Infra-red imaging of bulk water and water–solid interfaces under stable and metastable conditions

Superheated water has been studied by infrared spectroscopy to examine whether the special ability of liquid water to undergo such a metastable state corresponds to the development of peculiar inter-molecular networking under tension. As the best technique to superheat water is to trap the liquid inside micro-cavities in solids (the so-called “fluid inclusions”), the role of the water–solid interfaces to stabilize the adjoining liquid is also explored with the same infra-red micro-spectroscopy tool. The key signal is the intra-molecular OH stretching band, sensitive to the networking in the probed material. The sample of choice is liquid water occluded inside a quartz cavity of micrometric size, synthesized in laboratory from pure quartz and milli-Q water. The stretching band of the superheated water shows no significant spectral difference from that of a bulk “normal” water, which means that the molecular properties of the superheating liquid are quite similar to those of the stable bulk liquid. Liquid water is readily “superheatable” but retains its “normality” under these special conditions. Additionally, this result establishes a firm ground to justify that the properties of the former are predicted extrapolating the usual (though empirical) equation of state of the latter. The infra-red signals of the water–solid interfaces are more complex. The water–solid interfaces blue-shift the signal, affecting differently the three sub-bands of the OH-stretching. This effect was unexpected since the micro-IR spectroscopy probes volume beyond what is classically assigned for the interfacial properties. In addition, the interfacial signature is clearer under superheating than under the saturation conditions, which offers an interesting (and unexpected) way to interpret the special stability of the occluded metastable water. These encouraging results give confidence on the potentialities of the high-resolution micro-spectroscopy to get insights into the molecular basis of macroscopic properties.