Adsorption of a Cationic Surfactant onto Cellulosic Fibers I. Surface

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Articles Adsorption of a Cationic Surfactant onto Cellulosic Fibers I. Surface Charge Effects Sabrine Alila,† Sami Boufi,*,† Mohamed Naceur Belgacem,‡ and Davide Beneventi‡ LMSE, Faculte´ des Sciences de Sfax, BP 802-3018, Sfax, Tunisia, and LGP2-UMR5518, Ecole Franc¸ aise de Papeterie et des Industries Graphiques, INPG, BP 65, F-38402 St. Martin d’He` res, France Received February 9, 2005. In Final Form: April 5, 2005 The adsorption of four cationic surfactants with different alkyl chain lengths on cellulose substrates was investigated. Cellulose fibers were used as model substrates, and primary alcohol groups of cellulose glycosyl units were oxidized into carboxylic groups to obtain substrates with different surface charges. The amount of surfactant adsorbed on the fiber surface, the fiber ζ-potential, and the amount of surfactant counterions (Cl-) released into solution were measured as a function of the surfactant bulk concentration, its molecular structure, the substrate surface charge, and the ionic strength. The contribution of each of these parameters to the shape of the adsorption isotherms was used to verify if surfactant adsorption and self-assembly models usually used to describe the behavior of surfactant/oxide systems can be applied, and with which limitations, to describe cationic surfactant adsorption onto oppositely charged cellulose substrates.

Introduction The adsorption of surfactants at the solid-liquid interface has been a subject of considerable interest and plays an important role in controlling a variety of interfacial processes such as flotation,1,2 paper manufacturing and recycling,3 flocculation and dispersion,1,2 soil remediation,4 and adsorption of contaminants in water treatment.4,5 Although the behavior of surfactants and their selfassociation in bulk solution are well understood phenomena,4 the behavior of amphiphilic surfactant molecules at the solid-liquid interface is a complex phenomenon which depends on the properties of the solid surface, the ionic strength of the solution, the surfactant structure, the pH, and the temperature.6-13 Even though the concept of * To whom correspondence should be addressed. Fax: 00 33 4 76 82 69 33. E-mail: [email protected]. † Faculte ´ des Sciences de Sfax. ‡ INPG. (1) Rosen, M. Surfactants in Emerging Technologies; Marcel Dekker: New York, 1987. (2) Myers, D. Surfactants Science and Technology; VCH: New York, 1992. (3) Beneventi, D.; Carre´, B.; Gandini, A. Colloids Surf., A 2001, 189, 65. (4) Christian, S. D.; Scamehorn, J. F. Solubilization in Surfactant Aggregates; Surfactant Sciences Series 38; Marcel Dekker: New York, 1995. (5) Wang, Y.; Banziger, J.; Dubin, P. L.; Fillippelli, G.; Nuraje, N. Environ. Sci. Technol. 2001, 35, 2608. (6) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103, 219-304. (7) Goloub, T. P.; Koopal, L. K. Langmuir 1997, 13, 673. (8) Fuerstenau, D. W. J. Colloid Interface Sci. 2002, 256, 79. (9) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 463. (10) Chandar, P.; Somasundaran, P.; Turro, N. J. J. Colloid Interface Sci. 1987, 117, 31. (11) Fan, A.; Somasundaran, P.; Turro, N. J. Langmuir 1997, 13, 506. (12) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 2649.

surfactant self-assembly at the solid-liquid interface has been well admitted since it was first proposed by Gaudin and Fuerestenau,14,15 the exact shape of these structures is still a subject of debate. During the past decade, various kinds of experimental methods such as AFM,16,17 NMR,18,19 ellipsometry,20 fluorescence,11,21 and FT-IR22,23 have been used to give evidence of the adsorbed domains of ionic surfactants at a hydrophilic oxide surface. Using pyrene as a fluorescence probe, the micelle-like aggregation of the adsorbed surfactant molecules has been confirmed.11 Likewise, the aggregation number of the adsorbed surfactant aggregates at the surface has been determined by using time-resolved florescence quenching.21 Electron spin resonance (ESR) spectroscopy suggests that the adsorbed surfactant monomer is oriented vertically at the surface,24 and a structural transition of the adsorbed cationic surfactant from random adsorption to a more organized structure has been shown by combining ATR polarization spectroscopy with contact angle and ζ-potential measurements at silica surfaces.25 On behalf of these recent investigations, several morphol(13) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 2660. (14) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90. (15) Somasundaran, P.; Fuerstenau, D. W. Trans. AIME 1972, 252, 275. (16) Heather, N. P.; Gregory, G. W.; Srinivas, M.; Ilhan, A. A. Langmuir 1999, 15, 1685. (17) Sharma, B. G.; Basu, S.; Sharma, M. M. Langmuir 1996, 12, 6506. (18) So¨derlind, E.; Stilbs, P. Langmuir 1993, 9, 1678. (19) Quist, P. Q.; Soderlin, E. J. Colloid Interface Sci. 1995, 172, 510. (20) Tiberg, F.; Jo¨nsson, B.; Lindman, B. Langmuir 1994, 10, 79. (21) Stro¨m, C.; Hansson, P.; Jo¨nsson, B.; So¨derman, O. Langmuir 2000, 16, 2469. (22) Li, H.; Tripp, C. P. Langmuir 2002, 18, 9441. (23) Singh, P. K.; Adler, J. J.; Rabinovich, Y. I.; Moudgil, B. M. Langmuir 2001, 17, 468. (24) Esumi, K.; Matoba, M.; Yamanaka, Y. Langmuir 1996, 12, 2130. (25) Huang, L.; Somasundaran, P. Colloids Surf., A 1996, 117, 235.

10.1021/la050367n CCC: $30.25 © 2005 American Chemical Society Published on Web 07/26/2005

Surfactant Adsorption onto Cellulosic Fibers

ogy patterns of the adsorbed aggregates have been proposed: monolayer14,15,26 (hemimicelle), bilayer27 (admicelle), spherical,28 and cylindrical17 surface aggregates. These organized structures can be continuous along the surface, or they may form patches of aggregated domains. Adsorption isotherms usually provide information concerning the onset of surfactant self-assembly occurring when the solution concentration and the surface excess are altered. The common feature of experimental isotherms with surface aggregation is their shape, which can be divided into three distinct regions,8,15,26,27-30 corresponding to different states of the adsorbed phase. In region I, located in the low concentration domain, ionic surfactants are adsorbed individually on a localized site with opposite sign through an ion exchange mechanism. Adsorption is sparse with no interaction among adsorbed surfactant molecules. Up to a critical surfactant concentration, which corresponds to the boundary between regions I and II, adsorbed monomers begin to associate through lateral interactions, inducing the formation of monolayered (hemimicelle) and bilayered (admicelle) aggregates. This concentration, called the hemimicelle concentration (hmc), occurs well below the surfactant cmc. The generation of surfactant aggregates boosts monomer adsorption and explains the abrupt rise in the S-shaped isotherm. A further increase in the surfactant concentration leads to the progressive stabilization of surfactant adsorption up to the saturation of the interface near the cmc. The driving force behind surfactant aggregation is the transfer of surfactant alkyl chains from an aqueous to a hydrocarbon medium (hemimicelle and admicelle core) during selfassociation. This attractive interaction is counteracted by electrostatic repulsion forces between charged headgroups within surfactant aggregates, and when dispersive and electrostatic-repulsive forces are balanced, a saturation plateau is attained, viz., region III. Although their interpretation is simple, S-shaped isotherms do not allow clear identification of monolayer to bilayer phase transitions; indeed, the detailed description of surfactant self-assembly onto oppositely charged surfaces provided by theoretical models31-33 predicts a five-region adsorption isotherm, Figure 1, where surface saturation occurs via a hemimicelle and admicelle formation sequence. The general five-region adsorption isotherm has a three-region shape when monomer-to-monolayer and monolayer-to-bilayer transitions occur in a narrow concentration range, which seems to be the case for most surfactant/oxide systems. In this work, we pursue our investigation on the interactions of cationic surfactants with a cellulose substrate. In a previous work we showed that, even if cellulose is a poorly charged substrate, cationic surfactants are adsorbed in a relatively high amount.34-36 Their (26) Koopal, L. K.; Lee, M. E.; Bo¨hmer, M. R. J. Colloid Interface Sci. 1995, 170, 85. (27) Harwell, J. H.; Hoskin, J.; Schechter, R. S.; Wade, W. H. Langmuir 1985, 1, 251. (28) Adler, J. J.; Singh, P. K.; Patist, A.; Rabinovich, Y. I.; Shah, D. O.; Moudgil, B. M. Langmuir 2000, 16, 7255. (29) Gouloub, T. P.; Koopal, L. K.; Bijsterbosch, B. H.; Sidorova, M. P. Langmuir 1996, 12, 3188. (30) Fuerestenau, D. W.; Herrara-Urbina, R. In Surfactant-Based Separation Process; Scamehorn, J. F., Harwell, J. H., Eds.; Surfactant Sciences Series 33; Marcel Dekker: New York, 1989; pp 259-320. (31) Łajtar, L.; Narkiewicz-Michałek, J.; Rudzinski, W. Langmuir 1994, 10, 3764. (32) Li, B.; Ruckenstein, E. Langmuir 1996, 12, 5052. (33) Drach, M.; Łajtar, L.; Narkiewicz-Michałek, J.; Rudzinski, W.; Zaj_c, J. Colloids Surf., A 1998, 145, 243. (34) Aloulou, F.; Boufi, S.; Chakchouk, M. Colloid Polym Sci. 2004, 282, 699.

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Figure 1. Five-region general adsorption isotherm foreseen by theoretical models for the surfactant adsorption on an oppositely charged surface. Dotted and thin lines represent the partitioning of surfactant molecules in the three possible adsorbed states, viz., monomer, hemimicelle, and admicelle.

corresponding isotherms exhibited three to five characteristic regions, depending on the surfactant structure, which were interpreted in terms of surfactant aggregation at the solid-liquid interface. The purpose of this work was to analyze the effect of the cellulose charge on the adsorption behavior of cationic surfactants. The surface charge of cellulose fibers was modified by a soft oxidation treatment37,38 which allowed the selective conversion of the primary alcohol groups of the glycosyl unit into carboxylic groups which remain fully ionized at pH higher than 6. Materials and Methods Materials. Cellulose fibers used in this study were a bleached soda pulp from the Tunisian annual plant esparto (alfa, Stipa tenacissima). They were highly porous and had a specific surface area in a dry state of 3 m2 g-1. Their morphology was determined by averaging the length and width of 7380 fibers characterized with optical microscopy. The mean fiber length and width were found to be 0.75 mm and 14.2 µm, respectively, thus corresponding to an aspect ratio of 52. The carboxylic content of the original fibers was 45 µmol g-1. Chemicals used to oxidize cellulose fibers, namely, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy radical), sodium bromide (NaBr), and 12% sodium hypochlorite solution, were of laboratory grade and used without further purification. Four commercial, analytical grade, cationic surfactants, namely, octadecyltrimethylammonium bromide (C18), hexadecylpyridinium chloride (C16), tetradecyltrimethylammonium bromide (C14), and dodecylpyridinium chloride (C12), were dissolved in distilled water to obtain high-concentration mother solutions. Their critical micelle concentration (cmc) in distilled water (25 °C) determined by conductometry was 2.1 × 10-4, 7.5 × 10-4, 3.4 × 10-3, and 1.2 × 10-2 mol L-1 for C18, C16, C14, and C12, respectively. Oxidation Procedure of Cellulosic Fibers. Oxidation of cellulose fibers was carried out in the following conditions. Fibers (10 g) were dispersed in distilled water (500 mL). TEMPO (25 mg) and NaBr (250 mg) were added to the suspension, and the pH was adjusted to 10 by addition of a 0.5 mol L-1 NaOH solution. A 12% sodium hypochlorite solution was added to the suspension over 30 min, and the resulting suspension was then stirred for 2 h. The pH of the suspension was continuously adjusted to 10 (35) Aloulou, F.; Boufi, S.; Belgacem, N.; Gandini, A. Colloid Polym Sci. 2004, 283, 344. (36) Aloulou, F.; Boufi, S.; Beneventi, D. J. Colloid Interface Sci. 2004, 280, 350. (37) De Nooy, A. E.; Besemer, A. C.; Van Bekkum, H. Carbohydr. Res. 1995, 269, 89. (38) Chang, P. S.; Robyt, J. F. J. Carbohydr. Chem. 1996, 15, 819.

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Alila et al. Table 1. Carboxylic Content and Crystallization Degree of Oxidized Cellulose Samples

abbreviation

Va

carboxyl contentb (µmol g-1)

carboxyl contentc (µmol g-1)

crystallization degreed (%)

cellulose-0 cellulose-150 cellulose-300 cellulose-600

0 15 32 65

39 145 294 567

45 155 304 585

65 70 79 82

a Volume of 12% hypochlorite solution added to 10 g of fiber suspension. b Carboxyl content determined by methylene blue sorption. c Carboxyl content determined by the conductometric titration method d Crystallization degree determined by X-ray powder diffraction.

Figure 2. TEMPO-mediated oxidation of cellulose primary hydroxyl groups to carboxyl groups. by addition of the NaOH solution. The oxidation was stopped by adding 100 mL of ethanol, and oxidized fibers were filtered and washed two times with a 1 mol L-1 NaOH solution to remove traces of residual lignin. The recovered fibers were washed with distilled water until the pH of the filtrate was close to 7. Cellulose oxidation by TEMPO is depicted in Figure 2. The content of carboxylic moieties in the oxidized fibers was evaluated by using two techniques: (i) conductometric titration of a 1% fiber suspension with 10-3 mol L-1 HCl39 (details in the Supporting Information) and (ii) methylene blue (MB) sorption, which is based on the replacement of carboxylic counterions by the cationic dye.40 The X-ray diffraction patterns were measured with an X-ray diffractometer using Cu KR radiation at 40 kV and 30 mA. The crystallization degree is obtained by comparing the area under the crystalline peaks and the amorphous curve according to the usual method.41 Adsorption Isotherms. The adsorption isotherms of cationic surfactants were carried out at 25 °C using the depletion method. An appropriate volume of a high-concentration mother solution was added to a 1% cellulose fiber suspension to obtain surfactant concentrations ranging from 10-5 to 8 × 10-3 mol L-1. The pH of the suspension was not affected by the addition of cationic surfactant, and the pH was close to 7.5-8 for all cellulose samples. The suspension was then stirred for 24 h at room temperature to reach adsorption equilibrium. Dispersions were then centrifuged at 2500 rpm for 15 min, and the amount of free surfactant in the supernatant was determined by potentiometric titration using a surfactant-selective electrode for the dosage of C14 and C18 and by UV absorption at 260 nm in the presence of C16 and C12 surfactants bearing a pyridinium ion. Chloride Ion Dosage. The concentration of free chloride ion in solution was determined by precipitation with Ag+ according to a well-reported analytic technique.42 An excess amount of Ag+ solution was added to the chloride ion solution, and the excess of Ag+ was back-titrated with thiocyanate solution after removal of the precipitated AgCl. Surfactant Dosage Using a Surfactant-Selective Electrode. The surfactant concentration was determined potentiometrically by using a cation-selective electrode. The electrode was prepared as described by Ishibashi;43 its sensitive element was a 0.4 mm thick and 1 cm diameter membrane of poly(vinyl (39) Katz, S.; Beatson, R.P.; Scallan, A.M. Sven. Papperstidn. 1984, 87 (6R), 48. (40) Rohrsetzers, S.; Kovacs, P.; Kabai-Faix, M.; Papp, J.; Volgy, P. Cell. Chem. Technol. 1995, 29, 65. (41) Rabek, J. F. Experimental Methods in Polymer Chemistry: Application of Wide-angle X-ray Diffraction (WAXS) to the Study of the Structure of Polymer; Wiley-Interscience: Chichester, U.K., 1980; p 505. (42) Harris. D. C. Exploring Chemical analysis; W. H. Freeman: New York, 1997; p 112. (43) Ishibashi, N.; Masadome, T.; Imato, T. Anal. Sci. 1986, 2, 487.

chloride) (PVC) plasticized by bis(2-ethylhexyl phthalate) (20/ 80, w/w) containing 1% surfactant carrier complex. The latter was prepared by stoichiometric mixing of the anionic surfactant and hexadecyltrimethylammonium chloride in water, followed by its recrystallization in a diethyl ether-dichlormethane mixture (50/50, v/v). The membrane potential was measured with the following concentration cell assembly:

The reference solution was a 10-3 mol L-1 solution of the surfactant studied and 5 × 10-3 mol L-1 KCl solution as an ionic strength buffer. Before use, the electrodes were conditioned overnight in a 10-3 mol L-1 surfactant solution. The electromotive force (EMF) of the electrochemical cell was stable to within 1 mV. The electrodes gave a Nerstian response for surfactant concentrations from the cmc down to 5 × 10-6 mol L-1:

E ) E0 + p log [surfactant] where p is the experimental slope of the EMF (E) versus log [surfactant] plot at 25 °C and E0 is the EMF corresponding to a surfactant concentration of 1 mol L-1. The slope observed was in a range of 50-65 mV. The validity of the dosage method was confirmed by a spectroscopic-UV dosage in the presence of hexadecylpyridinium chloride. The difference in the concentration determined via the two analytical methods did not exceed 5%. ζ-Potential. A commercial ζ-potential analyzer (Malvern 2000) was used to measure the electrophoretic mobility of cellulose fibers in the aqueous suspension. Measurements were conducted on the suspension fine fraction, which was obtained after filtration of the original suspension through a 45 µm screen. To avoid the fluctuation of ζ-potential due to ionic strength changes generated by the increase in the surfactant concentration, the aliquot sample was diluted with a concentrated solution of KCl to buffer the ionic strength to 10-2 mol L-1. Four consecutive measurements were taken for each sample at room temperature and averaged. The ζ-potential was calculated from electrophoretic mobility data by using the Smoluchowski equation. We verified in a preliminary study that ζ-potential measurements repeated on the whole suspension by using the streaming potential technique (Mu¨tek SZP06 with a 40 µm screen electrode) matched with ζ-potential values obtained by electrophoresis, when measurements were conducted on the suspension fine fraction.

Results and Discussion Three samples of oxidized cellulose fibers were prepared by soft oxidation with a TEMPO/NaBr/NaClO system. Their carboxyl content, expressed as moles per gram of anhydroglycosyl units, is reported in Table 1. 13 C solid NMR and FT-IR spectroscopic analysis did not reveal a significant evolution in the chemical structure of the oxidized fibers, excluding the generation of carboxylic groups. FT-IR spectra of oxidized samples, presented in the Supporting Information, revealed a significant intensification of the CdO (∼1732 cm-1) and C-O

Surfactant Adsorption onto Cellulosic Fibers

Figure 3. Adsorption isotherm of C12 onto oxidized cellulose fibers according to a linear-log scale at pH 7.5-8 and in the absence of any added salts.

Figure 4. Adsorption isotherm of C16 onto oxidized cellulose fibers according to a linear-log scale at pH 7.5-8 and in the absence of any added salts.

(∼1050 cm-1) bands with an increase in the carboxylic content. X-ray diffraction patterns of the original and oxidized fibers shown in the Supporting Information revealed the presence of three peaks, 2θ ) 15°, 17°, and 22.7°, which confirmed that only cellulose I was present and no conversion of the supermolecular structure or polymorphic forms occurred during the oxidation treatment.44 These results are in agreement with the high selectivity of the TEMPO oxidation procedure, which allows the specific oxidation of primary alcohol groups of the glycosyl unit into carboxylic groups.45 However, a slight increase in the crystallization degree is noted after the oxidation (Table 1); the higher the carboxylic content, the higher the crystallization degree. The adsorption isotherms of C12 and C16 surfactants on cellulose fibers carrying three different amounts of carboxylic groups, viz., 150, 300, and 600 µmol g-1, are shown in Figures 3 and 4. When cellulose 300 was used as the substrate, C12 and C16 adsorption isotherms clearly displayed a five-region shape. In region I, the amount of (44) Ishikawa, A.; Okano, T.; Surgiyama, J. Polymer 1997, 38, 463. (45) da Silva Perez, D.; Montanari, S.; Vignon, M. R. Biomacromolecules 2003, 4, 1417.

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adsorbed surfactant was low and did not exceed 30-40 µmol g-1. When the free surfactant concentration reached a critical level, a significant increase in adsorption (onset of region II) was noted. This critical level was attained at a concentration close to 3.1 × 10-4 and 10-4 mol L-1 for C12 and C16, respectively. The adsorption was slowed as the adsorption attained a certain level (region III), and then when the surfactant concentration was further increased, it increased again (region IV) until it reached a constant level (region V). At first glance it seems astonishing that the adsorption remains low and increases slowly in region I despite the presence of a relatively high amount of available negative sites on the surface. In this region surfactant monomers are adsorbed via electrostatic interaction between surfactant headgroups and carboxylic sites on the cellulose surface. Hydrocarbon tails are supposed to lie flat on the substrate to minimize their contact area with water molecules. The relatively high surface occupied by the adsorbed species reduces the possibility of adsorption of other surfactant molecules. The steep increase in the slope of adsorption isotherms in region II is a result of a change in the conformation of adsorbed molecules from a flat to a perpendicular orientation, where alkyl chains form highly packed aggregates, minimizing their contact area with water molecules and therefore increasing the entropy of the system. In this region the electrostatic interaction between the charged headgroups and carboxylic sites on the surface is still present since the adsorbed amount is not sufficient to neutralize the surface charge. Surfactant molecules are still adsorbed with headgroups facing toward the surface, and self-association leads to the formation of monolayered aggregates. Region III appears when the adsorbed amount reaches about 50-75% (depending on the surfactant and surface charge density) of the total carboxylic sites. The decrease in the slope of the isotherm could be attributed to the progressive neutralization of the cellulose surface and to the subsequent (i) reduction of the electrostatic surface/ headgroup attraction and (ii) increase of the repulsion between surfactant headgroups. The restart of adsorption in region IV occurs when the substrate charge is close to neutral (the amount of adsorbed equivalents is close to the corresponding density of the negative carboxylic sites). The adsorption in this domain may occur with a bilayer conformation, with surfactant headgroups facing into the solution. The solution activity of the surfactant is now sufficient to overcome the electrostatic repulsion among headgroups and to continue the growth of the aggregated domains. The stabilization of the adsorption in region V may be attributed to the saturation of the bilayer. Except for C16-cellulse-150, this region occurs at a surfactant concentration lower than or close to the cmc. When we compare the different cellulose substrates and the two surfactants, we note that the slope of region II becomes much steeper as the surface charge density and the surfactant chain length increase. Likewise, the extent of region III follows the same trend. These results are consistent with our explanation. Indeed, with an increase in the surface charge density, the distance between adsorbed surfactant molecules is reduced and the possibility for lateral hydrophobic tail-tail interaction is more promoted. Moreover, with an increase in the chain length, the hydrophobic attraction between surfactant tails is more intense. These two effects can explain the transition from a five- to a three-region adsorption isotherm when the cellulose surface charge is increased from 300 to 600 µmol g-1. We can suppose that the hydrophobic contribution to both C12 and C16 adsorption was boosted by the

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Figure 5. Adsorption isotherm of C16 surfactants onto cellulose-300 and cellulose-150 and the corresponding evolution of the released amount of chloride (Cl) counterion.

Figure 6. Adsorption isotherm of C12 and C16 surfactants onto cellulose-600 and the corresponding evolution of the released amount of chloride (Cl) counterion.

high surface charge and monolayered and bilayered aggregates grew simultaneously, thus inducing the superposition of regions II, III, and IV. This hypothesis on the mechanisms of surfactant selfassembly at the surface of cellulose fibers was based on the different phenomena reported to take place in the interaction of ionic surfactants with oxide surfaces. Since the validity of this adsorption model for surfactant/ cellulose systems has not been proven yet, in this work, we will try to provide further arguments to support the extension of this model to surfactant/cellulose systems. In Figures 5 and 6, the amount of chloride counterion released by the adsorbed surfactant is superimposed to the adsorption isotherms of C16 and C12 surfactants on cellulose-150, -300, and -600, thus showing the following. (i) For the two surfactants, the amount of released counterion followed the same trend of the adsorption isotherm until adsorbed equivalents became close to the surface charge capacity. Up to this level, the chloride ion was no longer released in the solution and the surfactant was adsorbed with its counterion. (ii) In the presence of C16, the fraction of the adsorbed surfactant with its counterion was close to 50%. (iii) In the presence of cellulose-150 and -300, the plateau level of the released counterion coincided with the onset of region III, which means that the C16 adsorption in this region was no longer subjected to the electrostatic attraction of the charged sites on the surface as the

Alila et al.

Figure 7. ζ-potential and adsorption of cellulose-150 as a function of C12 and C16 solution concentration at pH 7.5-8.

surfactant was adsorbed with its counterion and no exchange with other counterions took place. (iv) With C12, the surfactant adsorption proceeded through an exchange mechanism even in region III. The fraction of the surfactant adsorbed with its counterion was relatively low compared to that of C16; it was about 10% and 20% for cellulose-300 and -600, respectively. These results suggest that the C12 surfactant was adsorbed as a monolayer (hemimicelle) whereas C16 was adsorbed as a bilayer (admicelle) which was generated in region IV of the isotherm. Presumably, the adsorption of C12 on a charged cellulosic substrate occurs predominantly head-on, with the compensation of the surfactant charge by the surface charge. Adsorption is mainly driven by electrostatic interaction. On the other hand, strong hydrophobic interaction associated with the long alkyl chain of C16 leads to the formation of a second layer with the surfactant headgroups facing toward the solution. The comparison between chloride ion and C16 adsorption isotherms confirmed that the switch from a five- to a threeregion adsorption isotherm obtained when the cellulose charge was increased from 300 to 600 µmol g-1 can be associated with a nearly simultaneous self-assembly of C16 into monolayered and bilayered aggregates. To further support the difference in the configuration of C12 and C16 surfactants on a cellulose substrate, the variation of the ζ-potential of cellulose fibers was measured as a function of the surfactant bulk concentration. The comparison of the cellulose ζ-potential with the corresponding adsorption isotherm of C12 and C16 on cellulose-150 and -300, Figures 7 and 8, shows the following. (i) The evolution of the ζ-potential with surfactant concentration displayed the same transition points observed on the adsorption isotherms. (ii) At a given adsorbed amount of C12 and C16 surfactants, cellulose-300 displayed a ζ-potential lower than that of cellulose-150. This difference is consistent with the higher density of negatively charged sites on cellulose-300. (iii) In the presence of C12, the ζ-potential at the saturation of the cellulose surface was close to zero. (iv) With C16 the reverse in the sign of the ζ-potential occurred in region III. As the onset of this region coincided with the isoelectric point (iep) of the substrate, we could suppose that the electrostatic repulsion between the cationic surface and headgroups caused the surfactant adsorption to decrease in region III. Nevertheless, ad-

Surfactant Adsorption onto Cellulosic Fibers

Figure 8. ζ-potential and adsorption of cellulose-300 as a function of C12 and C16 solution concentration at pH 7.5-8.

sorption continues to operate as the result of the hydrophobic effect of the surfactant tail. The positive value of the ζ-potential and its continuous increase in regions IV and V are in agreement with the hypothesis of the generation of a bilayer. (v) The ζ-potential value in region V with C16 increased as the substrate charge density increased. This result provides another confirmation of the growth of a second layer. The analysis of ζ-potential measurements gave us additional information to describe more precisely phenomena associated with the adsorption of a cationic surfactant on charged cellulose fibers. The adsorption of the C12 surfactant was mainly driven by electrostatic interactions, and the hydrophobic effect intervened essentially to promote the self-assembly of adsorbed monomers, namely, the transition from a flat to a perpendicular conformation. The adsorption is limited to a monolayer. The addition of four CH2 units in the alkyl chain length increased the contribution of the hydrophobic interaction, overcoming repulsion forces among the cationic surface and surfactant headgroups and promoting the growth of a second layer (region IV). The evolution of the adsorption isotherm with ionic strength has been studied with C16 and cellulose-150. Figure 9 shows the adsorption isotherm at pH 7 for C16 on cellulose-150 in the presence of different concentrations of KCl. At a low amount of adsorbed surfactant (lower than 20 µmol g-1) adsorption decreased with increasing salt concentration, and it was enhanced above this level. Likewise, the amount of adsorbed surfactant at the onset of region III and its slope increased with salt addition. At low surfactant concentration, region I, surfactant headgroups and cellulose surface charges are screened by the increase in the ionic strength, thus provoking a decrease in cellulose-surfactant interactions and adsorption. At higher surfactant concentration, viz., in regions II, III, and IV, surfactants form aggregates on the cellulose surface where cationic headgroups are closely packed. The charge screening due to the increase of the ionic strength reduces the repulsion between the headgroups, thus enhancing the formation of closely packed aggregates and surfactant adsorption. The contribution of ionic strength to surfactant self-assembly was also shown by the progressive transition from a five- to a three-region adsorption isotherm when the ionic strength was increased from 0 to 10-2, Figure 9.

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Figure 9. Adsorption isotherms of C16 SDBS onto cellulose150 at pH 7.5-8 and three different ionic strengths (addition of KCl).

Figure 10. Adsorption isotherm of C12, C14, C16, and C18 cationic surfactants onto cellulose-150 at pH 6.5-7 and in the absence of any added salts.

The influence of the alkyl chain length on the surfactant adsorption is shown in Figure 10. The increase in the alkyl chain length led to a shift of the adsorption isotherms toward low concentrations and to a continuous increase in the maximum amount of adsorbed surfactant. C12 gave a three-region adsorption isotherm which was interpreted as reflecting the generation of mainly monoloayered structures. C14 and C16 had five-region isotherms; we can suppose that the increase in the alkyl chain length favored the generation of bilayered aggregates. The three-region isotherm obtained for C18 shows that strong hydrophobic interactions among C18 alkyl chains enabled the electrostatic repulsion forces to be overcome, thus inducing a continuous transition from region I to region IV (a hemimicelle and an admicelle are generated simultaneously). These trends are in agreement with the hypothesis of surfactant self-association into monolayered and bilayered structures. Indeed, the increase of the number of CH2 groups in the surfactant tail strengthened hydrophobic interactions among alkyl chains, and induced a decrease in the free energy of hemimicellization, as in the well-known correlation between the increase in surfactant chain length and the corresponding decrease of its cmc2. Despite the low surface area measured for the cellulose fibers used in this study (3 m2 g-1), maximum adsorbed amounts of surfactant were relatively high compared to those of the charged oxide substrate, which displays such adsorption levels only with specific surface areas higher

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Figure 11. Possible adsorption isotherms and aggregation states of cationic surfactants at the surface of negatively charged cellulose fibers. These isotherms and aggregation states are consistent with models proposed for surfactant/oxide systems.31-33 (a) Three-region adsorption isotherm with monolayer aggregation, i.e., C12. Dispersive interactions among C12 molecules are too weak to overcome electrostatic repulsion and to allow bilayer packing. (b) Five-region adsorption isotherm with monolayer and bilayer aggregation, i.e., C14 and C16. Dispersive interactions among surfactant molecules are strong enough to allow the formation of a bilayer. The monolayer to bilayer transition is retarded (presence of region III). (c) Three-region adsorption isotherm with monolayer and bilayer aggregation, i.e., C18. Strong dispersive interactions boost surfactant self-assembly, monolayer and bilayer aggregates form simultaneously, and region III (b) disappears.

than 200-300 m2 g-1.46 This result could be justified only if we consider that surfactant molecules were adsorbed not only at the external surface of the fibers but also inside their porous structures thanks to the fiber swelling occurring during fiber oxidation. In other words, the cellulose surface available for surfactant adsorption in the water-swollen state was much larger than that measured for the dry substrate. Conclusions The adsorption and self-assembly of cationic surfactants on oxidized cellulose displayed a complex behavior depending on the cellulose charge, the surfactant molecular structure, and the ionic strength of the solution. Although we did not rationalize experimental results with a rigorous mathematical treatment, the contribution of these variables to surfactant adsorption and aggregation was in agreement with tendencies which can be predicted by DLVO theory. (i) Adsorption of cationic surfactants proceeded via electrostatic and dispersive interactions whose intensity (46) Thakulsukaant, C.; Lobban, L. L.; Osuwan, S.; Waritswat, A. Langmuir 1997, 13, 4595.

was linked with the charge density of the substrate and the length of the surfactant hydrocarbon chain. (ii) The increase in the negative charge density of cellulose improved electrostatic forces promoting surfactant adsorption and self-assembly, thus allowing the dense packing of adsorbed molecules into monolayered and bilayered aggregates. (iii) The alkyl chain length affected surfactant selfaggregation especially near the saturation of the substrate. The increase in the surfactant adsorption density and in the surface charge observed when the alkyl chain length was increased from 12 to 18 C atoms was associated with the progressive transition from monolayer to bilayer aggregates. With C12 dispersive interactions are not strong enough to prevail over electrostatic repulsion and after the neutralization of the surface charge adsorption decreases quickly; only monolayered aggregates are supposed to form. With C18 strong dispersive interactions promote surfactant adsorption and aggregation even after surface charge reversal; bilayered aggregates are supposed to form. (iv) The ionic strength influenced adsorption by screening the cellulose surface charge and the surfactant ionic head. At low surfactant concentration, when adsorption

Surfactant Adsorption onto Cellulosic Fibers

is driven by electrostatic forces, adsorption was retarded when the ionic strength was increased. At high surfactant concentration, when electrostatic forces among surfactant headgroups hinder a dense molecular packing on the substrate, adsorption was boosted when the ionic strength was increased. The negative charge of the cellulose substrate, the alkyl chain length, and ionic strength affected in similar ways surfactant adsorption and self-assembly. Indeed, an increase in one of these parameters was reflected by an increase in surfactant adsorption and aggregation with a consequent variation in the shape of the adsorption isotherm, which switched from a five- to a three-region curve. A sum of possible surfactant configurations at the

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cellulose-water interface and adsorption isotherms is given in Figure 11. Acknowledgment. This work was financially supported by the Tunisian “Ministe`re de la Recherche Scientifique, technologique et de developpement des compe´tences” and by the International Foundation for Science through Grant W/3358-1. Supporting Information Available: Details pertaining to conductometric titration, FT-IR characterization, and X-ray characterization. This material is available free of charge via the Internet at http://pubs.acs.org. LA050367N