Gibbs Free Energy and Reaction Rate Acceleration in and on Microdroplets

Adrian F Tuck

doi:10.3390/e21111044

Abstract

Recent observations show that many reactions are accelerated in microdroplets compared to bulk liquid and gas media. This acceleration has been shown to feature Gibbs free energy changes, ΔG, that are negative and so reaction enabling, compared to the reaction in bulk fluid when it is positive and so reaction blocking. Here, we argue how these ΔG changes are relatable to the crowding enforced by microdroplets and to scale invariance. It is argued that turbulent flow is present in microdroplets, which span meso and macroscales. That enables scale invariant methods to arrive at chemical potentials for the substances involved. G and ΔG can be computed from the difference between the whole microdroplet and the bulk medium, and also for individual chemical species in both cases, including separately the microdroplet’s surface film and interior, provided sufficiently fine resolution is available in the observations. Such results can be compared with results computed by quantum statistical mechanics using molecular spectroscopic data. This proposed research strategy therefore offers a path to test its validity in comparing traditional equilibrium quantum statistical thermodynamic tests of microdroplets with those based on scale invariant analysis of both their 2D surface and 3D interior fluid flows.

Highlights

The observed acceleration of chemical reaction rates in microdroplets, and in many cases their transformation from positive to negative Gibbs free energy changes (∆G), has implications for chemistry across many areas, including basic kinetics, aerosols in both the laboratory and atmosphere, prebiotic chemistry, synthetic routes and industrial processes [1,2,3,4,5,6,7,8]
Basic principles expounded were that they and other statistical thermodynamic variables were obtainable by the methods of generalized scale invariance, with attractive forces being responsible for the production of organization, while repulsive ones effectuated dissipation and the necessary entropy production
Microdroplets are common in a wide variety of processes and can occur in fluid media having an extensive range of scales, ranging from laboratory reactors to planetary atmospheres; aerosols in

Summary

Introduction

The observed acceleration of chemical reaction rates in microdroplets, and in many cases their transformation from positive to negative Gibbs free energy changes (∆G), has implications for chemistry across many areas, including basic kinetics, aerosols in both the laboratory and atmosphere, prebiotic chemistry, synthetic routes and industrial processes [1,2,3,4,5,6,7,8]. We discuss in more detail how ∆G is influenced by the processes determining the rate of a reaction when it is taking place in and on a microdroplet, as contrasted to its value in the bulk liquid or gas. The implications of this phenomenon for the chemical future have been emphasized [6,7]. The determination of entropy and Gibbs energy for open, non-equilibrium systems by analysis of observed scale invariance has been proposed and exemplified [9]. Dilution of energy density will minimize G and enable whatever chemical and fluid mechanical processes will lead the microdroplet to a stationary state, or in a few Entropy 2019, 21, 1044; doi:10.3390/e21111044 www.mdpi.com/journal/entropy

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