Nanosize effect on the deliquescence and the efflorescence of sodium chloride particles

The deliquescence and efflorescence relative humidity values of 6- to 60-nm NaCl particles were measured using a tandem nano-Differential Mobility Analyzer. The deliquescence relative humidity (DRH) increased when the dry particle mobility diameter decreased below approximately 40 nm. The efflorescence relative humidity (ERH) similarly increased. For example, the DRH and ERH of 6- nm particles were 87% and 53%, respectively, compared to 75% and 45% for particles larger than 40 nm. Power law fits describing the nanosize effect are: DRH(d(m)) = 213 d(m)(-1.6) + 76 and ERH(d(m)) = 213 d(m)(-1.65) + 44, which are calibrated for 6 < d(m) < 60 nm with less than 1% RH uncertainty and where dm is the dry particle mobility diameter ( nm). Two independent methods were used to generate the aerosol particles, namely by vaporizing and condensing granular sodium chloride and by electrospraying a high-purity sodium chloride aqueous solution, to investigate possible effects of impurities on the results. The DRH and ERH values were the same within experimental uncertainty for the particles generated by the two methods. The physical explanation for the nanosize effect of increasing DRH and ERH for decreasing dry particle mobility diameter is that the free energy balance of NaCl increasingly favors smaller particles (i.e., those without water) because the surface areas and hence surface free energies per particle are less for small, anhydrous particles than for bloated, aqueous particles.