STRUCTURE OF MONOLAYERS OF TETRAETHYLENE GLYCOL MONODODECYL ETHER ADSORBED ON SELF-ASSEMBLED MONOLAYERS ON SILICON - A NEUTRON REFLECTIVITYSTUDY

We have used neutron reflection to study, for the first time, the stru cture of a surfactant layer adsorbed at the hydrophobic solid/water in terface. Isotopic labeling of the water and of the hydrophobic self-as sembled layer of octadecyltrichlorosilane (OTS) was first used to char acterize protonated and deuteriated hydrophobic layers on the silicon oxide on the (111) face of silicon. This is the first time the structu re and composition of such layers has been examined under water. Some water penetration into both the silicon oxide layer and the hydrophobi c layer was observed. In the former case this is attributed to roughne ss of the oxide layer and in the latter to imperfections in the OTS la yer. The thickness of the OTS layer was found to be 24 +/- 2 Angstrom, in agreement with other measurements in air. The adsorption isotherm of the surfactant tetraethylene glycol monododecyl ether (C(12)E(4)) w as measured using two independent measurements (different isotopic com positions) on the deuteriated OTS layer. The surfactant was found to r each a constant excess at the critical micelle concentration (cmc) ver y similar to that at the air/water interface, i.e., an area per molecu le of about 50 Angstrom(2). The thickness of the surfactant layer was also found to be similar to that at the air/water interface and decrea sed rapidly with decreasing coverage. Estimates of the angle of tilt o f the surfactant molecules from the surface normal were 53 +/- 10 degr ees and 75 +/- 10 degrees at the highest and lowest coverages, respect ively. Isotopic labeling of the two halves of the surfactant molecule was used to show that the molecule is partially oriented with the ethy lene glycol groups pointing outward toward the aqueous solution. At th e cmc the thicknesses of the two halves of the surfactant molecule wer e both found to be 10 +/- 2 Angstrom, to be compared with fully extend ed chain lengths of 16.9 (alkyl chain) and 14.2 Angstrom (ethylene gly col chain). Some penetration of the OTS layer by the surfactant was ob served at the highest surfactant coverages.