Formation of an Icosahedral Structure during the Freezing of Gold Nanoclusters: Surface-Induced Mechanism

The freezing behavior of gold nanoclusters was studied by employing molecular dynamics simulations based on a semi-empirical embedded-atom method. Investigations of the gold nanoclusters revealed that, just after freezing, ordered nano-surfaces with a fivefold symmetry were formed with interior atoms remaining in the disordered state. Further lowering of temperatures induced nano-crystallization of the interior atoms that proceeded from the surface towards the core region, finally leading to an icosahedral structure. These dynamic processes explain why the icosahedral cluster structure is dominantly formed in spite of its energetic metastability.Comment: 9 pages, 4 figures(including 14 eps-files

Thermodynamics of Binary Two-Dimensional Phases on Nearly Perfect Substrates

We present some results on -stability and phase transitions of binary two-dimensional phases on the surfaces of nearly perfect substrates. A simple theoretical model allows to get insight into the mutual enhancement of the condensation of the adsorption partners, resulting in the formation of two-dimensional mixture, and into the subsequent demixting of the layers, when the concentration of one of the components in the bulk phase continues to increase. Both continuous and discontinuous demixing can be expectad, and other phase transformations, like two-dimensional solidification and melting, and/or different structural transitions, are possible when the molecules of the adsorbates have different shape and the substrate is not fully isomorphic with them. Some of the peculiarities regarding the physical realizations of the proposad model are shown on the example of the coadsorption of methane and krypton on exfoliated graphite

Self-assembly of protein amyloid: a competition between amorphous and ordered aggregation

Protein aggregation in the form of amyloid fibrils has important biological and technological implications. Although the self-assembly process is highly efficient, aggregates not in the fibrillar form would also occur and it is important to include these disordered species when discussing the thermodynamic equilibrium behavior of the system. Here, we initiate such a task by considering a mixture of monomeric proteins and the corresponding aggregates in the disordered form (micelles) and in the fibrillar form (amyloid fibrils). Starting with a model on the respective binding free energies for these species, we calculate their concentrations at thermal equilibrium. We then discuss how the incorporation of the disordered structure furthers our understanding on the various amyloid promoting factors observed empirically, and on the kinetics of fibrilization.Comment: Minor changes in the presentatio

Surface melting of nanoscopic epitaxial films

By introducing finite size surface and interfacial excess quantities, interactions between interfaces are shown to modify the usual surface premelting phenomenon. It is the case of surface melting of a thin solid film s deposited on a planar solid substrate S. More precisely to the usual wetting condition of the solid s by its own melt l, necessary for premelting (wetting factor F<0), is adjoined a new quantity G describing the interactions of the l/s interface with the s/S interface. When G>0 this interface attraction boosts the premelting so that a two stage boosted surface premelting is foreseen: a continuous premelting, up to roughly half the deposited film, is followed by an abrupt first order premelting. When G<0 these interfaces repell each other so that premelting is refrained and the film remains partly solid above the bulk melting point (overheating) what is called astride melting. Elastic stress modifies both types of melting curves. Bulk and surface stresses have to be distinguished.Comment: 65 pages, 16 figure

Oriented crystal growth on organic monolayers

Ordered organic substrates influence the crystallisation of minerals and different crystal morphologies and polymorphs can be stabilised by varying the properties of the substrates. The mechanisms behind this crystallisation control are not always apparent; however in recent years results from molecular modelling studies have led to an increased level of understanding. We present a review of the experimental evidence for crystallisation control by organic self-assembled monolayers and discuss the modelling methods that have been used to study these effects. We give an overview of the contribution modelling has made to the field of mineral crystallisation on organic substrates. The focus is on calcium carbonate because of its importance as a biomineral and, consequently, the large number of experimental and modelling studies that have been performed for this mineral

On Some Properties Of The Two-dimensional Phases Condensed On A Foreign Substrate. Ii. Surface Roughness Of Condensed Layers And Variation Of The Surface Free Energy During Multiple Layer Adsorption

The roughness of the surface resulting from a multiple-layer adsorption is treated as a function of the temperature and the supersaturation. It is shown that the supersaturation Δμ(i) v7, at which the number of vacancies and admolecules of the i-th top-layer are equal, is the arithmetic mean of the equilibrium supersaturations Δμ(i) 0 and Δμ(i + 1) 0 of the i-th and (i + 1)-th layers respectively, contrary to a three-dimensional phase, where Δμv7 and Δμ0 are identical (nil). The further use of Δμ(i) v7, as origin for the supersaturation scale, leads to reduced equations, permitting to calculate easily roughness and surface free energy variations in the stability range (Δμ(i) 0 &lt; Δμ &lt; Δμ(i + 1) 0) of the i-th layer. The total free surface energy variation due to multiple layer adsorption is decomposed into two parts: the first, due to the deposition of i compact layers, and the second, due to the roughness of the top monomolecular layer. A method is proposed for determining the absolute specific adhesion energy between two substances (A and B) in the case when only the positions of the steps of the experimental step-wise isotherm of A on B are known. The method is applied to the adhesion energy xenon/graphite. © 1976.61193108Mutaftschiev, (1976) Surface Sci., 60Champion, Halsey, (1953) J. Phys. Chem., 57, p. 646Singleton, Halsey, (1954) J. Phys. Chem., 58, p. 1011Hill, (1946) J. Chem. Phys., 14, p. 263Mutaftschiev, (1965) Adsorption et Croissance Cristalline, p. 231. , CNRS, ParisTemkin, (1966) Crystallization Processes, p. 15. , Consultants Bureau, New YorkFrumkin, (1925) Z. Physik. Chem., 116, p. 466Fowler, Guggenheim, (1960) Statistical Thermodynamics, p. 466. , CambridgeBragg, Williams, The Effect of Thermal Agitation on Atomic Arrangement in Alloys (1934) Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 145, p. 699Bragg, Williams, The Effect of Thermal Agitation on Atomic Arrangement in Alloys. II (1935) Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 151, p. 540Born, Stern, (1919) Sitz. Ber. Preuss. Akad. Wiss., 48, p. 901Thomy, Duval, (1965) Adsorption et Croissance Cristalline, p. 81. , CNRS, ParisQuentel, Rickard, Kern, (1973) Vide, 28, p. 65J. Regnier and B. Mutaftschiev, to be publishe