Evaporation Step Progressive Concentration

In a typical thin film formation by liquid deposition, the solvent is eliminated by evaporation, which is the process whereby molecules in the liquid state gain sufficient energy to overcome the surface tension barrier to enter the gaseous state [6]. As is very well known, evaporation is fester at high temperatures and for low surface tension liquids because it is associated with a higher vapor pressure. Typical evaporation times during thin film formation vary from a few seconds to several hours, depending on the parameters. This period is usually addressed as the tunable steady state [7,8]. The presence of nonvolatile solutes, such as precursors, tends to reduce the capacity for evaporation. For ideal solutions, Raoult found that the ratio of the partial vapor pressure of a component of 

Pick s first law of diffusion can be used when the concentration within the diffusion volume does not change with respect to time. In thin film formation, this is never the case and one must use Pick s second law of diffusion (Eq. (9.5)) that takes into account the evolution of the concentration within the diffusion volume with respect to time (non-steady or continually changing state diffusion), where c is the concentration, t is the time, and x is the position along the x-axis 

Three cases must be differentiated evaporation-equilibration takes place (i) in a sealed chamber, (ii) in an open chamber, or (iii) in a chamber where air of controlled composition constantly flows. In a sealed chamber, the escaping molecules accumulate in the vapor phase up to saturation, and the net flux of evaporation thus stops. During thin film formation in a sealed chamber, only a certain quantity of liquid is deposited on the substrate and evaporates. If the quantity of the volatile molecules is lower than that needed to saturate the volume of atmosphere, evaporation proceeds and the quantity of molecules (S) remaining in the film is fixed by the relative vapor pressure. Eventually, the film composition will include a proportion of volatile species that is in equilibrium with the quantity that could evaporate. In open chambers equipped with exit windows, the composition of the chamber atmosphere is in equilibrium with the external atmosphere. Assuming that the diffusion in the vapor phase is very fost, the relative vapor pressure inside the chamber (Pa) is constant, and the rate of evaporation is governed by the difference of vapor pressure in the chamber (PJ and at the solution surfece (Pg) (that is to say, at a distance of A from the surface as a first approximation). The net flux of evaporating species across the border, in the x-direction that is normal to the surfece, is then given by the Knudsen equation  

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Juice extracted from cane or beet undergoes further purification steps, including precipitation, absorption, crystallisation and evaporation, which remove nonsugars and progressively concentrate the sucrose solution. The final step is crystallisation of sucrose from the syrup. This mixture of sucrose and liquor, known as the massecuite , is then centrifuged, and the crystals are washed and dried to a moisture content of 0.02% w/w and stored (Beesley, 1990). 

For some apphcations, eg, foam mbber, high soHds (>60%) latices are requited. In the direct process, the polymerization conditions are adjusted to favor the production of relatively large average particle-size latices by lowering the initial emulsifier and electrolyte concentration and the water level ia the recipe, and by controlling the initiation step to produce fewer particles. Emulsifier and electrolyte are added ia increments as the polymerization progresses to control latex stabiUty. A latex of wt% soHds is obtained and concentrated by evaporation to 60—65 wt % soHds. 

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