Influence of Dissolved Gas on the Interaction between Hydrophobic Surfaces in Water

Abstract

We analyze how the interaction between two hydrophobic surfaces in water depends on the amount of dissolved gas. At equilibrium there is, for a truly hydrophobic surface with contact angle θ > 90°, a long range attractive force. The attraction is due to the formation of a gaseous film between the surfaces. For a completely degassed sample the gas consists of water vapor and the range of the force is of order 500 nm at ambient conditions. In the other extreme where the water is saturated with gas with respect to the atmosphere the range of the equilibrium force is determined by the hydrostatic pressure and it diverges as the distance between the surface setup and the air−water interface goes to zero. It is generally considered that the formation of a gaseous film is a kinetically very slow process so that one in practice stays on the metastable liquid branch until an instability appears at short separations. For the particular case of formation and stability of oil in water emulsions in the absence of stabilizer a degassing will affect at least two crucial steps in the process. In the formation of droplets, new interfaces are formed and separated from short distances. The presence of dissolved gas, in both oil and water, facilitates the formation of gas bubbles between the surfaces. Furthermore, the lifetime of gas bubbles adsorbed on the interface is substantially prolonged in the presence of dissolved gas and they can act as nucleation centers for the formation of a gaseous lens between two oil droplets at re-encounters.