Hygroscopic Growth and Deliquescence of NaCl Nanoparticles Coated with Surfactant AOT

Abstract

Aerosolized nanoparticles of NaCl coated with variable amounts of surfactant AOT were generated by electrospraying AOT/NaCl aqueous solutions, followed by neutralizing and drying the resulting particles. A tandem differential mobility analyzer was used to select a narrow size distribution of particles with mobility equivalent diameters below 20 nm and monitor their hygroscopic growth as a function of relative humidity. Effects of the particle size and relative amount of surfactant on the hygroscopic growth of NaCl were studied. For pure NaCl nanoparticles, the deliquescence relative humidity (DRH) increased as the particle size was decreased, in full agreement with previous measurements. Below the DRH the NaCl nanoparticles had an equivalent of one-four monolayers of water adsorbed on the surface. The addition of a sub-monolayer AOT coating reduced the DRH and suppressed the hygroscopic growth of the NaCl core. At AOT coverage levels exceeding one monolayer, a clear deliquescence transition was no longer discernible. The Zdanovskii-StokesRobinson (ZSR) model failed to predict the observed growth factors of mixed AOT/NaCl nanoparticles reflecting a large contribution of the interfacial interactions between NaCl and AOT to the total free energy of the particles. There were indications that AOT/NaCl nanoparticles prepared by the electrospray aerosol source were enhanced in the relative mass fraction of AOT in comparison with the solution from which they were electrosprayed.

Hygroscopic Growth and Deliquescence of NaCl Nanoparticles Mixed with Surfactant SDS

“…Results were attributed to interactions between NaCl-H 2 O-Na + and -O 3 SO-R at the surface that were comparable to the magnitude of the Na-Cl binding energy. 5 These results suggest the surfactants affect the deliquescence of NaCl in a size-dependent way and motivate the present investigation of hygroscopicity of mixed SDS/NaCl nanoparticles.…”

“…In our previous experiments with the AOT/NaCl particles, 5 an AOT enhancement was also observed, however, not to the extent as for the SDS/NaCl particles. These differences likely reflect stronger surfactant properties of SDS relative to AOT.…”

“…The relative S(2p)/Cl(2p) XPS signal also grows with the SDS weight fraction ( Figure 4); however, converting the measured S(2p)/Cl(2p) ratio into the absolute SDS wt % is not straightforward for a number of reasons (see below). The actual thickness of the SDS film can be estimated using a simple model, 5 which assumes core-shell morphology and the volume of each component is represented by bulk densities (F NaCl and F SDS ) and amounts of NaCl and SDS. Equation 7 shows how the core NaCl diameter (d NaCl ) is calculated, where w t is NaCl weight percent and d m is the overall particle diameter.…”

“…10,12 Numerous studies have addressed the effects of organic material on the hygroscopic properties of inorganic particles. 5,[14][15][16][17][18][19][20][21] Researchers often assume that organic/inorganic particles will phase segregate into a "core-shell" morphology with an organic "shell" coating an inorganic crystalline "core" as shown in the illustration in Figure 1. Observations show that the organic coating does not always inhibit growth of the soluble inorganic core, even for cases in which the organic material comprised the majority of the particle mass.…”

“…5 The term A(Kn, v ) has a unique solution in each flow regime which satisfies the relationship in eq 5, where d e (0) is the envelop equivalent, often referred to as volume equivalent diameter under dry conditions. It was found that the experimentally measured GF m could be predicted reasonably well by use of Kö hler theory for NaCl particles between 6 and 60 nm, provided the correct flow regime was used for a particle of a given Knudsen number.…”