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

We explore the dependence of the evaporation coefficient of water from aqueous droplets on thecomposition of a surface film, considering in particular the influence of monolayer mixed componentfilms on the evaporative mass flux. Measurements with binary component films formed from long chainalcohols, specifically tridecanol (C₁₃H₂₇OH) and pentadecanol (C₁₅H₃₁OH), and tetradecanol(C₁₄H₂₉OH) and hexadecanol (C₁₆H₃₃OH), show that the evaporation coefficient is dependent on themole fractions of the two components forming the monolayer film. Immediately at the point of filmformation and commensurate reduction in droplet evaporation rate, the evaporation coefficient is equalto a mole fraction weighted average of the evaporation coefficients through the equivalent singlecomponent films. As a droplet continues to diminish in surface area with continued loss of water, themore-soluble, shorter alkyl chain component preferentially partitions into the droplet bulk with theevaporation coefficient tending towards that through a single component film formed simply from theless-soluble, longer chain alcohol. We also show that the addition of a long chain alcohol to an aqueoussucrosedroplet can facilitate control over the degree of dehydration achieved during evaporation. Afterundergoing rapid gas-phase diffusion limited water evaporation, binary aqueous-sucrose droplets showa continued slow evaporative flux that is limited by slow diffusional mass transport within the particlebulk due to the rapidly increasing particle viscosity and strong concentration gradients that areestablished. 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 ofwater has decreased accordingly so that bulk phase diffusion arrest occurs in the monolayer coatedparticle, the droplet is found to have lost a greater proportion of its initial water content. A greaterdegree of slowing in the evaporative flux can be achieved by increasing the chain length of the surfaceactive alcohol, leading to a greater degree of dehydration.