Aggregation and crystallization of small alkanes.

This article presents a computer simulation study of the aggregation and ordering behavior of short n-alkane chains using a united atom polyethylene model and the Stochastic Approximation Monte Carlo (SAMC) method to determine the density of states and thermodynamics across temperatures. The study identifies first-order aggregation transitions and subsequent low-temperature ordering transitions, with small aggregates exhibiting quaternary structure formation analogous to peptide folding, while larger aggregates (seven or more chains) form lamellar crystalline structures typical of bulk alkanes. Aggregation temperatures extrapolated to the thermodynamic limit and ambient pressure closely match experimental boiling points, and crystallization transition temperatures also agree well with experimental data. The analysis reveals a two-step ordering process in small aggregates due to distinct surface and core ordering, which merges into a single transition in the thermodynamic limit. The work demonstrates the effectiveness of SAMC and Fisher zeros analysis in characterizing phase transitions in alkane aggregates and provides insights into the structural motifs and energetics underlying their phase behavior.