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J. Phys. Chem. C 2008, 112, 2618-2623
Ice Chromatographic Characterization of Thin Liquid Layer at the Interface between Water-Ice and Organic Solvent Yuiko Tasaki and Tetsuo Okada* Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan ReceiVed: NoVember 2, 2007; In Final Form: NoVember 21, 2007
The quasi-liquid layer (QLL) on the surface of water-ice has been studied by ice chromatography, in which water-ice particles are used as a stationary phase. We have, in previous papers, demonstrated that solutes are retained on the ice stationary phase with hexane-based mobile phases and have revealed that the hydrogen bonds between a solute and the dangling bonds on the surface of water-ice are responsible for retention at such low temperatures as -12.0 to -5.0 °C. However, increasing the temperature allows the shift of the principal retention mechanism from the hydrogen-bond adsorption to the partition into the QLL; this has been confirmed by drastic changes in the retention of probe solutes. The threshold temperature, at which the retention shift occurs, depends on the concentration and type of a polar component added to the mobile phase. Thus, the QLL developed in the present system contains not only water but also a polar component added to the mobile phase. This phenomenon occurs at a temperature much lower than that predicted from the lowering of the freezing point due to the dissolution of a polar component in water and, thus, is a surface process rather than a bulk phase transition.
Introduction Surface premelting occurs at temperatures lower than the bulk melting point of a solid material and is, in general, interpreted on the basis of a thermodynamic consideration.1-3 Even though the temperature of the entire system is kept lower than the melting point of a solid material, the total free energy can be reduced by covering the solid surface with a liquid-like layer, which is called a quasi-liquid layer (QLL) or a structure transition layer. Water-ice in contact with the water vapor is the case and has been well investigated to reveal the physicochemical nature of a QLL.4-6 A QLL on water-ice is ubiquitous on the earth and is involved in various natural phenomena. The growth of snow crystals and their shapes, for example, are explained by the different wettability of the facets of the waterice crystal.7,8 The difference in the thermodynamic stability between the crystal facets leads to the difference in the velocity of the QLL development and results in the characteristic shapes of snow crystals. Another phenomenon related to the QLL of water-ice is the generation of a thunderstorm. Dash et al. pointed out that its key step is the charge transfer between water-ice particles, which occurs through the QLL when they collide with each other in the atmosphere.9 Moreover, it is widely believed that frost heave is caused by the mass transport through a QLL.10,11 The interaction between sediments and water-ice causes a force imbalance, and water flows along the pressure gradient; the ground is heaved up in this way. Apart from these natural processes, the QLL on water-ice is of fundamental interest and has been studied by various techniques such as IR,12 AFM,13,14 NMR,15,16 helium backscattering,17,18 and MD simulation.19-21 Although these studies have revealed some features of the QLL, there is still a debate on the thickness of the QLL and its temperature dependence. It is a general consensus that the QLL begins to grow at a * Corresponding author. Phone and Fax: +81-3-5734-2612. E-mail:
[email protected].
particular temperature and becomes thicker as the temperature of water-ice increases. However, there is a substantial disagreement in the reported thickness values, which strongly depend on the systems studied. The surface of water-ice exposed to water vapor has been most widely studied because of the relevance to the natural phenomena noted above. The interfaces between water-ice and a solid substrate and between water-ice crystals have also been examined. Such work has suggested that the substance in contact with water-ice affects the property of the QLL. From this perspective, though water-ice/liquid interfaces deserve detailed investigation, there are few relevant studies because appropriate approaches have not been available. We have developed ice chromatography, in which water-ice particles are used as a chromatographic stationary phase. In our previous work,22 we reported that the molecule having two or more polar groups can be retained on the water-ice stationary phase over the temperature range of -12 to -5 °C by the hydrogen bonds between the polar groups of a solute and the dangling bonds (i.e., O-site and OH-site) on the surface of waterice. In addition, we devised methods to modify the adsorption ability of the stationary phase and succeeded in varying solute retention.22,23 Chromatography has been widely used for analytical separation and preparative isolation of various materials,24-26 and it is also known to be useful to study the molecular interactions occurring at the interface between the stationary phase and the mobile phase.27,28 Ice chromatography is thus expected to be a powerful tool for probing the water-ice/liquid interface. In the present work, we have investigated the QLL developed on the surface of water-ice in contact with an organic solvent. As described above, it is believed that the QLL exists near the melting point of water-ice. Ice chromatographic retention over the temperature range of -12 to -5 °C was well explained only by hydrogen-bonding adsorption, suggesting that the contribution from the QLL to the entire retention is negligible. On the basis of ice chromatographic experiments at higher temperatures, which are just below the melting point of
10.1021/jp7105605 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/31/2008
Liquid Layer at Water-Ice/Organic Solvent Interface
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2619
water-ice, we will show the important involvements of the QLL in the retention of solutes and will reveal some properties of the QLL. Experimental Section The chromatographic system was composed of a Tosoh model HPLC pump DP-8020, a Rheodyne injection valve equipped with a 100 µL sample loop, a Shimadzu model multichannel photodiode detector SPD-M10AVP, and an ADVANTEC model low-temperature bath TBT220DA. Water-ice particles were prepared by introducing sprayed water droplets directly into liquid nitrogen. Ice particles with sizes smaller than 75 µm were collected by sieving. The particles were put into a PEEK column (7.5 mm i.d. × 150 mm length) immersed in liquid nitrogen, and they were tapped down with a PTFE rod. The packed column was then transferred to the low-temperature bath maintained at an appropriate temperature ranging from -6.0 to -0.1 °C. The operation temperature was measured just outside of the column with a digital thermometer equipped with a Pt100 thermoresister; the temperature fluctuation was smaller than 0.1 °C. The surface area of the ice stationary phase was determined by the BET adsorption of nitrogen. The sample loop and a solvent reservoir were also put into the low-temperature bath. The mobile phase was precooled by passing it through a coiled tube connected between the pump and the sample injector. An ultrasonically deaerated hexane/polar-solvent mixture was used as the mobile phase. Tetrahydrofuran (THF) or diethyl ether (DEE) was added to hexane as a polar component in the mobile phase. Small pieces of water-ice were put in the mobile phase to saturate it with water at an operating temperature. Solutes tested were phenol, hydroquinone, resorcinol, benzo-18-crown-6 (B18C6), and dibenzo-24-crown-8 (DB24C8). Most of the solutes have two or more polar groups, which are necessary for the measurable retention on the water-ice stationary phase.22,23 The crown ethers were synthesized according to the literature.29 The concentrations of solutes were 10-80 µM. Reagents of the highest grade available were used as received. The partition coefficients of polar mobile-phase components between water and hexane were determined at 0 °C. The concentrations of the polar components in both phases were measured by gas chromatography with acetonitrile as an internal standard. The partition of solutes between water and hexane containing a polar component was evaluated with UV absorption spectrometry. The details of the partition coefficient measurements are described in Supporting Information. Results and Discussion Effect of the QLL on the Ice Chromatographic retention. As mentioned above, it is known that the QLL on the waterice surface exists just below its melting point and becomes thicker as the temperature rises. In our previous work, we confirmed that the QLL does not affect ice chromatographic retention of a solute at the temperature of
