Liquid water at — 45°C, the effects of confinement

J.Dore (Univ. of Kent, Canterbury), C.Haggenmüller (Univ. München), P.Behrens (Univ. Hannover), H.Fischer (LURE, Orsay).

Characteristics of bulk water

Water has a number of distinctive properties which distinguish it from other molecular liquids. Earlier neutron diffraction measurements have shown that the hydrogen-bonding interaction has a direct influence on the structural characteristics. At low temperatures, particularly in the super-cooled region, the orientational correlations are enhanced [1] and the system seems to be evolving towards the continuous random network which characterises the structure of low-density amorphous ice [2], made by vapour deposition onto a cold substrate plate at < 100 K. At ambient pressures there appears to be a limit, the homogenous nucleation temperature, to the stability of the super-cooled phase which is —45°C for H₂O and —40°C for D₂O. Special techniques using water droplets in emulsions have been developed to study deeply super-cooled D₂O water over a temperature regime extending down to —30°C. The main diffraction peak, Q₀(T), changes systematically to lower values [3] as shown in Fig. 1; the corresponding density change is given in the inset. The nature of the eventual transition to the ice phase remains controversial and several different theories have been advanced. The situation is puzzling because the glass-transition temperature and the upper limit to the stability of bulk amorphous ice are at much lower temperatures, in the region of 140 K.


Water/ice in mesoporous sol-gel silicas

The properties of water are substantially modified when the liquid is contained in the voids of a mesoporous solid. Neutron diffraction studies [4] of water in sol-gel silicas have shown that the water readily super-cools between —15 and —25°C (depending on the pore size) and subsequently nucleates to produce cubic ice rather than the hexagonal ice formed in the bulk phase. The super-cooled liquid phase shows enhanced hydrogen-bonding over bulk water at the same temperature indicating that the confinement has a direct effect on the hydrogen-bond structure and the nucleation process. Unfortunately, sol-gel derived silicas have rather broad pore-sized distributions and a poorly defined pore topology.

Water and ice nucleation in M41S silicas

Only recently, a new form of ordered mesoporous silica has become available. It has a well defined geometrical structure based on a regular array of cylindrical pores with narrow pore-size distributions and diameters typically in the range 25-35 Å. The MCM silicas [5] have different geometries categorised as M41S (hexagonal) and M48S (cubic) and are shown schematically in Fig. 2. These new silicas provide a superb means of extending the investigation of water in confined geometries and have produced some remarkable and unexpected results.

Neutron diffraction measurements [6] indicated that the water confined in MCM silicas super-cooled to even lower temperatures, down to — 45°C, before nucleation, as shown in the variable temperature plot of Fig. 1. Furthermore, the liquid phase could be re-gained by re-heating the sample to just above the nucleation point and showed no hysteresis. This behaviour is totally different to that observed in the previous studies and represents a reversible phase transition between two states that are both metastable with respect to the bulk phase! The MCM silicas therefore provide a suitable system for the production of liquid water at temperatures well below those normally available and open up new possibilities for studying liquid water under extreme low temperature conditions.


Characteristics of confined water

The reasons for this unusual behaviour are thought to depend on the interaction of the water molecules with the surrounding pore walls which have a low density of surface silanol groups. The largely hydrophobic surface causes an effective isolation of the water volume from the surrounding solid matrix. The hydrogen-bond connectivity is expected to be well developed with a disordered tetrahedral geometry such that the structure could be more accurately described as a gel network than a molecular liquid, as shown schematically in Fig. 3.

Clearly, it will be interesting to study other characteristics, such as the dynamics, of this ‘strange’ form of liquid water. This investigation also has implications for the properties of water at other more complex interfaces, particularly in a biosciences context.

Water has a relatively simple molecular conformation involving just three atoms but the complexities of its collective behaviour remain a major challenge for current scientific understanding.

[1] D.C. Steytler, J.C. Dore and C.J. Wright, Mol. Phys. 48 (1983) 1031. [2] M.R. Chowdhury, J.C. Dore and J.T. Wenzel, J. Non. Cryst. Solids 53 (1984) 247, J.C. Dore and D.M. Blakey, J. Mol. Liq. 65 (1995) 85. [3] M.C. Bellissent-Funel, J. Teixeira, L. Bosio, J.C. Dore,and P. Chieux, Europhys Letts. 2 (1986) 241. [4] J.M. Baker, J.C. Dore and P. Behrens, J. Phys. Chem. B101 (1997) 6226. [5] P. Behrens and G.D. Stucky, Angew. Chem. Int. Ed. Engl. 32 (1993) 696. [6] J.C. Dore, P. Behrens, C. Haggenmüller and H.E. Fischer, submitted to Phys. Chem. Chem. Phys.