Dynamic properties of water in porous Vycor glass studied by dielectric techniques

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Journal of Non-Crystalline Solids 171 (1994) 201-207

Dynamic properties of water in porous Vycor glass studied by dielectric techniques P. Pissis a,,, j. Laudat b, D. Daoukaki a, A. Kyritsis c a Department of Physics, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece b Institute of Physics, Charles University, Ke Karlovu 5, 12116 Prague 2, Czech Republic c Department of Physics, University of Athens, Panepistimioupolis, Zografou, 15711 Athens, Greece Received 15 April 1993; revised manuscript received 9 February 1994

Abstract

The structure and dynamic properties of water in porous Vycor glass of 40 ,~ mean pore diameter, as well as their changes with temperature, were studied by dielectric relaxation spectroscopy and dc conductivity measurements. The mobility of water molecules in the pores was reduced compared with that of bulk water by a factor which decreased with increasing water content. A significant fraction of water in the first monolayer is transformed to a glassy state at about 170 K. The formation of water clusters around primary hydration sites started at about 0.02 g(H20)/g (dry sample) where a transition to long-range connectivity among water molecules was observed.

1. Introduction The structure and dynamic properties of liquids confined in small volumes of porous media, such as silica glasses, have been studied using a variety of techniques [1-14]. These studies include observations of liquid-solid phase transitions and restriction of molecular mobility of confined liquids. Special techniques have been used to distinguish between physical and chemical contributions to the changes observed in the properties of confined liquids compared with their bulk properties [6]. The former are due to geometrical confinement of the liquids in pores, while the

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latter arise from interaction of liquid molecules with surface sites. Dielectric relaxation spectroscopy (DRS), thermally stimulated depolarization (TSDC) and polarization (TSPC) and dc conductivity were used to study the effects of confinement of water in porous Vycor glass.

2. Experimental We investigated Corning Vycor glass No. 7930 with porosity (defined as the volume of voids divided by the total volume) q~ = 0.28, internal surface area S = 250 m 2 / g and the average pore diameter is 40 A. The samples were 15 mm diameter 0.5-2.0 m m thick cylinders. They were carefully dried for 24 h at 120°C in wlcuum. The

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P. Pissis et al. /Journal o f Non-Crystalline Solids 171 (1994) 201-207

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samples were hydrated over saturated salt solutions in closed jars. The water content, h, defined as g of water per g of dry sample, was determined by weighing the samples prior to and after the dielectric measurements. The water content varied between 0.02 and 0.22 (_+ 0.005). Thermally stimulated depolarization (TSDC), polarization (TSPC) and dc conductivity were measured in the range 77-300 K as described elsewhere [15,16]. AC dielectric measurements at 25°C in the frequency range 5 Hz-10 GHz were carried out using network analyzers (HP 3577B and 8510B) [17].

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Fig. 2. T S D C ( ), TSPC ( - - - ) and dc conductivity ( . . . . . ) thermograms for Vycor glass at h = 0.07. Polarization temperature, Tp = 190 K, polarization time, tp = 15 min; polarizing field, E o = 1.67 × 105 V / m ; stainless steel electrodes; sample thickness 0.7 mm.

3. Results Fig. 1 shows the effective complex dielectric permittivity, E = E ' - g ' , of a Vycor glass sample at 25°C with two different water contents, h = 0.07 and 0.21 in the frequency range 5 Hz-10 GHz determined with a parallel plate condenser method in the range 4 Hz-200 MHz and with a reflection method in the range 0.1-10 GHz [17]. We observed a loss peak in the k H z - M H z frequency region which shifts by more than three decades to higher frequencies with increasing h. At frequencies lower than the peak frequency, both ~' and E" increase with increasing h. At higher frequencies, c" starts to increase again to give a second loss peak in the GHz frequency

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region. Measurements at several water contents show that in this frequency region with increasing water content both E' and c" increase at each frequency, while the loss peak shifts to slightly higher frequencies. In Fig. 2 we compare the TSDC, TSPC and dc conductivity measured on a sample with h = 0.07. These results are typical for samples at h > 0.05. The TSDC shows two peaks at about 130 K (low-temperature (LT) peak) and 170 K (intermediate (IT) peak) and a complex high-temperature (HT) band. The TSPC shows, in agreement with TSDC, the LT peak and the rise to the IT peak. At temperatures higher than about 170 K, the dc conductivity dominates over the polarization, in agreement with the dc conductivity showing a dramatic increase of conductivity around 170 K and coinciding with the TSPC. The HT band was not further studied. Fig. 3 shows TSDC measured on a Vycor sample at different values of h. The LT peak appears for water contents higher than about 0.020-0.025. With increasing h it shifts slightly to higher temperature, the shift being more pronounced at low values of h. The area under the peak, which is proportional to the depolarization charge and is thus a measure of the number of relaxing units contributing to the peak [15], increases continuously with h, the rate of increase being, however,

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lower at lower water contents. The temperature position of the IT peak is fixed for h > 0.05. At lower values of h, the IT peak shifts dramatically to higher temperatures. The magnitude of the IT peak increases systematically with increasing water content, h, for h < 0.05 (Fig. 3) and it changes unsystematically with h for larger values of h. The dc conductivity of a Vycor sample at various fixed water contents was measured as a function of temperature at a heating rate of 0.05 K / s by measuring the current flowing through the sample at an applied dc field E = 1.44 × 106 V / m . In Fig. 4 we show the corresponding Ar-

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rhenius plots at four water contents. The I - T plots at various fixed water contents, h, were used to construct the plot shown in Fig. 5, i.e., the dc current (a measure of the dc conductivity) against h at four different temperatures. The dc conductivity in Figs. 4 and 5 increases with temperature and at each temperature increases dramatically with water content for h < 0.06 and then becomes practically constant. The changes of conductivity with water content at h > 0.06 in Fig. 5 probably reflect changes in the quality of the electrical contacts between the stainless steel electrodes and the sample in different temperature runs. This is not surprising, since the sample was pressed between the plates of the measuring capacitor without evaporated electrodes on the surfaces, in order to allow water to go in and out of the sample. The Arrhenius plots in Fig. 4 show a slight curvature; thus two values of activation energy, E, could be calculated at each value of h, one for temperatures lower than about 210 K and the other for higher temperatures. Fig. 6 shows the dependence of these two energy values on h. It can be seen that for h > 0.05 the activation energy is larger for higher temperatures. The results are less conclusive at lower water contents where the values of E are significantly scattered because of the rapidly decreasing current (Fig. 5). For comparison, Fig. 6 shows values of the activation energy of the TSDC IT peak, calculated by fitting the expression for the depolarization cur-

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P. Pissis et al. /Journal of Non-Crystalline Solids 171 (1994) 201-207

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rent density [15,16]. For h > 0.05, the values of E of the TSDC IT peak are close to those of the conductivity at T < 210 K.

4. Discussion These results provide strong evidence for the following model of water in Vycor glass pores. The formation of water clusters around the primary hydration sites starts before the first monolayer is completed. Water in the pores is restricted in its mobility compared with bulk water. With increasing water content, it becomes more bulk-like. At low temperatures, water in the clusters crystallizes to ice, while at least a fraction of the water in the first monolayer transforms to a glassy state. The ac loss peak at 1-10 GHz at 25°C (Fig. 1) and the LT TSDC and TSPC peaks at about 130 K (Figs. 2 and 3) are due to relaxation of H 2 0 molecules in glass pores in the liquid and solid water phases, respectively. The ac loss peak in the k H z - M H z frequency region (Fig. 1) is attributed to a proton hopping transport process of percolation type (conductivity relaxation [18]), similar to processes measured on hydrated protein powders [19,20]. The TSDC and TSPC IT peaks (Figs. 2 and 3) are also attributed to this process. The freezing-in of this relaxation at about 170 K (Fig. 3) is due to a glass transition of water in the first monolayer, in agreement with the

freezing-in of the dc conductivity in the same temperature region (Figs. 2 and 5). The ac loss peak due to the relaxation of water molecules at 25°C is at lower frequencies (1-10 GHz, Fig. 1) than in bulk (free) water (19.2 GHz [2]), suggesting restricted mobility of the water molecules in glass pores. This result is in agreement with measurements using small-angle X-ray scattering [2], M6ssbauer spectroscopy [4], subpicosecond optical birefringence [5], forced Rayleigh scattering [6] and nuclear magnetic resonance spectroscopy [9]. The extent of restriction, defined here as the ratio of the frequencies of the ac loss peaks in bulk water and in water in the glass pores, decreases with increasing water content from about 20 at low water contents to about 2 at h = 0.22. This restriction is lower than in most studies, where the extent of restriction has always been computed by comparing the measured value of a property of water in the glass pores with that in bulk water. Please note, however, that the degree of restriction, apart from being dependent on the property measured [1], decreases with increasing pore radius [5], increasing filling factor [9] and increasing temperature of measurement [5]. The decrease of restriction with increasing filling factor appears in our measurements as a shift of the high-frequency loss peak to higher frequencies with increasing water content (Fig. 2). This is consistent with the sorbed molecules condensing in the smallest pores first [3], as well as with the sorbed molecules becoming more free (bulk-like) with increasing distance from the pore surface [4,5,13]. The ac loss peak in the k H z - M H z frequency region (Fig. 1) shifts to higher frequencies with increasing water content while its magnitude increases. Bearing in mind that the dc conductivity at room temperature increases continuously with increasing h [12], these results strongly support the attribution of the loss peak to a conductivity relaxation [18]. A relaxation with the characteristics described above has been measured by other investigators in silica-water systems [11-14] and ascribed either to hopping of protons [13] or, at low water contents, to reorientation of water molecules in ice-like structures [12]. Our results at low temperatures will provide further evidence

P. Pissis et aL /Journal of Non-Crystalline Solids 171 (1994) 201-207

for the first interpretation. A dielectric relaxation in the frequency region of our ac loss peak (at room temperature) and with similar dependence on water content has been measured in hydrated protein powders and ascribed to hopping of protons [19] of percolation type [20]. The LT TSDC and TSPC peaks at about 130 K for h > 0.05 and at lower temperature for h < 0.05 (Fig. 3) are attributed to the reorientation of water molecules in ice microcrystals [15,16]. The peaks appear only for water contents higher than 0.020-0.025, suggesting that clusters of H-bonded water molecules start forming around primary hydration sites at about this water content, i.e., before a complete mort•layer (h = 0.10) is formed [13]. Support for this comes from the fact that the rate of increase of the magnitude of the LT TSDC peak with water content, h, increases with h (Fig. 7). We point out that the magnitude of this peak is a measure of the number of water molecules contributing by their reorientation to it, i.e., of the number of water molecules in the frozen clusters. In several water-containing systems, the LT TSDC peaks has been found to appear for water contents, h, higher than a critical one, hc, and its magnitude has been found to increase linearly with h [15]. This has been interpreted as follows. For h < h c, the water molecules are bound at primary hydration sites, while for h > h c all the water in excess of h c is organized in clusters [15]. With this interpretation in mind, the results shown

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in Fig. 7 indicate that water sorbed in the glass pores in excess of 0.020-0.025 is partly bound in the first monolayer and partly organized in clusters. It follows that the sorbed water molecules up to h--0.020-0.025 are bound at primary hydration sites at the pore surfaces while at higher water contents an increasing fraction of water molecules is in frozen clusters. We have no explanation for the shift of the LT TSDC peak to lower temperatures with decreasing h for h < 0.05 (Fig. 3). The temperature position and shape of the TSDC IT peaks allow calculation of the activation energy, E, and pre-exponential factor, %, in the Arrhenius equation ~ - ( T ) = % exp(E/kT), where ~- is the relaxation time, T the temperature and k Boltzmann's constant [15,16] and, thus, the frequency of the corresponding loss peak at the temperature of our ac measurements (25°C). The result is that at h <0.05 the TSDC IT peak, which shifts to lower temperatures with increasing water content (Fig. 4), i.e., the corresponding relaxation mechanism becomes faster, corresponds to the ac loss peak in the k H z - M H z frequency region (Fig. 1). As an example, the IT TSDC peak in a Vycor glass sample with h = 0.037 is characterized by a peak temperature Tm = 187 K (Fig. 3) and activation energy E = 0.54 eV (Fig. 6). It follows that the pre-exponential factor, %, is 4.7 × 10-14 s and the frequency of the corresponding loss peak at 25°C is 758 Hz. For comparison, the measured ac loss peak was found to be at 1.2 kHz (after subtracting the dc conductivity). For h > 0.05, the ac k H z - M H z loss peak continues shifting to higher frequencies with increasing h (Fig. 1) while the corresponding TSDC IT peak does not (Fig. 3). This suggests freezing-in of the conductivity relaxation giving rise to the ac and TSDC peaks at about 170 K. Strong support for the attribution of the TSDC IT peak to conductivity relaxation comes from the fact that the activation energy of the IT peak has been found to be practically equal to that of the dc conductivity at low temperatures (Fig. 6) [18]. Moreover, the dramatic decrease of dc conductivity in the vicinity of about 170 K (Fig. 2) provides further evidence for a glass, or glass-like, transition of water in the first monolayer, in

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agreement with similar results and interpretations for different liquids in porous glasses [7,8,10,12,14] and for different hydrated materials [8,10,12,22,23]. The dc conductivity giving rise to the conductivity relaxations as measured by the ac k H z - M H z loss peak and the T S D C IT peak is probably due to thermally activated hopping of protons [13] with activation energies of about 0.6-0.7 eV (Fig. 6). At temperatures higher than about 170 K, the dc conductivity increases rapidly with water content over several orders of magnitude and becomes constant for h > 0.06 (Fig. 5), suggesting hopping of protons of percolation type along threads of hydrogen-bonded water molecules [2]. Hall et al. [12] reported a significant increase in the conductance of a silica/sorbed water system at water contents > 0.023. From the density (dry) p = 1.5/cm 3 of the Vycor glass used and its porosity q~ = 0.28, the volume accessible to water can be calculated as V o = q~/p = 0.19 cm3/g. The first monolayer coverage can be calculated as A S / V o = 0.39, where A = 0.3 mm is the thickness of the water monolayer and S = 250 m 2 / g is the internal surface area of the Vycor glass. With a maximum amount of water content hmax=0.25, the first monolayer is covered by h = 0.39 × 0.25 = 0.1. The conduction threshold for two-dimensional site percolation is 0.45 [20], which corresponds to a water content of 0.044. The results in Fig. 6 indicate a lower threshold for conduction, at about h = 0.02. The factor of about 2 between the measured and calculated conduction thresholds is within the knowledge of the true parameters used in the calculation. Moreover, low conduction thresholds have often been found in percolating systems and attributed to details of the sample topology and the conduction mechanism which was not taken into account in the site percolation calculations [17].

5. Conclusions

(1) Reorientation of water molecules in 40 .~ diameter pores of a Vycor glass at 25°C is restricted, compared with that of bulk (free) water,

by a factor decreasing from about 20 to about 2 with increasing water content. (2) The critical water content for clustering of water molecules around primary hydration sites is h = 0.020-0.025, well below first monolayer coverage, h = 0.10. (3) The results suggest a transition to longrange connectivity among water molecules in the pores at about h = 0.02. (4) Water in the first monolayer transforms to a glassy state at about 170 K (during cooling). The authors gratefully acknowledge the helpful collaboration of R. Pelster (ac measurements) and the support from Volkswagen-Stiftung, from the General Secretariat for Research and Technology of the Ministry of Industry, Energy and Technology of Greece and from the Federal Ministry for Strategy Planning of the Czech Republic.

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