Langmuir 2009, 25, 881-890
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Cetylpyridinium Chloride at the Mica-Water Interface: Incomplete Monolayer and Bilayer Structures Artur Meleshyn Center for Radiation Protection and Radioecology (ZSR), Leibniz UniVersita¨t HannoVer, Herrenha¨user Str. 2, 30419 HannoVer, Germany ReceiVed July 29, 2008. ReVised Manuscript ReceiVed NoVember 7, 2008 Monte Carlo simulations of the interface between the cleaved surface of muscovite mica and aqueous cetylpyridinium chloride (CPCl) solution at ambient conditions are reported. Simulation results reveal that monolayer or bilayer aggregates of CP+ ions at the muscovite-water interface remain incomplete up to a CP+ coverage compensating the negative charge of muscovite. It is predicted that at this CP+ coverage only a partial desorption of K+ ions occurs and the two aggregates can be distinguished with help of the X-ray reflectivity technique. Formation of inner-sphere and outer-sphere adsorption complexes of CP+ ions at distances of ∼3 Å and ∼5 Å, respectively, from the surface is observed. Despite an increasing adsorption of CP+ ions, the structure of the adsorbed water film is largely preserved within ∼5 Å from the surface. A strong decrease of water density beyond this distance and formation of “adsorbed K+”-Cl- ion pairs result in coadsorption of Cl- in an amount equivalent to 1/4 of the negative charge of muscovite as close as ∼4.3-4.8 Å to the surface for the incomplete bilayer aggregate. For the incomplete monolayer aggregate, no segregation between K+ and CP+ ions and a displacement of K+ ions into the adsorption sites ∼1.6 Å from the surface are observed.
1. Introduction Extensive experimental studies of adsorption of quaternary ammonium surfactants to the mica-solution and silica-solution interfaces were carried out in the two last decades using atomic force microscopy (AFM),1-5 surface force apparatus (SFA),6-8 electrokinetic technique,9 Fourier transform infrared (FTIR) spectroscopy,10,11 X-ray photoelectron spectroscopy,8 and optical reflectometry.3,5,12 These studies have revealed that quaternary ammonium surfactants partition to an interface modifying interfacial properties at very low bulk concentrations. Cetyltrimethylammonium (CTA+) as well as cetylpyridinium (CP+) ions form incomplete monolayer and bilayer aggregates on the silica surface at very low surfactant surface coverage in the presence of water.10,11 At increasing solution concentrations and with no added salt, CTA+ and CP+ ions form cylindrical aggregates on the mica and discoids on the silica surface.1-5 Due to the partition to the interface, CP+ ions form cylindrical aggregates at the mica-water interface well below the critical micelle concentration (cmc) in the bulk solution.1 After some time (at the scale of hours up to one day), however, the cylindrical aggregates were observed to evolve into flat bilayers, which was attributed to a slow transport of rival cations out of the surfactant film.1,2 (1) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602. (2) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160. (3) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548. (4) Davey, T. W.; Warr, G. G.; Almgren, M.; Asakawa, T. Langmuir 2001, 17, 5283. (5) Atkin, R.; Craig, V. S. J.; Biggs, S. Langmuir 2001, 17, 6155. (6) Pashley, R. M.; Ninham, B. W. J. Phys. Chem. 1987, 91, 2902. (7) Kekicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids Surf. 1989, 40, 31. (8) Chen, Y. L.; Chen, S.; Frank, C.; Israelachvili, J. J. Colloid Interface Sci. 1992, 153, 244. (9) Scales, P. J.; Grieser, F.; Healy, T. W.; Magid, L. J. Langmuir 1992, 8, 277. (10) Kung, K.-H. S.; Hayes, K. F. Langmuir 1993, 9, 263. (11) Singh, P. K.; Adler, J. J.; Rabinovich, Y. I.; Moudgil, B. M. Langmuir 2001, 17, 468. (12) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. J. Colloid Interface Sci. 2003, 266, 236.
Despite this progress in the understanding of surfactant adsorption to a charged interface, many issues are still unresolved. The exact mechanism of surfactant adsorption remains difficult to quantify due to an inability to reveal the precise structure of the incomplete aggregates.5 Although AFM measurements can be employed to directly image adsorbed surfactant structures, the meaningful AFM data are obtained only above the cmc, as a surfactant layer must have headgroups facing solution to provide a repulsive force necessary for imaging the aggregates.12 The uncertainty about the structure of adsorbed surfactant layers complicates, in particular, experimental efforts to quantify the role of counterions, which is further compounded by the difficulty to experimentally detect the adsorbed counterions in situ.3 Additionally, the role of water in the mechanism of surfactant adsorption and aggregation is only poorly understood, as evidenced by the absence of the corresponding section in the excellent review by Atkin et al.,13 which devotes to water only a very small subsection about the energetics of adsorption. Although molecular simulations, as a complementary tool to experimental studies, have been generally proven to be successfully applicable for elucidation of structural issues at the atomic level, to the author’s knowledge, no theoretical study of quaternary ammonium surfactants at the mineral-water (or at least mineral-air) interface has been carried out so far. The present study aims to fill this gap in order to obtain structural information not readily available from experimental studies. In particular, it provides detailed discussion on the structure of the water film and of CP+ aggregates as well as the positions of the adsorbed species at the interface between muscovite mica and aqueous CPCl solution using Monte Carlo (MC) simulations as described in the next section.
2. Simulation Details A layer of 2M1-muscovite mica with the formula unit KAl2(Si3AlO10)(OH)2 consists of two tetrahedral sheets with one (13) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. AdV. Colloid Interface Sci. 2003, 103, 244.
10.1021/la802450q CCC: $40.75 2009 American Chemical Society Published on Web 12/11/2008
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Figure 1. A snapshot of a simulated equilibrium configuration of the CP+-modified muscovite viewed normal to the (100) plane. Ball and stick colors: gray (K+), green (Cl-), red (O), white (H), brown (C), light green (N), turquoise (Al), gold (Si).
out of four Si atoms substituted by Al, which sandwich an octahedral sheet with two out of three octahedrally coordinated positions occupied. The Al substitutions in the tetrahedral sheet are arranged in accordance with the Lo¨wenstein’s rule of avoidance of Al-O-Al linkages, so that hexagonal rings of Si4Al2 and Si5Al1 compositions are equally represented in the modeled muscovite layer. Six basal oxygen atoms bridging Si and Al atoms of the same hexagonal ring are in the vertices of the two equilateral triangles with side lengths of ∼4 Å and ∼5 Å featuring a ditrigonal cavity in the mica surface.14 Parameters of the muscovite unit cell were taken from the X-ray reflectivity study by Schlegel et al.15 Although the name “muscovite” refers to the natural K+ form of this clay mineral, “K+-muscovite” is used instead further in the text for clarity as opposed to “CP+modified muscovite” referring to the cleaved surface of muscovite with adsorbed CP+ ions. The simulation cell consists of the two muscovite layers of a total thickness of 20.059 Å separated from the next two layers through a cleavage along the plane of the interlayer K+ ions and pulling apart the cleaved surfaces to 100 Å (Figure 1). Hence, whereas the interlayer space between the two muscovite layers within the simulation box is identical with the bulk muscovite and contains K+ ions at a coverage corresponding to 2 K+ ions per unit cell area (Auc; Auc ) 46.72 Å2), their two external surfaces have a coverage of 1 K+/Auc as a result of cleavage. To proceed with simulations of CPCl at the muscovite-water interface, the (14) Kuwahara, Y. Phys. Chem. Miner. 2001, 28, 1. (15) Schlegel, M. L.; Nagy, K. L.; Fenter, P.; Cheng, L.; Sturchio, N. C.; Jacobsen, S. D. Geochim. Cosmochim. Acta 2006, 70, 3549.
Meleshyn
K+ ions near one of the cleaved muscovite surfaces were moved to positions 7.5 Å away from the surface. 278 water molecules corresponding to water coverage of ∼35 H2O/Auc were randomly distributed in a slab of thickness 30 Å near the same cleaved muscovite surface within the simulation cell enclosing eight unit cells and having lateral dimensions of ∼20.75 Å by ∼18.01 Å. A simulation cell with such lateral dimensions has been shown to be representative of the macroscopic mineral system and not influenced by the artificial long-range symmetry of the imposed periodic lattice.16 Considering that alkyl chains of CP+ ions at the mica-water interface, adsorbed either as monomers or as incomplete aggregates, preferentially adopt vertical orientation,10 these lateral dimensions were preserved for simulations involving CP+ ions. To simulate CP+-modified muscovite, CP+ coverages of 0.125 · n CP+/Auc, n∈[1, 8], corresponding to one up to eight CP+ ions per simulation cell were considered. The maximum coverage of 1 CP+/Auc represents the coverage necessary to compensate the negative layer charge at the cleaved muscovite surface. In the initial configuration, the smallest distance between the muscovite surface and an aromatic carbon atom of a CP+ ion was equal to 3.9 Å, whereas the distance between the muscovite surface and a nitrogen atom of a CP+ ion was equal to 6.25 Å. The alkyl chains of CP+ ions had all-trans conformations and were vertically oriented. Two series of simulations with the structure of the adsorbed CP+ layer corresponding to incomplete monolayer and bilayer aggregates were carried out. In the first series of simulations, CP+ ions (one up to eight) were uniformly distributed on the mica surface within the simulation cell with alkyl chains pointing away from the muscovite surface. In the second series of simulations, CP+ ions had the same lateral positions as in the first series of simulations. However, only three of them had the same orientations as well, whereas the remaining CP+ ions (one up to five) were rotated by 180° with alkyl chains pointing to the mica surface. For these CP+ ions, the largest distance between the muscovite surface and aromatic carbon atoms was equal to 36.1 Å. Furthermore, Cl- ions at coverages equivalent to those of CP+ ions were positioned 15 Å above the cleaved surface. The water content of the simulation box (278 H2O) was chosen on the basis of the following considerations. First, the thickness of the water film on K+-muscovite at this content equals ∼25 Å,17 which makes certain that the adsorbed CP+ ions (characterized by the end-to-end distance of ∼22.8 Å in the all-trans conformation) are within the water film at all considered CP+ coverages. This is additionally ensured by an increase of the water film thickness in response to an increase of CP+ content within it and by gauche conformations of alkyl chains of CP+ ions leading to a decrease of the end-to-end distance. Second, a comparison of preliminary simulations at water contents of 278, 370, and 463 H2O per simulation cell at CP+ coverages of 0.375 and 0.75 CP+/Auc has revealed no dependence of the interfacial structure on an increase of water content beyond 278 H2O per simulation cell (except for the accordingly increased water film thickness). This is in agreement with experimental observations that the structure of the inner layer of quaternary ammonium surfactant aggregates adsorbed on mica does not change upon retraction from bulk solution.1,8,18,19 Last but not least, an increase of water content from 278 to 463 H2O per simulation cell resulted in an increase of computing time by a (16) Skipper, N.; Chang, F.-R.; Sposito, G. Clays Clay Miner. 1995, 43, 285. (17) Meleshyn, A. J. Phys. Chem. C 2008, 112, 14495. (18) Fujii, M.; Li, B.; Fukada, K.; Kato, T.; Seimiya, T. Langmuir 2001, 17, 1138. (19) Mellott, J. M.; Hayes, W. A.; Schwartz, D. K. Langmuir 2004, 20, 2341.
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Figure 2. Atomic density profiles for (a) water oxygen and (b) water hydrogen (color box with scale atoms/Å3) as functions of the distance from the muscovite surface (abscissa) and CP+ coverage (ordinate). The lower panel of each graph shows results for monolayer arrangement of CP+ ions at coverages of 0.125 · n CP+/Auc, n∈[1, 8], whereas the upper panel shows results for bilayer arrangement of CP+ ions at coverages of 0.125 · n, n∈[4, 8]. Water coverage equals ∼35 H2O/Auc in all simulated systems. Data for K+-muscovite with a coverage of 0 CP+/Auc (1 K+/Auc) are shown in lower panels for comparison. A bar along the abscissa in (a) or (b) shows the extension of the water film between mica-water and water-air interfaces in a simulated structure as defined by the thickness of the hydrogen distribution. The simulated water film thickness increases upon an increase of CPCl content within it, which is represented by the correspondingly increasing bar thicknesses (along abscissa).
factor of ∼3 on an Intel Core 2 Quad 2.4 GHz CPU (a single simulation run with 278 H2O and one up to eight CP+ ions per simulation cell took two weeks to three months of CPU time). These considerations eventually led to the above choice of water content in the simulation cell. Accordingly, the concentration of CP+ ions in the interfacial solution within the simulation cell varies between 0.2 and 1.6 M for 1 and 8 CP+ ions, respectively. This concentration appears to be much higher than the cmc of 0.9 mM for CPCl in the bulk solution.4 However, as a result of the favorable interaction with the surface, the local concentration of surfactants on the surface strongly exceeds the bulk concentration.13 Indeed, the charge neutralization of mica due to the adsorption of quaternary ammonium surfactant ions, which results in the surface coverage of one surfactant ion per unit cell area (1 CP+/Auc or local concentration of 1.6 M in the present study), occurs at bulk concentrations of ∼1/100 of the cmc.8 Hence, the chosen content of the interfacial solution in the simulation cell can be considered as a reasonable approximation of that in the interfacial solution of quaternary ammonium surfactants in equilibrium with the bulk solution. This justifies the applicability of the simulated interfacial structures for a comparison with those suggested or observed in previous experimental studies of quaternary ammonium surfactant adsorption to mica. The bond lengths, angles, and partial charges on the atoms of a CP+ ion (modeled according to an all-atom approach) were taken as described elsewhere.20 All bond lengths within a CP+ ion and the bond angles within its headgroup, consisting of the pyridinium ring and R-methylene, as well as the other fourteen CH2 groups and the terminal CH3 group were not allowed to change during the simulations. This approximation was made under the assumption that the corresponding changes can be neglected when studying the dependence of the interfacial structure on CP+ coverage and CP+ aggregation type. Conformational changes of a simulated CP+ ion take place only because of the changes in bond and torsion angles formed by the atoms of at least two neighboring groups. The Monte Carlo simulations based on the Metropolis acceptance/rejection rule were carried out in canonical, constant(20) Meleshyn, A.; Bunnenberg, C. J. Phys. Chem. B 2006, 110, 2271.
NVT ensemble with the temperature fixed at 298 K. Threedimensional periodic boundary conditions were applied to the simulation cell to model the interface between a muscovite platelet and water. Mineral layers were considered as rigid bodies with atomic charges assigned according to Skipper et al.16 The imposed rigidity of the mica layers is a reasonable approximation considering that mineral atoms show relaxations of
