Imaging Heterophase Molecular Materials in the Environmental SEM
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DETECTOR
Figure 1. Differential pumping vacuum system in the ESEM. Ion (IP), diffusion (DP) and mechanical roughing (RP) pumps are used to create a pressure gradient in the column.
Figure 2. Gas cascade amplification of secondary electrons in the ESEM. A positive bias voltage on the detector creates an electric field with the specimen. Electrons are accelerated and have ionizing collisions in the gas.
The key to performing experiments on heterophase molecular materials is simultaneously stabilizing each of the phases present. Because these instruments typically operate with a few torr of water vapor in the specimen chamber, liquid water can be thermodynamically stabilized by cooling the specimen to a few degrees Celsius according to the phase diagram in Figure 3. While the most obvious application is to image fresh biological tissues, any fluid-containing or indeed fully fluid specimen may be examined, provided appropriate conditions are maintained. Furthermore, by deliberately moving into any of the single-phase fields in Figure 3, controlled hydration, dehydration or sublimation can be achieved. In general, the chamber gas can be considered to be at room temperature Tr, whereas the specimen temperature T, is determined by
the stage cooling. Under these conditions, Cameron & Donald show that the evaporation (or condensation) rate R (in molecules per unit area per second) for water is given as: R=(kTs Yl• ex•_• 2'-•)n l U•ks f_ -
TrT-s
w X(TS)_
(1)
where k is Boltzman's constant, m is the mass of a water molecule, n1 is the number density of liquid water, F is the heat of vaporization of a single water molecule, Pw is the partial pressure of water vapor in the specimen chamber, and X(Ts) is saturated vapor pressure corresponding to temperature Ts. The evaporation rate as a function of chamber water vapor pressure is plotted is Figure 4 for a range of specimen temperatures. Equation 1 can be used to control solvent removal during dynamic experiments such as paint drying, film formation, freeze drying, etc. It should be remembered, though, that even when the evaporation rate is zero, the equilibrium is dynamic. Specifically, molecules are still striking and escaping from the specimen surface at a rate that increases with temperature. It is not strictly necessary that a phase be thermodynamically stable in order to observe it in the ESEM. It is also acceptable if the rate of solvent gain/loss is negligible on the time-scale 212
of an experiment. Many water-oil emulsions, for example, can be imaged readily because the evaporation rate of the oil is negligible. Thus, as long as the chamber conditions favor the stabilization of liquid water, the emulsion is effectively stable. Equation 1 gives insight into how the thermodynamics and kinetics of the experiment can be altered. The most influential factor in Equation 1 is the exponential dependence on the latent heat of vaporization. Any alteration to the system that increases this value will reduce the evaporation rate. Alternatively, the equilibrium vapor press
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