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Adsorption of acridine orange at C8,18/water/acetonitrile interface

Liquid chromatography is one of the most widely used separation techniques but there is still many questions concerning the organization and dynamics of the stationary phase structure. Acridine orange (3,6-dimethylaminoacridine) is a fluorescent adsorbate that has been well characterized at chromatographic interfaces. Its adsorption behavior at the hydrocarbon/solvent interface is investigated using Molecular Dynamics simulations. The diffusion coefficient D of acridine orange in pure solvent was also calculated and was found to be 4 times smaller at the water/C18 interface (D=0.022 10-4 cm2/s) than in bulk water (D=0.087 10-4 cm2s), in qualitative agreement with experiment. Detailed analysis of the solvent structure showed that the transport properties of acridine were primarily governed by the solvent distribution above the functionalized surface. The solvent structure, in turn, was largely determined by the surface consisting of the silica layer, the alkyl chains and their functionalization.

Structures and dynamics of protonated clusters


The structures and infrared spectra of protonated ammonia clusters NH4+(NH3)n, for n<=8, are investigated using density functional-theory (DFT) calculations and semiempirical DFT/molecular dynamics simulations. For n<5 the clusters are found to be mostly stable up to 100 K, while the larger clusters (n>=5) isomerize. Temperature effects are taken into account by performing ab initio molecular dynamics simulations with the computationally tractable self-consistent charges density functional tight-binding method. The infrared spectra at 10 K for the most stable isomers for n=3–8 compare qualitatively with predissociation experiments, and using a common scaling factor almost quantitative agreement is found. For n>=6 the notion of multiple isomers present under the experimental conditions is supported. Of the 13 stable structures for n=8 only three are found to survive at 100 K. All other clusters isomerize. Cluster structures are inferred from the analysis of the cumulative radial distribution function of the ammonia molecules surrounding the NH4+ core. The infrared spectra are found to be typical for the structure of the clusters, which should help to relate the experimentally measured infrared spectra to the number and identity of the contributing isomers. For clusters that reorganize to a more stable isomer during the dynamics, the infrared spectrum is generally similar to that of the stable isomer itself. The clusters are found to preferably form globular structures, although chain-like arrangements are also among the low-energy configurations.


The structures and dynamics of protonated water clusters H+(H2O)n (n=2-8, 17, 21) are investigated by using DFT/B3LYP and ab initio Molecular Dynamics. For each cluster size, the different structural minima are calculated with the Self-Consistent Charges Density Functional Tight-Binding (SCC-DFTB) method and at the B3LYP/6-31G** level. Temperature effects are taken into account by performing SCC-DFTB/MD simulations at 50 and 100K. The changes of structure are analyzed from radial distribution functions. At lower temperature, the preferred structures are the ones with a minimum dangling H atoms forming a network structure. At higher temperatures, chain like structures are preferred and most of the isomers undergo an isomerization which is reflected in a change of the infrared spectra and radial distribution function. The infrared spectra are discussed in view of recent experimental data [Science 304, 1137 (2004)]. As the cluster size increases, experimental spectra display a characteristic shifting and splitting of the vibration bands around 3400 cm-1 which are related to H-bond forming patterns. This is reproduced by the present simulations and can be related to the existence of multiple isomers contributing to the spectra, which is difficult to understand from experiment alone.