Internal structure, hygroscopic and reactive properties of mixed sodium methanesulfonate-sodium chloride particles

Internal structures, hygroscopic properties and heterogeneous reactivity of mixed CH₃SO₃Na/NaCl particles were investigated using a combination of computer modeling and experimental approaches. Surfactant properties of CH₃SO₃^- ions and their surface accumulation in wet, deliquesced particles were assessed using molecular dynamics (MD) simulations and surface tension measurements. Internal structures of dry CH₃SO₃Na/NaCl particles were investigated using scanning electron microscopy (SEM) assisted with X-ray microanalysis mapping, and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The combination of these techniques shows that dry CH₃SO₃Na/NaCl particles are composed of a NaCl core surrounded by a CH₃SO₃Na shell. Hygroscopic growth, deliquescence and efflorescence phase transitions of mixed CH₃SO₃Na/NaCl particles were determined and compared to those of pure NaCl particles. These results indicate that particles undergo a two step deliquescence transition: first at ∼69% relative humidity (RH) the CH₃SO₃Na shell takes up water, and then at ∼75% RH the NaCl core deliquesces. Reactive uptake coefficients for the particle-HNO₃ heterogeneous reaction were determined at different CH₃SO₃Na/NaCl mixing ratios and RH. The net reaction probability decreased notably with increasing CH₃SO₃Na and at lower RH.