Ice chromatography. Characterization of water-ice as a chromatographic stationary phase

Water-ice has been characterized as a stationary phase for liquid chromatography. Solutes having two or more polar groups are retained on this stationary phase with THF/hexane as the mobile phase, suggesting that multipoint interactions are required for measurable solute retention. Chromatographic separation of phenols or crown ethers on water-ice is possible. The ice surface is expected to provide two different adsorption sites coming from the OH and O dangling bonds. Although the solute partition into the quasiliquid layer is also considered, the dependence of the retention times on the THF concentration implies that the interaction of solutes with the water-ice surface rather than the partition into the quasiliquid layer is responsible for solute retention. A retention model suggests that the number of adsorption sites for a crown ether depends on its ring size, whereas two sites are involved for the retention of phenols having two hydroxyl groups. Although hydroxyl groups can act as both a hydrogen bond donor and an acceptor, the interaction with the ice OH sites, which are exposed to the surroundings in comparison with the ice O sites, is more important. However, when an acyclic polyether is added to the mobile phase, its adsorption onto the water ice surface allows the creation of the O sites that phenols can approach without steric hindrance. In the presence of the polyethers adsorbed on the ice surface, the retention of phenols is enhanced, whereas crown ethers become less retained due to the competitive adsorption of the polyethers.