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Cetylpyridinium Aggregates at the Montmorillonite- and Muscovite-Water Interfaces: A Monte Carlo Study of Surface Charge Effect Artur Meleshyn :: :: Center for Radiation Protection and Radioecology (ZSR), Leibniz Universitat Hannover, Herrenhauser Strasse 2, 30419 Hannover, Germany Received January 7, 2009. Revised Manuscript Received February 24, 2009 Monte Carlo simulations of the interface between the external surface of montmorillonite and aqueous cetylpyridinium chloride (CPCl) solution at ambient conditions are reported and compared with the preceding simulation study of muscovite. Simulation results reveal that a segregation of inorganic ions into the regions near the montmorillonitewater and bilayer-water interfaces leads to a nearly complete displacement of water molecules from the intermediate region of the bilayer aggregate. Such segregation does not occur for muscovite because of a considerably higher surface concentration of inorganic cations compensating its mineral charge. The presence of hydrated inorganic ions in the interfacial region containing the aliphatic part of the bilayer aggregate on muscovite leads to a more compact aggregate structure in agreement with a previously experimentally observed effect of added electrolyte. It also results in a lateral segregation of hydrophilic and hydrophobic clusters in this region in agreement with earlier experimental observations of striped surfactant structures on the muscovite surface. A configuration of the surfactant adsorption complex is found to strongly depend on the water content at the mineral-aggregate interface and suggested to be controlled by the types of inorganic counter- and co-ions present there. A transformation between characteristic configurations of the surfactant adsorption complexes is proposed as an explanation for previous experimental observations of the slow secondary surfactant adsorption.
1. Introduction Montmorillonite and muscovite are two clay minerals of high environmental and industrial importance characterized by very similar structures of the basal surface and, at the same time, by significant differences in the charges and crystallinities of their mineral layers. The first difference makes montmorillonite and muscovite very good candidates for studying the effects of the mineral surface charge on the interfacial solution structure and on the surfactant aggregation at the mineral-solution interface. Since the negative surface charges of montmorillonite and muscovite, contrary to those of high-purity silica, are compensated by exchangeable surface cations, a comparison between these two minerals and silica can greatly facilitate the interpretation of experimental observations related to the surface charge effects. However, the low crystallinity of montmorillonite and its ability to adsorb surfactants in the interlayer spaces render difficult or impossible studies of the adsorbed structure on its external surface with atomic force microscopy (AFM), which has been successfully applied for muscovite1-5 and silica6-9 and provided the greatest advance in the study of surfactant adsorption at the mineral-solution interface.10 (1) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (2) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602. (3) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160. (4) Davey, T. W.; Warr, G. G.; Almgren, M.; Asakawa, T. Langmuir 2001, 17, 5283. (5) Mellott, J. M.; Hayes, W. A.; Schwartz, D. K. Langmuir 2004, 20, 2341. (6) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548. (7) Atkin, R.; Craig, V. S. J.; Biggs, S. Langmuir 2000, 16, 9374. (8) Liu, J.-F.; Min, G.; Ducker, W. A. Langmuir 2001, 17, 4895. (9) Atkin, R.; Craig, V. S. J.; Biggs, S. Langmuir 2001, 17, 6155. (10) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103, 244.
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These studies have revealed that quaternary alkyl ammonium surfactants adsorbed on the silica or muscovite surface aggregate in a periodic structure resembling meandering stripes with a characteristic period, morphology, and stability of aggregates depending on type of mineral surface, electrolyte concentration, length of alkyl chain, type of headgroup, and type of counterion. However, quantitative analysis and interpretation of these effects are strongly complicated by ambiguities with respect to possible aggregate morphologies inherent to the AFM data as well as by the speculative nature of the surfactant layer adsorbed to the mineral-aggregate interface within the aggregated structure.10 This shows a need for an additional source of the data on the adsorbed structure at the mineral-aggregate interface in extension of the available experimental methods. Simulation studies have been successfully used to obtain detailed structural information about aggregates of quaternary alkyl ammonium surfactants either intercalated in the interlayer spaces of muscovite11 and montmorillonite12,13 or adsorbed on the cleaved muscovite surface.14 To the author’s knowledge, however, the aggregation structure of these surfactants at the silica-water interface or at the interface between aqueous solution and the external surface of montmorillonite has not been theoretically studied yet. The present study aims to partially fill this gap and to provide a detailed discussion on the structure of the water film and cetylpyridinium (CP+) aggregates on the surface of Na+-montmorillonite using Monte Carlo (MC) simulations. Furthermore, a comparison of the simulation results (11) (12) 2700. (13) (14)
Heinz, H.; Castelijns, H. J.; Suter, U. W. J. Am. Chem. Soc. 2003, 125, 9500. Tambach, T. J.; Boek, E. S.; Smit, B. Phys. Chem. Chem. Phys. 2006, 8, Meleshyn, A.; Bunnenberg, C. J. Phys. Chem. B 2006, 110, 2271. Meleshyn, A. Langmuir 2009, 25, 881.
Published on Web 4/7/2009
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for CP+-aggregates adsorbed on the external montmorillonite surface with those reported in the preceding study14 of CP+-aggregates adsorbed on the cleaved muscovite surface aims to provide the basis for a quantitative analysis of the surface charge effect on the interfacial structure on the atomic scale. The next section gives a description of the simulation details, whereas the corresponding section of the preceding simulation study14 gives an extensive discussion of the underlying system model.
2. Simulation Details A mineral layer with the formula unit Na0.375(Al1.625 + Fe3+ 0.125Mg0.25)(Si3.875Al0.125)O10(OH)2 corresponds to a Na montmorillonite of Wyoming-type and consists of 2 tetrahedral sheets with 1 out of 32 Si atoms substituted by Al, which sandwich an octahedral sheet with 2 out of 3 octahedrally coordinated positions occupied by Al, Fe3+, and Mg. In a tetrahedral sheet, 6 basal oxygen atoms bridging Si and Al atoms of the same hexagonal ring are in the vertices of the 2 equilateral triangles with side lengths of ∼4.0 A˚ and ∼4.9 A˚ featuring a ditrigonal cavity in the montmorillonite surface. Coordinates of layer atoms were calculated according to the method of Smoliar-Zviagina.15 Tetrahedral substitutions were equally distributed between the two mineral half-layers according to the requirement of their uniform distribution in the modeled mineral layer. Octahedral substitutions were uniformly distributed between cis octahedral sites in accordance with the mineral layer symmetry C2.16 The simulation cell of thickness of 106.6 A˚ contains one montmorillonite layer of thickness16 of 6.6 A˚ (starting from the dehydrated montmorillonite with the layer spacing of 9.6 A˚, the thickness of its Na+-containing interlayer space was increased from 3 A˚ to 100 A˚ to prepare the simulation cell). This ensures that, as a result of the application of three-dimensional periodic boundary conditions to the simulation cell to model the interface between the external basal surface of a montmorillonite platelet and surfactant aggregates, two neighboring montmorillonite layers are separated by 100 A˚ (compare with Figure 1 in the preceding study14). Whereas an interlayer space of a Na+montmorillonite particle contains Na+ ions at a coverage corresponding to 0.75 Na+ ions per unit cell area (Auc; Auc = 46.36 A˚2), each of its two external surfaces have a coverage of 0.375 Na+/Auc. To simulate CP+-modified montmorillonite, referring to the external surface of montmorillonite with adsorbed CP+ ions in the present study, CP+ coverages of 0.125n CP+/Auc, n∈[1, 8], corresponding to one to eight CP+ ions per simulation cell were considered in accordance with the preceding study14 of CP+-modified muscovite. The maximum coverage of 1 CP+/Auc is a factor of 2.66 higher than that necessary to compensate the negative surface charge at the external montmorillonite surface and corresponds to 133% of the cationic exchange capacity (CEC) of Na+-montmorillonite. To proceed with simulations of CPCl at the montmorillonitewater interface, the Na+ ions near one of the external surfaces of the modeled montmorillonite platelet were moved to positions 7.5 A˚ away from the surface. As discussed in the preceding paper,14 278 water molecules corresponding to water coverage of ∼35 H2O/Auc were randomly distributed in a slab of thickness of 30 A˚ near the same surface within the simulation cell enclosing 8 unit cells and having lateral dimensions of ∼20.65 A˚ by ∼17.96 A˚. A simulation cell with such lateral dimensions has been shown to be representative of the macroscopic mineral system and not to be influenced by the artificial long-range symmetry of the (15) Smoliar-Zviagina, B. B. Clay Miner. 1993, 28, 603. (16) Tsipursky, S. I.; Drits, V. A. Clay Miner. 1984, 19, 177.
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imposed periodic lattice.17 In the initial configuration, the smallest distance between the montmorillonite surface and an aromatic carbon atom of a CP+ ion was equal to 3.9 A˚, whereas the distance between the montmorillonite surface and a nitrogen atom of a CP+ ion was equal to 6.25 A˚. The alkyl chains of CP+ ions had all-trans conformations and were assigned a vertical orientation.14 Two series of simulations with the structure of the adsorbed CP+ layer corresponding to monolayer and bilayer arrangements were carried out. In the first series of simulations, CP+ ions (1 up to 8) were uniformly distributed on the montmorillonite surface within the simulation cell with alkyl chains pointing away from the surface. In the second series of simulations, CP+ ions had the same lateral positions as in the first series of simulations. However, only 3 of them had the same orientations as well, whereas the remaining CP+ ions (1 up to 5) were rotated by 180° with alkyl chains pointing to the montmorillonite surface. For these CP+ ions, the largest distance between the muscovite surface and aromatic carbon atoms was equal to 36.1 A˚. Furthermore, Cl- ions at coverages equivalent to those of CP+ ions were positioned 15 A˚ above the surface. 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.13 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. Conformational changes of a simulated CP+ ion take place only due to 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 NVT ensemble with a temperature fixed at 298 K. Mineral layers were considered as rigid bodies with atomic charges assigned according to Skipper et al.17 and were not allowed to move as prescribed by the chosen ensemble. During the simulations, displacement and rotation moves for water molecules and displacement moves for K+, Cl-, and CP+ ions were allowed. To simulate the conformational changes of CP+ ions, the configurational-bias Monte Carlo (CBMC) method18,19 was applied. In a CBMC move, a randomly chosen CP+ ion was regrown group by group (with all atoms within a group treated explicitly) starting from a randomly chosen group and proceeding in the headgroup (C5H5NCH2) or terminal group (CH3) direction. Trial orientations of grown groups were generated according to the algorithm described by Vlugt et al.19 The TIP4P model20 was used for water. To calculate the potential energies of interactions between ions, water molecules, and mica layers, the OPLS-AA force field equations and potential parameters21 and the procedure by Skipper et al.17 for mineral layers were applied. Intramolecular bending, torsion, and nonbonded interactions for CP+ ions were calculated as described in detail elsewhere.13 The cutoff distance of 9 A˚ and the all-image convention were adopted for the short-range interactions, and the Ewald technique as modified for systems with slab geometry22 was applied to handle the long-range Coulomb interactions. During the first ∼3 105 MC moves, only water (17) Skipper, N.; Chang, F.-R.; Sposito, G. Clays Clay Miner. 1995, 43, 285. (18) Frenkel, D.; Smit, B. Understanding Molecular Simulation: From algorithms to applications; Academic Press, San Diego, 1996. (19) Vlugt, T. J. H.; Martin, M. G.; Smit, B.; Siepmann, J. I.; Krishna, R. Mol. Phys. 1998, 94, 727. (20) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. J. Chem. Phys. 1983, 79, 926. (21) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. J. Am. Chem. Soc. 1996, 118, 11225. (22) Yeh, I.-C.; Berkowitz, M. L. J. Chem. Phys. 1999, 111, 3155.
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Figure 1. Atomic density profiles for (a) water oxygen and (b) water hydrogen (color box is scaled in atoms/A˚3) as functions of the distance from the montmorillonite surface (abscissa) and CP+ coverage (ordinate). The lower panel of each graph shows results for the monolayer arrangement of CP+ ions at coverages of 0.125n CP+/Auc, n∈[1, 8], whereas the upper panel shows results for the bilayer arrangement of CP+ ions at coverages of 0.125n, n∈[4, 8]. The water coverage equals ∼35 H2O/Auc in all simulated systems. Data for Na+-montmorillonite with a coverage of 0 CP+/Auc (1 Na+/Auc) are shown in the lower panels for comparison. A bar along the abscissa in (a) or (b) shows the extension of the water film between montmorillonite-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 corresponding increasing bar thicknesses (along the abscissa).
molecules were allowed to move, ensuring that the structure of the adsorbed water film is adjusted to the ion content and that the ions become hydrated before the position of an ion is changed in a MC move or the conformation of a CP+ ion is changed in a CBMC move. A mean of ∼3.3 108 MC moves in a simulation run with a mean of ∼5.9 106 MC moves for sampling of potential energies and structural properties (atomic density profiles, lateral atomic densities, and radial distribution functions) were made in the two simulation series. The atomic density profile represents a distribution of atomic positions weighted by their residence probabilities in the slices of interfacial solution with a thickness of 0.05 A˚, normalized by the volume of these slices and projected on a plane perpendicular to the montmorillonite surface. The lateral atomic density represents a distribution of atomic positions weighted by their residence probabilities within a specified interfacial region, normalized by the simulation cell area and projected on the montmorillonite surface. Radial distribution functions were calculated according to the definition by Allen and Tildesley.23
3. Results and Discussion 3.1. Structure of Water Film in the Direction Normal to the Surface. The effect of CP+ adsorption to the montmorillonite-water interface on the interfacial water structure can be analyzed with help of the data presented in Figure 1. This figure reveals that upon a gradual increase of CP+ coverage the local water density decreases gradually in the interfacial region 3 A˚ < z < 25 A˚, where z is the distance from the basal surface of montmorillonite. As a result of this decrease, the water layering24 characteristic for the mineral-water interface is strongly depressed at the highest simulated coverage of 1 CP+/Auc. Furthermore, the local water density drops below half of its bulk density in the range 10 A˚ < z < 23 A˚ or from 20 A˚ < z < 25 A˚ for bilayer or monolayer arrangements of CP+ ions, respectively (Figure 2). Moreover, for the bilayer arrangement it drops below one-tenth of the bulk density in the interfacial (23) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon: Oxford, 1987. (24) Israelachvili, J. N.; Pashley, R. M. Nature (London) 1983, 306, 249.
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region 14 A˚ < z < 19 A˚ at a coverage of 1 CP+/Auc (Figure 1, upper panels; Figure 2). Figure 1a shows further that no strongly adsorbed water layer is formed on montmorillonite at ∼1.7 A˚ from the surface. This is different than for muscovite14,25 and is due to a factor of 8 lower density of Al/Si tetrahedral substitutions in the simulated montmorillonite as compared to muscovite. The first water layer is adsorbed at 2.6-2.7 A˚ from the montmorillonite surface, which is the position of the second adsorbed water layer on muscovite.14,25 The position of the corresponding first peak of the water hydrogen density at 1.7-1.8 A˚ (Figure 1b) indicates that water molecules of the first adsorbed layer are singly hydrogen-bonded to basal oxygens of montmorillonite. The number of water molecules adsorbed within the first water layer varies in the range 2.3-2.9 H2O/Auc for CP+-modified montmorillonite as compared to 3.3 H2O/Auc for Na+-montmorillonite. This is again very different to muscovite,14 for which the number of water molecules in the same interfacial region with z e 3.2 A˚ increases from 3.2 H2O/Auc at 0.125 CP+/Auc to 4.0 H2O/Auc at 1 CP+/Auc as compared to 3.4 H2O/Auc for K+muscovite. As a further consequence of the lower tetrahedral (as well as overall) charge density in montmorillonite as compared to muscovite, the second water layer is adsorbed at 5.5-5.8 A˚ (coverages below 0.5 CP+/Auc in Figure 1a), whereas the corresponding water layer is adsorbed at ∼4.2 A˚ for CP+-modified muscovite and at ∼4.7 A˚ for K+-muscovite.14 Upon an increase of the CP+ coverage above 0.375 CP+/Auc, the local water density within the second water layer adsorbed on montmorillonite drops below half of its bulk value (Figures 1 and 2). As can also be seen from Figures 1 and 2, the local water densities in water films for monolayer and bilayer arrangements at a coverage of 1 CP+/Auc are comparable to that on Na+montmorillonite for z values above ∼28 A˚ and ∼36 A˚, respectively. A comparison of the data for the interaction energies between water monomers and the entire water film for Na+- and CP+-modified montmorillonites in Figure 8a,b leads to the same conclusion. Indeed, these energies are comparable to the (25) Meleshyn, A. J. Phys. Chem. C 2008, 112, 14495.
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Figure 2. Atomic density profiles for water oxygen at a water coverage of ∼35 H2O/Auc as functions of the distance from the surface of Na+-montmorillonite (1 Na+/Auc) and CP+-modified montmorillonite (for monolayer and bilayer arrangements at a coverage of 1 CP+/Auc). The atomic density for water oxygen in bulk water is shown for comparison.
Figure 3. Average potential energies (kJ/mol per simulation box) of the interaction between water monomers and the water film (one symbol for one water molecule) related to their average distance z (A˚) from the surface of (a) Na+-montmorillonite (1 Na+/Auc), (b) monolayer arrangement (1 CP+/Auc), and (c) bilayer arrangement (1 CP+/Auc). The average energy of -84.26 kJ/mol of the interaction between water monomers and the water environment in bulk TIP4P water results in the value of -42.13 kJ/mol for the internal energy of TIP4P water.20
corresponding value of -84.26 kJ/mol in bulk TIP4P water20 in the regions 8 A˚ < z < 21 A˚ on Na+-montmorillonite (Figure 3a) and 28 A˚ < z < 33 A˚ for monolayer arrangement on CP+-montmorillonite (Figure 3b). Increases of these energies at the interface with air, occurring as a result of an increasing number of dangling OH bonds, are very similar for Na+montmorillonite (z > 21 A˚) and CP+-montmorillonite (z > 33 A˚) as well. According to energy data in Figure 3c, the perturbation of water structure for the bilayer arrangement is significant in the region with z values below 36-37 A˚, which agrees with the estimate made above using local water densities. Furthermore, it can be seen in Figure 3 that adsorption of water molecules within the first three layers on the montmorillonite surface (z < 6 A˚) weakens their interaction with the other water molecules. This increase becomes much stronger if water molecules are participating in the first hydration shells of Na+ ions, Langmuir 2009, 25(11), 6250–6259
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as can be seen in Figure 3a for inner-sphere and outer-sphere adsorption complexes of Na+ ions (energies above 0 and -40 kJ/mol, respectively, in Figure 3), as well as Cl- and CP+ ions (energies above -40 kJ/mol in Figure 3b,c). 3.2. Structure of CP+ Aggregates in the Direction Normal to the Surface. The changes in the interfacial water structure as discussed above can be understood with the help of the data on density distributions of aromatic and aliphatic carbons of CP+ ions in Figure 4. This figure reveals that the bilayer-water interface is positioned at 35-36 A˚ (Figure 4a, upper panel), which explains the above observation of an increase of the local water density in the interfacial region beyond ∼36 A˚ for the bilayer CP+ arrangement on montmorillonite up to values comparable with those on Na+-montmorillonite. The position of the monolayer-water interface cannot be unambiguously determined within the simulated range of CP+ coverages. However, a consideration of the content of aliphatic carbons on the montmorillonite surface as presented in Figure 5 in addition to the data on local water density in Figures 1 and 2 suggests that the presence of the total of one aliphatic carbon per unit cell area in the region 28 A˚ < z < 35 A˚ (Figure 5) at a coverage of 1 CP+/Auc does not lead to a significant perturbation of the water structure in this region. This conclusion is also supported by the data on interaction energies between water monomers (Figure 3b). It can be then estimated on the basis of the data in Figures 1, 4b, and 5 that the monolayer-water interface on montmorillonite is positioned at z ≈ 28 A˚ at a coverage of 1 CP+/Auc. Furthermore, it can be seen from Figure 4 that the gradual decrease of the local water density in the interfacial region with z e 10 A˚ can be attributed mainly to the increasing content of aromatic carbons at the montmorillonite-water interface. Its strong decrease in the interfacial region with z > 10 A˚, on the contrary, can be assumed to result from an increase of the content of aliphatic carbons in this region as documented in Figure 5. A comparison of the data for a coverage of 1 CP+/Auc in Figures 2 and 5 indicates, however, that the latter assumption does not suffice to explain the strong drop in the local water density discussed for Figures 1 and 2. Indeed, the local water density for the bilayer arrangement is significantly lower than that for the monolayer arrangement in the region 11 A˚ < z < 20 A˚ (Figure 2), whereas the content of aliphatic carbons for the bilayer arrangement shows significant excess over that for the monolayer arrangement in the region 16 A˚ < z < 23 A˚ (Figure 5), which is shifted with respect to the former region by 3-5 A˚. Moreover, the latter excess has a maximum at z ≈ 20 A˚, where the local water densities are about the same for the two arrangements. Hence, other factor(s) should be responsible for the discussed strong drop of the local water density. In order to elucidate this, the two remaining constituents of the montmorillonite-water-CPCl system, Na+ and Cl- ions, should be taken into consideration. Their equilibrium positions in the interfacial solution dependent on the CP+ coverage are presented in Figure 6. This figure reveals that a segregation of inorganic ions into two layers takes place for the bilayer arrangement in accordance with the two layers of positively charged head groups of CP+ ions (Figures 4a, 6a, upper panels): (1) Na+ ions compensating the negative charge of montmorillonite as well as Cl- ions associated with the head groups of CP+ ions adsorbed to the montmorillonite-water interface, and (2) Cl- ions associated with the head groups of CP+ ions at the bilayer-water interface. The first layer of inorganic ions is restricted to the interfacial region with z < 11 A˚ at CP+ coverages above 0.625 CP+/Auc and the second one to the DOI: 10.1021/la804311w 6253
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Figure 4. Atomic density profiles for (a) aromatic and (b) aliphatic carbon atoms of CP+ ions (color box is scaled in atoms/A˚3) as functions of the distance from the montmorillonite surface (abscissa) and CP+ coverage (ordinate). A black bar along the abscissa in (a) or (b) shows the extension of the water film between montmorillonite-water and water-air interfaces in a simulated structure. Other details as in Figure 1.
Figure 5. Cumulative contents of aliphatic carbons as functions of the distance z (A˚) from the montmorillonite surface for the monolayer arrangement at coverages of 0.25 3 n CP+/Auc, n∈[1, 4], and the bilayer arrangement at a coverage of 1 CP+/Auc.
region with z > 23 A˚ at all simulated CP+ coverages. For the monolayer arrangement, no such segregation occurs, and the positions of inorganic ions are primarily limited to the interfacial region with z < 20 A˚ (with at most 0.125 Cl-/Auc or 0.125 Na+/Auc beyond this region). For this reason, at a coverage of 1 CP+/Auc, the region 11 A˚ < z < 20 A˚ contains no Na+ and Cl- ions for the bilayer arrangement as opposed to 0.25 Na+/Auc and 0.5 Cl-/Auc for the monolayer arrangement. Since the two layers of head groups of CP+ ions are separated by ∼21 A˚ with the positions of aromatic carbons limited to the regions z < 6 A˚ and z > 27 A˚, such a segregation and the increasing concentration of Cl- ions near the bilayer-water interface (Figure 6a, upper panel) lead to a nearly complete displacement of water molecules out of the discussed region 11 A˚ < z < 20 A˚ and into the hydration shells of these Cl- ions. This conclusion obtains further support from a comparison of the water content within the aliphatic part of bilayer aggregates formed at 1 CP+/Auc on montmorillonite and muscovite, for which a similar segregation of inorganic ions has been observed in the preceding study.14 At this CP+ coverage, the boundaries of the two segregated layers are defined by the positions of Clions at z ≈ 10 A˚ and z ≈ 25 A˚ for montmorillonite or at z ≈ 14 A˚ and z ≈ 22 A˚ for muscovite. The difference in the upper boundary of the first segregated layer of Cl- ions can be explained by the adsorption of CP+ ions ∼5 A˚ farther from the mineral surface for muscovite as compared to montmorillonite (compare Figure 4 in the preceding study14 and Figure 4 in the present study). Although only 0.125 Cl-/Auc are 6254
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positioned at z ≈ 14 A˚ on muscovite, the values of ∼7 and ∼25 for coordination numbers in, respectively, the first and the second hydration shells of a Cl- ion (Figure 7) result in additional 0.875 up to 3.125 H2O/Auc in the region 11 A˚ < z < 20 A˚. This additional water suffices to maintain a contiguous water network throughout the bilayer aggregate, which results in a closer approach of 0.125 Cl-/Auc of the second segregated layer of Cl- ions to the surface of muscovite as compared to montmorillonite and in a further increase of water content in the region 11 A˚ < z < 20 A˚. In agreement with the above reasoning, the water content in this region equals 2.1 and 6.6 H2O/Auc for montmorillonite and muscovite, respectively. Figure 8 visualizes interfacial structures in the region 11 A˚ < z < 20 A˚ at a coverage of 1 CP+/Auc on montmorillonite as well as on muscovite and demonstrates the relative abundance of water molecules and inorganic ions for the monolayer arrangement (Figure 8a) as compared to the bilayer arrangement (Figure 8b) on montmorillonite. Indeed, for the bilayer arrangement only a small portion of this interfacial region remains available to water because of its complete depletion with respect to inorganic ions and a related aggregation of aliphatic chains of CP+ ions. This aggregation is characterized by a strong decrease of the conformational order of the aliphatic chains and by an increase of the concentration of terminal methyl groups within or near the lower boundary of the region 11 A˚ < z < 20 A˚. For the monolayer arrangement, however, a different mode of aggregation occurs as a result of the formation of a honeycomb-like structure consisting of inorganic ions and their (incomplete) hydration shells interconnected through a hydrogen-bonded water network (Figure 8a). A large part of water molecules in this water network accepts and donates a total of four hydrogen bonds due to a (partial) repetition of the thin interfacial layers containing honeycomb-like ordered water molecules in the direction normal to the mineral surface. As a result of this aggregation mode, triads of the highly conformationally ordered middle parts of aliphatic chains of CP+ ions (consisting of seven up to eight methylene groups) can be formed within the honeycomb-like holes in the discussed interfacial region. It can be further concluded from Figure 8 that monolayer and bilayer aggregates of CP+ ions on the montmorillonite surface, similar as those on the muscovite surface,14 are incomplete at the highest simulated coverage of 1 CP+/Auc. This is indicated in their structures by a presence of voids (partially) filled out by Langmuir 2009, 25(11), 6250–6259
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Figure 6. Atomic density profiles for (a) Cl- and (b) Na+ ions (color box is scaled in atoms/A˚3) as functions of the distance from the montmorillonite surface (abscissa) and CP+ coverage (ordinate). A black bar along the abscissa in (a) or (b) shows the extension of the water film between montmorillonite-water and water-air interfaces in a simulated structure. Other details as in Figure 1.
Figure 7. Radial distribution functions and coordination numbers for water oxygens around Cl- ions for the bilayer CP+ arrangement on muscovite and montmorillonite at a coverage of 1 CP+/Auc.
water and each having an area larger than that occupied by an aliphatic chain within the respective aggregate. The lateral pattern and the conformational order of the aliphatic parts of monolayer aggregates on montmorillonite and muscovite are very similar in accordance with the similar contents of water and inorganic ions. However, they differ strongly for bilayer aggregates on these minerals, as can be seen by a comparison of Figure 8b and c. The formation of a contiguous water cluster within the aliphatic part of the bilayer aggregate on muscovite is another manifestation of the effect of Cl- ions as discussed above. This observation disputes the suggestion by Velegol et al.6 that the difference between the peak-to-peak distances as measured for aggregates arranged as meandering stripes on mica and silica is alone due to a larger edge-to-edge aggregate spacing on silica. Albeit the lateral length of the simulated system is by a factor of 2.5-5 smaller than characteristic peak-to-peak distances measured in AFM studies of self-assembly of quaternary alkyl ammonium surfactants on mineral surfaces,1-9 the effect of the presence of inorganic ions in the aliphatic part of the bilayer aggregate as demonstrated in Figure 8b,c is obvious: It leads to a more compact bilayer aggregate. This conclusion agrees very well with the experimental observation of Velegol et al.6 that the peak-to-peak distance on silica decreases from 10 ( 1 nm without added salt to 8 ( 1 nm with added 10 mM KBr. Furthermore, it explains the fact that the latter value is very similar to the peak-to-peak distance of 7 nm on muscovite Langmuir 2009, 25(11), 6250–6259
without added salt,3 as K+ ions, displaced from the muscovite surface in the course of adsorption of quaternary alkyl ammonium surfactants, slowly diffuse (partially in pairs with counterions) through the adsorbed aggregate and into the bulk solution.27 A higher concentration of inorganic ions can possibly increase the compaction degree of a bilayer aggregate (micelle size) to some extent, and very likely, it can lead to a higher water content and to a larger lateral dimension of the hydrophilic cluster (edge-to-edge spacing) in the interfacial region containing the aliphatic part of the bilayer aggregate. This suggests that a change in the peak-to-peak distance as a result of the change in the electrolyte concentration or mineral layer charge can be contributed by changes in both micelle size and edge-to-edge spacing. The above reasoning also suggests an alternative explanation to the observation3 of a striped structure for dodecyltrimethylammonium bromide (DTAB) and a flat bilayer structure for cetyltrimethylammonium bromide (CTAB) on muscovite, which is inconsistent with a packing parameter argumentation as discussed by Ducker and Wanless.3 Indeed, the striped structure for DTAB can be explained by a reduced segregation of Br- ions into the two interfacial regions near mineral-water and bilayer-water interfaces as a result of the smaller chain length and correspondingly the smaller thickness of the DTAB bilayer aggregate as compared to the CTAB bilayer aggregate. Such reduced segregation as well as higher Br- concentration in the interfacial solution (see discussion by Ducker and Wanless3) apparently leads to the presence of hydrated Br- ions in the aliphatic part of the DTAB bilayer aggregate, which is not the case for the CTAB bilayer aggregate. Another interesting implication of the structure in Figure 8c (albeit on the small scale) is that the aggregates arranged as meandering stripes on the muscovite surface need not necessarily be of a cylindrical type as suggested by Manne and Gaub.1 Actually, meandering stripes observed in AFM studies1-9 may just represent stripes of a bilayer aggregate formed as a result of the lateral segregation of hydrophilic and hydrophobic clusters in the interfacial region containing the aliphatic part of the bilayer aggregate. Their transformation2 to flat bilayers within up to 24 h can then be explained by a diffusion of the inorganic (26) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Design 2000, 14, 123. (27) Chen, Y. L.; Chen, S.; Frank, C.; Israelachvili, J. J. Colloid Interface Sci. 1992, 153, 244.
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Figure 9. Cumulative contents of nitrogens as functions of the distance z (A˚) from the montmorillonite surface for the monolayer arrangement at coverages of 0.25n CP+/Auc, n∈[1, 4], and the bilayer arrangement at coverages of 0.875 and 1 CP+/Auc.
Figure 8. Snapshots of equilibrium interfacial structures in the region 11 A˚ < z < 20 A˚ at a coverage of 1 CP+/Auc for (a) the monolayer arrangement and (b) the bilayer arrangement on montmorillonite, as well as (c) the bilayer arrangement on muscovite viewed normal to the (100) plane. The structure in (c) represents a snapshot of the averaged interfacial structure in Figure 9b in the preceding study.14 Ball and stick colors: gray (Na+), green (C), red (O), white (H), turquoise (N), gold (Cl). Visualizations were made with the help of the MOLDEN software package.26
ions and the related displacement of water out of the interfacial region containing the aliphatic part of the bilayer aggregate as discussed above. In agreement with a previous proposal by 6256
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Lamont and Ducker,2 this explanation additionally highlights the role of water in such transformation and does not require a change in the aggregate curvature following the diffusion of inorganic ions out of the surfactant film. Further studies are surely necessary to prove whether such a suggestion is valid, and if so, why such segregation leads to the patterns observed in the experimental studies. 3.3. Adsorption Structure of CP+ Ions on the Montmorillonite Surface. To proceed with a discussion of the adsorption structure of CP+ ions, it is beneficial to be able to differentiate between the CP+ ions adsorbed to the montmorillonite surface and those having no close contact to the mineral surface despite being part of the adsorbed aggregate. The latter CP+ ions will be further referred to as detached ones. It has been shown in the preceding study that CP+ ions can be adsorbed to the micawater interface through the formation of inner-sphere or outersphere complexes.14 A consideration of the structure of the latter complex then allows an estimation of the maximum separation between the mineral surface and the adsorbed CP+ ions. Indeed, water molecules adsorbed on the montmorillonite surface are characterized by the maximum z value of ∼3.2 A˚ (Figures 1a and 2), whereas the first hydration shell is limited to the region of ∼4.2 A˚ around the pyridinium ring of the CP+ ion.14 The latter value can also be used as a good proxy for the maximum distance between basal oxygens and aromatic carbons of specifically adsorbed CP+ ions.14 Hence, CP+ ions with at least one aromatic carbon characterized by a z value smaller than ∼4.2 A˚ or ∼7.4 A˚ can be considered to be adsorbed as inner-sphere or outer-sphere complexes, respectively, on the montmorillonite surface. Taking further into account that the maximum carbon-carbon or nitrogen-carbon distance in the model of the pyridinium ring of the CP+ ion equals 2.76 A˚, CP+ ions with at least one aromatic carbon (or nitrogen) characterized by a z value larger than ∼7.0 A˚ or ∼10.2 A˚ can be considered to be adsorbed as outer-sphere complexes or to be detached from the montmorillonite surface, respectively. An application of the above limits identifies 0.125 CP+/Auc at coverages of 0.375, 0.625, and 1 CP+/Auc as well as of 0.25 CP+/Auc at a coverage of 0.875 CP+/Auc as surfactants detached from the surface (Figure 4a, lower panel). An identification of inner-sphere and outer-sphere complexes can be made based on the data in Figure 4a in a similar way at lower coverages. At higher coverages, it is more straightforward, if the data on the cumulative nitrogen distribution in Figure 9 are additionally taken into account. It follows from these data that, for the monolayer aggregate, 0.125, 0.375, 0.375, and 0.5 CP+/Auc are adsorbed as inner-sphere complexes at coverages Langmuir 2009, 25(11), 6250–6259
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Figure 10. Lateral atomic densities (color boxes are scaled in atoms/A˚2) for sodium and chloride ions as well as water hydrogen, water oxygen, and aromatic carbon atoms at the montmorillonite-water interface for (a) monolayer and (b) bilayer CP+ arrangements at a coverage of 1 CP+/Auc. Lateral atomic densities were sampled for water oxygen and water hydrogen within the first peak of the corresponding atomic density profiles, for Na+ ions and aromatic carbons of CP+ ions within 7 A˚, as well as for Cl- ions within 12 A˚ from the montmorillonite surface. The circles are silicon atoms, the crossed circles are aluminum atoms, the triangles are basal oxygen atoms (only structural ions and basal oxygens of the tetrahedral sheet at the interface with water film are shown).
of 0.25, 0.5, 0.75, and 1 CP+/Auc, respectively. For muscovite, the inner-sphere adsorption has been observed only for 0.125 CP+/Auc at a coverage of 1 CP+/Auc.14 This strong difference is a result of the stronger water adsorption on muscovite as compared to that on montmorillonite as discussed in section 3.1. Similarly to muscovite, however, aromatic carbons of adsorbed CP+ ions are separated by at least 3 A˚ from the mineral surface (except for 0.125 CP+/Auc at coverages of 0.25 and 0.5 CP+/Auc with minimum z value of 2.75 A˚) as can be seen in Figure 4a. The latter observation requires a thorough consideration as to the reason of the weak penetration of pyridinium rings into ditrigonal cavities of the montmorillonite surface. A simple estimation shows that, for the limiting case of a pyridinium ring adsorbed in a ditrigonal cavity with the carbon-nitrogen C2 symmetry axis normal to the surface, the distances between basal oxygens and aromatic carbons do not fall below the value of 3 A˚ allowed in the first hydration shell of the latter,14 if the carbon on the C2 axis is positioned at least 2.1 A˚ above the cavity center. For the similar configuration with the carbon positioned at a distance of 3 A˚ from the surface, which is characteristic for montmorillonite (Figure 4a), the distances between basal oxygens and aromatic carbons vary in the range 3.8-5.4 A˚ and, hence, well above the most probable distance of 3.5 A˚ in the first hydration shell of aromatic carbons.14 For muscovite, a stronger penetration of the pyridinium rings into ditrigonal cavities could be suggested to be sterically hindered by water molecules strongly adsorbed at ∼1.7 A˚ from the surface and belonging to the first hydration shells of adsorbed CP+ ions.14 Such water molecules are, however, absent on montmorillonite as discussed in section 3.1. It then seems that the weak penetration of pyridinium rings into ditrigonal cavities of montmorillonite and muscovite in the simulated coverage range should be attributed to the preferential coordination of pyridinium rings to interfacial water molecules as compared to basal oxygens. Coordination of aromatic carbons of adsorbed CP+ ions to basal oxygens and water molecules of the first adsorbed water layer can be analyzed with help of the data in Figure 10. This figure documents the competition among CP+ ions, Na+ ions, and water molecules for adsorption sites on the montmorillonite surface and reveals peculiarities of the adsorption structure of CP+ ions in addition to those discussed above. The first of these Langmuir 2009, 25(11), 6250–6259
peculiarities suggests an additional criterion for a differentiation between inner-sphere and outer-sphere adsorption complexes of CP+ ions on the montmorillonite surface: The formation of an inner-sphere complex leads to a displacement of water molecules from their adsorption sites above a contiguous surface region containing at least four basal oxygens. Indeed, in agreement with the estimations based on Figures 4a and 9 this criterion allows an identification as inner-sphere complexes of four CP+ ions per simulation cell (or 0.5 CP+/Auc) at (x,y) ∼ (6,4), (12,7), (12,12), and (19,16) for the monolayer arrangement (Figure 10a, a basal oxygen at (x,y) ∼ (0.5,2.5) should be taken into account for the latter complex) as well as three CP+ ions per simulation cell at (x,y) ∼ (4,14), (7,1), and (11,7) for the bilayer arrangement (Figure 10b). Second, a nearly vertical orientation of the adsorbed pyridinium rings is a preferred one in the simulated coverage range. This orientation apparently ensures the most favorable mutual arrangement of the adsorbed species, which are primarily CP+ ions and water, albeit outer-sphere complexes of Na+ ions should also be taken into account for a CP+ ion at (x,y) ∼ (11,7) (Figure 10b). With this orientation, a specifically adsorbed CP+ ion exclusively occupies a basal surface area equivalent to that of a ditrigonal cavity (23.2 A˚2), and it can be adsorbed in the positions near a Si site instead of (or along with) those near a ditrigonal cavity center. Third, a nearly horizontal orientation of the adsorbed pyridinium rings can lead to a displacement of water molecules from a contiguous basal surface region containing as many as eight basal oxygens and having an area equivalent to that of about one and a half ditrigonal cavities (Figure 10b). For a pyridinium ring, a transition from a nearly vertical to a nearly horizontal orientation, which occurs only at the highest simulated coverage of 1 CP+/Auc for the bilayer arrangement, leads to a decrease of the minimum z value for nitrogens from ∼5.5 A˚ to ∼4.2 A˚ (Figure 9) and for aliphatic carbons from 6.5-7 A˚ to ∼5 A˚ (Figure 4b), respectively. Figure 4a further indicates that such a transition is not accompanied by a change of the minimum distance between aromatic carbons and the montmorillonite surface. A comparison between the interfacial structures at CP+ coverages of 0.875 CP+/Auc and 1 CP+/Auc can be used to investigate possible reasons for the formation of an inner-sphere DOI: 10.1021/la804311w 6257
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adsorption complex of a CP+ ion with a (nearly) horizontal orientation of its pyridinium ring. Although the content of ionic species within the interfacial region z < 11 A˚ is identical for both structures (0.375 Na+/Auc, 0.375 Cl-/Auc, 0.375 CP+/Auc; compare Figures 4a, 6, and 10b), the nitrogens of the adsorbed pyridinium rings in the former structure are on average ∼1 A˚ farther from the surface than those for the latter one (Figure 9). An inspection of the structures reveals further that an adsorbed pyridinium ring contains on average 14.67 oxygens in its first hydration shell (with some oxygens hydrating simultaneously two pyridinium rings, as can be seen in Figure 10b) in each of these two systems. Out of these oxygens, 12.67 are contributed by water and 2.0 by the basal surface at 0.875 CP+/Auc as compared to, respectively, 10 and 4.67 at 1 CP+/Auc. Thus, in the first hydration shells of pyridinium rings, 2.67 H2O per adsorbed CP+ ion or, equivalently, 1 H2O per Auc are replaced by basal oxygens upon an increase of the CP+ coverage from 0.875 to 1 CP+/Auc. On one hand, the water content in the first adsorbed water layers decreases by only 0.3 H2O/Auc from 2.7 to 2.4 H2O/Auc at 0.875 and 1 CP+/Auc, respectively. Therefore, an exchange of water adsorbed on the montmorillonite surface by pyridinium rings can be excluded as a major reason for the strong water depletion in their first hydration shells. This is in accordance with the above conclusion on the preferential coordination of pyridinium rings to interfacial water molecules as compared to basal oxygens. On the other hand, the water content within the region z < 11 A˚, beyond which a nearly water-free region occurs for the bilayer arrangement at 1 CP+/Auc as discussed in the previous section, decreases by exactly 1 H2O/Auc from 11.2 to 10.2 H2O/Auc at 0.875 and 1 CP+/Auc, respectively. Apparently, this decrease results in a deficit of water molecules available for hydration of the adsorbed pyridinium rings. This deficit is suggested to be compensated by basal oxygens through a closer approach of CP+ ions to the montmorillonite surface in the first step and, if insufficient, through a transition from a (nearly) vertical to a (nearly) horizontal orientation of the pyridinium rings in the second step. This suggestion is equivalent to the conclusion that the hydration of the inorganic species (Na+ ions, contact and solvent-separated Na+-Cl- ion pairs, Cl- ions participating in CP+-Cl- ion pairs) is preferred to that of pyridinium rings in the region z < 11 A˚ at the montmorillonite-water interface. Considering that Cl- ions within this region form ion pairs with either Na+ or CP+ ions, this agrees with the lower hydration enthalpy of -556 ( 17 kJ/mol for a CP+-Cl- ion pair (estimated from the changes in the simulated total potential energy upon increases in CPCl coverage) as compared to that of -783.3 kJ/mol for a Na+-Cl- ion pair.28 It then follows that a presence of ion pairs hydrated more strongly than Na+-Cl- ion pairs in the adsorbed layer of the bilayer aggregate may strongly facilitate the formation of the inner-sphere adsorption complex of CP+ ions with (nearly) horizontal orientations of their pyridinium rings. The hydration enthalpy of a Li+-Cl- ion pair is higher than that of a Na+-Cl- ion pair and equals -897.8 kJ/mol.28 It is possible that the observation29 of the slow secondary surfactant adsorption in the presence of 10 mM LiCl on the silica surface is a manifestation of an aggregate transformation allowing for a closer approach of adsorbed headgroups to the silica surface (28) Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen, M. H.; Earhart, A. D.; Coe, J. V.; Tuttle, T. R.Jr. J. Phys. Chem. A 1998, 102, 7787. (29) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. J. Colloid Interface Sci. 2003, 266, 236.
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(e.g., a transformation from an outer-sphere to an inner-sphere complex). The fact that this effect was observed for LiCl but not for NaCl or KCl electrolytes29 suggests that the presence of an ion pair at the mineral-water interface can activate such a transformation on the silica surface only if its hydration enthalpy is higher than that of a Na+-Cl- ion pair. If so, the observation that the slow secondary surfactant adsorption has been observed for 0.6 mM CTAB7 as well as for 0.2 mM and 0.55 mM CPBr with no added electrolyte,9 but not in the presence of 10 mM KBr,7,9 agrees well with the proposed mechanism, as the hydration enthalpy of a K+-Br- ion equals28 -669.0 kJ/mol and is lower than that of a Na+-Clion pair (as well as of a K+-Cl- ion pair). Another implication of the above considerations is that, at the other equal conditions, an organic cation with the same aliphatic chain as a CP+ ion but a less hydrated headgroup than the pyridinium ring may form inner-sphere adsorption complexes on the mineral surface more readily. 3.4. Surface Charge of the CP+-Modified Montmorillonite. It has been shown experimentally that upon an adsorption of CP+ ions the surface charge of montmorillonite particles increases in a monotonic manner, becomes positive after passing the point of zero charge, and reaches positive values, which exceed the absolute value of the initial negative surface charge of Na+-montmorillonite particles by a factor of ∼2.30,31 In these experiments, the surface charge has been measured by a titration with charge compensating cationic or anionic polyelectrolytes exchanging inorganic cations or anions, respectively, from the external montmorillonite surface. Contact angle measurements on CP+-modified montmorillonites at varying CP+ coverages have additionally revealed that the surface of montmorillonite particles becomes hydrophobic upon an adsorption of CP+ ions in amounts corresponding to ∼10% up to 60-80% of the cation exchange capacity (CEC) of Na+-montmorillonite, and it becomes hydrophilic again at higher CP+ contents.31 An analysis of these experimental results supported by the simulated data discussed in previous sections yields insights into the process of CP+ adsorption on the external montmorillonite surface, which are otherwise very difficult to obtain because of the lacking ability to differentiate between the adsorption of quaternary alkyl ammonium ions on the external surface of montmorillonite particles and that into their interlayer spaces in the experimental studies. Indeed, it follows from the discussion for Figures 1 and 4 in section 3.2 that the surface of CP+-modified montmorillonite particles can become hydrophobic only for the monolayer arrangement of CP+ ions at coverages of at least 0.625-0.75 CP+/Auc, for which the local water density near the monolayerwater interface on the aggregate side becomes significantly lower than that on the aqueous solution side. The mean number of interlayer spaces per montmorillonite particle is characterized by the most probable value32 of 10 and the maximum value33 of 50, which agrees with published transmission electron microscopy images of CTA+-modified montmorillonites.34 Taking into account that the interlayer content of 0.75 CP+/Auc corresponds to 100% CEC (see section 2), these values suggest that, for a hydrophobic montmorillonite particle with 0.75 CP+/ Auc adsorbed on the external surface, its mean interlayer CP+ content should not exceed ∼1% up to ∼8% CEC for its total (30) (31) (32) (33) (34)
Dultz, S.; Bors, J. Appl. Clay Sci. 2000, 16, 15. Schampera, B.; Dultz, S. Clay Miner. (accepted). Iwasaki, T.; Watanabe, T. Clays Clay Miner. 1988, 36, 73. Shomer, I.; Mingelgrin, U. Clays Clay Miner. 1978, 26, 135. Lee, S. Y.; Kim, S. J. Clay Miner. 2002, 37, 465.
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4. Conclusions
Figure 11. (a) Radial distribution functions and (b) coordination numbers for Cl- ions around Na+ ions and nitrogen atoms (N) of CP+ ions for monolayer and bilayer CP+ arrangements on montmorillonite at a coverage of 1 CP+/Auc.
CP+ content to equal ∼10% CEC as observed in contact angle measurements.31 Further CP+ adsorption occurs primarily in the interlayer spaces of montmorillonite as indicated by increasing layer spacings.13,30 The accompanying decrease of the absolute values of the negative surface charge of montmorillonite observed30,31 in the CP+ content range below 60-80% CEC is likely due to a reduced availability of inorganic cations adsorbed at the external surface, which results from increased coagulation of montmorillonite particles characterized by the monolayer arrangement of the adsorbed CP+ ions.35 An additional reduction of the availability of inorganic cations within monolayer aggregates can be assumed to result from their strong association with Cl- ions in the contact ion pairs (Figure 11). The reversal of the surface charge at CP+ contents of 60-80% CEC and its subsequent monotonic increase at higher CP+ contents,30,31 on the contrary, is suggested to occur due to the formation of a bilayer aggregate and a related monotonic increase of Cl- concentration at the bilayer-water interface (Figure 6a, the upper panel). At this interface, Cl- ions preferentially form solvent-separated ion pairs with positively charged pyridinium rings of CP+ ions (Figure 11) and can be assumed to be readily exchanged by an anionic polyelectrolyte as used in the discussed experiments. This suggestion agrees very well with the observation that the surface of montmorillonite particles becomes hydrophilic again at CP+ contents above 60-80%.31 (35) Janek, M.; Lagaly, G. Colloid Polym. Sci. 2003, 281, 293.
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Upon an increase of the CP+ content at the montmorillonitewater interface up to the highest simulated coverage of 1 CP+/ Auc, the local water density in the interfacial region containing several terminal groups of aliphatic chains drops below half of its bulk value near the aggregate-water interface for the incomplete monolayer aggregate and below one-tenth of its bulk value in the middle part of the incomplete bilayer aggregate. The nearly complete displacement of water molecules from this interfacial region of the bilayer aggregate is a result of a segregation of inorganic ions into the two neighboring interfacial regions near montmorillonite-water and bilayer-water interfaces. Such segregation does not occur for muscovite because of its considerably higher mineral layer charge and accordingly considerably higher surface concentration of compensating inorganic cations. The presence of inorganic ions in the aliphatic part of the bilayer aggregate leads to a more compact bilayer aggregate in agreement with the experimental observations of the added electrolyte effect on the characteristic dimensions of the periodic structures self-assembled on silica and muscovite surfaces.3,6 The simulation results indicate (albeit at the small scale) that the aggregates arranged as meandering stripes on the muscovite surface need not necessarily be of a cylindrical type as suggested by Manne and Gaub1 and may represent stripes of a bilayer aggregate formed as a result of the lateral segregation of hydrophilic and hydrophobic clusters in the interfacial region containing its aliphatic part. In agreement with a previous proposal by Lamont and Ducker,2 the transformation of meandering stripes to a flat bilayer within up to 24 h is suggested to follow a diffusion of the inorganic ions and the related displacement of water out of this interfacial region. On the montmorillonite surface, CP+ ions are preferentially adsorbed as inner-sphere complexes. A deficit of water molecules available for the hydration of the pyridinium rings adsorbed at the mineral-water interface at increased CP+ contents is compensated by basal oxygens through a closer approach of CP+ ions to the montmorillonite surface in the first step, and, if insufficient, through a transition from a (nearly) vertical to a (nearly) horizontal orientation of the pyridinium rings in the second step. Such a closer approach of adsorbed headgroups to the mineral surface is suggested to be controlled by the type of the inorganic ion pair at the mineral-water interface. This mechanism is argued to be possibly responsible for the observation of the slow secondary surfactant adsorption29 in the presence of 10 mM LiCl as well as of its cancelation7 in the presence of 10 mM KBr on the silica surface. An analysis of experimental30,31 and simulation data suggests that the monolayer arrangement of CP+ ions is characteristic for the external montmorillonite surface at the total CP+ contents not exceeding 60-80% CEC, whereas the bilayer arrangement of CP+ ions is formed on montmorillonite at the higher CP+ coverages. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under Project No. ME 3128/1-1. I thank two anonymous reviewers for helpful comments and H. Wicke for reading the manuscript.
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