Eur. Phys. J. E (2015) 38: 72
https://doi.org/10.1140/epje/i2015-15072-9

Regular Article

Bubble formation in water with addition of a hydrophobic solute

¹ Department of Chemistry, Tokyo Metropolitan University, 192-0397, Hachioji, Tokyo, Japan
² Department of Physics, Kyoto University, 606-8502, Kyoto, Japan

^* e-mail: okamotor@tmu.ac.jp
^** e-mail: onuki@scphys.kyoto-u.ac.jp

Received: 1 April 2015
Revised: 20 May 2015
Accepted: 27 May 2015
Published online: 7 July 2015

Abstract

We show that phase separation can occur in a one-component liquid outside its coexistence curve (CX) with addition of a small amount of a solute. The solute concentration at the transition decreases with increasing the difference of the solvation chemical potential between liquid and gas. As a typical bubble-forming solute, we consider O₂ in ambient liquid water, which exhibits mild hydrophobicity and its critical temperature is lower than that of water. Such a solute can be expelled from the liquid to form gaseous domains while the surrounding liquid pressure is higher than the saturated vapor pressure p _cx. This solute-induced bubble formation is a first-order transition in bulk and on a partially dried wall, while a gas film grows continuously on a completely dried wall. We set up a bubble free energy ΔG for bulk and surface bubbles with a small volume fraction ϕ. It becomes a function of the bubble radius R under the Laplace pressure balance. Then, for sufficiently large solute densities above a threshold, ΔG exhibits a local maximum at a critical radius and a minimum at an equilibrium radius. We also examine solute-induced nucleation taking place outside CX, where bubbles larger than the critical radius grow until attainment of equilibrium.