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Adsorption of Cationic Cellulose Derivative/Anionic Surfactant Complexes onto Solid Surfaces. II. Hydrophobized Silica Surfaces Eiji Terada,† Yulia Samoshina,‡ Tommy Nylander,*,‡ and Bjo¨rn Lindman‡ Hair Care Labs, Kao Corporation, Bunka 2-1-3, Sumida-ku, Tokyo, 131-8501, Japan, and Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, Box 124, SE-221 00 Lund, Sweden Received January 9, 2004. In Final Form: April 23, 2004 The effect of the anionic surfactant SDS (sodium dodecyl sulfate) on the adsorption behavior of cationic hydroxyethyl cellulose (Polymer JR-400) and hydrophobically modified cationic cellulose (Quatrisoft LM200) at hydrophobized silica has been investigated by null ellipsometry and compared with the previous data for adsorption onto hydrophilic silica surfaces. The adsorbed amount of LM-200 is found to be considerably larger than the adsorbed amount of JR-400 at both surfaces. Both polymers had higher affinity toward hydrophobized silica than to silica. The effect of SDS on polymer adsorption was studied under two different conditions: adsorption of polymer/SDS complexes from premixed solutions and addition of SDS to preadsorbed polymer layers. Association of the surfactant to the polymer seems to control the interfacial behavior, which depends on the surfactant concentration. For the JR-400/SDS complex, the adsorbed amount on hydrophobized silica started to increase progressively from much lower SDS concentrations, while the adsorbed amount on silica increased sharply only slightly below the phase separation region. For the LM-200/SDS complex, the adsorbed amounts increased progressively from very low SDS concentrations at both surfaces, and no large difference in the adsorption behavior was observed between two surfaces below the phase separation region. The complex desorbed from the surface at high SDS concentrations above the critical micelle concentration. The reversibility of the adsorption of polymer/ SDS complexes upon rinsing was also investigated. When the premixed polymer/SDS solutions at high SDS concentrations (>5 mM) were diluted by adding water, the adsorbed amount increased due to the precipitation of the complex. The effect of the rinsing process on the adsorbed layer was determined by the hydrophobicity of the polymer and the surface.
1. Introduction Mixed cationic polymer/anionic surfactant systems are extensively used in formulations of personal care products, one major application being hair shampoo. The role of cationic polymers in hair shampoo is not only to control rheological properties of hair shampoo solutions but also to improve hair condition, like combability, texture, softness, hair shine, resistance to damage, and reduction of split ends.1-3 There is no doubt that such improvements in hair condition are brought by polymer adsorption onto hair surfaces. In general, hair surfaces are covered with cuticle cells.4 Each cuticle cell is composed of exocuticle and endocuticle, surrounded by a thin hydrophobic membrane, epicuticle. Thus, natural (untreated) hairs have hydrophobic surfaces; however, hair surfaces become hydrophilic when hair fibers are treated with some chemical compounds.5 For instance, hydrogen peroxide, included in bleaching products and permanent coloring products as a main * To whom correspondence should be addressed. Tel: +46 46 2228158. Fax: +46 46 2224413. E-mail: Tommy.Nylander@ fkem1.lu.se. † Kao Corp. ‡ Lund University. (1) Reich, C. Hair Cleansers, 2nd ed.; Reich, C., Ed.; Marcel Dekker: New York, 1997; Vol. 68. (2) Lochhead, R. Y. Soap Cosmetics Chem. Specialties 1992, 68, 42. (3) Hunting, A. L. L. Cosmetics Toiletries 1984, 99, 57-60. (4) Feughelman, M. Mechanical Properties and Structure of AlphaKeratin Fibers. Wool, Human Hair and Related Fibers; UNSW Press: Sidney, 1997. (5) Molina, R.; Comelles, F.; Julia, M. R.; Erra, P. J. Colloid Interface Sci. 2001, 237, 40-46.
ingredient, turns hair surfaces markedly hydrophilic due to oxidation of cystine amino acid residues in the hair fibers and/or partial removal of fatty acids belonging to epicuticle. Therefore, a comparative study of adsorption of polymer/surfactant complexes on hydrophobic surfaces and on hydrophilic surfaces is of considerable practical interest, and a fundamental understanding of the phenomena involved is required for the efficient and successful application of these systems. The adsorption of complexes formed from oppositely charged polymers and surfactants on the liquid-solid interface has been previously reported.6-12 Shubin et al. investigated the effect of sodium dodecyl sulfate (SDS) on the structure of hydrophobically modified cationic cellulose layers adsorbed on hydrophilic surfaces (mica and silica).6,7 Dedinaite et al. reported on the interfacial properties of poly[2-(propionyloxy)-ethyl]trimethylammonium chloride and SDS on mica surfaces.9 They found that the adsorption behavior of the polymer was strongly affected by the oppositely charged surfactant. In general, oppositely charged polymers and surfactants interact strongly with (6) Shubin, V.; Petrov, P.; Lindman, B. Colloid Polym. Sci. 1994, 272, 1590-1601. (7) Shubin, V. Langmuir 1994, 10, 1093-1100. (8) Dedinaite, A.; Claesson, P. M.; Bergstrom, M. Langmuir 2000, 16, 5257-5266. (9) Dedinaite, A.; Claesson, P. M. Langmuir 2000, 16, 1951-1959. (10) Fielden, M. L.; Claesson, P. M.; Schillen, K. Langmuir 1998, 14, 5366-5375. (11) Claesson, P. M.; Fielden, M. L.; Dedinaite, A.; Brown, W.; Fundin, J. J. Phys. Chem. B 1998, 102, 1270-1278. (12) Ananthapadmanabhan, K. P.; Mao, G. Z.; Goddard, E. D.; Tirrell, M. Colloids Surf. 1991, 61, 167-174.
10.1021/la049922w CCC: $27.50 © 2004 American Chemical Society Published on Web 06/30/2004
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Langmuir, Vol. 20, No. 16, 2004 6693 Table 1. Physical Properties for Cationic Cellulose Derivatives as Specified by the Manufacturer polymer trade name
ratio of the cationic monomers (τ) (mol %)
Mw
refractive index increment (dn/dc)a (cm3/g)
JR-400 LM-200
29 3
400 000 100 000
0.144 0.152
a
Figure 1. Molecular structures of the cationic cellulose derivatives. See Table 1 for values of the ratio of the cationic monomers (τ) in the polymers.
each other via electrostatic interactions. This interaction may lead to an associative phase separation of a polymer/ surfactant complex.13 Further surfactant addition leads to a redissolution of the complex. That is, the solubility of the polymer changes dramatically by adding an oppositely charged surfactant. Solvency effects always play a crucial role in material adsorption;14 therefore a surfactant strongly affects the adsorption of an oppositely charged polymer. In the present paper, we mainly report on the adsorption of cationic cellulose derivative/SDS complexes onto hydrophobized silica. In addition, we compare the adsorption of the complexes on hydrophobized silica with our previous results for silica.15 The cationic cellulose derivatives are cationic hydroxyethyl celluloses (Polymer JR-400) and hydrophobically modified cationic hydroxyethyl cellulose (Quatrisoft LM-200). Cationic cellulose derivatives are typical polymers used in hair shampoo formulations due to their functional properties and also because they are nontoxic and highly biodegradable. For personal care products in general and especially for hair shampoos, not only the adsorption of the active compounds but also the changes in the adsorbed layer during rinsing with water are highly relevant. In this study, we therefore present experimental data on the adsorption of the polymer/ surfactant complexes as well as on the reversibility of the complex adsorption upon rinsing. The adsorption of complexes was measured by in situ null ellipsometry. This technique can provide us with an adsorbed amount as well as an average thickness and a refractive index of an adsorbed layer as a function of time. 2. Experimental Section 2.1. Materials and Chemicals. The polymers (trade names: Polymer JR-400 and Quatrisoft LM-200) are N-trimethylammonium and N,N-dimethyl-N-dodecylammonium derivatives of hydroxyethyl cellulose, respectively. The molecular structures of the monomers are given in Figure 1, and the physical properties of the polymers are shown in Table 1.7,16 Hydrophobized silica surfaces were prepared by exposing oxidized, cleaned, and plasma-treated silicon slides having an approximately 30 nm thick SiO2 layer to a low-pressure (13) Goddard, E. D. Interactions of Surfactants with Polymers and Proteins; Ananthapadmanabhan, K. P., Lindman, B., Thalberg, K., Ed.; CRC Press: Boca Raton, FL, 1993. (14) Holmberg, K.; Jo¨nsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution, 2nd ed.; John Wiley & Sons Ltd.: Chichester, U.K., 2002. (15) Terada, E.; Samoshina, Y.; Nylander, T.; Lindman, B. Langmuir 2004, 20, 1753-1762 (16) Harrison, I. M.; Meadows, J.; Robb, I. D.; Williams, P. A. J. Chem. Soc., Faraday Trans. 1995, 91, 3919-3923.
Determined as specified in ref 15.
atmosphere of octyldimethylchlorosilane for 24 h. After reaction, the slides were sonicated in (i) ethanol and (ii) tetrahydrofuran five times each. Finally, they were cleaned with ethanol again and stored in absolute ethanol prior to use. To avoid an air film sticking to the hydrophobic surface, ethanol was pumped through the cuvette before the salt solution was added. Note that reproducible measurements could not be obtained without this intermediate step. The ethanol was then rinsed off by a continuous flow of salt solution, and prior to the start of adsorption measurements, the surface was allowed to stabilize in salt solution for at least 1 h. For more detailed information about other reagents and silica cleaning protocols, the reader is referred to our previous study.15 2.2. Ellipsometry. The ellipsometry measurements were conducted on an automated Rudolph Research thin-film null ellipsometer, type 43603-200E. All the measurements were performed at λ ) 401.5 nm, under agitation with a magnetic stirrer in a temperature-controlled cuvette (25 ( 0.1 °C), equipped with Teflon tubes that allowed continuous rinsing with 10 mM NaCl solution. The time resolution of the instrument allowed measurements every 2-3 s, which enabled us to follow the kinetics of the adsorption. After characterization of the bare hydrophobized silica surface in air and in the salt solution, a known amount of sample (cationic polymer solution, cationic polymer-anionic surfactant mixed solution, etc.) was injected into the cuvette, which originally contained 5 mL of 10 mM NaCl solution, and the ellipsometric angles Ψ and ∆ were recorded continuously until plateau values were reached. When the adsorption of the polymer or the polymer/surfactant complex had reached plateau values, rinsing with 10 mM NaCl aqueous solution or water was carried out in order to investigate the reversibility of the adsorption. The rinsing solution flowed through the cuvette at a rate of 2 mL/min. A detailed description of the experimental procedure and the model used for evaluation of the data can be found in our previous publication.15 All experiments were repeated at least two times, and the deviation from the mean values of the adsorbed amount and adsorbed layer thickness was ×b15% and ×b110% or less, respectively.
3. Results 3.1. Adsorption of Cationic Polymers on Hydrophobized Silica. Figure 2a,b shows the adsorbed amount and the layer thickness as a function of the polymer concentration for JR-400 and LM-200 on hydrophobized silica. To give more insight on the role of the surface nature in adsorption processes, the data for the adsorption of JR-400 and LM-200 on (hydrophilic) silica, from our previous study,15 are also inserted. Both polymers showed a higher adsorbed amount and a thicker adsorbed layer on hydrophobized silica than on silica. In all cases, plateau values of the adsorbed amount were established already above a polymer concentration of 10 ppm. Additionally, on both surfaces, LM-200 had a higher affinity toward the surfaces than JR-400. Adsorbed amounts of JR-400 and LM-200 on hydrophobized silica versus time are shown in Figure 3. JR-400 showed a sharp increase in the adsorbed amount within the first few minutes, and then the adsorbed amount approached a plateau value. On the other hand, LM-200 adsorption required almost 4 h to reach a plateau. Qualitatively, the kinetics of the adsorption process showed two different stages: a rapid increase in adsorption
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Figure 2. (a) The adsorbed amount as a function of polymer concentration for the cationic polymers, on hydrophobized silica and on silica. (b) The layer thickness as a function of polymer concentration for the polymers on hydrophobized silica and on silica. Squares indicate LM-200, and circles indicate JR-400. Filled symbols indicate data for the hydrophobized silica, and open symbols data for silica. The NaCl concentration was 10 mM. Lines are drawn just to guide the eye.
Figure 3. The kinetics of JR-400 and LM-200 adsorption on hydrophobized silica from aqueous solutions with polymer concentration of 100 ppm. The NaCl concentration was 10 mM.
within the first minutes, followed by a slow adsorption to reach the plateau. On silica, a similar adsorption trend was observed: the adsorption kinetics of JR-400 was significantly faster than that of LM-200. Additionally, it is important to note that no desorption of JR-400 and LM-200 was observed when the polymer
Terada et al.
solution was replaced with pure 10 mM NaCl solution (data not shown). These results reveal the irreversibility of the adsorption of the cationic polymers upon dilution. 3.2. Adsorption of JR-400/SDS Complexes on Hydrophobized Silica. To investigate interfacial polymer/ surfactant interactions, the anionic surfactant SDS was added to preadsorbed JR-400 layers on hydrophobized silica and on silica. In these experiments, JR-400 was first added to reach the bulk concentration of 100 ppm. After steady state (the plateau of the adsorbed amount) was reached, different volumes of SDS stock solution with the desired concentration were injected sequentially to reach the desired final concentration in the bulk after each surfactant addition. The system was equilibrated until a plateau in the adsorbed amount was reached before the next SDS addition was made. Figure 4a,b shows the changes in the adsorbed amount and the layer thickness of preadsorbed JR-400 layers when SDS was added to the bulk polymer solution. On hydrophobized silica, the layer thicknesses showed almost the same trend as in the case of silica: the layer thickness was not changed significantly in the preprecipitation region but expanded sharply in the postprecipitation region. On the other hand, the adsorbed amount on hydrophobized silica shown versus SDS concentration was different from the adsorption on silica and was found to increase progressively from very low SDS concentrations (>0.003 mM). Taking into account the fact that SDS can adsorb on hydrophobized silica,17 this progressive increase in the adsorbed amount is expected to be due to SDS adsorption alone and/or supplementary adsorption of JR-400/SDS complexes. Figure 4c shows the effect on the adsorption of SDS to preadsorbed JR-400 layers on hydrophobized silica when no polymer is present in the bulk solution; that is, the polymer solution was replaced with polymer-free aqueous solution before SDS addition. In this case, no progressive increase in the adsorbed amount from very low SDS concentrations was observed. Therefore, we conclude that the increase in the adsorbed amount in the pre-precipitation region shown in Figure 4a is due to the supplementary adsorption of the polymer/surfactant complex from the bulk solution, rather than the adsorption of SDS alone. It should be stressed that no significant difference in the results was observed when SDS was added in one step rather than stepwise addition to reach the same final bulk concentration. That is, the same adsorption plateau level was obtained independently of the method of reaching the desired SDS concentration. The adsorbed amount and the adsorbed layer thickness on hydrophobized silica, when the JR-400 and SDS stock solutions were premixed before being added to the cuvette, are shown in Figure 5a,b. At low SDS concentrations, below the phase separation region, the adsorbed amount was almost the same as when SDS was added to preadsorbed JR-400 layers. However, at high surfactant concentrations (>5 mM), the adsorbed amounts and the layer thicknesses were markedly smaller than when SDS was added to preadsorbed polymer layers. The adsorbed amount and the layer thickness were almost the same as the values obtained for the adsorption of SDS at the corresponding concentrations. That is, from a premixed surfactant/polymer, SDS completely inhibits the adsorption of JR-400 molecules on the surface at high SDS concentrations above the critical micelle concentration (cmc). 3.3. Adsorption of LM-200/SDS Complexes on Hydrophobized Silica. Figure 6a,b shows the effect on (17) Montgomery, M. E.; Wirth, M. J. Langmuir 1994, 10, 861-869.
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Figure 5. (a) The amount of the complex adsorbed from premixed JR-400/SDS solutions on hydrophobized silica (filled squares) and the effect of SDS addition to preadsorbed JR-400 layers on hydrophobized silica (open circles). (b) The adsorbed layer thickness of the complex adsorbed from premixed JR400/SDS solutions on hydrophobized silica (filled squares) and the effect of SDS addition on the adsorbed amount to preadsorbed JR-400 layers on hydrophobized silica (open circles). The polymer concentration was fixed at 100 ppm. The NaCl concentration was 10 mM.
Figure 4. (a) Effect on the adsorbed amount and (b) effect on the layer thickness of SDS addition to preadsorbed JR-400 layers on hydrophobized silica and on silica. The polymer concentration was fixed at 100 ppm, and the NaCl concentration was 10 mM. The polymer was not removed from the bulk solution before SDS addition. Filled squares show the data for hydrophobized silica, and open circles show the data for silica. (c) Effect on the adsorbed amount of SDS addition to preadsorbed JR-400 layers on hydrophobized silica with the polymer being removed from the bulk solution before SDS addition.
the adsorbed amount and the layer thickness of SDS addition to preadsorbed LM-200 layers on hydrophobized silica and on silica. Contrary to the JR-400/SDS system, no large difference in the adsorbed amount below the phase separation region was observed between the two types of surfaces. In both cases, the adsorbed amount increased progressively from very low SDS concentrations with the maximum adsorbed amount obtained just before the phase
separation region. The layer thickness increased sharply with SDS concentrations above 0.1 mM, both on hydrophobized silica and on silica, prior to the phase separation. This layer expansion in the pre-precipitation region was not observed in the JR-400/SDS system (see Figure 4b). In the post-precipitation region, the adsorbed amounts decreased with increasing SDS concentrations, that is, LM-200/SDS complexes desorbed from both surfaces at high SDS concentrations above the cmc. In addition, the thickness of the adsorbed layers remains higher than at low (
