Dynamics of Water in Hydrophobic Confinement

1. Dynamics of Water in Hydrophobic Confinement FIGURE 2: a) the temperature dependence of D in hydrophobic MCM-41-SM1-18. The continuous red line represents the values obtained for bulk water. The dashed black line is the fit to the data with a VFT law. The inset b) compares the average characteristic translational relaxation times of water in MCM-41-SM1-18 with those of water in hydrophilic MCM-41-S-14. FIGURE 1: Measured NSE values of the intermediate scattering function, plotted semilogarithmically as a function of sqrt(Q2 t), lie on straight lines whose slopes depend only on temperature, indicating a distribution of diffusive processes. A. Faraone1, K.-H. Liu2, C.-Y. Mou2, Y. Zhang3, and S.-H. Chen3 DMR-0454672 DE-FG02-90ER45429 1NIST and U. Md., 2National Taiwan U., 3MIT To better understand the dynamics of water near a hydrophobic surface, we have studied water confined in a hydrophobically modified nanoporous silica matrix, MCM-41-SM1-18, with ≈15 Å pore dimension. Confinement prevents crystallization, enabling measurements from 300 K to 210 K. This is important because the anomalous behavior of water can only be understood by probing water in deeply supercooled states. To follow the time evolution of the water dynamics, we collected data using three CHRNS spectrometers; DCS, HFBS, and NSE. At high temperature T the measured average diffusion coefficient, D, is similar to that of bulk water. In the deeply supercooled region, the diffusion coefficient of bulk water tends to decrease with decrease in T, much faster than for water in MCM-41-SM1-18. Figure 2(b) compares the average characteristic relaxation times 0 for water in hydrophilic MCM-41-S (pore diameter ≈14 Å), and in hydrophobically modified MCM-41-SM1-18. At high T, 0is the same forboth samples but in the deeply supercooled region  as in super-cooled bulk water H2O molecules confined in hydrophilic sieves show a much stronger dependence of 0 on T. Fitting the data using a Vogel-Fulcher-Tammann (VFT) law, we find that the fragility parameter is higher for water in MCM-41SM1-18 than in MCM-41-S, suggesting that hydrophobically confined water behaves as a stronger glass-former. These findings suggest that water confined in a hydrophilic matrix is qualitatively more similar to bulk water than water confined by hydrophobic walls. A. Faraone, K.-H. Liu, C.-Y. Mou, Y. Zhang, and S.-H. Chen, J. Chem. Phys., 130, 134512 (2009).