Dispersion Stability of Colloids in Sub- and Supercritical Water

Formation of amorphous silica nanoparticles and its impact on permeability of fractured granite in superhot geothermal environments

Superhot geothermal environments in granitic crusts of approximately 400–500 °C are a frontier of geothermal energy. In the development of such environments, there is a concern of a reduction of permeability of fractured granite due to the formation of fine particles of amorphous silica induced by the phase change from subcritical water to supercritical water or superheated steam. However, the formation of silica particles and a resultant reduction in permeability have not been demonstrated to date. Therefore, experiments were conducted on the formation of amorphous silica particles with various combinations of temperature (430–500 °C) and pressure (20–30 MPa), in which the phase change of Si-containing water from liquid to either supercritical fluid or vapor was induced. Amorphous silica nanoparticles occurred under all conditions with smaller particles for higher temperature. The permeability of fractured granite was also observed to decrease significantly within several hours during injection of the particles into rock at 450 °C and 30 MPa under a confining stress of 40 MPa, with slower permeability reduction at a smaller number of particles or in the presence of larger aperture fractures. The present study suggests that the nanoparticles are likely to form and destroy the permeability in superhot geothermal environments, against which countermeasures should be investigated.

Free-Radical Polymerization of Acrylic Acid under Extreme Reaction Conditions Mimicking Deep-Sea Hydrothermal Vents

Free-radical polymerization with a thermochemical initiator, which usually takes hours to complete, was dramatically accelerated under reaction conditions mimicking the deep-sea hydrothermal vents, where reaction mixtures were only briefly exposed to ultrahigh temperatures under pressure. In tests using acrylic acid and potassium persulfate, poly(acrylic acid) (M n = 2.1 × 104, Đ = 2.73) was obtained in 5.2 s with the monomer conversion of 60.3% in water at 200 °C and 25 MPa without using any catalysts. The process that we call heat-shock-induced polymerization may pave the way for an entirely new strategy in reaction engineering for developing extremely fast, green, and scalable processes for polymer synthesis.

Anomalous long-range repulsion between silica surfaces induced by density inhomogeneities in supercritical ethanol

Anomalous long-range repulsion, extending over several micrometres, emerged between silica surfaces around the ridge of density fluctuations in supercritical ethanol at temperatures and pressures near the gas/liquid critical point (T(c) = 241 °C, P(c) = 6.14 MPa). Analysis shows that augmentation of ethanol density around silica surfaces in the presence of density fluctuations facilitates dissociation of silanol groups, leading to long-range electrostatic repulsion in the nonpolar medium.

Supercritical water: a fascinating medium for soft matter

Properties of water change dramatically at high temperatures and high pressures near and/or above the critical point ( = 374 °C, = 22.1 MPa). The dielectric constant, for example, decreases from 78 at 25 °C and 0.1 MPa to 6 at the critical point, the value of which is comparable to that of 1-dodecanol. As fascinating characteristics of soft matter rely on unique properties of ambient liquid water, the change should have significant impacts on soft matter. However, our knowledge of soft matter under such extreme conditions is virtually nonexistent. In this article, properties of colloidal dispersions in water at high temperatures and high pressures are described. Implications of the findings for geological processes in deep-subsurface are also discussed.