Discontinuous Structural Transitions in Fluids with Competing Interactions

TL;DR

This study investigates how competing interactions influence structural transitions in fluids, revealing complex behaviors and the impact of dimensionality through exact and approximate analyses of 1D and 3D systems.

Contribution

It introduces a versatile potential model with stepped interactions and compares 1D and 3D systems to elucidate the effects of confinement and interaction features on structural transitions.

Findings

Discontinuous oscillation crossover lines can occur in temperature-density space.

Fisher-Widom lines appear for certain attractive-repulsive step combinations.

Dimensionality affects the temperature range and nature of structural transitions.

Abstract

This paper explores how competing interactions in the intermolecular potential of fluids affect their structural transitions. This study employs a versatile potential model with a hard core followed by two constant steps, representing wells or shoulders, analyzed in both one-dimensional (1D) and three-dimensional (3D) systems. Comparing these dimensionalities highlights the effect of confinement on structural transitions. Exact results are derived for 1D systems, while the rational function approximation is used for unconfined 3D fluids. Both scenarios confirm that when the steps are repulsive, the wavelength of the oscillatory decay of the total correlation function evolves with temperature either continuously or discontinuously. In the latter case, a discontinuous oscillation crossover line emerges in the temperature--density plane. For an attractive first step and a repulsive second step, a Fisher--Widom line appears. Although the 1D and 3D results share common features, dimensionality introduces differences: these behaviors occur in distinct temperature ranges, require deeper wells, or become attenuated in 3D. Certain features observed in 1D may vanish in 3D. We conclude that fluids with competing interactions exhibit a rich and intricate pattern of structural transitions, demonstrating the significant influence of dimensionality and interaction features.