Effect of Surfactant Layers on the Size Changes of Aerosol Particles as a Function of Relative Humidity

We present water sorption/desorption curves for NaCl and carbon black particles coated with individual organic surfactants. The surfactants used in this study were Tween 80 (a polyoxyethylene sorbitan ester and nonionic surfactant), dodecyl sulfate, sodium salt (an anionic surfactant), and azelaic acid (an organic surfactant). The sorption curves show the change in mass of the coated particles as a function of relative humidity and were determined using an electrodynamic particle balance. Our results show that a Tween 80 coating on a NaCl particle lowers the deliquescence relative humidity from 75% for an uncoated NaCl particle to 70–73%, depending on the thickness of the coating. Another effect of the coating is a decrease in the maximum relative mass gain (M/M₀) at the deliquescence point. M/M₀ ranges from 4.0 to 4.1 for the uncoated NaCl particle while M/M₀ for the coated NaCl particle ranges between 1.75 and 3.5, again depending on the thickness of the coating. For the carbon black/surfactant systems, our results suggest that while uncoated carbon black particles are hydrophobic, a surfactant coating on carbon black particles increases the hygroscopicity of the carbon black particles. Carbon particles coated with Tween 80 showed a relative mass gain of 1.2 at 90% relative humidity; carbon particles coated with dodecyl sulfate, sodium salt showed a relative mass gain of 1.1–1.2 at 90% relative humidity; and carbon particles coated with azelaic acid showed a relative mass gain of 1.4 at 86% relative humidity.