Langmuir 2000, 16, 3093-3100
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Binding of Sodium Dodecyl Sulfate to Some Polyethyleneimines and Their Ethoxylated Derivatives at Different pH Values. Electromotive Force and Microcalorimetry Studies Y. Li,† S. M. Ghoreishi,‡ J. Warr,§ D. M. Bloor,‡ J. F. Holzwarth,*,†,⊥ and E. Wyn-Jones*,†,‡ Fritz-Haber Institut der Max-Planck Gesellschaft, Faradayweg 4-6, D-14195 Berlin-Dahlem, Germany, Division of Chemical Sciences, Science Research Institute, University of Salford, Salford M5 4WT, U.K., and Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral L63 3JW, U.K. Received July 27, 1999. In Final Form: November 14, 1999 Electromotive force and isothermal titration microcalorimetry measurements have been carried out to study the interactions of sodium dodecyl sulfate (SDS) with polyethyleneimines (PEI’s) and some ethoxylated PEI’s at different pH’s. In all cases the polymers show a remarkable affinity toward SDS. The SDS concentration at the onset of binding decreases with decreasing pH. At low pH’s phase separation similar to that observed for strong polyelectrolyte/oppositely charged surfactant systems occurs presumably as a consequence of the protonation of the N atoms in the polymer to form a polycation. The SDS range over which phase separation occurs decreases as the size of the ethoxylated chains increases, and for the polymer with the longest ethoxylated chain no precipitation occurs. In many cases where phase separation occurs, as more SDS is added the polymer/surfactant complex often resolubilizes and binding proceeds until the polymer becomes saturated with bound SDS. For any given polymer at this limiting stage in the binding, the number of surfactant molecules bound per mole of polymer is independent of pH. In addition both the charged and uncharged N atoms as well as the pendant ethoxylated chains make significant contributions to the binding process.
Introduction The interactions between polymers and surfactants have been the subject of many fundamental studies in physical chemistry.1,2 The reason for this interest is that polymer/ surfactant mixtures are used in the processing of colloidal systems and are also the ingredients in many cosmetic and detergent products. Recently we have carried out a number of studies on sodium dodecyl sulfate (SDS)/neutral polymer interactions.3-10 We have found that when electromotive force (EMF) measurements carried out with a dodecyl sulfate selective electrode are complemented by isothermal titration calorimetry (ITC) the combined results lead to a very powerful insight into the binding * Corresponding authors. † Fritz-Haber Institut der Max-Planck Gesellschaft. ‡ University of Salford. § Port Sunlight Laboratory. ⊥ TEL: +49 30 8413 5516. FAX: +49 30 8413 5385. E-mail:
[email protected]. (1) Goddard, E. D. Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 22 (2), 85. (3) Robb, I. D. Anionic Surfactants. Surf. Sci. Ser. 1981, 11, 109. (4) Li, Y. Ph.D. Thesis, University of Salford, 1998. (5) Ghoreishi, S. M.; Li Y.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 4380. (6) Ghoreishi, S. M.; Li, Y.; Holzwarth J. F.; Khoshdel, E.; Warr, J.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1999, 15, 1938. (7) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 6326. (8) Painter, D. M.; Bloor, D. M.; Takisawa, N.; Hall, D. G.; WynJones, E. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2087. (9) Takisawa, N.; Brown, P.; Bloor, D.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2099. (10) 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.
behavior.5-7 In soluble polyelectrolyte/oppositely charged surfactant systems, cationic surfactant electrodes have also been used successfully by a number of workers to provide binding isotherms and details of the binding mechanism.11-13 These systems have been the subject of some recent comprehensive reviews.12,13 In the present work we have used EMF and ITC methods to study the binding of SDS to polyethyleneimines (PEI’s) and their various polyethoxylated derivatives. The motivation for the present work stems from an earlier study of the binding of surfactants to various generations of poly(1,4-diaminobutane) dendrimers, which showed that these relatively new oligomers have remarkable binding powers toward SDS compared to linear nonionic polymers.6 In general, the linkages between the N atoms in polyethyleneimines are similar to those in the internal cavity of diaminobutane dendrimers. The PEI’s are polymers with a large number of amine groups and their chemical structure is simple in the sense that each nitrogen atom is joined to another nitrogen via the ethylene (-CH2CH2-) linkage.14 The resulting polymer is branched and globular with a generic structure as shown in Scheme 1. These branched polyethyleneimine chains contain primary (P), secondary (S), and tertiary (T) nitrogens in the ratio P:S:T of 1:2:1.14 In the polyethoxylated derivatives, each hydrogen atom attached to a primary and secondary nitrogen in Scheme 1 is replaced by an ethoxylated chain (11) Hayawaka, K.; Kwak, J. C. T. Cationic Surfactants. Surf. Sci. Series 1981, 11, 109. (12) Hansson, P.; Lindmann, B. Curr. Opini. Colloid Interface Sci. 1996, 1, 604. (13) Linse, P.; Piculell, L.; Hansson, P. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Surfactant Science Series 77; Marcel Dekker: New York, 1998; pp 193-238. (14) BASF literature on LUPASOL.
10.1021/la9910172 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/17/2000
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Langmuir, Vol. 16, No. 7, 2000 Scheme 1: Structure of Polyethyleneimines
Scheme 2: Structure of Ethoxylated Polyethyleneimines
Scheme 3: Polymer Coding with Molecular Weights in Square Brackets
of general structure -(CH2CH2O)n-H according to Scheme 2. The polymer used in the present work originates from two PEI polymers of molecular weights 2000 and 800. For each PEI base of specific molecular weight, the substitutable hydrogens on the primary and secondary nitrogens are replaced by ethoxylated chains that contain on average values of n of 3, 7, and 20 repeat units, respectively. Hence the samples used in this work are coded as indicated below in Scheme 3 (the calculated molecular weights are in square brackets). These polymers are pure in the sense that they do not contain excessive amounts of impurities (
