Structure of Monolayers of Tetraethylene Glycol Monododecyl Ether Adsorbed on Self-Assembled Monolayers on Silicon:  A Neutron Reflectivity Study

Abstract

We have used neutron reflection to study, for the first time, the structure of a surfactant layer adsorbed at the hydrophobic solid/water interface. Isotopic labeling of the water and of the hydrophobic selfassembled layer of octadecyltrichlorosilane (OTS) was first used to characterize protonated and deuteriated hydrophobic layers on the silicon oxide on the (111) face of silicon. This is the first time the structure and composition of such layers has been examined under water. Some water penetration into both the silicon oxide layer and the hydrophobic layer was observed. In the former case this is attributed to roughness of the oxide layer and in the latter to imperfections in the OTS layer. The thickness of the OTS layer was found to be 24 ( 2 Å, in agreement with other measurements in air. The adsorption isotherm of the surfactant tetraethylene glycol monododecyl ether (C 12E4) was measured using two independent measurements (different isotopic compositions) on the deuteriated OTS layer. The surfactant was found to reach a constant excess at the critical micelle concentration (cmc) very similar to that at the air/water interface, i.e., an area per molecule of about 50 Å 2 . The thickness of the surfactant layer was also found to be similar to that at the air/water interface and decreased rapidly with decreasing coverage. Estimates of the angle of tilt of the surfactant molecules from the surface normal were 53 ( 10°and 75 ( 10°at the highest and lowest coverages, respectively. Isotopic labeling of the two halves of the surfactant molecule was used to show that the molecule is partially oriented with the ethylene glycol groups pointing outward toward the aqueous solution. At the cmc the thicknesses of the two halves of the surfactant molecule were both found to be 10 ( 2 Å, to be compared with fully extended chain lengths of 16.9 (alkyl chain) and 14.2 Å (ethylene glycol chain). Some penetration of the OTS layer by the surfactant was observed at the highest surfactant coverages.

Adsorbed and free lipid bilayers at the solid-liquid interface

“…On the other hand, with the UV-ozone treatment the oxide layer was surprisingly good: low value of thickness and great homogeneity. We systematically looked for water contamination, which is usually found in oxides grown in aqueous solutions [10], but did not detect any. The same silicon blocks, without lipids, have been investigated in four water contrasts [13]: all results indicate the same roughness, thickness and homogeneity of oxide.…”

“…A bilayer suspended in water is stable but position control is not sufficient for reflectivity measurements. The solution consists in grafting lipid layers on silicon [10], or physically adsorbing them on a hydrophilic oxide [11]. This allows an in-situ, non destructive investigation of thickness, roughness, hydration, fluctuations and composition of phospholipid bilayers in a liquid medium, as well as their interaction with proteins.…”

The Denaturation of Lysozyme Layers Adsorbed at the Hydrophobic Solid/Liquid Surface Studied by Neutron Reflection

“…However, in practice it is much more important to maximize the reflected signal from the protein layer. We have demonstrated previously that the use of deuterated octadecyltrichlorosilane (d-OTS) can highlight the adsorbed protein substantially (28). The origins of this enhancement are not simple to explain but lie partly in the very high scattering length density of the densely packed OTS layer and the generation of strong interference in the reflectivity from the composite layer of OTS and protein.…”

Heat-treatment and Displacement of Adsorbed Lysozyme Layers

“…[1][2][3] In particular, efforts have been directed to the investigation of conformational changes undergone by the protein molecules upon adsorption using a variety of techniques including ellipsometry, 4,5 neutron reflection, [6][7][8] circular dichroism, 2 Fourier Transform InfraRed Spectroscopy in the Attenuated Total Reflection mode (FT IR-ATR), [9][10][11][12] and differential scanning calorimetry. 13,14 Also of considerable interest are any structural changes undergone by a pre-adsorbed protein layer under external factors, as the heating and the displacement by small molecules.…”

“…The value of concentration is consistent to the values found by other authors, and as found in these studies, was slightly below in maximally dense mono-layer. 7,8 The hydrophilic surfaces were prepared by performing the RCA 1 standard cleaning procedure at 75 C, as described in our previous papers. 4,6 The silicon oxide layers were characterised by ellipsometry and the average measured thickness was 14 AE 4 # A. Ellipsometry measures the changes in polarisation of an electromagnetic wave reflected from the experimental system, and has been thoroughly described in other works.…”