The influence of the surface composition of mixed monolayer films on the evaporation coefficient of water.

Rachael E H Miles,Jonathan P Reid,James F Davies

doi:10.1039/c6cp03826c

Abstract

We explore the dependence of the evaporation coefficient of water from aqueous droplets on the composition of a surface film, considering in particular the influence of monolayer mixed component films on the evaporative mass flux. Measurements with binary component films formed from long chain alcohols, specifically tridecanol (C13H27OH) and pentadecanol (C15H31OH), and tetradecanol (C14H29OH) and hexadecanol (C16H33OH), show that the evaporation coefficient is dependent on the mole fractions of the two components forming the monolayer film. Immediately at the point of film formation and commensurate reduction in droplet evaporation rate, the evaporation coefficient is equal to a mole fraction weighted average of the evaporation coefficients through the equivalent single component films. As a droplet continues to diminish in surface area with continued loss of water, the more-soluble, shorter alkyl chain component preferentially partitions into the droplet bulk with the evaporation coefficient tending towards that through a single component film formed simply from the less-soluble, longer chain alcohol. We also show that the addition of a long chain alcohol to an aqueous-sucrose droplet can facilitate control over the degree of dehydration achieved during evaporation. After undergoing rapid gas-phase diffusion limited water evaporation, binary aqueous-sucrose droplets show a continued slow evaporative flux that is limited by slow diffusional mass transport within the particle bulk due to the rapidly increasing particle viscosity and strong concentration gradients that are established. The addition of a long chain alcohol to the droplet is shown to slow the initial rate of water loss, leading to a droplet composition that remains more homogeneous for a longer period of time. When the sucrose concentration has achieved a sufficiently high value, and the diffusion constant of water has decreased accordingly so that bulk phase diffusion arrest occurs in the monolayer coated particle, the droplet is found to have lost a greater proportion of its initial water content. A greater degree of slowing in the evaporative flux can be achieved by increasing the chain length of the surface active alcohol, leading to a greater degree of dehydration.

Highlights

We explore the dependence of the evaporation coefficient of water from aqueous droplets on the composition of a surface film, considering in particular the influence of monolayer mixed component films on the evaporative mass flux
Such suppressions are consistent with measurements of the impact of monolayer films on the reactive uptake coefficients of species such as N2O516–18 and broadly consistent with the structures and permeabilities of films expected from molecular dynamics simulations.[14,19,20,21,22]
Quantifying the mass transport of water, and volatile components more generally, through organic monolayer films on droplet surfaces and from amorphous particles is of importance to fields as diverse as atmospheric chemistry, spray drying and drug delivery to the lungs

Summary

Introduction

We explore the dependence of the evaporation coefficient of water from aqueous droplets on the composition of a surface film, considering in particular the influence of monolayer mixed component films on the evaporative mass flux. Condensed films formed from long-chain alcohols can slow the rate of water evaporation, reducing the evaporative mass flux by many orders of magnitude.[1,2] Further, the high solute concentrations reached in evaporating aqueous droplets containing sucrose can lead to particles of high viscosity (4107 Pa s), significantly enhancing water retention within a particle due to slow bulk phase diffusion, and leading to particles that continually lose water over many hours or days.[3,4,5,6,7]. Simulations indicated that, prior to film collapse, increasing surface pressure leads to the ‘‘squeezing out’’ of the bromoalkane from the film

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Conclusion