Adsorption of quaternary ammonium chlorides with various hydrocarbon chain lengths to glass

Contact angles, measured with various liquids, have been employed to calculate the surface free energies of glass after adsorption of quaternary ammonium chlorides with a variable hydrocarbon chain length 8n16. The thickness of the adsorbed layers has been determined ellipsometrically. A clear relation is observed between the measured parameters and the hydrocarbon chain lengthn, if only the extremesn=8 andn=16 are considered. Surface free energies decrease from 138 erg.cm^–2 for clean glass to 101 and 64 erg.cm^–2 forn=8 andn=16, respectively, at the highest concentration tested (7.5 mM). The adsorbed layer thickness of C₈ amounts to approximately 50 % of the thickness observed for C₁₆. No clear relation between the measured parameters is observed for the intermediate hydrocarbon chain lengths, which presumably reflects the many configurations possible in these adsorbed layers. It is envisaged that adsorption of C₈ as well as C₁₆ is restricted to a monolayer, which is completed at approximately 2 mM. In the case of C₈ electrostatic repulsion between the polar headgroups will inhibit further adsorption, whereas in the case of C₁₆ the van der Waals attraction from the adsorbed layer and the glass will probably not be sufficient to stimulate further adsorption.