The surface tension of surfactant-containing, finite volume droplets

Abstract

Surface tension influences the fraction of atmospheric particles that become cloud droplets. Although surfactants are an important component of aerosol mass, the surface tension of activating aerosol particles is still unresolved, with most climate models assuming activating particles have a surface tension equal to that of water. By studying picoliter droplet coalescence, we demonstrate that surfactants can significantly reduce the surface tension of finite-sized droplets below the value for water, consistent with recent field measurements. Significantly, this surface tension reduction is droplet size-dependent and does not correspond exactly to the macroscopic solution value. A fully independent monolayer partitioning model confirms the observed finite-size-dependent surface tension arises from the high surface-to-volume ratio in finite-sized droplets and enables predictions of aerosol hygroscopic growth. This model, constrained by the laboratory measurements, is consistent with a reduction in critical supersaturation for activation, potentially substantially increasing cloud droplet number concentration and modifying radiative cooling relative to current estimates assuming a water surface tension. The results highlight the need for improved constraints on the identities, properties, and concentrations of atmospheric aerosol surfactants in multiple environments and are broadly applicable to any discipline where finite volume effects are operative, such as studies of the competition between reaction rates within the bulk and at the surface of confined volumes and explorations of the influence of surfactants on dried particle morphology from spray driers.

Model for estimating activity coefficients in binary and ternary ionic surfactant solutions

“…We have shown that with a parametrization of the surfactant CMC as function of the inorganic salt content in the solution, the CISA model can be used to estimate the water activity and mean ionic coefficients of both surfactant and inorganic salts in solutions spanning concentrations from highly dilute conditions up to several times the surfactant CMC. Both water activity and surface tension (as a function of surface or bulk surfactant activity, depending on the equation of state) play important roles in controlling the solution-air interface equilibrium of atmospheric droplets (Malila and Prisle 2018;Lin et al 2018;Bzdek et al 2020). Their proper estimation is therefore essential for predicting the formation of clouds and their radiative effects in the atmosphere (Prisle et al 2012).…”

“…The amphiphilic nature leads to a preferential adsorption of dissolved surfactant molecules at the solutionair interface. The ensuing partitioning surfactant solute between solution bulk and interface is particularly important in microscopic systems with relatively large surface areas, such as atmospheric droplets (cloud, fog, mist and rain) (Prisle et al 2010;Malila and Prisle 2018;Lin et al 2020;Bzdek et al 2020).…”

The effect of biological particles and their ageing processes on aerosol radiative properties: Model sensitivity studies

“…This assumption is likely justified under many conditions due to the 210 strong dilution of internally mixed aerosol particles near droplet activation. There are numerous studies that postulate that surfactants in aerosol particles might influence the surface tension sufficiently to significantly change their CCN activity (Bzdek et al, 2020;Facchini et al, 1999;Lowe et al, 2019;Nozière et al, 2014).…”

“…We conclude that the mass concentration of biosurfactants needs to be quantified in order to better explore the biosurfactant effect on CCN activation of small particles. Given that the surface concentration of surfactants is likely higher than the bulk concentration (Bzdek et al, 2020;Lowe et al, 2019;Ruehl et al, 2016) as assumed here, even a smaller mass fraction of biosurfactants than calculated by Equation 1 might be sufficient to decrease the surface tension of small aqueous BAP and the corresponding critical 540 supersaturation. However, also for the concept of surface partitioning of biosurfactants, rather than for a bulk concentration, our conclusions hold true on the limited impact of surface tension suppression on CCN activation of supermicron BAPs.…”

“…Equation 1 implies that the concentration of biosurfactant on the surface is the same as in the bulk. Recent studies suggest that the surface concentration of surfactants is higher than the bulk concentration (Bzdek et al, 2020;Lowe et al, 2019;Ruehl et al, 2016). Thus, a smaller amount of https://doi.org/10.5194/acp-2020-781 Preprint.…”