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J. Phys. Chem. B 2002, 106, 10292-10295
Comparison of the Orientational Dynamics of Water Confined in Hydrophobic and Hydrophilic Nanopores Alessandra Scodinu and John T. Fourkas* Eugene F. Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467 ReceiVed: June 20, 2002; In Final Form: August 22, 2002
The collective orientational dynamics of water confined in hydrophilic and hydrophobic nanopores have been studied using ultrafast optical Kerr effect spectroscopy. In both types of pores the dynamics depend on pore size and are significantly slower than in the bulk liquid. The slowest dynamics are attributed to molecules at the pore surfaces and are faster in hydrophobic pores than in hydrophilic pores. The dynamics of molecules in the centers of the pores are also inhibited as compared to the bulk liquid but are not influenced by the nature of the pore surfaces. These results suggest that the chemical nature of a solid/water interface affects the structural and dynamic properties of water over only a very short distance.
The behavior of liquids can change radically upon confinement in regions with dimensions on the order of a few molecular diameters.1 The structure and dynamics of nanoconfined water in particular have received considerable attention, due in no small part to the importance of confined water in the structure and function of biomolecules.2-4 It is important in both biological and other contexts to understand the differences between water confined in hydrophilic and hydrophobic environments.5,6 Although it is known from simulations that the changes in going from a hydrophilic environment to a hydrophobic one can be significant,7,8 the difficulties inherent in introducing water into a hydrophobic nanocavity have limited the experiments reported to date almost entirely to the study of water in hydrophilic nanopores. Here we report what is to our knowledge the first comparative study of the orientational dynamics of water in saturated hydrophobic and hydrophilic nanopores of well-defined size and geometry. Nanoconfinement is known to have several significant structural effects on water.7-12 As is the case with other liquids, water exhibits densification and layering near walls, whether they be hydrophobic or hydrophilic. Near hydrophilic walls the number of water-water hydrogen bonds decreases in concert with the number of nearest neighbors, although additional hydrogen bonds can form with the walls.8 Near hydrophobic walls there is an increase in water-water hydrogen bonding relative to the decrease in number of nearest neighbors that propagates approximately 0.6 nm out from the interface.7,8 In both cases, the properties of the confined water become more like those of the bulk with increasing distance from the pore surfaces. Hydrogen bonding of water to hydrophilic pore surfaces dominates its dynamic behavior, significantly decreasing the rates of translational and orientational diffusion as compared to the bulk.8,13 The orientational correlation time decreases monotonically in moving away from the pore surfaces, as the effects of the fixed hydrogen bonding sites at the pore surfaces are felt only over a limited distance. However, even in the centers of sizable pores the orientational correlation time can be modestly larger than in the bulk.7,8 * To whom correspondence should be addressed. E-mail:
[email protected].
At the surfaces of hydrophobic pores two conflicting effects come into play to influence the dynamic behavior of water. On one hand, the hydrophobic surfaces can act as a “lubricant”. For example, the orientational dynamics of hydrogen-bonded organic liquids confined in hydrophobic pores have been observed to be similar to those of the bulk liquid, although in hydrophilic pores the dynamics of the same liquids are inhibited significantly.3,14,15 Indeed, in the supercooled regime the dynamics of these liquids in hydrophobic pores can be significantly faster than those in the bulk if the dynamic correlation length in the liquid is greater than the pore size.14,15 By the same token, the translational diffusion of water can be enhanced near hydrophobic surfaces.8 On the other hand, the increase in density and in hydrogen bonding near hydrophobic surfaces has been observed in a simulation to lead to a modest inhibition of the orientational dynamics of water.8 Which of these effects wins out probably depends as well on factors such as the pore size and geometry. We have employed optical-heterodyne-detected optical Kerr effect (OKE) spectroscopy16 to study the orientational dynamics of water confined in hydrophilic and hydrophobic nanoporous sol-gel glasses17 at 293 K. OKE spectroscopy is the timedomain analogue of Rayleigh-wing scattering18 and can be used to measure the collective orientational correlation function of transparent liquids. Our experimental apparatus has been described in detail elsewhere.19 Monolithic nanoporous silica sol-gel glass samples were prepared as described previously.19 For the studies reported here, samples with average pore radii of 1.25, 2.95, and 4.98 nm were employed. BET measurements indicated that the distribution of pore sizes in each sample varied by approximately (10% from the average pore size. The samples were polished to high-optical-quality disks that were approximately 1 mm thick and 8 mm in diameter. In their raw state, the porous glass samples have approximately 2.5 surface hydroxyl groups per nm2,17 and so they are highly hydrophilic. To study the effects of confinement in hydrophilic pores, each sample was immersed in a 2 mm path length optical cell filled with ultrapure water and allowed to imbibe the liquid for at least 24 h to saturate the pores with water. The bulk liquid remained in the sample cells during data collection, but the pump and probe laser beams were aligned carefully so that their
10.1021/jp026349l CCC: $22.00 © 2002 American Chemical Society Published on Web 09/18/2002
Letters
J. Phys. Chem. B, Vol. 106, No. 40, 2002 10293 TABLE 1: Average Collective Orientational Times (ps) as a Function of Pore Radius hydrophilic hydrophobic
Figure 1. Natural logarithm of Ccoll(τ) for bulk water and water confined in hydrophilic pores. The dotted line through the bulk data is a biexponential fit with time constants of 0.81 and 2.7 ps, and the bottom plot is a magnified view of the residuals of the fit (not on a logarithmic scale). The data sets have been displaced from one another for clarity.
crossing volume was completely within the porous glass. All data were obtained at 293 K. After data collection in the hydrophilic pores was completed, the samples were dried carefully and then treated with chlorotrimethylsilane to make the pore surfaces hydrophobic.20 On the basis of IR absorption measurements, the conversion of the surface hydroxyl groups was estimated to be greater than 98%. As an additional indication of the hydrophobicity of these samples, after this treatment the samples floated on water and could not be filled with this liquid directly. However, the samples could be filled with methanol readily, and the methanol could then be exchanged with water by soaking the samples in several successive samples of ultrapure water. To monitor the removal of methanol from the pores, CD3OH was employed for the initial filling of the pores, and the disappearance of CD3 vibrational modes upon exchange of methanol with water was monitored with IR absorption spectroscopy. After the CD3OH could no longer be detected, at least one more soak in water was performed. On the basis of our detection limit for CD3OH, we estimate conservatively that at least 95% of the methanol was removed by exchange. The OKE decay is proportional to the negative time derivative of the collective orientational correlation function (Ccoll(τ)).21 Thus, this correlation function can be obtained from the negative integral of the signal with respect to time. Integration of the signal has the additional advantage of damping high-frequency noise, allowing us to obtain useful data at longer delay times than would otherwise be possible. However, it is also necessary to determine the constant of integration. As long as the decay has a well-defined functional form (such as an exponential), the constant of integration can be determined readily, but in other cases it may not be able to be determined uniquely. For the data reported here it proved possible to determine the constant of integration uniquely, so we will discuss the orientational correlation functions rather than the OKE decays. Figure 1 shows Ccoll(τ) in the bulk liquid and in hydrophilic pores. The long-time tail of the correlation function in the bulk liquid can be fit reasonably well to a biexponential function with time constants of 2.7 and 0.81 ps (Figure 1), in good agreement with the decay times reported at this temperature by Winkler et al.22 The slower decay constant is expected to reflect the collective orientational correlation time, although Winkler et al. have pointed out that the close correspondence of this number with the orientational correlation time derived from NMR experiments suggests that the collective and singlemolecule orientational correlation times are identical in this liquid.22 The source of the faster decay is a matter of some
bulk
4.98 nm
2.95 nm
1.25 nm
2.7 2.7
6.1 5.1
15.7 9.9
22.6 19.7
debate and will not be considered in detail here. To determine average collective orientational times 〈τor〉 in the hydrophilic and hydrophobic pores, the Ccoll(τ) data for τ > 2 ps were fit to biexponential decays. The values of 〈τor〉 are listed in Table 1. Although 〈τor〉 in the hydrophobic pores is smaller than that in the hydrophilic pores, on average the relaxation in the hydrophobic pores is only about 25% faster than that in the hydrophilic pores. Close inspection of the residuals in the biexponential fit to Ccoll(τ) for both bulk and confined water reveals systematic deviations, suggesting that the orientational relaxation is not truly exponential, which is not surprising for a liquid with the complex character of water. A log-log plot of Ccoll(τ) for bulk water (Figure 2) reveals that another reasonable empirical description of the decay is a pair of power laws, t-b. The powerlaw exponent b for times greater than approximately 1.5 ps is 2.0, whereas for times between 0.1 and 1 ps the exponent is 0.66. A number of different functional forms have been explored for fitting dynamical data for water confined in hydrophilic pores. For instance, Crupi et al. were able to fit Rayleigh-wing data on water confined in sol-gel glasses to a HavriliakNegami profile, which is essentially the frequency-domain equivalent of a stretched exponential, exp(-(t/τ)β).23 Gallo, Rovere, and Spohr fit the intermediate scattering function of simulated supercooled water confined in a glass pore to the sum of a stretched exponential and a Gaussian decay.24 Our confinedwater data do not fit well to either of these functions. Although the collective orientational correlation function would not necessarily be expected to have the same dynamics as the intermediate scattering function for supercooled confined water, it is surprising that our data cannot be fit in the same manner as the Rayleigh-wing spectrum. We believe that the reason for this disparity is that OKE spectroscopy gives higher signal-tonoise ratios for low-frequency (i.e., long-time) features, whereas Rayleigh-wing spectroscopy is more sensitive to high-frequency features. Thus, our data provide a more stringent test for models of the long-time behavior of the collective orientational correlation function. The empirical power-law description that was used above for bulk water also works well for the pore data out to times in the neighborhood of 15 ps (Figure 1), after which the confined decays appear to be exponential. The power-law exponents from fits to this function are listed in Table 2. Whether or not this empirical model has direct physical significance, it is a useful means of making a direct comparison of the dynamics in hydrophilic and hydrophobic pores. In the 2.95 nm and 4.98 nm pores two power-law decays are observed, the earlier of which has an exponent of 0.45 and the latter of which has an exponent that is smaller than that of bulk water and becomes smaller when the pore size is decreased or when the pore surface is hydrophobic. In the 1.25 nm pores a single power-law decay is observed, and we assume from this that the earlier process cannot be observed once the exponent for the later process is less than 0.45. At first glance the comparison between the results in hydrophilic and hydrophobic pores seems contradictory, in that 〈τor〉 is smaller in hydrophobic pores than in hydrophilic pores whereas b is smaller in hydrophobic pores than in hydrophilic
10294 J. Phys. Chem. B, Vol. 106, No. 40, 2002
Letters
Figure 2. log-log plot of Ccoll(τ) for water in the bulk and in hydrophilic pores (solid lines) and power-law fits to the data (dashed lines). The data sets have been displaced from one another for clarity.
TABLE 2: Values of b for Early Time and Later Time Power-law Decays as a Function of Pore Radius hydrophilic hydrophobic
bulk
4.98 nm
2.95 nm
0.66 2.0 0.66 2.0
0.46 0.97 0.49 0.80
0.42 0.64 0.45 0.56
1.25 nm 0.33 0.07
pores. It is important to recognize that 〈τor〉 is dominated by the slowest relaxation, and in particular will be influenced strongly by the relaxation that occurs at times longer than those for which the data can be described well by a power law. On the other hand, the power-law fits describe the relaxation on shorter time scales. In light of the simulations discussed above, these results can be interpreted by assuming that all of the water in the pores has dynamics that are inhibited compared to the bulk, but that the molecules near the pore walls exhibit additional dynamic inhibition. As predicted, the inhibition of dynamics near the walls of hydrophobic pores is less than that in hydrophilic pores. The inhibition of dynamics of molecules that are not near the pore walls depends strongly on the pore curvature. Although the dynamic inhibition in the centers of hydrophobic pores appears significantly stronger than that in hydrophilic pores, it is important to recognize that the procedure used to make the pore surfaces hydrophobic also decreases the effective pore diameter. To determine whether this effect can account for the smaller values of b observed in the hydrophobic pores, we plotted b as a function of pore curvature for both the hydrophilic and hydrophobic pores, adjusting the effective curvature of the hydrophobic pores to account for the addition of the hydrophobic groups to the surfaces. If we assume that the thickness of the hydrophobic layer is ≈0.6 nm, the data for the hydrophilic and hydrophobic pores fall on a single curve (Figure 3). We therefore believe that the power-law exponent does not depend on the nature of the surface, but rather only on the pore curvature. A thickness of 0.6 nm is slightly larger than might be expected for a trimethylsilyl group, so it is also possible that a surface layer with increased hydrogen bonding in the hydrophobic pores contributes in part to the observed reduction in pore radius. In either case, the dynamics of water are affected by the nature of the pore walls over only a very short distance from the surfaces. It is worth noting that whereas both experiments13 and simulations8 on water confined in slit pores have suggested that the water dynamics resemble those of the bulk within a few molecular diameters of a hydrophilic surface, we see a strong inhibitory effect on the dynamics even in the centers of the largest pores used here. This observation suggests that the geometry of the confining environment can play a strong role in determining the structure and dynamics of water over
Figure 3. Power-law exponents for the later time decay of Ccoll(τ) as a function of pore curvature. The triangle is for the bulk liquid, the circles are for hydrophilic pores, and the squares are for hydrophobic pores. The radii of the hydrophobic pores have been decreased by 0.6 nm compared to their values before surface modification.
relatively large distances, even though the nature of the confining surfaces is not so important. In conclusion, we have made what are, to our knowledge, the first measurements of the dynamics of water in saturated hydrophobic pores. In agreement with simulations, the relaxation of water near pore walls is less inhibited in hydrophobic pores than in hydrophilic pores. The relaxation in the pore centers is significantly slower than in the bulk and is increasingly inhibited with decreasing pore radius but does not depend on the nature of the pore walls. These results imply that any structural or dynamic effects that arise from the chemical nature of a solid/ water interface propagate only a very short distance into the liquid. Acknowledgment. This work was supported by the National Science Foundation, Grant CHE-0073228. J.T.F. is a Research Corp. Cottrell Scholar and a Camille Dreyfus Teacher-Scholar. References and Notes (1) Dynamics in Small Confining Systems IV; Drake, J. M., Grest, G. S., Klafter, J., Kopelman, R., Eds.; Materials Research Society: Warrendale, PA, 1999; Vol. 543, p 372. (2) Bhattacharyya, K.; Bagchi, B. J. Phys. Chem. A 2000, 104, 10603. (3) Crupi, V.; Majolino, D.; Migliardo, P.; Venuti, V. J. Phys. Chem. A 2000, 104, 11000. (4) Bellissent-Funel, M. C. J. Phys.-Condes. Matter 2001, 13, 9165. (5) Allen, T. W.; Kuyucak, S.; Chung, S. H. J. Chem. Phys. 1999, 111, 7985. (6) Beckstein, O.; Biggin, P. C.; Sansom, M. S. P. J. Phys. Chem. B 2001, 105, 12902. (7) Lee, C. Y.; McCammon, J. A.; Rossky, P. J. J. Chem. Phys. 1984, 80, 4448. (8) Lee, S. H.; Rossky, P. J. J. Chem. Phys. 1994, 100, 3334. (9) Hartnig, C.; Witschel, W.; Spohr, E.; Gallo, P.; Ricci, A.; Rovere, M. J. Mol. Liq. 2000, 85, 127. (10) Ricci, M. A.; Bruni, F.; Gallo, P.; Rovere, M.; Soper, A. K. J. Phys.-Condes. Matter 2000, 12, A345. (11) Gallo, P.; Ricci, M. A.; Rovere, M. J. Chem. Phys. 2002, 116, 342. (12) Fouzri, A.; Dorbez-Sridi, R.; Oumezzine, M. J. Chem. Phys. 2002, 116, 791. (13) Woessner, D. E. J. Magn. Reson. 1980, 39, 297. (14) Gorbatschow, W.; Arndt, M.; Stannarius, R.; Kremer, F. Europhys. Lett. 1996, 35, 719. (15) Arndt, M.; Stannarius, R.; Groothues, H.; Hempel, E.; Kremer, F. Phys. ReV. Lett. 1997, 79, 2077. (16) Righini, R. Science 1993, 262, 1386. (17) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990. (18) Kinoshita, S.; Kai, Y.; Yamaguchi, M.; Yagi, T. Phys. ReV. Lett. 1995, 75, 148. (19) Loughnane, B. J.; Farrer, R. A.; Scodinu, A.; Reilly, T.; Fourkas, J. T. J. Phys. Chem. B 2000, 104, 5421. (20) Loughnane, B. J.; Scodinu, A.; Fourkas, J. T. J. Phys. Chem. B 1999, 103, 6061.
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