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Langmuir 1989,5, 1439-1441
Notes Melting of Ice in Silica Pores W. Drost-Hansen
Laboratory for Water Research, University of Miami, Coral Gables, Florida 33124 Frank M. Etzler' Surface and Colloid Science Section, Institute of Paper Science + Technology,' Atlanta, Georgia 30318 Received February, 24, 1989. In Final Form: May I, 1989
Introduction The properties of water near solid surfaces are known to differ from those of the bulk. Earlier, we have reviewed the nature of vicinal (interfacial) water.'-7 Among the many unusual properties of vicinal water, it is notable that the heat capacity of water in 14-nm-diameter silica pores is 25-30% greater than the bulk heat capacitys98 and that the density is 2-376 lower than the bulk d e n ~ i t y .Also, ~ measurements by other investigators indicate that the viscosity of water between quartz plates is 2-20 times that of the bulk viscosity and is nonN e w t ~ n i a n .Additionally, ~ it appears that the properties of vicinal water, to a good first approximation, are independent of the specific physicochemical details of the surface.'^^ In an attempt to correlate some of the properties of vicinal water, Etzlers has proposed a statistical thermodynamic model for water confined to narrow pores. The model considers water both in terms of bond percolation, as discussed by Stanley and Teixeira," and a bimodal single-particle enthalpy distribution calculated earlier by Stey." Etzler's model is able to correlate some thermodynamic and dynamic properties of water in silica pores. From the model, it appears that vicinal water is similar to water in the supercooled region or under negative pressure. This implies that hydrogen bonding between water molecules is enhanced near solid surfaces. Recently, from measurements of the density of water in silicas of various pore diameters, Etzler and Fagundus7 have estimated that vicinal structuring extends at least 3-5 nm from surfaces and that this structuring decays in an approximately exponential manner with distance. The distance over which vicinal structuring occurs, as calculated from the density measurements, appears Formerly Institute of Paper Chemistry, Appleton, Wisconsin. (1) Drost-Hansen, W. Ind. Eng. Chem. 1969,61,10. (2) Drost-Hansen, W. In Chemistry of the Cell Interface; Brown, H. D., Ed.; Academic Press: New York, 1971;Part B. (3) Etzler, F. M.; Drost-Hansen, W. In Cell Associated Water; DrostHansen, W., Clegg, J. S., Eds.; Academic Press: New York, 1979. (4) Drost-Hansen, W. In Biophysics of Water; Franks, F., Ed.; Wiley and Sons: New York, 1982. (5) Etzler, F. M.;Drost-Hansen, W. Croat. Chem. Acta 1983,56, 563. (6) Etzler, F. M.J. Colloid Interface Sci. 1983,92,43. (7) Etzler, F. M.;Fagundus, D. M. J. Colloid Interface Sci. 1987, 115, 513. (8) Etzler, F. M.Langmuir 1988,4,878. (9) Peschel, G.;Adlfinger, K. H. J. Colloid Interface Sci. 1972,34, 505. (10) Stanley, H. E.; Texeira, J. J. Chem. Phys. 1980,73,3404. (11) Stey, G.C. Ph.D. Thesis, University of Pittsburgh, 1967.
to be consistent with the properties of water in montmorillonite,12the apparent properties of intracellular water as discussed by Clegg,13*14and the disjoining pressure of water between quartz plates as measured by Peschel et al.15 More recently, Israelachvili and coworkers have observed "hydration forces" which extend over similar distances.16 Here we report our observations concerning the melting of ice frozen in the pores of a silica gel (14-nm pore diameter). This gel is the same material used in a variety of experiments in our l a b o r a t ~ r i e s . ~ The ~ ~ ~obser~',~~ vations are discussed in terms of our current view of vicinal water structure.
Experimental Section In this study, the melting of ice frozen in 14-nm-diameter silica gel pores (Grace Code 62) was investigated via differential scanning calorimetry (DSC). A Perkin-Elmer DSC 2 was used to collect the data reported here. Calibration of the DSC was performed by using standard procedures. Bulk ice samples gave normal melting points and the expected melting profiles. In this paper, we concentrate our discussion on the unusual melting behavior exhibited by porous silica nearly saturated with water. Typical results are seen in Figures 1 and 2. Enough water was placed in the sample such that 70-80% of the pore volume was filled. In order to minimize any possible instrumental artifacts, low heating rates were used throughout this study. Most runs were made from 0.31 to 2.5 OC/min. (See Williams andHirschlO for a careful study of scan rate influence on melting profiles.) Results and Discussion Figures 1 and 2 contrast the melting of ice in silica pores with the melting of bulk ice. The figures suggest that the melting of ice in silica pores is complex. Some of the differences between the curves shown are undoubtedly due to variations in the precise amount of pore water contained in each sample; however, the figures do suggest that a portion of the ice begins melting at a temperature a few degrees above 0 "C. Our observation is apparently not unique. For instance, Anderson and Tice2Ohave studied the melting of ice in clays and found melting temperatures as high as 4 "C. The studies by Low and also by Etzler suggest that a number of other thermodynamic properties of water in clays and in silicas are nearly identical. I t is thus not surprising that the melting behavior of ice in these two systems is similar. Earlier investigations concerningthe properties of water in silica pores suggest that the water differs from the bulk in that hydrogen bonding between water molecules is enhanced by propinquity to solid surfaces. Vicinal water ~
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(12) Low, P. F. Soil Sci. Am. J. 1979,43,652. (13) Clegg, J. S.Am. J.Physiol. 1984,246,R133. (14) Clegg, J. S.Cell Biophys. 1984,6,153. (15) Peschel, G.;Belocheck, P.; Muller, M. M.; Muller, M. R.;Koing, R.Colloid Polym. Sci. 1982,260,444. (16) Isrealachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1985. Isrealachvili, J. N. Acc. Chem. Res. 1987,20,415. (17) Etzler, F. M.;Liles, T. L. Langmuir 1986,2,797. (18) Hurtado, R.M.;Drost-Hansen, W. In Cell Associated Water; Drost-Hansen, W., Clegg, J. S., Eds.; Academic Press: New York, 1979. (19) Williams. R.J.: Hirsh. A. G. Crvo.-Lett. 1986,7, 146. (20) Anderson, D. M.;Tice, A. R.Personal communication.
0743-7463/89/2405-1439~01.50/0 0 1989 American Chemical Society
1440 Langmuir, Vol. 5, No. 6, 1989
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,
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Notes
250
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Figure 1. Typical DSC scan (heating) of the melting of normal bulk ice. As expected, the onset of the peak is found a t the normal melting point of 273 K.
290
310
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-
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thus appears to be similar to water at low temperatures or under negative pressure. It appears that it may be useful to consider the analogy between vicinal water and water at negative pressures (i.e., water in tension). Experimental investigation of water under negative pressure has been limited for obvious experimental reasons. Recently, however, Henderson and Speedyz1have experimentally studied the melting of ice and solid D20 under negative pressure. Their results suggest that the Claperyon line for the ice-water phase transition may be correctly extrapolated into the negative pressure region. Furthermore it is possible to extrapolate some measured properties of water into the region of negative pressure from a suitable equation of state. Figures 3 and 4 show the heat capacity and density of water under negative pressures as extrapolated by using an equation of state determined by Bradshaw and Schleicher.22 Heat capacities were calculated from the equation of state by using the following relation:
C,(P,T)= C,”
+
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1
1
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270
272
(21) Henderson, S. J.; Speedy, R. J. J. Phys. Chem. 1987, 91, 3069. (22) Bradshaw, A.; Schleicher, K. E. J. Chem. Eng. Data 1986, 31,
v)
c-z
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200 KELVIN
276 200 KELVIN
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C,” is the heat capacit at 1bar and the temperature of interest. Etzler et al.1have shown that water in silica gel poresexhibits a decreased density and increased heat capacity compared to bulk water. Thus density measurements of vicinal water suggest that water in 14-nmdiameter silica gel pores is similar to water at -300 to -600 bar. Henderson and Speedy’s results, which are partially shown in Figure 5, suggest that water a t such pressures should melt between +2 and +4 “C. Speedy’s observation for water under negative pressure are consistent with our observations of the melting behavior of water in silica pores. It appears that “stretched ice” may occur in porous materials and be responsible for the complex melting behavior. Here the term “stretched ice” may simply refer to normal ice Ih at negative pressure. We propose tentatively that either or both of the following arguments may explain the observation of elevated melting points for ice in the porous matrix: (1)the water in the pores of the silica gel is in tension; (2) the water and/or the ice in the pore has a structure which is modified by the proximity to the surface. 189.
w
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204
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290
Figure 2. Four DSC scans (heating) of ice frozen in 7-nm-diameter silica gel pores. From top to bottom, scan rates are 2.5, 1.25, 0.62, and 0.31 K/min. Note peak with melting (onset) near 3-5 “C.
Consider the first of these possibilities: in most of the DSC runs, the ratio of silica gel to water was fairly large. In other words, the amount of water present was insufficient to fill all the pores completely. (Typically only 7 0 4 0 % of the pores was filled with water.) Thus, in many individual pores, water will be in tension as the result of being contained between two free airlwater minisci. Assuming that the ice formed (upon freezing of the water in the capillary) is ice I, (see discussion below) dP,/
Langmuir, Vol. 5, No. 6, 1989 1441
Notes
vincingly demonstrated by Roedder= from a study of freezing of aqueous inclusions in minerals. Assuming circular capillaries with a diameter of 14 nm (the pore diameter employed in the present experiments), the pressure calculated from the Kelvin equation is about -400 bars. From the data by Henderson and Speedy, this corresponds to a melting point elevation of +3 "C. This is certainly of the correct order of magnitude for the observed melting temperature increase. Unfortunately, there are some problems with this facile explanation. In the study by Anderson and Tice in which elevated melting temperatures were observed,there were no "rigid walls" (in the clay/water mixture). Thus, I -660 : -PO 0 it is difficult to see by what means a significant tension P BAR could be created. Figure 3. Heat capacity of bulk water extrapolated into negThe second possible explanation for the observed results ative pressure region:22upper curve, 0 OC;lower curve, 25 O C . is that the structure of the water and/or the ice in the Note large heat capacities have been observedfor water in pores. silica pores differ from the bulk structures. As indicated in the Introduction, a great deal of evidence exists for notable structural effects due to proximity to a surface. Furthermore, as discussed, for instance, by Dore and co-w0rkers,2~neutron diffraction studies on water frozen in porous silica suggest that the ice formed may be not the usual ice Ih. Thus, it is not unreasonable to assume that the solid formed may not melt at 0 "C. As the density of liquid water in pores has a lower density than normal bulk water, the elevated melting point may alternatively or additionally result from the difference between the state of pore water and that of bulk water. I
I
I
8
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1
Summary P BAR
Figure 4. Density of water extrapolated into negative pressure region.22 At 0 bar: upper curve, 0 "C;lower curve, 25 "C.
Note the density of water in silica pores has been found to be less than the bulk density.
T C
Figure 5. Melting pressure of water as a function of temper-
ature, redrawn from experimentaldata by Henderson and Speedy. d T is negative along the ice-water line as the result of the density difference corresponding to the normal pressure melting of ice, observed with increasing pressure. By the same token, the melting point must increase when the ice is subject to negative pressures. This was con-
We have investigated the melting behavior of ice in silica pores; it appears that at least a fraction of the ice contained in the silica pores melts at a temperature above 0 "C. Our observation appears not to be entirely unique: similar behavior has been observed for water in clays and in mineral inclusions. Earlier studies concerningthe properties of water in silica pores suggest that pore water is similar to water under negative pressures. Henderson and Speedy found that the observed melting points of ice, frozen under negative pressure, were consistent with extrapolations from equations of state and occurred at temperatures above 0 "C. It is our suggestion that at least some of the pore ice is structurally similar to "negative pressure ice" either due to tension or due to structural changes induced by proximity to the pore wall with the result that the ice melts at an elevated temperature.
Acknowledgment. We thank Dr. R. J. Speedy for sharing his data before publication. Most of the experimental work was carried out while W.D.H. was a summer Guest Researcher a t the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL) in Hanover, NJ. We also thank Allen Tice of CRREL for valuable discussions and help with the
DSC. Registry No. H,O, 7732-18-5. (23) Roedder, E. Science 1967,155, 1413. (24) Steytler, D. C.; Dore, J. C.; Wright, C.J. J . Phys. Chem. 1983, 87, 2458.
