Langmuir 2007, 23, 11587-11596
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Adsorption States of Amphipatic Solutes at the Surface of Naturally Hydrophobic Minerals: A Molecular Dynamics Simulation Study Hao Du and J. D. Miller* Department of Metallurgical Engineering, 135 S. 1460 E., 412 William C. Browning Building, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed May 31, 2007. In Final Form: July 30, 2007 An initial molecular dynamics simulation study regarding interfacial phenomena at selected naturally hydrophobic surfaces is reported. Simulation results show that, due to the natural hydrophobicity of graphite and talc basal planes, the cationic surfactant dodecyltrimethylammonium bromide preferentially adsorbs at these surfaces through hydrophobic interactions. When a model dextrin molecule is considered, the simulation results suggest that the hydrophobic interaction between the naturally hydrophobic surfaces of graphite, talc basal plane, and sulfur and the hydrophobic moieties (C-H and methylene groups) in the dextrin molecule plays a significant role in dextrin adsorption at these surfaces. The hydroxyl group in the dextrin molecule also contributes to its adsorption at the talc basal plane surface. In contrast, dextrin was not found to adsorb at talc edge surfaces.
Introduction Naturally hydrophobic minerals such as graphite and talc are common gangue minerals found in sulfide ores and are difficult to separate due to their tendency to float together with valuable sulfide minerals. Despite the relatively successful use of the polysaccharide group of chemicals [dextrin, guar gum, carboxymethylcellulose (CMC), etc.] as flotation depressants for these naturally hydrophobic minerals, the nature of the adsorption processes remains in debate. Consequently, the adsorption of amphipatic solutes at naturally hydrophobic minerals such as coal/graphite, talc, and sulfur is of interest to many researchers, and substantial research has been reported.1-16 Talc, having the chemical formula Mg3(Si4O10)(OH)2, is composed of three layers. Its middle layer is a brucite layer consisting of a magnesium-oxygen/hydroxyl octahedral, while the two outer layers are composed of silicon-oxygen tetrahedra. These three-layer sheets are bonded to each other only by van der Waals forces, so the layers are capable of slipping easily over one another, which accounts for the soft character of talc and its smoothness. The basal surfaces of the three-layer elementary structure do not contain hydroxyl groups or active sites, which provides the basal plane of talc with a natural hydrophobicity and accounts for its floatability.1 Different from talc, graphite * Corresponding author. E-mail:
[email protected]. (1) Fuerstenau, D. W.; Huang, P. XXII Int. Miner. Process. Congr. 2003, 1034. (2) Balajee, S. R.; Iwasaki, I. Trans. Am. Inst. Mining, Metall. Petrol. Eng. 1969, 244, 407. (3) Haung, H.-H.; Calara, J. V.; Bauer, D. L.; Miller, J. D. Recent DeV. Sep. Sci. 1978, 4, 115. (4) Healy, T. W. J. Macromol. Sci., Chem. 1974, 8, 603. (5) Jenkins, P.; Ralston, J. Colloids Surf., A: Physicochem. Eng. Aspects 1998, 139, 27. (6) Liu, Q.; Laskowski, J. S. J. Colloid Interface Sci. 1989, 130, 101. (7) Liu, Q.; Laskowski, J. S. Int. J. Miner. Process. 1989, 26, 297. (8) Liu, Q.; Laskowski, J. S. Int. J. Miner. Process. 1989, 27, 147. (9) Miller, J. D.; Laskowski, J. S.; Chang, S. S. Colloids Surf. 1983, 8, 137. (10) Morris, G. E.; Fornasiero, D.; Ralston, J. Int. J. Miner. Process. 2002, 67, 211. (11) Pugh, R. J. Int. J. Miner. Process. 1989, 25, 131. (12) Pugh, R. J. Int. J. Miner. Process. 1989, 25, 101. (13) Rath, R. K.; Subramanian, S.; Laskowski, J. S. Processing of Hydrophobic Minerals and Fine Coal, Proceedings of the UBC-McGill Bi-Annual International Symposium on Fundamentals of Mineral Processing, 1st; Vancouver, BC, Canada, Aug. 20-24, 1995, p 105. (14) Rath, R. K.; Subramanian, S.; Laskowski, J. S. Langmuir 1997, 13, 6260. (15) Wie, J. M.; Fuerstenau, D. W. Int. J. Miner. Process. 1974, 1, 17. (16) Brossard, S. K.; Du, H.; Miller, J. D. J. Colloid Interface Sci. In press.
and sulfur are elemental crystals with elements of low polarity, which together with their structural organization account for their hydrophobic properties. It has been suggested by Healy4 and later Pugh11,12 that chemical, electrostatic, hydrogen-bonding, and hydrophobic interactions are the major mechanisms that govern polymer adsorption at mineral surfaces. Studies regarding the adsorption behavior of dextrin at molybdenite,15 coal,3,9,17 and sulfur16 surfaces have demonstrated the significance of hydrophobic interactions in adsorption processes. Later research also has confirmed that hydrophobic interactions play a critical role in polysaccharide adsorption at talc surfaces.1,5,10 In contrast, other researchers suggest that hydrogen bonding, which happens between the hydroxyl groups in organic molecules and the hydrogen-bonding sites at mineral surfaces, is the major reason for adsorption.2,14 Chemical interactions, related to mineral surface metallic sites and consequent complexation effects, have also been considered to contribute to the adsorption mechanism.6-8 A substantial number of experimental techniques have been used to understand the surface chemistry of natural hydrophobic minerals (molybdenite, graphite, coal, talc, and sulfur), including flotation tests, particle characterization, titration, adsorption, microcalobrimerty, spectroscopy, and electrophoretic measurements.2,3,6-8,13-16,18-22 Recently, molecular dynamics simulation (MDS) has been recognized to be an important tool that can be used to explore water/water and water/mineral interactions and to elucidate the structure of water at mineral surfaces, providing detailed information and fundamental understanding on issues (17) Miller, J. D.; Lin, C. L.; Chang, S. S. Coal Prep. (London) 1984, 1, 21. (18) Belardi, G.; Rice, D.; Marabini, A. Processing of Hydrophobic Minerals and Fine Coal, Proceedings of the UBC-McGill Bi-Annual International Symposium on Fundamentals of Mineral Processing, 1st; Vancouver, BC, Canada, Aug. 20-24, 1995, p 363. (19) Oliveira, J. F.; Gomes, L. M. B. Processing of Hydrophobic Minerals and Fine Coal, Proceedings of the UBC-McGill Bi-Annual International Symposium on Fundamentals of Mineral Processing, 1st; Vancouver, BC, Canada, Aug 2024, 1995, p 341. (20) Shortridge, P. G.; Harris, P. J.; Bradshaw, D. J. Polymers in Mineral Processing, Proceedings of the UBC-McGill Bi-Annual International Symposium on Fundamentals of Mineral Processing, 3rd; Quebec City, QC, Canada, Aug 22-26, 1999, p 155. (21) Maelhammar, G. Colloids Surf. 1990, 44, 61. (22) Steenberg, E.; Harris, P. J. Surface-chemical and mineralogical properties relevant to the flotation of talc and other layer silicates. Counc. Miner. Technol., Rep. 1985.
10.1021/la701604u CCC: $37.00 © 2007 American Chemical Society Published on Web 10/11/2007
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such as mineral surface potential, surfactant/macromolecule adsorption, and mineral floatability.23-29 In the past decade, much research based on MDS has been reported on the study of water structures, as well as the dynamic and thermodynamic characteristics for water/mineral systems.30-37 Also efforts have been made to establish force fields for selected minerals.28,29,38,39 In terms of water structure at naturally hydrophobic surfaces, our previous MDS suggests that, due to the absence of electron donor/ acceptor sites at naturally hydrophobic mineral such as graphite and talc basal plane surfaces, water molecules interact weakly with the surface atoms and arrange themselves randomly some distance (∼3 Å) from the surface,27 which is in good agreement with their hydrophobic character. In this regard, an initial study to examine the adsorption states of selected amphipatic solutes at selected naturally hydrophobic mineral surfaces using MDS was conducted, aiming at providing further information regarding adsorption mechanisms, especially examining the significance of hydrophobic interactions. Initially, the adsorption of a cationic surfactant dodecyltrimethylammonium bromide (DTAB) at a graphite surface has been simulated, since this adsorption process has been well-recognized to involve hydrophobic interactions. In this way, the effectiveness of MDS methodology for the study of surfactant adsorption can be assessed. Then, the behavior of DTAB at a hydrophobic talc basal plane surface has been studied and the significance of hydrophobic interactions examined for this case. For the adsorption of polysaccharide polymers, the behavior of a model dextrin molecule at graphite, talc, and sulfur surfaces has been investigated. The different adsorption states of dextrin at these surfaces are discussed in terms of the molecular structure of dextrin, mineral surface hydrophobicity, surface atom polarity, and surface nanoroughness. Finally, some influential factors affecting the adsorption of dextrin at naturally hydrophobic mineral surfaces are summarized and discussed. Simulation Details The MDS program DL_POLY_21440,41 was used for this study. Simple cubic cells containing water molecules, surfactants, and interested minerals were defined with periodic boundary conditions. The initial configurations of the minerals were constructed using lattice parameters provided by American Mineralogist Crystal Structure Database.42,43 For simulations involving the surfactant DTAB, the generic force field DREIDING44 was used with the atom charge distribution shown in Figure 1. The adsorption of DTAB molecules at the graphite (0001) surface and the talc basal plane (23) Heinz, H.; Vaia, R. A.; Krishnamoorti, R.; Farmer, B. L. Chem. Mater. 2007, 19, 59. (24) Nalaskowski, J.; Abdul, B.; Du, H.; Miller, J. D. Can. Metall. Q. 2007. (25) Du, H.; Rasaiah, J. C.; Miller, J. D. J. Phys. Chem. B 2007, 111, 209. (26) Du, H.; Miller, J. D. J. Phys. Chem. B. Submitted for publication. (27) Du, H.; Miller, J. D. Int. J. Miner. Process. 2007. (28) Heinz, H.; Koerner, H.; Anderson, K. L.; Vaia, R. A.; Farmer, B. L. Chem. Mater. 2005, 17, 5658. (29) Heinz, H.; Suter, U. W. J. Phys. Chem. B 2004, 108, 18341. (30) Lee, S. H.; Rasaiah, J. C. J. Phys. Chem. 1996, 100, 1420. (31) Rustad, J. R.; Felmy, A. R.; Bylaska, E. J. Geochim. Cosmochim. Acta 2003, 67, 1001. (32) Wang, J.; Kalinichev, A. G.; Kirkpatrick, R. J. Geochim. Cosmochim. Acta 2004, 68, 3351. (33) Kalinichev, A. G.; Kirkpatrick, R. J. Chem. Mater. 2002, 14, 3539. (34) Rustad, J. R. ReV. Mineral. Geochem. 2001, 42, 169. (35) Gallo, P.; Rapinesi, M.; Rovere, M. J. Chem. Phys. 2002, 117, 369. (36) Spohr, E.; Hartnig, C.; Gallo, P.; Rovere, M. J. Mol. Liq. 1999, 80, 165. (37) Lee, S. H.; Rossky, P. J. J. Chem. Phys. 1994, 100, 3334. (38) Cygan, R. T.; Liang, J.-J.; Kalinichev, A. G. J. Phys. Chem. B 2004, 108, 1255. (39) Cygan, R. T. ReV. Mineral. Geochem. 2001, 42, 1. (40) Smith, W.; Forester, T. R. J. Mol. Graphics 1996, 14, 136. (41) Forester, T., T.; Smith, W. 1995. (42) Gruner, J. W. 1934. (43) Perdikatsis, B.; Burzlaff, H. Z. Kristallogr. 1981, 156, 177.
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Figure 1. Schematic drawing of a DTAB molecule with charge distribution. In the figure, charges for identical atoms are only listed once for clarity.
Figure 2. Schematic drawing of a dextrin molecule composed of two R-D-glucopyranose monomers with charge distribution. In the figure, charges for identical atoms are only listed once for clarity.
(001) surface was simulated to investigate the organization of the surfactant at these mineral surfaces. For the dextrin/water/mineral system, a molecule composed of five R-D-glucopyranose monomers was used to represent the dextrin molecule, and the interaction of dextrin with the graphite (0001) surface, the talc basal plane (001) surface, and the rhombic sulfur (111) surface was studied. A force field from CHARMM45,46 for R-D-glucopyranose monomer combined with DREIDING was used to account for the intermolecular interactions, and the charge distribution for atoms in the “dextrin” molecule is provided in Figure 2. The simple point charge (SPC) water model47 was used for this study, and the recently developed CLAYFF force field for talc39 was the basis for the simulation of talc. The CLAYFF force field in conjunction with the SPC water model has been demonstrated to be suitable for the description of hydrated and multicomponent mineral systems and the investigation of interfacial phenomena involving aqueous solutions.48-50 Each simulation was performed in a rectangular simulation cell with three-dimensional periodic boundary conditions applied. The size of the simulation cells as well as the mineral slab thickness and the number of water molecules in each simulation cell is summarized in Table 1. The intermolecular potential parameters for water molecules and minerals are listed in Table 2. The pair potential between particles was a combination of the Lennard-Jones and electrostatic interactions expressed as φ(rij) ) 4ij
[( ) ( ) ] σij rij
12
-
σij rij
6
+
qiqj rij
(1)
(44) Mayo, S. L.; Olafson, B. D.; Goddard, W. A., III. J. Phys. Chem. 1990, 94, 8897. (45) Brooks, B. R.; Bruccoleri, R. E.; Olafson, B. D.; States, D. J.; Swaminathan, S.; Karplus, M. J. Comput. Chem. 1983, 4, 187. (46) MacKerell, A. D., Jr.; Bashford, D.; Bellott, M.; Dunbrack, R. L.; Evanseck, J. D.; Field, M. J.; Fischer, S.; Gao, J.; Guo, H.; Ha, S.; Joseph-McCarthy, D.; Kuchnir, L.; Kuczera, K.; Lau, F. T. K.; Mattos, C.; Michnick, S.; Ngo, T.; Nguyen, D. T.; Prodhom, B.; Reiher, W. E., III; Roux, B.; Schlenkrich, M.; Smith, J. C.; Stote, R.; Straub, J.; Watanabe, M.; Wiorkiewicz-Kuczera, J.; Yin, D.; Karplus, M. J. Phys. Chem. B 1998, 102, 3586. (47) Berendsen, H. J. C. Comput. Phys. Commun. 1987, 44, 233. (48) Wang, J.; Kalinichev Andrey, G.; Kirkpatrick, R. J. Geochim. Cosmochim. Acta 2005, 562. (49) Wang, J.; Kalinichev, A. G.; Kirkpatrick, R. J. Earth Planet. Sci. Lett. 2004, 222, 517. (50) Wang, J.; Kalinichev, A. G.; Kirkpatrick, R. J. Abstracts of Papers, 227th ACS National Meeting; Anaheim, CA, March 28-April 1, 2004, GEOC.
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Table 1. Summary of Simulation Cell Parameters cell dimensions, Å
system DTAB-graphite dextrin-graphite DTAB-talc dextrin-talc dextrin-sulfur
no. of water solid solid length width height molecules layers thickness 39.7 39.7 43 43 40
41.1 41.1 36 36 42
40 50 40 30 30
1562 1667 1426 912 1090
3 3 1 1 1
∼12 ∼12 ∼10 ∼10 ∼10
Table 2. Potential Parameters for Water/Water and Water/Ion Interactions atom
σio (Å)
io (kcal/mol)
charge (q)
ref
S (sulfur) C (graphite) O (water) H (water) hydroxyl hydrogen hydroxyl oxygen bridging oxygen apex oxygen silicon magnesium
3.906 3.283 3.169 0 0 3.169 3.169 3.169 3.706 5.909
0.0515 0.1160 0.1550 0 0 0.1550 0.1550 0.1550 1.8 × 10-6 9 × 10-7
0 0 -0.820 0.410 0.425 -0.950 -1.050 -1.2825 2.10 1.360
61 62 63 63 38, 39 38, 39 38, 39 38, 39 38, 39 38, 39
where rij is the distance between particle i and j, σ and are the size parameter and energy parameter, respectively, and qi is the charge of the ith atom (or ion). The simulation was performed in a NVT ensemble using Hoover’s thermostat.51 The leapfrog method with a time step of 1 fs was used to integrate the particle motion. The Ewald sum has been used to account for the electrostatic interactions. A final simulation time of 1 ns (106 steps each of 1 fs) including a 500 ps equilibration period was performed. The final results were analyzed on the basis of the production of a 500 ps simulation time after the equilibration period.
Results and Discussion In the following sections, the adsorption states of DTAB molecules and dextrin molecules at selected natural hydrophobic surfaces are reported and discussed with respect to the mineral surface properties and adsorption characteristics. 1. Cationic Surfactant Adsorption. 1.1. DTAB at the Graphite Surface. The DTAB adsorption process at a hydrophobic graphite surface is described in Figure 3. Initially, 24 DTAB ions are sandwiched between graphite surfaces with their polar head groups facing the substrates (Figure 3A). Due to the strong hydrophobic interaction between the substrates and the DTAB tails, spontaneous reorientation of surfactant molecules is observed, as seen in Figure 3B,C. The alkyl chain of the DTAB ion spreads at the graphite surface, and the polar amine head groups stretch out into the aqueous phase, leading to a hydrophilic state at the graphite surfaces. Also, a spherical micelle structure appears to be formed in bulk water, as observed early in the dynamic adsorption process (Figure 3B). Eventually, complete adsorption of the DTAB surfactant at the graphite surfaces is achieved (Figure 3D), suggesting a strong affinity of the DTAB molecules for the graphite substrate. The MDS of DTAB ions at the graphite surface suggests that hydrophobic interaction between the natural hydrophobic graphite surfaces and the hydrophobic tail of the DTAB ion is vital for the surfactant adsorption process to occur. This observation is consistent with the results from previous studies regarding the significance of the hydrophobic interaction for the structurally similar surfactant cetyltrimethylammonium bromide (CTAB) in adsorption at naturally hydrophobic mineral surfaces using surface (51) Melchionna, S.; Ciccotti, G.; Holian, B. L. Mol. Phys. 1993, 78, 533.
characterization techniques such as contact angle measurements and atomic force microscope imaging.52-55 Figure 4 illustrates the changes in water contact angle at a graphite surface as a function of CTAB concentration.53,55 It is obvious that the hydrophobicity of graphite decreases significantly when the CTAB concentration is above the critical micelle concentration (cmc).53,55 In the case of graphite, this is due to self-assembly of well-ordered ∼5 nm diameter hemicylindrical micelle structures at the graphite surface, as seen in Figure 552,53. Such amazing organization is due to the templating effect of the graphite surface, which is not taken into consideration in the MDS. The alkyl chain of the CTAB molecule stays in close contact at the graphite substrate with head groups stretching out into the aqueous phase, in good agreement with simulation results. 1.2. DTAB at the Talc Basal Plane Surface. Snapshots of DTAB surfactant ions at the talc basal plane surface (001) are presented in Figure 6 for different simulation times. The initial configuration of DTABs was such that the polar head groups were facing the talc surface. Similar to DTAB/graphite system, after a short period of simulation, the surfactant molecules reorient themselves so that their hydrophobic tails move toward the surface and displace water molecules, as clearly represented in Figure 6B. Our previous study regarding the water structure at talc basal plane surfaces has suggested that, due to the absence of specific hydrogen-bonding-donor and/or -acceptor sites on the basal plane of talc, the interaction between water molecules and the basal plane is weak.24,27 The relatively weak water/talc interaction on the molecular scale is the origin of the macroscopic hydrophobic character of the basal plane surface. The observed spontaneous reorientation of DTAB ions is due to the hydrophobic nature of the talc basal plane, which favors the adsorption of the DTAB hydrophobic tails and the displacement of interfacial water molecules.27 As time evolves, the hydrophobic interaction between the substrate and the tail of the surfactant is so dominating that almost all water molecules are excluded between them, as shown in Figures 6C and 4D. Further study of the surfactants in Figure 6C,D reveals that, at equilibrium, the DTABs form a hemispherical-like micelle structure, with the hydrophobic tails sticking to the substrate and the polar heads in contact with surrounding water molecules. Due to the orientation of adsorbed DTAB ions, a hydrophilic surface is produced. This observation from MDS compliments the literature, in which contact angle measurements using dodecyl ammonium acetate as surfactant show that surfactant adsorption at the basal plane decreases the contact angle from above 60° to around 30°.1,24 2. Dextrin Adsorption. 2.1. Dextrin at the Graphite Surface. The behavior of the dextrin molecule at a graphite surface is presented in Figure 7. It is noticed that, first, the dextrin molecule remains attached to the graphite surface at one end initially (Figure 7A,B), indicating its strong affinity for the graphite surface. Second, the dextrin molecule reaches full contact with the graphite surface as time evolves (Figure 7B-D), suggesting that a complete spreading or occupation of the dextrin molecule at the graphite surface is an energetically stable state. In order to understand the dextrin organization at the graphite surface, the properties of the graphite surface as well as the structure of the dextrin molecule have to be considered. As (52) Paruchuri, V. K.; Nalaskowski, J.; Shah, D. O.; Miller, J. D. Colloids Surf., A: Physicochem. Eng. Aspects 2006, 272, 157. (53) Bakshi Mandeep, S.; Kaura, A.; Miller, J. D.; Paruchuri, V. K. J. Colloid Interface Sci. 2004, 278, 472. (54) Paruchuri, V. K.; Nguyen, A. V.; Miller, J. D. Colloids Surf., A: Physicochem. Eng. Aspects 2004, 250, 519. (55) Miller, J. D.; Paruchuri, V. K. Recent Res. DeVel. Surf. Colloids 2004, 1, 205.
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Figure 3. MDS snapshots of the adsorption states of 24 DTAB molecules at the graphite surface for different simulation times. The snapshots are taken at 0, 200, and 500 ps and 1 ns for A, B, C, and D, respectively. The color representations are as follows: red, oxygen atoms; white, hydrogen atoms; blue, nitrogen atoms; green, bromide atoms; and light blue, carbon atoms.
mentioned previously, the graphite, composed of inactive neutral carbon atoms, is naturally hydrophobic and is not able to hydrogen bond with water molecules. Consequently, water molecules are excluded from the graphite surface. The dextrin molecule, on the other hand, shows a dual characteristic in that the presence of hydroxyl groups provides this molecule with plenty of hydrogenbonding sites and enables the dextrin molecules to hydrogen bond with surrounding water molecules. At the same time, the low polarity of the methylene groups as well as the C-H groups in the dextrin molecule produces hydrophobic character, and these groups drive the molecule to escape from the water phase.
When interacting with the hydrophobic graphite surface, the hydrophobic moieties of the dextrin molecule preferentially stay in contact with the graphite surface, as suggested by Figure 7; when compared to hydroxyl groups, the methylene and the C-H groups in the dextrin molecule are significantly closer to the graphite surface. The hydrophilic hydroxyl groups favor hydrogen bonding with water molecules. As a compromise, the dextrin molecule reorients to expose as many hydrophobic moieties as possible to the hydrophobic graphite surface, while the majority of the dextrin hydroxyl groups hydrogen bond with water molecules. In conclusion, part of the dextrin molecule adsorbs
Adsorption States of Amphipatic Solutes
Figure 4. Change of graphite contact angle as a function of cationic surfactant CTAB concentration (data are take from ref 55).
Figure 5. AFM image of adsorbed CTAB at a graphite surface using the soft contact imaging technique. CTAB molecules form linear parallel hemicylindrical surface micelles at the graphite surface. The alkyl chain of CTAB stays in contact with the substrate and the polar head groups stick into the water phase. (scan size 200 nm and solution concentration 6 mM). The insertion is a schematic representation of hemicylindrical micelles at a hydrophobic surface (data are taken from ref 55).
at the graphite surface due to hydrophobic attraction, while the remainder of the molecule remains hydrated by the water phase through hydroxyl-water hydrogen bonding, as observed in Figure 7. The particle density distributions along the graphite surface normal are plotted in Figure 8. As expected, due to the natural hydrophobicity of the graphite, water molecules are excluded from the surface, as indicated by the zero water density at the graphite surface. Similar to previous reports in the literature,32,56 the primary water density peak is located at about 3.5 Å away from the surface, and this distance is larger than the distance between hydrogen-bonded water/water molecules, which is approximately 2.8 Å.30,57-60 This result demonstrates the weak (56) Wang, J.; Kalinichev, A. G.; Kirkpatrick, R. J. Earth Planet. Sci. Lett. 2004a, 222, 517. (57) Rasaiah, J. C. NATO ASI Ser., Ser. B 1988, 193, 89. (58) Lynden-Bell, R. M.; Rasaiah, J. C. J. Chem. Phys. 1997, 107, 1982. (59) Dang, L. X.; Smith, D. E. J. Chem. Phys. 1993, 99, 6950. (60) Dang, L. X.; Rice, J. E.; Kollman, P. A. J. Chem. Phys. 1990, 93, 7528. (61) Philpott, M. R.; Goliney, I. Y.; Lin, T. T. J. Chem. Phys. 2004, 120, 1943.
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interaction between water molecules and the graphite surface. The density distribution of atoms in the dextrin molecule is of special interest, in that it provides further information regarding the behavior of this molecule at the graphite surface. A close examination of Figure 8 reveals a sharp hydrogen density peak located at ∼2.4 Å away from the graphite surface, which is attributed mainly to the C-H or the methylene groups, suggesting that the dextrin molecule is adsorbed at the graphite surface through a hydrophobic interaction between the surface and the hydrophobic moieties in the molecule, as previously discussed. In contrast, the primary hydroxyl hydrogen peak is ∼3.5 Å away from the graphite surface, due to the absence of hydrogen-bonding sites at the graphite substrate; thus, polar hydroxyl groups in the dextrin molecules are excluded from the substrate. The ability of the dextrin molecule to be adsorbed at a hydrophobic surface through hydrophobic interactions and at the same time to form a hydrogen-bonding network with water molecules in the aqueous phase enables it to behave like a bridge connecting the hydrophobic graphite surface with surrounding water molecules. In this way, the naturally hydrophobic graphite surface is made hydrophilic. 2.2. Dextrin at Talc Basal Plane Surface. The simulation of a dextrin molecule at a talc basal plane surface is presented in Figure 9. It is noticed that the adsorption state is very similar to that observed at a graphite surface. Initially, the dextrin molecules are positioned in the middle of the solution (Figure 9A). As the simulation time evolves, the dextrin organization migrates toward the talc basal plane surface (Figure 9A,B) and remains in full contact with the talc surface once it reaches such a position (Figure 9C,D), indicating its strong affinity for the basal plane. As previously discussed, because of the absence of polarity, the talc basal plane surface is hydrophobic; thus, hydrophobic interaction is proposed to be one of the major reasons for the dextrin adsorption.1,5,10,26 On the other hand, despite the fact that the talc basal plane surface oxygen atoms are not ideal hydrogenbonding sites, because of the negative charge these oxygen atoms carry, they can weakly hydrogen bond with water molecules; thus, compared to the carbon atoms at the graphite surface, these tetrahedral oxygen atoms at the talc surface are able to accommodate hydroxyl groups in the dextrin molecule to some extent. Consequently, both the hydrophobic moieties and the hydroxyl groups are stabilized at the talc basal plane surface, as suggested by Figure 9C,D which show that both C-H/methylene and hydroxyl groups are very close to the substrate. In summary, hydrophobic interactions as well as attraction between the talc basal plane surface oxygen atoms and hydrogen atoms in the hydroxyl groups contribute to dextrin adsorption at the talc basal plane surface. In contrast, it has been shown that dextrin does not adsorb at the hydrophilic talc edge surface.27 Figure 10 is the particle number density plot for the talc system. Similar to the graphite system, there is a discontinuity of particle density distribution due to the exclusion of water molecules from the interfacial region by the hydrophobic talc basal plane surface. It is interesting to notice that there is a dextrin oxygen peak closer to the talc basal plane surface when compared to the carbon peaks. This is in good agreement with our previous discussion regarding the significance of the interaction between dextrin hydroxyl groups and talc surface oxygen atoms. The hydrogen density distribution further reveals that the hydrogen atoms in the dextrin hydroxyl groups are much closer to the talc basal plane surface (the primary peak is about 2.0 Å away from the (62) Vaitheeswaran, S.; Yin, H.; Rasaiah, J. C. J. Phys. Chem. B 2005, 109, 6629. (63) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. J. Phys. Chem. 1987, 91, 6269.
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Figure 6. MDS snapshots of the adsorption states of nine DTAB molecules at the talc basal plane surface for different simulation times. The snapshots are taken at 0, 200, and 500 ps and 1 ns for A, B, C, and D, respectively. The color representations are as follows: red, oxygen atoms; white, hydrogen atoms; blue, nitrogen atoms; purple, bromide atoms; yellow, silicon atoms; green, magnesium atoms; and light blue, carbon atoms.
substrate) than the hydrogen atoms in the C-H groups, the primary peak of which is about 2.8 Å away from the substrate. This observation supports our conclusion that the interaction between the substrate and the polar hydroxyl groups in the dextrin contribute to the adsorption of the dextrin molecule. Typical hydrogen bonding is the interaction between polarized water oxygen atoms and polarized water hydrogen atoms. In contrast, the talc basal plane surface oxygen atoms, which participate in the silicon/oxygen tetrahedral structure, are nonpolar. Therefore, the hydrogen bonding between the talc basal plane surface oxygen atoms and hydroxyl groups of the dextrin appears to be due to electrostatic interactions and is weaker than typical hydrogen bonding.24,27,48 Further, the incapability of the talc basal plane surface oxygen atoms to form traditional hydrogen bonds has been partially compensated by the low mobility of the dextrin molecule, which facilitates the electrostatic interaction between the hydrogen atoms of the dextrin hydroxyl group and the talc basal plane surface oxygen atoms. In summary, the weak hydrogen bonding between the talc basal plane surface oxygen atoms and the hydroxyl groups of the dextrin due to the electrostatic interaction plays a significant role regarding dextrin adsorption at the talc basal plane surface.
2.3. Dextrin at the Sulfur Surface. The adsorption of dextrin molecules by sulfur has been reported to be similar to that observed for talc and coal.16 Hydrophobic interaction has been proposed to be the major adsorption mechanism.16 In this study, a rhombic sulfur mineral has been cut along the (111) plane to keep the integrity of each S8 molecule. Our simulation results, as illustrated in Figure 11, suggest that the dextrin molecule behaves very differently than it does at the graphite and talc basal plane surfaces. The adsorption of the dextrin molecule at the sulfur surface is achieved by the attachment of a portion of the molecule, and a complete adsorption has not been observed during the entire simulation time (Figure 11B-D). Due to the similarity of the sulfur and graphite substrate, which are composed of nonpolar atoms, the significantly weakened adsorption state is understood by considering the structural difference between the sulfur and graphite surfaces. It is known that rhombic sulfur is built up from cyclooctasulfur (S8) molecules. Unlike graphite, which can be cleaved perfectly along its major cleavage plane (0001), the cleavage of a rhombic sulfur crystal is not able to produce an atomically flat surface due to its unique structure. A close examination of the sulfur surface (111) used in the simulation shows that the surface sulfur atoms are not
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Figure 7. MDS snapshots of the adsorption states of dextrin molecule (five dextrose monomers) at the graphite surface for different simulation times. The snapshots are taken at 0, 200, and 500 ps and 1 ns for A, B, C, and D, respectively. The color representations are as follows: red, oxygen atoms; white, hydrogen atoms; and light blue, carbon atoms.
Figure 8. Relative equilibrium density distribution of selected atoms along the normal to the graphite surface.
positioned in the same plane. As a consequence, atomic notches occur between each row of S8 molecules, as illustrated in Figure 11. This atomic nanoroughness provides structural barriers to the adsorption of dextrin hydrophobic moieties at the surface
while simultaneously remaining in an energetically stable molecular configuration. On the other hand, small water molecules are capable of dynamically occupying these surface voids, thus providing additional energy barriers for the dextrin adsorption.
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Figure 9. MDS snapshots of the adsorption states of dextrin molecule (five dextrose monomers) at the talc basal plane surface for different simulation times. The snap shots are taken at 0, 200, and 500 ps and 1 ns for A, B, C, and D, respectively. The color representations are as follows: red, oxygen atoms; white, hydrogen atoms; yellow, silicon atoms; green, magnesium atoms; and light blue, carbon atoms.
Figure 10. Relative equilibrium density distribution of selected atoms along the normal to the talc basal plane surface.
Figure 12 is the particle number density distribution normal to the sulfur surface. It is noticed that some of the hydrogen atoms in water are accommodated at the sulfur surface, as indicated by the significant hydrogen number density at the sulfur surface. Close examination of the dextrin particle number density distribution plot reveals a carbon peak and a carbon-bonded hydrogen peak, assigned to the methylene group in the dextrin, very close to the sulfur surface and confirms that the hydrophobic interactions between the substrate and the methylene groups in the dextrin molecule are the major reason for the adsorption.
2.4. Further Discussion. Figure 13 is the dextrin adsorption isotherm at selected naturally hydrophobic mineral surfaces (coal, talc, molybdenite, and sulfur), and it is evident that the isotherms at these naturally hydrophobic mineral surfaces follow the same trend, indicating a similar adsorption mechanism, which has been suggested to be hydrophobic interactions.3,9,15-17 The MDS results regarding adsorption states of dextrin at graphite, talc, and sulfur surfaces have suggested that there are several important factors that influence the adsorption processes. (a) Naturally Hydrophobic Surfaces. The hydrophobic interaction between naturally hydrophobic surfaces and the hydro-
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Figure 11. MDS snapshots of the adsorption states of dextrin molecule (five dextrose monomers) at the sulfur (111) surface for different simulation times. The snap shots are taken at 0, 200, and 500 ps and 1 ns for A, B, C, and D, respectively. The color representations are as follows: red, oxygen atoms; white, hydrogen atoms; yellow, sulfur atoms; and light blue, carbon atoms.
Figure 12. Relative equilibrium density distribution of selected atoms along the normal to the talc basal plane surface.
phobic moieties of the dextrin molecule plays a significant role in the adsorption process. (b) Presence of Charged Surface Atoms in the Talc Structure. The weak hydrogen bonds between the charged surface atoms and the polar hydroxyl groups in the dextrin also contribute to stabilization of dextrin molecules at talc basal plane surfaces. (c) Surface Nanoroughness of Sulfur at the Atomic LeVel. The atomic scale surface roughness produces energy and structural barriers and prevents the simultaneous stabilization of multiple hydrophobic moieties of dextrin molecules at the sulfur surface. Recent studies by Heinz et al.28 regarding the adsorption of amine surfactants at selected silicate mineral surfaces have suggested that on the basis of the charges and van der Waals parameters, the CLAYFF force field may have overestimated
the surface tension/energy of selected silicate minerals.28 As previously discussed in the Simulation Detail section, from comparison of swelling characteristics, far-infrared spectra, and mineral structures with experimental measurements, the CLAYFF force field has shown good promise to describe mineral/aqueous interfacial phenomena such as water structure and further surfactant molecule adsorption states.38 Our simulation results such as the talc wetting properties and selected organic molecules adsorption states at the talc basal plane surface have complimented previous experimental observations. Therefore, it is appropriate to use the CLAYFF force field to study surface chemistry of selected silicate/oxide mineral system. Nevertheless, further efforts to explore the significance of mineral surface energy on the surfactant molecule adsorption states deserve much attention.
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Figure 13. Dextrin adsorption at selected naturally hydrophobic mineral surfaces (data are taken from refs 9 and 16).
Conclusions MDS of adsorption states of the cationic surfactant DTAB at the graphite surface and the talc basal plane surface provides further information for the fundamental understanding of surfactant adsorption mechanisms at naturally hydrophobic surfaces. It is concluded from MDS that the hydrophobic characteristic of the graphite surface and the talc basal plane surface favors the hydrophobic interaction with the nonpolar alkyl chains of the cationic surfactant DTAB and renders the basal plane surface hydrophilic. These results are in agreement with the previously reported results that reveal a significant decrease in contact angle when cationic surfactants are present at sufficient concentration and with the results from soft contact AFM imaging of surface micelle structures. MDS regarding the adsorption of dextrin at selected naturally hydrophobic surfaces such as graphite, talc basal plane, and sulfur suggests that hydrophobic interactions between the substrates and the hydrophobic moieties in the dextrin molecule play a significant role in the adsorption processes. At the graphite surface, which is composed of neutral elemental carbon atoms, the dextrin
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molecule reorients to expose as many hydrophobic moieties as possible to the hydrophobic graphite surface while the majority of the dextrin hydroxyl groups hydrogen bond with water molecules by stretching into bulk aqueous phase. It is proposed that the dextrin molecule is attracted to and stabilized at the talc basal plane surface due to the hydrophobic attraction between the talc basal plane surface and the hydrophobic moieties of the dextrin molecule. The electrostatic interaction between the charged talc basal plane surface oxygen atoms and the hydrogen atoms in the hydroxyl groups also contribute to dextrin adsorption at the talc basal plane surface. In contrast, dextrin was not found to adsorb at talc edge surfaces. The behavior of a dextrin molecule at the rhombic sulfur (111) surface suggests a different adsorption state from that observed at the graphite and talc basal plane surfaces. The dextrin molecule attaches to the sulfur substrate through the hydrophobic interaction between the methyl group and the sulfur surface, while the remainder of the dextrin molecule stays in the aqueous phase. The decreased level of attachment of the dextrin molecule at the sulfur surface compared to that at the graphite and talc basal plane surfaces may be due to the inherent atomic roughness of the sulfur surface, which allows instantaneous accommodation of water molecules within the sulfur surface region. Consequently, significant energetic and structural barriers are produced for all the hydrophobic moieties of the dextrin molecule that inhibit interaction with the sulfur substrate. Nevertheless, the dextrin adsorption density is equivalent to that observed for graphite and talc. Acknowledgment. The authors are grateful to Prof. Jayendran C. Rasaiah for valuable discussions and grateful for the collaboration with Prof. Anh V. Nguyen, who helped to initiate the MDS research. The financial support provided by the Department of Energy, Basic Science Division Grant No. DEFG-03-93ER14315 is gratefully acknowledged. In addition, this study was prompted to some extent by collaborative research supported by NSF under grant nos. INT-0227583 and INT0352807. LA701604U
