Classification of the Chemical Physical Dependence

2 Classification of the Chemical Physical Dependence The Thermodynamics of Aerosol Interactions 

When the forces between a particle and another condensed species are treated, thermodynamics and statistical mechanics of the aerosol (particles plus gas) enter through temperature dependence of the interaction forces. However, actual aerosol particle interaction forces may be altered in a fundamental way if one or both of the particles or surfaces absorb molecules from the suspending gas. ASH et al. [5.4] considered nonionic systems in which the relative velocity of the particle and surface or other particle is sufficiently small, in relation to the rates of absorption and desorption, that absorption equilibrium is maintained as the particles move together, collide and then either adhere or separate. They, therefore, assume constant temperature for the entire aerosol system implying at least several nonabsorbing gas molecular collisions with the sorbent species between each sorbate interaction that is to say, the sorbate must be a minority ( 10 percent) species in the gas. By use of conventional equilibrium thermodynamics they derive the expression for the excess force (beyond van der Waals and electrostatic) between two bodies due to sorption as 

In any case, once these multipoles are known, the interaction energies can be computed from standard formulas of electrostatics. Multi pole-induced multipolar interactions are known as Debye (or Falkenhagen) forces and like Keesom forces are important for bodies possessing permanent moments when interacting with polarizable bodies. 

Besides the importance of the specific molecular species interacting with the particle, which properly falls under the domain of heterogeneous nucleation, particle chemistry is of fundamental importance in the domain of interaction forces. 

The interactions of uncharged species have been touched upon above. Since the van der Waals forces dominate these interactions, they will be discussed at length in the final section of this chapter. It suffices here to say that these forces arise from the frequency-dependent electric and magnetic susceptibilities of the interacting species, and it is precisely these susceptibilities which are responsible for the spectral properties of the molecules comprising the particle. Thus, molecular (or chemical) specificity of particle interaction forces is of central importance. From another standpoint, this can be understood by considering an individual molecule as the limiting case of a particle. Then for the intermolecular van der Waals force to be consistent as two such particles (molecules) approach to the point of orbital overlap, their interaction must reduce to the relevant chemical interaction force which is fundamentally dependent upon chemical specificity. 

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