Langmuir 1998, 14, 1697-1704
1697
Ellipsometric Study of the Adsorption of Hydrophobically Modified Polyacrylates at Hydrophobic Surfaces Ce´line Poncet,*,† Fredrik Tiberg,‡ and Roland Audebert† Laboratoire de Physicochimie Macromole´ culaire, Universite´ Pierre et Marie Curie, CNRS URA No. 278, ESPCI, 10 rue Vauquelin, F-75231 Paris Cedex 05, France; and Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-22100 Lund, Sweden Received October 20, 1997. In Final Form: January 19, 1998
The adsorption of hydrophobically modified polyelectrolytes (HMPEs) at hydrophobic surfaces was studied by ellipsometry. Both the adsorbed amount of polymer Γ and the layer thickness d were measured as a function of time. The HMPEs are polyacrylates carrying various fractions of grafted dodecyl chains. Hydrophobic interactions are responsible for the adsorption (unmodified polyacrylate does not adsorb at the surface). The surface anchoring of grafted dodecyl groups is coupled with the repulsion of charged acrylate groups. This results in the formation of large loops at the interface, separated by very short trains, since the probability to have two or more successive alkyl groups grafted at the polymer chain is small. As in the case of ordinary polyelectrolytes, the mean thickness of the adsorbed layer decreases with increasing solution ionic strength. Changes of the HMPE chemical composition (molar ratio, X, of alkyl units attached on the polyacrylate backbone) affect the adsorbed layer structure in a complex fashion. HMPEs with relatively few grafted chains form thick dilute adsorbed layers. Increasing the ratio of grafted chains to the number of polyelectrolyte monomers results in a monotonic decrease of the thickness and an increase of the HMPE density. However, the adsorbed amount exhibits a maximum at a relatively small ratio between 5% and 7%, above which the effect of the decrease of Γ caused by the decrease of the average loop size becomes larger than the opposing effect of the increasing surface density of anchors. We also discuss the shape of the adsorption isotherms, experimental evidence for the existence of a depletion layer located outside the adsorbed layer, and the time evolution of adsorption and desorption.
Introduction Hydrophobically modified polymers (HMPs) consist of hydrophilic polymer backbones onto which hydrophobic chains have been covalently attached. HMPs can be block or graft copolymers. As the hydrophobic segments tend to self-associate and form reversible intra- and intermolecular bonds,1,2 these polymers are also commonly called associating polymers. At high concentrations, the intermolecular bonds can result in the formation of network structures and thereby induce a large viscosity increase of for example aqueous solutions. Therefore, HMPs have found important practical applications as thickeners. They are currently used in several industrial products such as paints, food, and cosmetics. The rheological behavior of HMPs in aqueous solutions and colloidal dispersions is now rather well understood.1,2 The polymers used in this investigation are hydrophobically modified polyelectrolytes (HMPEs), that is, polyacrylates onto which dodecyl chains have been chemically grafted. HMPEs are an interesting subgroup of HMPs, which also display an associative behavior. However, the relatively long-range electrostatic repulsions in low-ionicstrength solutions tend to counteract the association between hydrophobic groups. Varying the electrolyte * To whom correspondence should be addressed. † Universite ´ Pierre et Marie Curie. ‡ Lund University. Present address: Institute for Surface ChemistrysYKI, P.O. Box 5607, S-114/86 Stockholm, Sweden. (1) Water Soluble Polymers: Synthesis, solution properties and applications; Shalaby, S. W., McCormick, C. L., Butler, G. B., Eds.; ACS Symposium Series 467; American Chemical Society: Washington, DC, 1991. (2) Special issue: Associating Polymers 1995, a collection of papers presented at the Associating Polymers 1995 meeting held in Loen, Norway. Colloids Surf. A: Physicochem. Eng. Aspects 1996, 112 (2-3).
concentration and/or the pH offers an interesting way of tuning the self-associative behavior exhibited by HMPEs.3-8 Homopolymers, especially polyelectrolytes, are traditionally used as dispersing agents.9 Because HMPEs are flexible molecules comprising both hydrophobic and hydrophilic units, these macromolecules are surface-active and thus potentially interesting as dispersing/stabilizing agents for colloidal dispersions and emulsions.10-15 As opposed to normal polyelectrolytes, they generally have an affinity for nonpolar surfaces, which makes them useful in many practical formulations. The length and the chemical nature of the segments can further be tailored to fit specific applications. (3) Awad, N. M.; Morawetz, H. J. Polym. Sci.: Polym. Phys. Ed. 1981, 19, 245. (4) Buscall, M. R.; Corner, T. Colloids Surf. 1986, 17, 39. (5) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988, 20, 577. (6) Magny, B.; Iliopoulos, I.; Audebert, R. Polym. Commun. 1991, 32, 456. (7) Ringsdorf, H.; Venzmer, J.; Winnick, F. M. Macromolecules 1991, 24, 1678. (8) Chang, Y.; McCormick, C. L. Macromolecules 1991, 24, 6121. (9) Napper, D. H. Polymeric stabilization of colloidal dispersions; Academic Press: London, 1983. (10) Lundbergh, D. J.; Glass, J. E. J. Coatings Technol. 1992, 64 (807), 53. (11) van de Steeg, H. G. M.; de Keizer, A.; Cohen-Stuart, M. A.; Bijsterbosch, B. H. Colloids Surf. A: Physicochem. Eng. Aspects 1993, 70, 77. (12) Hulde`n, M. Colloids Surf. A: Physicochem. Eng. Aspects 1994, 88, 207. (13) Meadows, J.; Williams, P. A.; Tanaka, R. Macromol. Rep. 1994, A31 (Suppls 6 & 7), 777. (14) Lochhead, R. Y.; Rulison, C. J. Colloids Surf. A: Physicochem. Eng. Aspects 1994, 88, 27. (15) Liang, W.; Bognolo, G.; Tadros, Th. F. Langmuir 1995, 11, 2899.
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1698 Langmuir, Vol. 14, No. 7, 1998
The adsorption of polymers and polyelectrolytes has been studied extensively, both experimentally and theoretically.10-23 HMP adsorption has, however, so far mainly been studied using nonionic polymers.24,25 To better understand the interfacial behavior of HMPEs, we have studied the adsorption of a series of hydrophobically modified polyacrylates with different grafting densities of hydrophobic dodecyl chains and different molecular weights. We have also varied the ionic strength of the solution and the HMPE concentration. The substrate used throughout the work was hydrophobized silica. The investigation was performed by time-resolved ellipsometry, which allows monitoring of the adsorbed layer properties with a time resolution suited for the adsorption/desorption processes studied in this work. With ellipsometry, the determination of thicknesses and refractive indices of thin adsorbed layers (
