Contact angles of surfactant solutions in oil solvents on low energy solid surfaces

Abstract

We have measured contact angles for a range of pure oils of different cohesive energy density and surfactant solutions in oil on polyethylene (PE) and polytetrafluoroethylene (PTFE) solid surfaces. For the pure oils, the measured contact angles are in approximate agreement with values calculated using dispersion force theory. Using measurements of surface tension and contact angle as a function of surfactant concentration, we have estimated surfactant adsorption isotherms at the oil/vapour surface and the ratio of the extents of adsorption at the oil/vapour and solid/oil surfaces. For alkylbenzenes in di-iodomethane (DIM) it is found that the extents of adsorption at the DIM/vapour and PTFE/DIM surfaces are similar whereas extents of adsorption at the PE/DIM surfaces are lower. n-Dodecyl pentaoxyethylene glycol ether (C₁₂E₅) adsorbs at the DIM/vapour surface but not at either the hexadecane (HD)/vapour or squalane (SQ)/vapour surfaces. For all the oils tested, C₁₂E₅ shows virtually zero adsorption at the solid/oil surface.