From Molecules to Droplets

We consider methods for investigating the interactions between aerosol particles and molecules and how to calculate properties of molecules interacting with aerosol particles. The basic models include a heterogeneous dielectric media approach and a quantum mechanical-classical mechanical approach. Both models describe the electronic structure of the molecule at the level of correlated electronic approaches or density functional theory approximations. 

Research Department, Danish Meteorological Institute, Lyngbyvej 100, DK-2100 Copenhagen 0, Denmark Department of Chemistry, University of Copenhagen, Universifetsparken 5, DK-2100 Copenhagen 0, Denmark 

The interaction between molecules in the gas phase and the surface of particles and droplets in the atmosphere is central to important phenomena such as ozone depletion in polar areas, hygroscopic growth of particles, aging, acid rain, particle growth and cloud droplet formation. 

Following the Montreal protocol and its amendments the use of CFCs and halons has been phased out as has the use of some of the first generation alternatives. New alternatives with shorter atmospheric lifetimes are being investigated in order to comply with stricter regulations on the use of greenhouse gases (GHG). 

We consider methods for describing how molecules interact with aerosol particles and how to obtain molecular properties and rate constants of relevance when studying the molecular level mechanisms for the formation of aerosol particles and how these provide the basis for heterogeneous chemistry. For understanding mass and heat transfer to and from aerosol particles we need to focus on the processes related to a gas molecule as it approaches the surface of an aerosol particle. A macroscopic property related to these processes is the sticking probabilities/ mass accommodation coefficients that are used when modelling evaporation. 

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From Molecules to Droplets Allan Gross, Ole John Nielsen and Kurt V. Milckelsen ... 

From Molecules to Droplets 365 In this expression we have defined the following terms ... 

In air, permanent solid particles (atmospheric aerosols) occur either from primary sources out of the atmosphere or from gas-to-particle conversion within the atmosphere. All trace matter shows a high variability in concentration because of chemical and microphysical processes. Liquid particles (cloud, fog and rain droplets), however, are not permanent in air and form and exist only under specific physicochemical conditions (the presence of condensation nuclei and water vapor saturation). The transition from molecules to droplets comprises many steps ... 

A solution of an analyte in a solvent can be sprayed (nebulized) from an electrically charged narrow tube to give small electrically charged droplets that desorb solvent molecules to leave ions of the analyte. This atmospheric-pressure ionization is known in various forms, the one most relevant to this section being called electrospray. For additional detail, see Chapters 8, 9, and 11. 

Ejfect of Gas Saturation in Scrubbing If hot unsaturated gas is introduced into a wet scrubber, spray particles will evaporate to cool and saturate the gas. The evaporating liquid molecules moving away from the target droplets will repel particles which might collide with them. This results in the forces of diffusiophoresis opposing particle... 

Consider an alchemical transformation of a particle in water, where the particle s charge is changed from 0 to i) (e.g., neon sodium q = ). Let the transformation be performed first with the particle in a spherical water droplet of radius R (formed of explicit water molecules), and let the droplet then be transferred into bulk continuum water. From dielectric continuum theory, the transfer free energy is just the Born free energy to transfer a spherical ion of charge q and radius R into a continuum with the dielectric constant e of water ... 

Several components of the organic phase contribute greatly to the character of the final product. The pore size of the gel is chiefly determined by the amount and type of the nonsolvent used. Dodecane, dodecanol, isoamyl alcohol, and odorless paint thinner have all been used successfully as nonsolvents for the polymerization of a GPC/SEC gel. Surfactants are also very important because they balance the surface tension and interfacial tension of the monomer droplets. They allow the initiator molecules to diffuse in and out of the droplets. For this reason a small amount of surfactant is crucial. Normally the amount of surfactant in the formula should be from 0.1 to 1.0 weight percent of the monomers, as large amounts tend to emulsify and produce particles less than 1 yam in size. 

The function of emulsifier in the emulsion polymerization process may be summarized as follows [45] (1) the insolubilized part of the monomer is dispersed and stabilized within the water phase in the form of fine droplets, (2) a part of monomer is taken into the micel structure by solubilization, (3) the forming latex particles are protected from the coagulation by the adsorption of monomer onto the surface of the particles, (4) the emulsifier makes it easier the solubilize the oligomeric chains within the micelles, (5) the emulsifier catalyzes the initiation reaction, and (6) it may act as a transfer agent or retarder leading to chemical binding of emulsifier molecules to the polymer. 

By solubilizing very viscous aqueous solutions of polyethylene glycol in AOT/iso-octane solutions, it has been observed that the polymer leads to a decrease in the intermi-cellar interactions and enhances the stability of very large droplets with R values ranging from 55 to 150. The largest reversed micelle may contain up to 200 polymer molecules [238],... 

Theoretically, the extrapolation from PSP to LSD as far as fallout is concerned is complicated slightly by the difference in molecular weights. The rate of evaporation of the small droplets can be very rapid as the surface curvature increases. The smaller die molecular weight of die dissolved molecule, die greater die number of particles for a given number of grams per milliliter. It has been assumed that this error is not significant at this point in our experimental framework. 

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