Interfacial Phenomena and Surface Forces of Hydrophobic Solids

At the molecular level the entropic “hydrophobic effect” is responsible for high interfacial energies between hydrophobic solids and aqueous liquids, the low solubility of apolar solutes in aqueous solvents, and self-assembly in biological processes, such as vesicle formation and protein folding. Although it is known that a strong attraction between apolar molecules exists at the molecular level, it is not clear how this force scales up to objects with dimensions in the range 100 nm–1 m. This work sets out to measure the forces between particles with a radius of about 10 µm. Because we can only measure the total force, which includes the van der Waals force and the electrostatic forces, it is important to isolate the effect of “hydrophobicity”. We do this by measuring for systems where the particles are very hydrophobic (water contact angle, θ ~110°) and the van der Waals and electrostatic forces are very small. Under these conditions we find that the total force is very small: it is similar to the van der Waals force at separations exceeding 5 nm.

Many early works on the hydrophobic force reported surface force at over 100 nm of separation. However, many of these strong, long-ranged attractive forces are likely caused by submicron interfacial bubbles, known as nanobubbles. Nanobubbles were imaged with an atomic force microscope to better understand their stability and dependence on solution properties, such as initial concentration of dissolved gas and changes in gas concentration. We found that nanobubbles still formed in degassed solutions and that lowering the dissolved gas concentration did not reduce the bubble size, implying that nanobubbles do not form from dissolved gas in the liquid phase or do not contain gas and are instead water vapor. Furthermore, addition of an oxygen scavenger agent, sodium sulfite, to a liquid phase that had been pressured with oxygen did not reduce bubble size which could be evidence that nanobubbles are impermeable to gas diffusion across the gas liquid interface, do not form from the dissolved gas in the surrounding liquid, or do not contain gas and are instead water vapor.