1248
Langmuir 2007, 23, 1248-1252
Co-adsorption of Carboxymethyl-Cellulose and Cationic Surfactants at the Air-Water Interface S. Trabelsi and D. Langevin* Laboratoire de Physique des Solides, UniVersite´ Paris Sud, Baˆ timent 510, 91405 Orsay, France ReceiVed August 3, 2006. In Final Form: October 25, 2006 We investigated the interaction between an anionic polyelectrolyte (carboxymethylcellulose) and cationic surfactants (DTAB, TTAB, and CTAB) at the air/water interface, using surface tension, ellipsometry, and Brewster angle microscopy techniques. At low surfactant concentration, a synergistic phenomenon is observed due to the co-adsorption of polyelectrolyte/surfactant complexes at the interface, which decreases the surface tension. When the surfactant critical aggregation concentration (cac) is reached, the adsorption saturates and the thickness of the adsorbed monolayer remains constant until another characteristic surfactant concentration, C0, is reached, at which all the polymer charges are bound to surfactant in bulk. Above C0, the absorbed monolayer becomes much thicker, suggesting adsorption of bulk aggregates, which have become more hydrophobic due to charge neutralization.
I. Introduction The study of the interaction between surfactants and polymers is an active field of research in colloidal science.1 The strong electrostatic interaction of oppositely charged surfactants and polymers leads to the formation of hydrophobic aggregates in bulk above a surfactant concentration called critical aggregation concentration (cac). This concentration is smaller than the critical micelle concentration (cmc) of the pure surfactant. The properties of bulk aggregates depend on the charge density and backbone rigidity of the polymer, on the chain length and concentration of the surfactant, on the ionic strength, and on the pH of the solutions, in some cases. Interactions between polymer and surfactant also lead to the formation of mixed layers at interfaces (liquid-solid, liquidliquid, or liquid-air). These complexes are of great interest in practical applications such as colloidal stabilization, wettability, adhesion, emulsions, and foam formation and stability.1,2 The aggregates formed in bulk and the mixed layers at interfaces are likely related.3 Experimentaltechniquessuchassurfacetension,4,5 ellipsometry,6-9 neutron reflectivity,10 and X-ray reflectivity11 have been used to understand the interfacial behavior of oppositely charged surfactants and polymers at the air-water interface. In previous studies, we investigated mixtures of cationic surfactants (CnTAB) with an anionic polysaccharide (carboxymethyl cellulose) in bulk.12 In this paper, we describe investigations of the mixed surface layers by using surface tension complemented by (1) Kwak, J. C. T. Polymer-Surfactant Systems; Surfactant Science Series; Marcel Dekker: New York, 1998; Vol. 77. (2) Bhattacharrya, A.; Monroy, F.; Langevin, D.; Argillier, J. F. Langmuir 2000, 16, 8727. (3) Delinaite, A.; Claesson, P. M. Langmuir 2000, 16, 1951. (4) Goddard, E. D. J. Colloid Interface Sci. 2002, 256, 228, and references therein. (5) von Klitzing, R.; Asnacios, A.; Langevin, D. Colloids Surf. A 2000, 167, 189. (6) Asnacios, A.; Langevin, D.; Argillier, J. F. Eur. Phys. J. B 1998, 5, 905. (7) McLoughlin, D.; Langevin, D. Colloids Surf. A 2004, 250, 79. (8) Monteux, C.; Williams, C. E.; Bergeron, V. Langmuir 2004, 20, 5367. (9) Naves, A. F.; Petri, D. F. S. Colloids Surf. A 2005, 254, 207. (10) Penfold, J. Curr. Opinion. Colloid Surf. 2002, 7, 139. Taylor, D. J. F.; Thomas, R. K.; Penfold, J. Langmuir 2002, 18, 4784. (11) Stubenrauch, C.; Albouy, P. A; von Klitzing, R.; Langevin, D. Langmuir 2000, 16, 3206. Ritacco, H.; Albouy, P. A.; Bhattacharyya, A.; Langevin, D. Phys. Chem. Chem. Phys. 2000, 2, 5243. (12) Guillot, S.; Delsanti, M.; Langevin, D. Langmuir 2003, 19, 230.
ellipsometry and Brewster angle microscopy experiments. We studied the influence of different parameters (polymer charge density, surfactant concentration, and chain length) on the surface properties. Finally, we relate the properties of bulk aggregates to the interfacial behavior. II. Experimental Section 1. Materials. We have used three cationic surfactants obtained from Aldrich (purity 99%) DTAB (dodecyltrimethylammonium bromide), TTAB (tetradecytrimethylammonium bromide), and CTAB (hexadecyltrimethylammonium bromide). These surfactants were recrystallized three times before use in acetone-ethanol (24: 1) solutions. Carboxymethylcellulose (carboxyMC) is an anionic water-soluble polymer derived from cellulose. These properties make it suitable for a wide range of applications in the food, pharmaceutical, and cosmetics industries. Cellulose is a polymer with β-D-glucose units. Each unit contains three hydroxyl groups that can be substituted with sodium-carboxyl groups: sodium carboxymethylcellulose is then obtained. The average number of hydroxyl groups substituted per glucose unit is known as the degree of substitution (DS). In this work, we have used three types of sodium carboxymethylcellulose (blanose 12m31P, 9M31F, and 7M31CF) supplied by Aqualon Hercules, with a minimum purity of 99.5%. These three polymers differ in their degrees of substitution (DS ) 1.23, 0.9, and 0.7, respectively). 2. Methods. 2.1. Surface Tension. The surface tension was measured by using an open frame version of the Wilhelmy plate to avoid the wetting problems of the classical plate. The measurements were carried out in a Teflon trough placed in a Plexiglas box with an opening for the tensiometer. After the mixed solutions were poured into the trough, surface tension took several hours to reach an equilibrium value. Measurements were taken every 30 min until a constant value of surface tension was attained. All measurements were performed at room temperature (22 ( 1 °C). 2.2. Ellipsometry. We used a Plasmos (SD 2300) rotating-analyzer ellipsometer operating at a wavelength of 633 nm and at an angle of 55°, close to the Brewster angle (53.1°). Solutions are poured into a Teflon trough sitting on a vibration-isolated table. Ellipsometric angles ψ and ∆ are converted into surface layer thickness d and surface layer refractive index ns. Here, the thickness of the adsorbed layer is small and difficult to extract from ellipsometry measurements since the accuracy on the refractive index ns is poor. The adsorbed amount Γ is independent of the choice for ns and can be calculated by using the formula derived by de Feijter et al.: Γ ) (ns - nb)d/(dn/dc), where nb is the
10.1021/la062296d CCC: $37.00 © 2007 American Chemical Society Published on Web 12/07/2006
Carboxymethyl-Cellulose and Cationic Surfactants
Figure 1. Surface tension as a function of DTAB concentration for mixed solutions containing 1 g/L carboxyMC with different degrees of substitution. Squares: DS ) 0.7; circles: DS ) 0.9; open circles: DS ) 1.23. refractive index of the bulk solution and dn/dc is the bulk refractive index increment.13 We have taken dn/dc ) n0 - nw, nw being the refractive index of water and n0 that of pure polymer or surfactant, which are close: n0 ) 1.44.12 Here, the solutions are very dilute, so nb ∼ nw. The value of ns depends on the amount of solvent present in the layer. In the case of DTAB and small polymer concentrations (
