Properties of Water Confined in an Amphiphilic Nanopore

View Sections. ACS2GO © 2018. ← → → ←. loading. To add this web app to the home screen open the browser option menu and tap on Add to hom...
1 downloads 0 Views 874KB Size
Download PDF
J. Phys. Chem. C 2008, 112, 16893–16897

16893

Properties of Water Confined in an Amphiphilic Nanopore Vitaly Kocherbitov* Biomedical Laboratory Science and Technology, Faculty of Health and Society, Malmo¨ UniVersity, SE-205 06 Malmo¨, Sweden ReceiVed: June 14, 2008; ReVised Manuscript ReceiVed: August 21, 2008

Molecular dynamics simulations of water and nitrogen confined in a model amphiphilic nanotube were performed. The nanotube has a diameter of 4 nm and consists of hydrophobic atoms and regularly placed OH groups. The results show that the density of water close to the nanotube walls is lower compared to the density in the center of the nanotube. The hydrogen bonded network of water molecules is practically intact compared with the bulk water. The simulation confirms that the experimentally observed low formal density of water in the nanopores (0.88 g/cm3) is due to formation of small unfilled cavities adjacent to the pore walls. Nitrogen molecules are localized primarily in the unhydrated cavities. The presence of nitrogen molecules is not the main reason for the decrease of water density in the nanotube. Introduction In most practical applications, nanomaterials are expected to be in contact with water. In close proximity to the nanomaterial especially when confined inside a nanopore, water is expected to have different properties than in the bulk. Hence, the nanomaterial-water interactions, including the properties of water confined in nanopores, attract broad scientific interest from chemists, physicists, and biologists. A large number of experimental studies1-5 and simulations6-19 dealing with properties of water in different types of pores have been presented. All simulations of water in nanopores published so far can be divided into two groups: simulations of water in hydrophobic6-13 and hydrophilic14-19 nanopores. The first group of simulations corresponds to the properties of water in carbon nanotubes; the second group corresponds to the properties of water in Vycor glass or in mesoporous silica materials like MCM-41. In reality, however, many types of nanopores have both hydrophilic and hydrophobic groups on the surface; in other words, they can be described as amphiphilic. Freshly calcined MCM-41, for example, has a surface density of OH groups of 1.6 nm-2 (as determined by sorption calorimetry1), which is much lower than the surface density that corresponds to a fully hydroxylated surface. Apart from hydrophilic OH groups, the rest of the surface of MCM-41 can be considered as rather hydrophobic. Moreover, surfaces of many biological nanosystems contain both hydrophilic and hydrophobic groups and patches; i.e., these surfaces are amphiphilic. The amphiphilic character of such molecules as lipids and surfactants leads to a great variety self-assembled structures20 that exist at different temperatures and water contents.21 In self-assembly of surfactants and lipids, the amphiphilic molecules arrange in structures where hydrophobic moieties aggregate and their contact with water is minimized. In the case of solid amphiphilic surfaces, however, the hydrophilic and hydrophobic groups are not mobile and cannot self-assemble. On the contrary, the interfacial water layer can adjust its properties according to the amphiphilic character of the surface. * Telephone: +4640 6657946. Fax: +4640 6658100. E-mail: [email protected].

Recently we proposed a model of hydration of MCM-41 pores based on the experimental observation that the density of water in the nanopores is lower than that in the bulk.1 According to this approach, water hydrates hydrophilic OH groups on the surface but leaves hydrophobic patches unhydrated. Small cavities adjacent to the silica walls may thus be formed, while the density of water in the center of the cylindrical pore is close to the density of bulk water. Based on results obtained using the method of desorption calorimetry,22 we also suggested that the cavities may contain molecules of air.1 Without using additional experimental data or simulations, it was, however, impossible to judge whether the molecules of air fill the cavities that exist in any case or if the air is the primary reason for their formation. Here we present a molecular dynamics simulation study of water confined in a 4 nm amphiphilic nanopore that has the surface density of hydrophilic groups close to that in calcined MCM-41. The densities of water used in the simulation are close to the density of water in the pores of MCM-41 at the end of the capillary condensation transition. To elucidate the role of air in the hydration of the amphiphilic surface of the nanotube, molecules of nitrogen were added in the system. Simulation Model In the model used here, an amphiphilic nanotube consists of 27 rings of 45 oxygen atoms. Every third ring contains both charged (-0.4) and uncharged oxygen atoms; other rings contain only uncharged oxygen atoms (that correspond to siloxane groups on the silica surface). In the rings that contain charged oxygen atoms, only every third oxygen atom is charged. Every charged oxygen atom is connected to a hydrogen atom that has the same but positive charge. All OH bonds are directed toward the center of the ring. The total length of the tube is 8.1 nm, and the diameter (calculated as the distance between centers of oxygen atoms) is 4.3 nm, which gives an inner diameter of the tube of about 4.0 nm (Figure 1). Values of parameters of bonded interactions between atoms of the tube were chosen in a way that prevents significant deformation of the tube during simulation. Full periodic boundary conditions were applied to an 8.0 × 8.0 × 8.1 nm simulation box. The tube was aligned along

10.1021/jp805247b CCC: $40.75  2008 American Chemical Society Published on Web 10/04/2008

16894 J. Phys. Chem. C, Vol. 112, No. 43, 2008

Kocherbitov

Figure 1. Model of an amphiphilic nanotube. White, hydrogen atoms; red, polar oxygen atoms; magenta, nonpolar oxygen atoms.

the z-axis. The potential parameters of the nanotube used in the simulation can be found in the Supporting Information. We should point out that the present model does not reproduce all features of MCM-41 silica. It is rather designed to mimic amphiphilic character of its surface: the presence of both hydrophobic patches and hydrophilic OH groups. In our recent sorption calorimetric study1 we calculated the density of OH groups in calcined MCM-41 as 1.6 nm-2. In the model presented here, the distances between closest OH groups are 0.9 nm, which gives OH group density about 1.2 nm-2. We also performed simulations with a modified model that contains 54 oxygen atoms per ring, thus giving an inter-OH distance of 0.75 nm and a density of OH groups of about 1.8 nm-2. The results of the two sets of simulations are qualitatively similar. If it is not stated otherwise, the presented results refer to simulations of the model that includes 45 oxygen atoms per ring. The SPCE model of water23 was used. Besides 2900-3100 water molecules, 30 nitrogen molecules were placed inside the tube. Each nitrogen molecule was represented by two bound uncharged Lennard-Jones spheres. The space outside the tube did not contain any molecules. Molecular dynamics simulations were performed using the GROMACS24,25 software package. Simulations were run at 300 K, and a time step of 2 fs was used. Long-range electrostatic interactions were treated using the particle mesh Ewald method. Each system was equilibrated during at least 2 ns, and production runs were 5 ns long.

Figure 2. Particle density maps in a 5 ns simulation of 2993 water and 30 nitrogen molecules in an amphiphilic nanotube. The densities are averaged along the z-axis including both hydrophilic and hydrophobic rings of the tube. Red, atoms of the nanotube (including hydrogen atoms); blue, water oxygen atoms; green, nitrogen atoms. Intensities of different colors are scaled independently.

Results and Discussion Distribution of Molecules of Nitrogen. Figure 2 shows density maps of water oxygen and nitrogen atoms in a nanotube filled with water that has a total density of 0.88 g/cm3. Hydrophilic oxygen atoms of the nanotube can be easily identified by the presence of hydrogen atoms close to every third oxygen atom. Most of the nitrogen molecules are located close to the surface of the nanotube, and especially high concentrations are seen in the hydrophobic regions of the tube surface. This confirms an earlier proposed idea that nitrogen molecules in pores of MCM41 filled with water are preferentially located close to the walls of the pores.1 The reason for this is the hydrophobic interactions driving nonpolar nitrogen molecules toward hydrophobic parts of the nanotube. Low concentrations of nitrogen in bulk areas and a discontinuous monolayer of nitrogen molecules confined between the nanotube surface and water were observed in most of the performed simulations. In a simulation with the lowest number of water moleculess 2900sthat corresponds to the total density of 0.852 g/cm3, formation of a bubble of nitrogen was observed (Figure 3). The

Figure 3. Snapshot of a molecular dynamics simulation of 2900 water molecules and 30 nitrogen molecules in a nanotube. The formal density of water is 0.852 g/cm3. In contrast with other simulations (with higher densities of water), a nitrogen gas bubble was formed.

bubble spanned up to 1 nm toward the center of the tube, and the hydrogen bonds between the surface and water were broken. Nonetheless, the amphiphilic silica surface is more hydrophobic than the surface of water; therefore, the formation of the bubble close to the wall (not in the center of the tube) is reasonable. Considering the formation of the bubble from a thermodynamic point of view, due to low compressibility of water the system could not withstand low pressure caused by the low density of water and a phase separation occurred. An increase

Water Confined in Amphiphilic Nanopore

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16895

Figure 4. Radial distribution functions of SPCE water oxygen with respect to the center of mass of the system in 5 ns simulations with 2900, 2993, and 3100 water molecules. Only x and y coordinates of oxygen atoms were used in calculations.

TABLE 1: Water Density and Hydrogen Bonds in Simulated Nanotube with a Radius of 2 nm H2O density, g/cm3 H2O molecules

N2 molecules

no. of H-bondsa

total

inner 1 nm

outer 1 nmb

2900 2993 2993 3000 3100 1728 (bulkc)

30 30 0 30 30 0

1.75 1.75 1.75 1.75 1.76 1.78

0.852 0.880 0.880 0.882 0.911 0.942

0.959 0.957 0.947 0.958 0.978 0.942

0.817 0.853 0.857 0.856 0.889 0.942

a The number of hydrogen bonds in the system divided by the number of water molecules. Calculated according to conditions r(O-O) < 0.35 nm and R(HOO) < 30°. b The density in the outer layer was calculated as 3Fout ) 4Ftot - Fin. c For comparison, properties of bulk SPCE water of density similar to that in the inner 1 nm (no nanotube, size of the simulation box 3.8 nm) are shown.

of the Gibbs energy of the system caused by formation of water-air interface is apparently lower than the increase of the Gibbs energy that could be caused by homogeneous stretching of the liquid water. At higher simulated densities of water (0.88 g/cm3 and above), the system could withstand corresponding pressures and no phase separations were observed. Density of Confined Water. In experimental studies of water confined in nanopores, it was shown that the density of water in such pores is lower than in the bulk.1 The density of water in 4 nm pores of both MCM-41 and Vycor glass is shown to be about 0.88 g/cm3. This phenomenon can be interpreted in two different ways. As a first alternative, the decrease of the water density can be homogeneous throughout the whole volume of confined water. As a second alternative, the decrease of the density can be more pronounced close to the walls of the nanotube due to a weaker hydration of the hydrophobic patches of the nanotube surface.1 If the density of water decreases homogeneously, then the Gibbs energy change during capillary condensation should include contributions not only from the decrease of the interfacial tension and from the change of water activity but also from the isothermal expansion of the liquid. Since the isothermal compressibility of liquid water is very low (45.2 × 10-11 Pa-1), the 12% decrease of density of water would imply

Figure 5. Densities of water in the inner and the outer parts of the nanotube as functions of the total density of water. Open symbols show a simulation without N2 molecules.

a large increase of Gibbs energy of the system. On the other hand, the decrease of the water density lowers the mass of water needed for filling the nanopore, which favors the capillary condensation. In order to choose between homogeneous and nonhomogeneous changes of water density in the pore, we compared the densities of water in the center of the pore and in the outer part of the pore. Figure 4 shows the radial distribution function of water oxygen atoms with respect to the center of mass of the system. The whole 2 nm radius of the tube can be divided into two rather different parts: inner 1 nm and outer 1 nm (the latter appears to be more structured). The density of water in the central part of the pore (inner 1 nm) Fin was calculated from the radial distribution function. The density in the outer part of the pore Fout was calculated using the total density of water in the pore:

3Fout ) 4Ftot - Fin

(1)

The results of the calculations are shown in Table 1. In all considered cases, the density of water in the inner part of the tube is higher than the total density, while the density in the outer part of the tube is lower. The difference in densities in the inner and outer parts of the tube is larger in the case of low total densities, while at higher total densities the difference is less pronounced. It would be reasonable to expect that at total densities close to 1 g/cm3 the difference in densities in the two parts of the tube will be rather small. One should note, however, that a part of the density decrease in the case of the first point in figure 5 arises from the presence of the air bubble (Figure 3). Still, the effect is seen even if this point is excluded from consideration. Interestingly, the density of water in the inner part of the tube in the case of air bubble formation does not reach 1 g/cm3, but has a value of 0.96 g/cm3. This density is close to the density in the center of the tube observed for the next point, which corresponds to the total density of 0.88 g/cm3sthe density measured in experimental studies.1 Structure of Confined Water and Hydrogen Bonding. In order to explore the impact of lowering of the water density on the structure of water, one should examine radial distribution functions and hydrogen bonding related properties. The density profiles of water in the nanotube (radial distribution function with respect to the center of mass of the system) are shown in

16896 J. Phys. Chem. C, Vol. 112, No. 43, 2008

Figure 6. Snapshot of molecular dynamics simulation of 2993 water molecules and 30 nitrogen molecules in a nanotube. The formal density of water is 0.880 g/cm3. The molecular surface of water is drawn in the slab mode using the Lee-Richards27 method embedded in the RasMol program. OH groups of the nanotube are shown as sticks. Approximate positions of the hydrophobic atoms of the nanotube are shown by straight lines.

Figure 4. They differ substantially depending on the number of water molecules in the nanotube. The most different shape of the density profile (observed in case of 2900 molecules of water) is, however, due to the formation of the air bubble that displaces the center of mass of the system. The O-O radial distribution function (data not shown) changes much less depending on the water content than the density profile does. The first and the second peaks of the O-O radial distribution function are at 2.75 and 4.5 Å, respectively, which are close to the values found in the bulk water. Confinement-induced changes in the hydrogen bonded network compared to the bulk water are even smaller. Distributions of O-O distances and O-H-O angles between hydrogen bonded atoms are practically identical in the bulk water and in the confined water. The number of hydrogen bonds per water molecule including the hydrogen bonds between water and OH groups of the pore is about 1.75 (or 3.5 if both donated and accepted bonds of a molecule are counted), which is close to that in the bulk water (Table 1). One should keep in mind that about one-third of the water molecules in this system can be considered as interfacial molecules and a loss for example of one hydrogen bond per interfacial water molecule would decrease the average number of hydrogen bonds substantially. The result presented here shows that creation of an interface between the amphiphilic surface and water does not lead to a significant loss of hydrogen bonds. Moreover, the geometry of the hydrogen bonds remains essentially intact. Unfilled Cavities. Intact geometry of hydrogen bonded network in combination with lower density of water in the outer layer of water can be explained by the presence of small unfilled cavities close to the pore walls. The presence of such cavities was predicted in our experimental study of hydration of MCM41.1 Figure 6 shows that the unfilled cavities adjacent to hydrophobic patches indeed exist. The unfilled cavities should be described as dynamic (they can change the size and even be filled with water for a short time). On average, the difference between hydration of hydrophilic and hydrophobic oxygen atoms of the nanotube can be quantified as follows. About 97% of hydrophilic oxygen atoms of the nanotube have a water oxygen atom within a 3.5 Å distance, while only 12% of hydrophobic atoms of the nanotube have a water oxygen atom within the same distance. Figure 6 shows that the interface between water and the amphiphilic surface is a combination of small (less than 1 nm) cavities. It is known that hydration of small and large (more than 1 nm) cavities in water involves two different mechanisms. In the first case the hydrogen bonds around a solute are preserved; in the second case they are broken.26 The hydrogen

Kocherbitov bonded network can go around small cavities, while to hydrate the large cavities the hydrogen bonded network needs to be broken. In the simulation presented here, the hydrogen bonds between interfacial water molecules are preserved by formation of small cavities. Hydration of small cavities involves a decrease of entropy of the sytem,26 which is in agreement with our calorimetric data1 that show a negative entropy of hydration of MCM-41. The density map of nitrogen (Figure 2) indicates that nitrogen molecules are mostly localized in the unhydrated cavities. However, the nitrogen molecules are not the primary reason for their formation since their impact on the distribution of water density in the nanotube is limited (see the data in Table 1). Conclusions A molecular dynamics simulation of a model amphiphilic nanotube filled with SPCE water and nitrogen has been performed. The results indicate the following: • The density of confined water in the outer layer (close to the walls of the tube) is lower than that in the center of the nanotube. • The hydrogen bonded network of water remains essentially intact upon confinement. • The decrease of the density is mostly caused by formation of unfilled cavities adjacent to hydrophobic patches of the amphiphilic nanotube. • In the presence of air, the cavities are filled with air molecules. The presence of air is, however, not the main reason for the formation of the cavities. Supporting Information Available: Potential parameters used in the simulation of the nanotube. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kocherbitov, V.; Alfredsson, V. J. Phys. Chem. C 2007, 111, 12906. (2) Smirnov, P.; Yamaguchi, T.; Kittaka, S.; Takahara, S.; Kuroda, Y. J. Phys. Chem. B 2000, 104, 5498. (3) Grunberg, B.; Emmler, T.; Gedat, E.; Shenderovich, J.; Findenegg, G. H.; Limbach, H. H.; Buntkowsky, G. Chem.sEur. J. 2004, 10, 5689. (4) Majolino, D.; Corsaro, C.; Crupi, V.; Venuti, V.; Wanderlingh, U. J. Phys. Chem. B 2008, 112, 3927. (5) Kittaka, S.; Ishimaru, S.; Kuranishi, M.; Matsuda, T.; Yamaguchi, T. Phys. Chem. Chem. Phys. 2006, 8, 3223. (6) Hummer, G.; Rasaiah, J. C.; Noworyta, J. P. Nature 2001, 414, 188. (7) Striolo, A.; Chialvo, A. A.; Gubbins, K. E.; Cummings, P. T. J. Chem. Phys. 2005, 122. (8) Liu, Y. C.; Wang, Q.; Lu, L. H. Chem. Phys. Lett. 2003, 381, 210. (9) Striolo, A.; Gubbins, K. E.; Gruszkiewicz, M. S.; Cole, D. R.; Simonson, J. M.; Chialvo, A. A. Langmuir 2005, 21, 9457. (10) Takaiwa, D.; Hatano, I.; Koga, K.; Tanaka, H Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 39. (11) Noon, W. H.; Ausman, K. D.; Smalley, R. E.; Ma, J. P. Chem. Phys. Lett. 2002, 355, 445. (12) Brovchenko, I. V.; Geiger, A.; Paschek, D. Fluid Phase Equilib. 2001, 183, 331. (13) Hennrich, F.; Arnold, K.; Lebedkin, S.; Quintilla, A.; Wenzel, W.; Kappes, M. M. Phys. Status Solidi B: Basic Solid State Phys. 2007, 244, 3896. (14) Brovchenko, I.; Oleinikova, A. J. Phys. Chem. C 2007, 111, 15716. (15) Gallo, P.; Rapinesi, M.; Rovere, M. J. Chem. Phys. 2002, 117, 369. (16) Gallo, P.; Rovere, M.; Spohr, E. J. Chem. Phys. 2000, 113, 11324. (17) Rovere, M.; Ricci, M. A.; Vellati, D.; Bruni, F. J. Chem. Phys. 1998, 108, 9859. (18) Spohr, E.; Hartnig, C.; Gallo, P.; Rovere, M. J. Mol. Liq. 1999, 80, 165. (19) Rovere, M.; Gallo, P. Eur. Phys. J. E 2003, 12, 77. (20) Seddon, J. M.; Templer, R. H. Polymorphism of Lipid-Water Systems. In Handbook of Biological Physics, Vol. 1: Structure and Dynamics of Membranes; Lipowsky, R., Sackmann, E., Eds.; Elsevier: New York, 1995; p 97.

Water Confined in Amphiphilic Nanopore (21) Kocherbitov, V. J. Phys. Chem. B 2005, 109, 6430. (22) Kocherbitov, V.; Wadso¨, L. Thermochim. Acta 2004, 411, 31. (23) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. J. Phys. Chem. 1987, 91, 6269. (24) Lindahl, E.; Hess, B.; van der Spoel, D. J. Mol. Model. 2001, 7, 306.

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16897 (25) Van der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.; Berendsen, H. J. C. J. Comput. Chem. 2005, 26, 1701. (26) Chandler, D. Nature 2005, 437, 640. (27) Lee, B.; Richards, F. M. J. Mol. Biol. 1971, 55, 379.

JP805247B