Comparative adsorption of argon and nitrogen for the characterisation of hydrophobized surfaces

A macroporous silica sample is progressively hydrophobized by grafting alkyl chains on its surface. Mono and tri-chloroalkylsilane molecules are used in order to vary the grafting density. After characterisation by infra-red, the adsorption of nitrogen or argon at 77 K is studied. The BET surface area derived from the nitrogen adsorption isotherm decreases with grafting coverage whereas that determined from argon adsorption slightly increases. The results are interpreted by the possible orientation of nitrogen molecule on polar surfaces. A method using the comparison between argon and nitrogen surface area is proposed to determine the fraction of hydrophobized surface.