engleski

On the Microheterogeneity in Neat and Aqueous Amides: A Molecular Dynamics Study

A molecular dynamics study of three amides, namely, formamide, N-methyl formamide, and dimethyl formamide, and their mixtures with water is conducted to sort out the nature of the local microstructure of both the pure amides and the aqueous mixtures. For pure amides, both the cluster distribution and pair structure factors are investigated. The former reveals no specific cluster formation for all three amides, while the latter shows that formamide has a weak prepeak at about k ≈ 1.32 &Aring ; reminiscent of that observed for water, thus enforcing the puzzle of the nature of the hydrogen-bonding self-clustering in this type of liquids. The analysis of aqueous mixtures shows that all three mixtures are strongly microheterogeneous at the nanometer/nanosecond scales. This is confirmed in two ways: the direct visual observation and through the analysis of the pair distribution functions. The observation of microheterogeneity through usually measured quantities, namely the Kirkwood-Buff integrals, is discussed in light of the present analysis, and these latter quantities are shown to be weak indicators of microheterogeneity mainly because all three aqueous amides have nearly ideal such integrals in contrast to displaying strong microheterogeneity. As a direct consequence of this is the fact two features are reported. First, the Kirkwood-Buff integrals of the amide-amide correlations are obtained in relatively good agreement with the experimental ones, and second those involving correlations with water are considerably affected by the microheterogeneity beyond the 15 &Aring ; range and need prohibitively larger statistics despite the large system sizes used in this study.

aqueous amides; Kirkwood-Buff integrals; microheterogeneity; MD study

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