Hygroscopic Growth and Deliquescence of NaCl Nanoparticles Mixed with Surfactant SDS

Abstract

Several complementary experimental and theoretical methodologies were used to explore water uptake on sodium chloride (NaCl) particles containing varying amounts of sodium dodecyl sulfate (SDS) to elucidate the interaction of water with well-defined, environmentally relevant surfaces. Experiments probed the hygroscopic growth of mixed SDS/NaCl nanoparticles that were generated by electrospraying aqueous 2 g/L solutions containing SDS and NaCl with relative NaCl/SDS weight fractions of 0, 5, 11, 23, or 50 wt/wt %. Particles with mobility-equivalent diameters of 14.0(±0.2) nm were size selected and their hygroscopic growth was monitored by a tandem nano-differential mobility analyzer as a function of relative humidity (RH). Nanoparticles generated from 0 and 5 wt/wt % solutions deliquesced abruptly at 79.1(±1.0)% RH. Both of these nanoparticle compositions had 3.1(±0.5) monolayers of adsorbed surface water prior to deliquescing and showed good agreement with the Brunauer−Emmett−Teller and the Frenkel−Halsey−Hill isotherms. Above the deliquescence point, the growth curves could be qualitatively described by Köhler theory after appropriately accounting for the effect of the particle shape on mobility. The SDS/NaCl nanoparticles with larger SDS fractions displayed gradual deliquescence at a RH that was significantly lower than 79.1%. All compositions of SDS/NaCl nanoparticles had monotonically suppressed mobility growth factors (GF_m) with increasing fractions of SDS in the electrosprayed solutions. The Zdanovskii−Stokes−Robinson model was used to estimate the actual fractions of SDS and NaCl in the nanoparticles; it suggested the nanoparticles were enhanced in SDS relative to their electrospray solution concentrations. X-ray photoelectron spectroscopy (XPS), FTIR, and AFM were consistent with SDS forming first a monolayer and then a crystalline phase around the NaCl core. Molecular dynamics simulations of water vapor interacting with SDS/NaCl slabs showed that SDS kinetically hinders the initial water uptake. Large binding energies of sodium methyl sulfate (SMS)−(NaCl)₄, H₂O−(NaCl)₄, and SMS−H₂O−(NaCl)₄ calculated at the MP2/cc-pVDZ level suggested that placing H₂O in between NaCl and surfactant headgroup is energetically favorable. These results provide a comprehensive description of SDS/NaCl nanoparticles and their properties.