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Notes: The effects of aqueous solutions of 2-hydroxyalkanoic acids and analogous materials on the extensibility of an in vitro model for human stratum corneum have been investigated. The increase in extensibility of stratum corneum caused by treatment with 2-hydroxy acid solutions reaches a peak at C8 in the case of the saturated molecules. Introducing unsaturation into the hydrocarbon chain shifts the point of maximum efficacy to a longer chain length. Further modifications to the molecular backbone, such as the replacement of a methylene group by heteroatoms, can improve significantly the relative plasticization obtained from aqueous treatment. Alteration of the acidic head group markedly influences the elasticity increase. The free acid is the most efficacious species. An increase in the pKa of HCA by the addition of polyol increases the extensibility improvement. Movement of the hydroxyl group from C-2 is not advantageous, but replacement with a 2-thio derivative is beneficial in terms of increased plasticization. The best systems for improving extensibility have a strong affinity for protein.〈section xml:id="abs1-2"〉〈title type="main"〉RésuméNous avons analysé les effets de solutions aqueuses d'acides 2-hydroxyalkanoïques et de matériaux similaires sur l'extensibilité in vitro d'un échantillon de stratum corneum humain. L'augmentation de l'extensibilité du stratum corneum générée par le traitement aux solutions des acides 2-hydroxy atteint un pic à C8 dans le cas de molécules saturées. L'introduction d'une insaturation au niveau de la chaîne d'hydrocarbone amène le point d'efficacité maximum à une chaîne plus longue. Des modifications apportées aux molécules principales, telles que le remplacement du groupe méthylène par des hétéroatomes peuvent améliorer de façon significative la plasticité relative obtenue par un traitement aqueux. L'altération du groupe acide de tête influence de façon notable l'augmentation de l'élasticité. L'acide libre est la plus efficace. Une augmentation du pKa de l'acide 2-hydroxy caprylique par addition de polyol accroît l'amélioration de l'extensibilité. Le deplacement du groupe hydroxy de C-2 n'est pas efficace mais le remplacement par des dérivés 2-thio améliore la plasticité. Les systémes qui améliorent au maximum l'extensibilité ont une affinité importante avec les protéines.

Notes: We have investigated visible photoluminescence (PL) from thin porous Si1−xGex alloy layers prepared by stain etching of molecular-beam-epitaxy-grown material. Seven samples with nominal Ge fraction x varying from 0.04 to 0.41 were studied at room temperature and 80 K. Samples of bulk stain etched Si and Ge were also investigated. The composition of the porous material was determined using x-ray photoemission spectroscopy and Rutherford backscattering techniques to be considerably more Ge rich than the starting epitaxial layers. While the luminescence intensity drops significantly with the increasing Ge fraction, we observe no significant variation in the PL wavelength at room temperature. This is clearly in contradiction to the popular model based on quantum confinement in crystalline silicon which predicts that the PL energy should follow the band-gap variation of the starting material. However, our data are consistent with small active units containing only a few Si atoms that are responsible for the light emission. Such units are present in many compounds proposed in the literature as the cause of the visible PL in porous Si.