Solid-fluid and solid-solid equilibrium in hard sphere united atom models of n-alkanes: rotator phase stability.

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• N-alkanes exhibit complex phase behavior, including rotator phases, crucial for understanding material properties.
• Previous models often simplified alkane chains, limiting their ability to capture nuanced phase transitions.

Purpose of the Study:

• To investigate the phase behavior of n-alkanes (up to C21) using a hard sphere united atom model.
• To explore the role of chain conformations and orientational order in the formation of solid phases, particularly rotator phases.

Main Methods:

• Utilized hard sphere united atom models for methyl and methylene groups with fixed bond lengths and angles.
• Incorporated gauche conformations to represent chain flexibility.
• Performed extensive thermodynamic property and order parameter calculations.

Main Results:

• The model predicts both a rotator phase and a more ordered solid phase, in addition to the fluid phase.
• Phase behavior, including the existence of rotator phases, depends on alkane chain length and parity (even/odd carbon number).
• The transition between solid phases is first-order with minimal density change.

Conclusions:

• Rotator phases can emerge in n-alkane models without explicit attractive forces or hydrogen atom treatment.
• The model provides a simplified yet effective representation of alkane phase behavior, consistent with experimental data.
• Chain flexibility and orientational order are key factors driving the formation of distinct solid phases in n-alkanes.