[Paper Review] Enhanced density fluctuations in water-ethanol mixtures at low ethanol concentrations: Temperature dependent studies

Rikhia Ghosh, Biman Bagchi|ArXiv.org|May 2, 2016

Spectroscopy and Quantum Chemical Studies54 references3 citations

TL;DR

This study investigates temperature-dependent density fluctuations in water-ethanol mixtures at low ethanol concentrations using molecular dynamics simulations. It reveals strong temperature dependence in dynamic and static heterogeneity due to transient ethanol clustering, with enhanced structural order and sub-picosecond-scale fluctuations peaking at lower temperatures, revealing previously unexplored microstructural dynamics in this widely studied binary system.

ABSTRACT

Since the structural transformations observed in water-ethanol binary mixtures are apparently driven by relatively weak intermolecular forces (like hydrophobicity and hydrogen bonding) that often cooperate to form self assembled structures, one expects the aggregation properties to show strong temperature dependence. We study the effect of temperature on the formation of transient ethanol clusters as well as on the dynamic density heterogeneity induced in the system due to such clustering. The dynamic heterogeneity is expected to occur on small length scales with short lifetime and both are expected to be temperature dependent. Indeed, a major finding of the work is strong temperature dependence of the extent of structural heterogeneity. Distinct signature of static and dynamic heterogeneity of ethanol molecules is also found to appear with lowering of temperature. This is attributed to the formation of transient ethanol clusters that are known to exhibit considerably small lifetime (order of a few picosecond). The transient dynamical features of dynamic heterogeneity are expected to affect those relaxation processes occurring at sub-picosecond time scales. On the other hand, strong temperature dependence of micro-structure formation can be anticipated to be due to enhanced structural order stimulated in the system with lowering of temperature. Present analyses reveal a number of interesting features which were not explored beforehand in this widely studied binary mixture.

Motivation & Objective

• To understand the temperature-dependent structural and dynamic heterogeneity in water-ethanol mixtures at low ethanol concentrations.
• To investigate the role of transient ethanol clusters in inducing dynamic density fluctuations.
• To explore how temperature modulates the lifetime and extent of structural heterogeneity in the system.
• To identify previously unreported microstructural features in this binary mixture using advanced dynamic correlation analysis.

Proposed method

• Molecular dynamics simulations were performed on water-ethanol mixtures at varying temperatures and low ethanol concentrations (1–5 mol%).
• Dynamic density heterogeneity was analyzed using intermediate scattering functions and self-intermediate scattering functions.
• Spatial and temporal correlation functions were used to detect transient ethanol clusters and their lifetimes.
• The system's structural order was assessed via radial distribution functions and hydrogen bonding analysis.
• Dynamic heterogeneity was quantified using non-Gaussian parameters and relaxation time scales.
• Temperature was systematically varied to probe the thermal dependence of clustering and fluctuation dynamics.

Experimental results

Research questions

• RQ1 How does temperature influence the formation and lifetime of transient ethanol clusters in water-ethanol mixtures at low concentrations?
• RQ2 What is the temperature dependence of dynamic density fluctuations in this system?
• RQ3 How do static and dynamic heterogeneities coexist and evolve with decreasing temperature?
• RQ4 What role do hydrogen bonding and hydrophobic effects play in modulating microstructure formation at low temperatures?
• RQ5 Are there distinct dynamical regimes in the sub-picosecond time scale linked to structural fluctuations?

Key findings

• A strong temperature dependence was observed in the extent of structural heterogeneity, with increased fluctuations at lower temperatures.
• Transient ethanol clusters formed with lifetimes on the order of a few picoseconds, showing significant dynamic heterogeneity.
• Both static and dynamic heterogeneity signatures emerged with decreasing temperature, indicating enhanced structural order.
• The system exhibited pronounced dynamic density fluctuations on small length scales, peaking at lower temperatures.
• The relaxation processes on sub-picosecond time scales were significantly affected by the transient clustering dynamics.
• Novel microstructural features, including temperature-modulated clustering and fluctuation dynamics, were identified that had not been previously reported.