Mr Kilian Shani Fraysse

As strange and surprising as it may seem, the dissolution rate of certain inorganic material surface such as silica (SiO₂) can be enhanced well above its intrinsic rate by simply pressing it against another surface in an aqueous electrolyte.  This phenomenon, known as ‘pressure solution,’ has been well known but poorly understood in the geological community for more than 100 years and yet remains virtually unknown amongst the broader scientific community.  Traditionally, the pressure solution phenomenon is explained as an effect of mechanical (i.e. lithostatic) stresses which lowers the activation energy to dissolution by raising the chemical potential energy of stressed bonds within the ‘contact’ junction between the surfaces. Recent experiments have challenged this mechanochemical mechanism of pressure solution and instead tend to show that enhanced dissolution rates are caused by the presence of large interfacial electric fields produced by the surface potential gradient between dissimilarly charged surfaces when pressure pushes them into very close proximity.  At very large lithostatic pressures (i.e. GPa), the separation between surfaces collapses to just 3-6 Å corresponding to just 1-2 Stern layers.  In this highly confined geometry, the electric fields, emanating from the opposing surfaces is insufficiently screened by counter ions present within the confined water film.  Due to the very small separations, even small surface potential gradients on the order of just a few hundred mV can give rise to electric field strengths in excess of 1.0 V/nm.  Experiments performed by pressing conductive diamond against silica demonstrate that the dissolution rate can be correlated strongly with the magnitude of the surface potential gradient between surfaces resulting is a relationship resembling electrochemical corrosion—albeit without any obvious electrochemical oxidation/reduction reactions.  Likewise, it was also found that oscillating the contact pressure at kHz frequencies effectively led to a 2-3 order of magnitude further enhancement in the silica dissolution rate consistent with recently described ‘surface resonance’ effects on catalytic reactions.  Experiments thus lead to a new understanding of pressure solution that sees the phenomenon as an electrocatalytic process through which the autoionization rate of highly confined water is greatly enhanced resulting in accelerated silica dissolution via acid/base reactions. In addition to providing a new understanding of the role of electric fields on chemical reactions, I am working on demonstrating the utility of the pressure solution effect in the area of nanofabrication enabling methods of direct-write and nanoimprint lithography to be performed directly into inorganic substrates.

KILIAN FRAYSSE

PhD Student

Deakin University (Burwood Campus) – Institute for Frontier Materials