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R Steitz

Publications and source records attributed to R Steitz.

5 recordsLinked to original sources
PubMed2007

Density depletion at solid-liquid interfaces: a neutron reflectivity study.

Neutron reflectivity experiments conducted on self-assembled monolayers (SAMs) against polar (water) and nonpolar (organic) liquid phases reveal further evidence for a density reduction at hydrophobic-hydrophilic interfaces. The density depletion is found at the interface between hydrophobic dodecanethiol (C12) and hexadecanethiol (C16) SAMs and water and also between hydrophilic SAMs (C12/C11OH) and nonpolar fluids. The results show that the density deficit of a fluid in the boundary layer is not unique to aqueous solid-liquid interfaces but is more general and correlated with the affinity of the liquid to the solid surface. In water the variation of pH has only minor influence, while different electrolytes taken from the Hofmeister series seem to increase the depletion. On hydrophobic SAMs an increase in density depletion with temperature was observed, in agreement with Monte Carlo simulations performed on corresponding model systems. The increase in the water density depletion layer is governed by two effects: the surface energy difference between water and the substrate and the chemical potential of the aqueous phase.

Journal Article↗
PubMed2004

Swelling behavior of self-assembled monolayers of alkanethiol-terminated poly(ethylene glycol): a neutron reflectometry study.

The swelling behavior of alkanethiol-terminated poly(ethylene glycol) with an average molecular weight of 2180 Da (i.e., approximately 45 ethylene glycol, EG, units) in contact with water was investigated by neutron reflectometry as a function of the morphology of the PEG-SH layer. Amorphous films at a low grafting density show significant swelling with an increase of the film thickness from approximately 25 A in the dry state to approximately 70 A in contact with D2O, which corresponds to a total water uptake of approximately 38 mass %. In contrast, quasi-crystalline monolayers exhibit only a small amount of water penetrating into the film (approximately 8 mass %) with a corresponding change of the layer thickness from approximately 110 to approximately 125 A. The water uptake per EG unit corresponds to the literature value of 1.5 for the amorphous layer and to only 0.25 in the case of the quasi-crystalline film.

Letter↗
PubMed1999

Neutron reflectivity studies of critical adsorption: the correspondence between a critical adsorption profile and specular neutron reflection.

For interfacial order parameter profiles which decay as Az(-mu), such as the composition profile of the noncritical interface of a binary liquid mixture at a critical end point, there is a reported one-to-one correspondence between the profile and the reflectivity which can be described by an analytical theory [S. Dietrich and R. Schack, Phys. Rev. Lett. 58, 140 (1987)]. Evidence for mu=0.53+/-0.02 for adsorption at the hydrophilic silicon/liquid surface of the mixture (2-butoxyethanol+deuterium oxide) near its lower critical end point, determined from neutron reflectivity measurements, is presented. This value is in good agreement with the theoretical prediction of mu=0.516+/-0.004. Further examination of the data permits the determination of the asymptotic surface enrichment scaling factor amplitude P0 approximately 0.11, which is not in agreement with the theoretical value P(0) = 0.94+/-0.05 and values determined by other experimental methods.

Journal Article↗
PubMed1998

The Evaporation Resistance of Mixed Monolayers of Octadecanol and Cholesterol.

The high resistance to water evaporation exhibited by monolayers of octadecanol is markedly reduced if small quantities of cholesterol are present, an effect that mirrors the behavior of contaminants in octadecanol monolayers. Surface pressure-area isotherms show that octadecanol and cholesterol are almost (but not entirely) immiscible. Grazing-incidence synchrotron X-ray diffraction (GIXD) of the floating monolayers shows that cholesterol produces no diffraction pattern and thus is amorphous, octadecanol gives a clear diffraction peak which can be deconvoluted into a major and a minor peak corresponding to a slightly distorted hexagonal packing of the alkyl chains, and the alcohol-rich mixtures give diffraction patterns like that of pure octadecanol. The latter result shows that cholesterol does not enter the octadecanol domains in the mixed monolayers. The evaporation data are explained by permeation through the irregularly packed domain boundary regions in a pure octadecanol monolayer and by the rapid increase in the size of these regions when cholesterol is added. Copyright 1998 Academic Press.

Journal Article↗