Surface tension predictions during hygroscopic growth and cloud droplet activation using a simple kinetic surfactant partitioning model

Surface active molecules present in ambient aerosols, including sea spray, have been suggested to reduce their surface tension during hygroscopic growth and lower the barrier to cloud droplet activation. Surfactants partition to air-aqueous interfaces, but in high surface-area-to-volume ratio aerosol droplets, the surfactant concentration in the droplet bulk can become depleted when an equilibrium is reached between the surface and bulk concentrations. During hygroscopic growth, the size- and composition-dependent bulk concentration of surface active material changes, impacting both the droplet’s water activity and surface tension, potentially affecting the barrier to cloud droplet activation. Here we use, for the first time, a computationally inexpensive simple kinetic model to predict the size- and concentration-dependent surfactant partitioning during hygroscopic growth. Köhler calculations are performed for two model surfactants (Triton X-100 and sodium octanoate) in different mixtures with NaCl and glucose (a soluble organic). For 50 nm dry diameter aerosol, surfactant mass ratios of 0.033 and 0.333 were not predicted to significantly lower the surface tension at activation. A surfactant mass ratio of 0.952 led to a predicted surface tension reduction throughout hygroscopic growth, only increasing to 63.5 mN/m for Triton X-100 and 61.9 mN/m for sodium octanoate at the critical radius. Copyright © 2025 American Association for Aerosol Research