Research

Nanodroplet Structure

How do molecules distribute themselves within a nanodroplet? Are they well-mixed, does one component enrich the surface, or is there complete phase segregation? These questions are important because the structure of an aerosols droplet strongly affects the heterogeneous chemistry of aerosol particles as well as nucleation, growth and evaporation kinetics.

Unlike solid particles, that can be captured and subjected to surface and cross-sectional microscopic analysis, liquid droplets must be examined in situ. Small Angle Neutron Scattering (SANS) is non-invasive technique that has been extremely useful for determining the structure of complex fluids. We decided to apply SANS to our aerosol samples, and first we had to prove that the idea works!

On the left is a raw 2-dimensional neutron scattering signal for a pure D₂O aerosol taken using the NG7SANS instrument at NIST. On the right is the averaged scattering spectrum I(q) as a function of the momentum transfer vector q. We determine the size and number density of the droplets by fitting the spectrum assuming a lognormal size distribution of well-mixed droplets. For compounds rich in deuterium aerosol-SANS works.

To look for structure in nanodroplets we work with systems that exhibit a bulk miscibility gap. On the left, the predicted scattering signal from a D₂O/n-butanol aerosol is compared to that from a H₂O/deuterated-butanol aerosol. Because the hydrogenated compounds contribute little to the scattering, the D₂O/n-butanol droplets scatter primarily from the deuterium rich core, while the H₂O/deuterated-butanol droplets scatter primarily from the deuterium rich shell. The predicted scattering spectra are quite different. On the right are the measured scattering spectra for these two systems and they exhibit all of the qualitative features predicted by a shell-core droplet model.  

We are continuing to work in this exciting new research area. If you want to see pictures of our experiments in action, please click on pictures from NIST. We are also featured in the 2001 NIST Annual Report, and under POSTERS you can find some 8.5x11" copies of recent posters to download

Look at NIST's annual report for 2001

   

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