Interaction between Ionic Surfactants and Glass Surfaces with

Jul 11, 2001 - dodecyltrimethylammonium chloride (DTAC) and/or sodium dodecyl sulfate (SDS), the electrostatic repulsion was observed above the half ...
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Langmuir 2001, 17, 4973-4979

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Interaction between Ionic Surfactants and Glass Surfaces with Covalently Attached Quaternary Ammonium Groups Kenichi Sakai, Kanjiro Torigoe, and Kunio Esumi* Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received February 5, 2001. In Final Form: May 2, 2001 Interaction forces between glass surfaces with covalently attached quaternary ammonium groups (XNm, m is the carbon number of alkyl chains) in aqueous ionic surfactant solutions have been studied by the colloidal probe atomic force microscopy. Surface preparation of glass substrates was characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and contact angles. In the absence of surfactants, almost zero forces were observed between XN12 and XN16 glass surfaces, respectively, while in the case of XN8 an electrostatic repulsive force could be detected. By the addition of surfactants, dodecyltrimethylammonium chloride (DTAC) and/or sodium dodecyl sulfate (SDS), the electrostatic repulsion was observed above the half critical micelle concentration of each surfactant. In addition, with increasing concentration of the surfactants, the magnitude of force curves increased and the range of repulsion extended. These results suggest that the headgroups of adsorbed surfactants orient toward the solution phase and the charging-up mechanism is supported in these systems. On the other hand, the range of repulsion between XN16 glass surfaces in the surfactant solutions was not always larger than that between the other samples (XN8 or XN12) because the grafted alkyl chains onto glass can adopt various conformations corresponding to their chain length.

Introduction Adsorption of surfactants and/or polymers on particles plays an important role in the interfacial phenomena at the solid/liquid interface. Therefore, adsorption characteristics have been very interesting subjects, and various adsorption models have been proposed by many workers, for example, the reverse-orientation model,1-3 the twostep model,4,5 and the small surface micelle model.6-8 However, the direct proof for adsorption phenomena on various surfaces is insufficient to establish the microscopic aspects. Pashley and Israelachvili are pioneers in the direct force measurement between mica surfaces with the surfactant adsorption using by the surface force apparatus (SFA).9,10 They found that the adsorption of hexadecyltrimethylammonium bromide (CTAB) takes place stepwise; a packed monolayer, a second monolayer, and a bilayer are formed on mica with increasing concentration of CTAB. In addition, Kekicheff et al.11 carried out the same experiment in detail and proposed the charging-up mechanism of admicelles. There are many reports focused on the adsorption characteristics for surfactants with various structures onto mica.12-14 Furthermore, Parker et al. constructed an (1) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90. (2) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 2649. (3) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 2660. (4) Harwell, J. H.; Hoskins, J. C.; Schechter, R. S.; Wade, W. H. Langmuir 1985, 1, 251. (5) Yeskie, M. A.; Harwell, J. H. J. Phys. Chem. 1988, 92, 2346. (6) Gu, T.; Huang, Z. Colloids Surf. 1989, 40, 71. (7) Gao, Y.; Du, J.; Gu, T. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2671. (8) Rupprecht, H.; Gu, T. Colloid Polym. Sci. 1991, 269, 506. (9) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169. (10) Pashley, R. M.; McGuiggan, P. M.; Horn, R. G.; Ninham, B. W. J. Colloid Interface Sci. 1988, 126, 569. (11) Kekicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids Surf. 1989, 40, 31. (12) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Brady, J.; Evans, D. F. J. Phys. Chem. 1986, 90, 1637.

original surface force apparatus15 and measured interaction forces between glass surfaces (instead of mica surfaces) in CTAB solution.16 However, in general, the SFA has difficulty in the adaptation to various systems because it can only measure the forces between optically transparent and atomically smooth surfaces. Evolving from the scanning tunneling microscope (STM), the atomic force microscope (AFM) is one of a group of devices generally referred to as scanning probe microscopes (SPMs).17 The AFM acquires topological mappings of nonconductive surfaces with subnanometer resolution. In 1995, adsorbed surfactant layers on solid surfaces were observed directly by Manne et al.18 for the first time. They demonstrated that self-assembled structures of surfactant molecules adsorbed are effected by several factors, for example, the surfactant geometry and the density of the electrostatic binding sites in the case of the adsorption from the ionic surfactant solution onto the hydrophilic surfaces.19,20 Furthermore, Ducker et al. investigated the counterion effects systematically in view of the variety of solid substrates,21 the competitive adsorption of rival cations,22,23 and the role of polarizability and charge for counterions.24 These reports concluded that the traditional picture of bilayer formation is evidently simplistic. At the same time, the AFM is a powerful tool to measure forces between the probe and the substrate as well as (13) Herder, C. E.; Claesson, P. M.; Herder, P. C. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1933. (14) Herder, P. C. J. Colloid Interface Sci. 1989, 134, 346. (15) Parker, J. L.; Yaminsky, V. V.; Claesson, P. M. J. Phys. Chem. 1993, 97, 7706. (16) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 1110. (17) Binnig, G.; Quate, C. F.; Gerber, G. Phys. Rev. Lett. 1986, 56, 930. (18) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (19) Manne, S. Prog. Colloid. Polym. Sci. 1997, 103, 226. (20) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685. (21) Liu, J.-F.; Ducker, W. A. J. Phys. Chem. 1999, 103, 8558. (22) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160. (23) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602. (24) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 4447.

10.1021/la010183i CCC: $20.00 © 2001 American Chemical Society Published on Web 07/11/2001

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imaging surface topology. Particularly, by using the “colloidal probe technique”, which was first developed by Ducker et al.,25,26 the interaction force between a colloidal probe attached to the cantilever and the sample surface can be measured. Bremmell et al.27 investigated the surface forces between a spherical glass particle and a smooth glass substrate as a function of the concentration of the cationic surfactant, hexadecylpyridinium bromide (CPB). At the low CPB concentration, the electrostatic repulsion between the bare surfaces was neutralized and a long-range attractive force appeared. With increasing concentration of CPB, the surfaces began to recharge and the electrostatic repulsion was reobserved. On the other hand, the adsorption of sodium dodecyl sulfate (SDS) on gold substrates deposited by the self-assembled monolayer of 2-aminoethanethiol hydrochloride was presented by Hu and Bard.28 Thus, the measurement of interaction forces between the substrates with modified surface in surfactant solution is of great importance for the wide application to various systems. The solid particles have been modified using various methods and utilized for immobilizing antimicrobial agents, packings for liquid chromatography columns, catalysts, and adsorbents. Suhara et al.29,30 synthesized fine silica powders with covalently attached quaternary ammonium groups bearing various alkyl chain lengths (XNm, m is the carbon number of alkyl chains) and investigated the adsorption behavior of anionic surfactants on XNm by measuring adsorption isotherms and ζ potentials. They concluded that in the case of the shorter alkyl chain length, an electrostatic attraction is predominant for the adsorption, while a hydrophobic interaction plays an important role in the case of the longer one. A similar adsorption behavior was observed for titanium dioxide with covalently attached quaternary ammonium groups by the addition of surfactants and/or water-soluble polymers.31-35 In this study, by using the colloidal probe atomic force microscopy we investigated the interaction force between glass surfaces with covalently attached quaternary ammonium groups by the addition of cationic surfactant, dodecyltrimethylammonium chloride (DTAC), or anionic surfactant, sodium dodecyl sulfate (SDS). Surface preparation of the glass spheres and/or plates was characterized by using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and contact angles. Experimental Section Materials. Glass spheres with 10-30 µm diameter were purchased from Polysciences Inc.. The surface roughness of these spheres is a very important factor for force measurements. We (25) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (26) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (27) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloids Surf. 1999, 146, 75. (28) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5418. (29) Suhara, T.; Kanemaru, T.; Fukui, H.; Yamaguchi, M. Colloids Surf. 1995, 95, 1. (30) Suhara, T.; Fukui, H.; Yamaguchi, M.; Suzuki, F. Colloids Surf. 1996, 119, 15. (31) Esumi, K.; Toyoda, H.; Goino, M.; Suhara, T.; Fukui, H. Langmuir 1998, 14, 199. (32) Esumi, K.; Toyoda, H.; Goino, M.; Suhara, T.; Fukui, H.; Koide, Y. J. Colloid Interface Sci. 1998, 202, 377. (33) Esumi, K.; Toyoda, H.; Suhara, T.; Fukui, H. Colloids Surf. 1998, 145, 145. (34) Esumi, K.; Sakai, K.; Torigoe, K.; Suhara, T.; Fukui, H. Colloids Surf. 1999, 155, 413. (35) Esumi, K. Colloids Surf. 2001, 176, 25.

Sakai et al. Scheme 1. Surface Preparation of the Glass Sphere with Covalently Attached Quaternary Ammonium Groups (XNm)

confirmed that the roughness is sufficient enough to measure the forces since the straight gradient of the diode response to the piezo expansion is obtained at the constant compliance region. As a further step, the glass spheres and the glass plates were washed with concentrated H2SO4 dissolving a small amount of KMnO4 under an ultrasonicator and then rinsed thoroughly with water. DTAC (Tokyo Kasei Co., Ltd.) was purified with acetone several times and SDS (Nacalai Tesque Inc.) with ethanol three times. The purity of these surfactants was confirmed by the absence of a minimum in the surface tension versus concentration curve. The critical micelle concentrations (cmcs) of these surfactants were 28 mmol dm-3 for DTAC and 2.6 mmol dm-3 for SDS in the presence of 10 mmol dm-3 NaCl, respectively. Other reagents used in all experiments were of analytical grade and the water was deionized through a Milli-Q Plus system. Surface Preparation of Glass Substrates with Covalently Attached Quaternary Ammonium Groups. The surface preparation of glass spheres with covalently attached quaternary ammonium groups was carried out by the procedure shown in Scheme 1. Glass spheres were contacted with 1,3,5,7tetramethylcyclotetrasiloxane (TMCTS) vapor in a vacuum oven at 80 °C for 16 h. The product was dried at 120 °C for 4 h to remove excess TMCTS and then reacted with p-(chloromethyl)styrene (CMS) in toluene in the presence of H2PtCl6 and tributylamine complex as a catalyst at 110 °C for 6 h. The product was washed with toluene and then dried at 110 °C in vacuo. By a further reaction with N,N-dimethyl-n-alkylamine in tetrahydrofuran at 65 °C for 4 h, final products with covalently attached quaternary ammonium groups (XNm; XN8, XN12, and XN16) were obtained. These products were washed with ethanol to remove excess N,N-dimethyl-n-alkylamine and then dried at 70 °C in vacuo. On the other hand, we also synthesized XNm glass spheres with a small amount of quaternary ammonium groups (XNm′). Surface preparation for XNm′ was also carried out from the CMS glass spheres by a reaction with N,N-dimethyl-nalkylamine under a mild condition (at 0 °C). In this examination, XN12′ was synthesized as one example of XNm′. The surface preparation of glass plates was carried out by the same procedure as described above. FT-IR Measurements. FT-IR spectra were recorded using a JASCO FT-IR Model 430 spectrophotometer in the range of

Interaction of Ionic Surfactants and Glass Surfaces 4000-400 cm-1 with 1 cm-1 resolution. KBr and the sample were mixed by the same fraction in order to attempt the quantitative analysis from the acquired FT-IR spectra. XPS Measurements. High-resolution XPS spectra were recorded using an ESCALAB MKII system (VG Scientific Inc.) with a monochromatic Mg KR X-ray source (1253.6 eV). The binding energy was calibrated using the Au (4f7/2) peak energy (84.0 eV) as the energy standard. The pass energy of the analyzer, the step size of the scan energy, and the irradiation time at each step were set at 80 eV, 0.1 eV, and 150 ms, respectively. All peaks were resolved by using a spectral processing program included in the XPS operating system. The qualitative analysis of the surface composition was estimated from the peak areas normalized by the relative sensitivity factor, the electron escape depth on the kinetic energy, and the apparatus transmission function. The relative sensitivity factors were 0.87, 1.00, and 1.77 for Si (2p), C (1s), and N (1s), respectively. Contact Angle Measurements. Contact angles were measured by using a FACE contact angle meter CA-DT type (Kyowa Interface Science Co., Ltd.). A small droplet of pure water was placed on a sample plate and the static contact angle was measured. This procedure was repeated for at least three different spots on each plate in order to eliminate the influence of any local differences in surface properties. The reproducibility of this measurement was within (1. Surface Force Measurements by AFM. Surface force measurements were carried out by using a commercially available AFM, the TMX2100 Explorer atomic force microscope (ThermoMicroscopes Inc.). In this device, a surface was held in a constant position while a probe displacement was achieved using a piezoelectric crystal and allowed to deflect as it senses the approaching surfaces. The deflection of the cantilever was monitored by movements of a laser beam reflected from the back of the cantilever across a split photodiode. The data were collected with a commercial silicon nitride cantilevers (Digital Instrument Inc.) with a spring constant of 0.58 N m-1 modified by attaching an original or a surface-treated glass sphere, as described by Ducker et al.25,26 It is necessary to convert the raw data (cantilever deflection versus piezo movement curve) to the normal force one (force versus separation curve) and then to define zeros of both force and separation. These regulations are also explained in the previous reports. However, the deflection detected by using TMX2100 Explore AFM has a gradient to some extent even when no force exists between the probe and the surface, because the probe travels to the Z-direction when attached to the piezoelectric crystal. Therefore, the baseline corrections were made in the determination of the zero force. The gradient of the diode response to the piezo expansion at the constant compliance was also calibrated by the deflection of the cantilever in terms of force. Finally, the force was normalized by the radius of the colloidal probe (Derjaguin approximation36). Prior to force measurements, the colloidal probe attached to the cantilever was immersed in ethanol overnight, followed by washing with purified water thoroughly, and finishing with rinses with water at 50 °C. The washing of a sample plate was carried out by the same procedure as that of the colloidal probe. After washing, both the probe and the plate were set at the fixed position of AFM and immersed in sample solution immediately. All measurements were performed after 30 min of equilibration in order to eliminate the drift of a force spectrum and carried out by using the thermomodule controller (MT862-04C12, Netsu Denshi Co., Ltd.) controlled at 25 °C.

Results and Discussion Characterization of Surface Preparation. Proceedings of surface preparation of glass spheres were characterized qualitatively from FT-IR spectra.29 On the other hand, the spectrum of XN12′ was apparently different from both spectra of CMS glass spheres and XN12 (data is not shown). For example, in the case of XN12′, the C-H stretching vibration derived from CMS was clearly observed, while in the case of XN12 the band disappeared. (36) Derjaguin, B. V. Kolloid Z. 1934, 69, 155.

Langmuir, Vol. 17, No. 16, 2001 4975 Table 1. Quantitative Analysis of Surface Modified Glass Spheres from XPS Survey Spectrum glass sphere unmodified TMCTS XN8 XN12 XN16 XN12′

binding energy (eV) Si (2p) N (1s) 103.6 102.4 102.4 102.6 102.4 102.4

402.5 402.7 401.3

Si (2p)/N (1s)

occupied area (Å2)

14.1 8.34 5.94

224 133 94.4

Table 2. Water Contact Angle of Glass Plates with Surface Modification glass plate unmodified (original) TMCTS CMS XN8

contact angle contact angle (deg) glass plate (deg) 5.4 105.9 94.5 66.5

XN12 XN16 XN12′

74.4 78.6 80.9

Therefore, we concluded that the glass surfaces with a small amount of quaternary ammonium groups covalently attached are successfully obtained. Table 1 shows the binding energy for Si (2p) and N (1s) from the XPS survey spectrum of surface-prepared glass spheres. The binding energy for Si (2p) was decreased from 103.6 to 102.4 eV with the TMCTS deposition on original glass surfaces, suggesting that the electron density on the silicon atom of original glass surfaces is lower than that of the TMCTS-deposited ones. On the other hand, no shifts of the binding energy for Si (2p) were observed by introducing quaternary ammonium groups on the glass surfaces. In addition, it was found that the prepared glass surfaces could be coated with TMCTS completely because the overlap is not detected in the narrow scanning for the Si (2p) region. By analyzing the XPS spectra, several quantitative suggestions could be acquired. Table 1 also shows the Si (2p)/N (1s) peak area ratios of XNm and the occupied area per one nitrogen atom assuming the molecular diameter of TMCTS (about 0.9 nm).37 The peak area of the silicon atom relating to that of the nitrogen was decreased with increasing chain length of grafted quaternary ammonium groups. This result is in agreement with the result from FT-IR. The limiting area occupied at the air/solution interface for dodecyltrimethylammonium ion (DTA+) is about 0.32 nm,2,20 so that the grafted alkyl chains exist with comparatively low density on the surface. On the other hand, the peak for N (1s) of XN12′ glass spheres was hardly detected under the experimental resolution of our apparatus, so the occupied area of each ammonium group grafted could not be estimated quantitatively. The surface preparation for the glass plates was characterized by measuring the static contact angles of water (θ). These results are shown in Table 2. The contact angles were increased with increasing chain length grafted. However, a linear relationship between the chain length and the cosθ was not observed. It is suggested that the hydrophobicity of XNm is affected by not only the chain length grafted but also the surface density of quaternary ammonium groups. In addition, the contact angle of the XN12′ glass plate was intermediate between that of the CMS glass plate and XN12, as expected. Interaction Forces between Unmodified Glass Surfaces. Prior to the force measurements between XNm glass surfaces, we investigated the interaction between original glass surfaces with the adsorption of DTAC. Each (37) Horn, R. G.; Israelachvili, J. N. J. Chem. Phys. 1981, 75, 1400.

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Figure 1. Normalized force curves between hydrophilic glass surfaces in aqueous DTAC solution containing 10 mmol dm-3 NaCl as a background electrolyte. Feed concentration of DTAC: (a) 0, (b) 0.5, and (c) 12 mmol dm-3.

solution contained 10 mmol dm-3 NaCl as a background electrolyte. In the absence of DTAC (Figure 1a), the electrostatic repulsion was observed and the best-fit parameters under the constant potential limit of the nonlinear Poisson-Boltzmann equation were 24 mV and 3.5 nm for the surface potential and the Debye length, respectively. The interaction force between glass surfaces was dramatically influenced by the addition of DTAC. In aqueous 0.5 mmol dm-3 (0.03 cmc) DATC solution with 10 mmol dm-3 NaCl, an attractive force was detected from the separation of 15 nm and then the surfaces experienced an adhesive contact (Figure 1b). Indeed, with an increasing concentration of DTAC the repulsive force was reobserved. Figure 1c shows the interaction between glass surfaces in 12 mmol dm-3 (0.86 cmc) DTAC solution as one example of force curves with the repulsion between adsorbed surfactant layers. These results can be interpreted by the charging-up mechanism of the adsorbed surfactant layer on surfaces, previously reported by many workers.11,14-16 The development of the AFM imaging technique has focused the self-assembly of surfactant molecules at the solid/liquid interface. However, the organization of the adsorbed surfactant layer has not been described from these results, since the force measurements observe the wide range of surface structure. Interaction Forces between Glass Surfaces with Covalently Attached Quaternary Ammonium Groups in Aqueous NaCl Solution. The interesting phenomena were observed from the force measurements using glass substrates with covalently attached quaternary ammonium groups. Figure 2 shows the normalized

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force curves between XNm glass surfaces in aqueous 10 mmol dm-3 NaCl solution. Almost zero forces were observed in the cases of both XN12 and XN16, which are probably due to either the hydrophobicity on these surfaces or the surface charge screening caused by the longer chains grafted. On the other hand, an electrostatic repulsion was observed between XN8 glass surfaces from the range of 3-5 nm, which was apparently different from the results of XN12 and/or XN16. This result suggests that in the case of XN8 (i) the shortest length of alkyl chains produces the hydrophilic surfaces despite the smallest amount of quaternary ammonium groups grafted and (ii) the electrostatic interaction caused by the existence of cationic parts dominates over the hydrophobic attraction. To compare these results, the interaction force between CMS glass surfaces that are pregrafted with quaternary ammonium groups was measured in aqueous NaCl solution. An obvious attraction was observed from the range of 5 nm, which is significantly smaller than that between the neutralized surfaces in the surfactant solution well below the cmc.27 Interaction Forces between Glass Surfaces with Covalently Attached Quaternary Ammonium Groups in Aqueous DTAC Solution. Figure 3 shows the normalized force curves between XN12 glass surfaces in DTAC solutions of various concentrations containing 10 mmol dm-3 NaCl as a background electrolyte. No significant differences in the absence and in the presence of 0.5 mmol dm-3 DTAC were observed from the force measurements. With increasing concentration of DTAC, the force-detectable distance and the magnitude of repulsion increased. Such a tendency was also observed for the unmodified glass surfaces in aqueous DTAC solutions, suggesting that the charging-up mechanism of DTAC adsorption is also supported in this system. In addition, the driving force for the adsorption of DTAC seems to be the hydrophobic interaction, because of the existence of either the hydrophobic CMS layer or the grafted dodecyl chains on the surfaces. As a result, the adsorbed surfactants orient their headgroups toward the solution phase. The results obtained in both XN8 and XN16 systems were similar to that in XN12. Figure 4 shows the normalized force curves between XNm glass surfaces in aqueous solution containing both 12 mmol dm-3 DTAC and 10 mmol dm-3 NaCl. It is very interesting to note that the range of repulsion between XN16 glass surfaces (about 9 nm separation) is smaller than that between XN12 surfaces (about 12 nm). Since the repulsive range is dependent on a few factors, we assumed that (i) the location of the hard wall on each XNm glass substrates is the same distance from the bare surfaces and (ii) the amounts of DTAC adsorbed on both XN12 and XN16 are not significantly different. Under this situation, the results can be interpreted that the intercalation of the adsorbed surfactants into the grafted hydrophobic layer takes place on XN16 glass surfaces. This phenomenon seems to be caused by the dynamic conformation of grafted hexadecyl chains in aqueous solution. The flexibility of grafted alkyl chains increases with increasing its length up to 16, and then the affinity of alkyl chains and the hydrophobic CMS layer becomes strong. Accordingly, DTAC adsorbs in the vicinity of the hydrophobic CMS layer, resulting in the decrease of the range of repulsion. Furthermore, it can be pointed out that the magnitudes of repulsion in the cases of both XN12 and XN16 glass surfaces are much larger than that in the case of XN8. This result is reasonable because the positively charged ammonium groups grafted on the only XN8 glass surface affect the interfacial surroundings.

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Figure 2. Normalized force curves between XNm glass surfaces in aqueous 10 mmol dm-3 NaCl solution: (a) XN8, (b) XN12, (c) XN16, and (d) CMS glass surfaces.

Figure 3. Normalized force curves between XN12 glass surfaces in aqueous DTAC solution containing 10 mmol dm-3 NaCl as a background electrolyte. Feed concentration of DTAC: (a) 0.5, (b) 7, (c) 12, and (d) 28 mmol dm-3.

Therefore, it can be explained that the DTAC adsorption taken place through the hydrophobic attraction is inhibited by the electrostatic repulsion between the surfactant headgroups and XN8 glass surfaces. Figure 5 represents the adsorbed layer structures of DTAC formed on XNm surfaces, schematically. As we have pointed out in the previous section, the organization of the adsorbed surfactant layer has not been directly observed from the force measurements. However, the AFM observation of the

cationic surfactant molecules on a solid surface suggests that the hemicylindical admicelles are formed on hydrophobic substrates, which is caused by the head-to-head and the tail-to-tail interactions.38 The consideration for the adsorbed layer structure formed on XNm has been somewhat complex, but we suppose that the hemispherical (38) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409.

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Figure 4. Normalized force curves between XNm glass surfaces in aqueous 12 mmol dm-3 DTAC solution containing 10 mmol dm-3 NaCl as a background electrolyte.

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Figure 6. Normalized force curves between XNm glass surfaces in aqueous 2.2 mmol dm-3 SDS solution containing 10 mmol dm-3 NaCl as a background electrolyte.

Figure 5. Schematic representation of DTAC adsorption on XNm.

or hemicylindical admicelles are formed around the alkyl chains grafted. Interaction Forces between Glass Surfaces with Covalently Attached Quaternary Ammonium Groups in Aqueous SDS Solution. In the next step, we measured forces between XNm glass surfaces in aqueous SDS solutions. The dependence on the surfactant concentration was not significantly different from that in aqueous DTAC solutions. Accordingly, it is suggested that the dynamic conformation of grafted alkyl chains on XNm in SDS solutions is similar to that in DTAC solutions; the range of repulsion between XN16s (about 6 nm separation) is significantly smaller than that between XN12 surfaces (about 9 nm). However, a clear distinction was observed between DTAC and SDS systems with respect to their magnitudes of repulsion. Figure 6 shows the interaction forces between XNm glass surfaces in aqueous 2.2 mmol dm-3 (0.86 cmc) SDS solution with 10 mmol dm-3 NaCl as a background electrolyte. Among three samples (XN8, -12, and -16), the magnitude of repulsion for XN8 glass surfaces was the largest and that for XN12 was almost the same as that for XN16. The driving force for the adsorption of SDS on these surfaces is mainly the hydrophobic attraction, which is the same as DTAC adsorption. Moreover, the additional force is required to interpret the adsorption phenomenon on XN8 glass surfaces. In this case, the adsorption of SDS is not only due to the hydrophobic interaction but also an electrostatic attraction between positively charged XN8 surfaces and the anionic headgroups of SDS. As a result, the adsorption of SDS is favored by these effects, contrary to the case of DTAC. The Effects of the Amount of Quaternary Ammonium Groups Grafted. Surface forces between glass substrates with covalently attached quaternary am-

Figure 7. Normalized force curves between glass surfaces with various densities of quaternary ammonium groups in aqueous 10 mmol dm-3 NaCl solution.

monium groups are changed as a function of not only the length of grafted alkyl chains but also the density of cationic parts on the surfaces. In the previous sections, we discussed the effects of the grafted chain length. However, it was very difficult to interpret the effect of the amount of grafted quaternary ammonium groups at the same time. Accordingly, to study the effect of the amount of quaternary ammonium groups grafted, two samples for XNm were used. One was XN12 and the other was XN12′, having a smaller amount grafted. Figures 7 and 8 show the normalized force curves between the glass surfaces with various densities of quaternary ammonium groups covalently attached in the absence (Figure 7) or presence (Figure 8) of SDS aqueous solution. In both cases the solution contained 10 mmol dm-3 NaCl. Two interesting tendencies can be seen by comparison of three curves (XN12, pre-XN12 (CMS), and XN12′): (i) the magnitude of repulsion for XN12′ is intermediate between that for XN12 and pre-XN12, and (ii) the force-detectable distance is almost the same range between XN12 and XN12′. These results suggest that the range of repulsion is not influenced by the density of quaternary ammonium groups but by the length of the

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hydrophobicity is probably due to the self-assembly of surfactants onto the alkyl chains grafted. Conclusions

Figure 8. Normalized force curves between glass surfaces with various densities of quaternary ammonium groups in aqueous 2.2 mmol dm-3 SDS solution containing 10 mmol dm-3 NaCl as a background electrolyte.

grafted alkyl chains. The fact that the interaction forces between CMS glass surfaces with the adsorption of surfactants are remarkably small despite their large

Colloidal probe atomic force microscopy has been used to measure the interaction forces between glass substrates with covalently attached quaternary ammonium groups in aqueous surfactant solutions. The driving force for the adsorption of DTAC/SDS seems to be a hydrophobic interaction between the surfactant tails and the grafted hydrophobic layer, whereas the electrostatic effect for the adsorption of the surfactants is the minor interaction. Surface forces are affected by the surfactant concentration, since the charging-up has occurred at the interface. At the same time, with the adsorption of surfactants the range of repulsion between XN16 surfaces is significantly shorter than that between XN12 surfaces, which suggests that the grafted hexadecyl chains tilt to their substrates. This colloidal probe method is a powerful technique to investigate the adsorption behavior of surfactants on various solids with the surface chemical modification. Acknowledgment. The authors thank Prof. Kazue Kurihara (Tohoku University) for helpful discussions and Dr. Tsuneo Suhara (Shiseido Co., Ltd.) for his technological advice on surface preparation. LA010183I