Structure of surfactant monolayers at the air-water interface determined by neutron reflection

The determination of the structure of a soluble surfactant monolayer, hexadecyl trimethyl ammonium bromide (C(16)TAB), using isotopic substitution in conjunction with neutron reflection is described. The roughness of the layer has been determined as has the separation of the various parts of the molecule in the normal direction. The molecule is strongly tilted with respect to the surface normal and the outer part of the molecule is more tilted than the inner part. The roughness is comparable with the projection of the molecule on the surface normal direction. In different C(n)TABs with n = 12 to 18 the thickness of the chain region of the monolayer is found to he almost independent of chainlength at 16 - 17 Angstrom. In monolayers of C(12)E(m), where E(m) is an ethylene glycol chain of in units, the thickness of the ethylene glycol chain region varies from about 8 Angstrom for m = 2 to 19 Angstrom when m = 8.