/VP is much less facile. However systematically introducing negatively charged AA groups into the polymer leads to a significant progressive increase in the binding. The electrostatic nature of the binding was confirmed by the addition of 0.1 mol dm-3 salt, which screens25 90% of the negative charges on the AA groups and prevents binding. This work has also enabled us to interpret the origin of the maximum found in ∆Hi during the ITC experiment as the self-aggregation of electrostatically bound TTAB monomers to form bound micellar aggregatessa process which is also accompanied by a conformational change in the polymer. Thus for the cationic surfactant the binding model can be rationalized in terms of the following sequence of equilibria denoted in (25) Holzwarth, J.; Ju¨rgensen, H. Bev. Bunsen-Ges. Phys. Chem. 1974, 78, 526.
6332 Langmuir, Vol. 15, No. 19, 1999
Li et al.
Scheme 2 rS + P S PrS
(1)
PrS S P(Sq)j
(2)
P(Sq)j + S S P(Si)k + SN
(3)
P(Si)k + S S P(Sn)m + SN
(4)
which represent the significant events during the binding process, where PrS is a complex with r individually
bound surfactant monomers; the other notations are as in Scheme 1. Acknowledgment. Y.L. would like to thank the MaxPlanck Gesellschaft for the provision of a postdoctoral research fellowship and Professor Josef Holzwarth for the opportunity to carry out the ITC experiments at the FritzHaber Institut. S.M.G. thanks The Islamic Republic of Iran for the provision of a maintenance award, enabling this work to be carried out under the supervision of Professor Evan Wyn-Jones, at the University of Salford. LA990342M
