Wetting behaviour and wetting transitions of alkanes on aqueous surfaces

The nature of the surface forces, acting in the wetting films of pentane, hexane and heptane on water and on aqueous NaCl solutions is discussed. It is shown that an important contribution to the surface forces originates from the low solubility of water in short-chain alkanes. The isotherms of water adsorption, the disjoining pressure and the excess free energy were calculated for wide range of temperatures and salinities of underlying aqueous solution. To account for the screening action of ions in the aqueous solution on the image-charge interactions in alkane films we have solved the set of Poisson equations with the appropriate boundary conditions and within the Debye-Hückel approximation applied for the electrolyte phase. The analysis of the influence of temperature and salinity on different components of the disjoining pressure allows the conclusion that both factors shift the total isotherm onto the region of positive values resulting in the wetting transition. Such transitions for the range of temperatures and salinities studied were detected for pentane and hexane films only. Achievement of the wetting transition in heptane requires higher salinities and temperatures. The nature of wetting transitions from partial to complete wetting and from thin alpha- to thick beta-films was analyzed on the basis of Derjaguin -Frumkin theory taking into account the behaviour of the disjoining pressure and the excess free energy isotherms. It was shown that the surface forces, considered here, provide in the range of vapour undersaturation two sequential (partial-to-complete followed by thin-to-thick films) wetting transitions which combine at saturation into a single transition. For the oversaturated vapours only partial-to-complete wetting transition may be described.