Relationships between the enthalpy and entropy changes resulting from perturbations of a system have been discussed in the literature for some time. Both positive correlations (compensation) and negative correlations (anti-compensation) between deltaH and DeltaS have been observed in various experimental contexts, including chemical reaction, physical association, solvation, and protein folding. Many examples have been demonstrated to be statistical artifacts, but some are genuine signatures of the perturbations in molecular characteristics. In particular, recent literature claims that compensation is a general feature of bimolecular associations arising from weak intermolecular interactions. We employ a statistical mechanical framework to predict the magnitude and direction of enthalpy-entropy correlation in bimolecular association. The theory links the macroscale thermodynamic correlation to the relationship between the intermolecular potential parameters. Using a harmonic approximation to the Lennard-Jones model and potential parameters taken from the literature, we show examples of both compensation and anti-compensation for gas-phase self-association among five homologous series. Furthermore, an aggregate presentation of data for 48 different chemical species shows no correlation in either direction, for the case of self-association in a dilute gas phase.

The structure and reducibility of ceria clusters was investigated using simulated annealing and interionic potentials, given by Sayle and co-workers (Sayle, T. X. T.; Parker, S. C.; Catlow, C. R. A. Surf. Sci. 1994, 316, 329), which include terms for ionic and van der Waals interactions, overlap repulsions, and ion polarizability. For Ce n O 2n , the lattice energies, pair-distribution functions, and angular-distribution functions were determined for the crystals at 0 K for n ) 2-20 and n ) 50. As expected, the lattice energy decreased with n. Only for the largest cluster was the fluorite structure clearly observed. This cluster also exhibited (111) surface facets, indicating that this is the most stable surface. The reducibility of the clusters was determined from the difference in lattice energies of Ce n O 2n and Ce n O 2n-1 , where charge neutrality was maintained by changing two Ce 4+ ions to Ce 3+ . The energy required to reduce the cluster generally increased with increasing cluster size, but large fluctuations were also observed. These results generally agree with the experimental observations that reduction of ceria is structure sensitive and that larger crystals are more difficult to reduce.

Utilizing polypeptide secondary structure as a means for controlling oxide pore architectures is explored. Poly-L-lysine is used as a model polypeptide as its folding behavior is well understood and compatible with the sol-gel chemistry of silica. Here, we show that silicas synthesized with poly-L-lysine in a R-helix conformation possess cylindrical pores that are approximately 1.5 nm in size, whereas silicas synthesized with poly-L-lysine in a β-sheet conformation possess larger pores, the size of which are a function of the poly-L-lysine concentration, or in other words the size of the aggregate. In both cases, highly porous materials are obtained. In-situ circular dichroism measurements of the synthesis mixtures show that the poly-L-lysine secondary structure is not perturbed during synthesis. Infrared spectroscopy of the as-synthesized materials is consistent with the poly-L-lysine retaining its secondary structure. Grand canonical Monte Carlo simulations were also performed to validate the interpretation of the experimental adsorption results. The experimental isotherms are consistent with simulated isotherms of cylindrical pores 1.3-1.7 nm in size, in good agreement with expected values. Our results suggest a new avenue for synthesizing porous oxides with highly tuneable pore sizes and shapes under mild conditions.

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