Forces between fluorocarbon surfactant monolayers: salt effects on the hydrophobic interaction

• Further studies of the hydrophobic attraction between fluorocarbon surfaces in water have been carried out, including (i) a comparison between surfaces prepared by Langmuir-Blodgett deposition and by adsorption from solution using the same surfactant and (ii) the effect of added electrolyte. In general, the surfaces prepared by deposition show the longest range of the attraction - measurable at 80-90 nm with an exponential decay length of about 15 nm at separations above 5 nm. In the case of adsorbed monolayers incomplete adsorption or additional adsorption to the hydrophobic monolayer surfaces often leads to an effective attraction that is of considerably shorter range. Under optimal conditions, however, adsorbed monolayers give an attraction of a similar range. In both cases the interaction becomes more attractive at small separations and appears to decay exponentially with a decay length of 2-3 nm below about 15 nm. The deposited fluorocarbon monolayers are unstable in sodium or potassium salt solution but remain intact in terapentylammonium bromide. With increasing concentration of this salt the range of the hydrophobic interaction decreases as more and more ions adsorb to the surfaces. The short-range interaction seems to be less sensitive to the presence of electrolyte - the adhesion at contact shows a much less dramatic decrease with increasing electrolyte concentration. Nevertheless, the measured electrolyte dependence of the hydrphobic interaction is at least partly due to the surfaces becoming less hydrophobic themselves, as reflected by their caviation bahaviour.

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