Hygroscopicity of amphiphilic nanoparticles: The role of size and composition

Direct and indirect effects of aerosols on radiative forcing are poorly understood therefore fundamental processes involved in water uptake on model atmospheric surfaces were examined experimentally and theoretically. Water uptake on NaCl nanoparticles (< 20 nm) with varying coverage of sodium dodecyl sulfate (SDS) was studied because key features, such as monolayer water-uptake, are amplified in nanoparticle hygroscopic growth curves compared to larger, micron size particles. Experimental techniques employed in this work are novel due to their ability to probe hygroscopic growth of nanoparticles in the monolayer coverage regime and bulk regime without the influence of a substrate. Nanoparticles were generated with an electrospray ionization-neutralization apparatus and their hygroscopic growth was quantified by exposure, varied from 1 to 70 seconds, to relative humidity (RH) and tandem-nano-differential mobility analysis techniques. As surfactant coverage level increases in the nanoparticles water uptake begins at lower RH values compared to NaCl and no prompt deliquesce transition is observed, contrary to observations of larger (> 60 nm) surfactant containing NaCl nanoparticles. Hygroscopic growth is continually suppressed as surfactant coverage increases. Variance in RH exposure time made no difference to experimental observations. Hygroscopic growth models are used to evaluate growth of NaCl and surfactant/NaCl nanoparticles. In the post-deliquescence regime, a modified version of Kohler theory is used to compare with NaCl nanoparticle growth and the Zdanovskii-Stokes-Robison model is used to evaluate surfactant/NaCl nanoparticle growth. In the pre-deliquescence regime, standard adsorption isotherms (Brunauer-Emmett-Teller and/or Frenkel-Halsey-Hill) are used to evaluate growth. A nanoparticle jet impactor has also been constructed for imaging and chemical analysis of nanoparticles. Atomic force microscopy images, X-ray photoelectron spectra and Fourier transform infrared spectra confirm the success and usefulness of the jet impactor. Characterizations of SDS/NaCl nanoparticles reveal the core-shell model, which is a commonly assumed morphology for organic-inorganic particles, may not be accurate for describing the morphology of high surfactant containing SDS/NaCl nanoparticles.