Relative rate of evaporation

Drawing Conclusions Assume that the three alcohols have approximately the same molar enthalpy of vaporization. What can you say about the relative rates of evaporation of the three alcohols ... 

This is why the salinity of seawater is nearly the same throughout the open ocean, varying by only a few parts per thousand. (As per Figure 3.3, 75% of seawater has a salinity between 34 and 35 %o.) The small degree of spatial variability is a consequence of geographic variations in the balance of evaporation versus precipitation in the surface waters. Recall that these surface waters are the source waters for intermediate and deep water masses. Since shifts in the relative rates of evaporation versus precipitation involve only addition or removal of water, the major ion ratios are unaltered. This is why the major ion ratios do not exhibit little if any spatial differences within the open ocean. 

Evaporation of Solvent-Water Blends. The evaporation of blends of solvents and water presents several problems, the most prominent of which is the fact that the relative rates of evaporation of solvents and water in a given blend vary greatly under conditions of varying humidity. Under dry conditions the water evaporates relatively rapidly, but under high humidity the water evaporates relatively slowly in comparison to solvent. Usually it is desired that the water and solvent evaporate at rates such that the residual liquid remains constant in composition (perfect solvent balance). Often, enrichment of solvent in the residual liquid is desired, but depletion of solvent in the residual liquid might lead to resin kick-out or loss of ability of the resin to coalesce into a smooth, continuous film. One means of minimizing premature loss of... 

Volatility refers to the relative rate of evaporation of materials to assume the vapor state. US 171.8... 

The Dow Chemical Company has a solvent related computer program, Chemcomp, which simulates evaporation profiles for solvent blends. Other features include the ability to measure the effects of humidity on evaporation, calculation of the Hansen solubility parameters for the solvent blend, and inclusion of data on the relative rate of evaporation of the blend at 90% as compared to a standard n-butyl acetate being equal to 1.0. More information on Chemcomp and related solvent modeling computer programs can be obtained from Dow at 1-800-258-2436... 

In the case of Fig. 32, the results are very sensitive to the deposition conditions, which indicates that the relative rates of evaporation and condensation rather than shear-induced restructuring are responsible for the behavior observed in Fig. 32. 

The previous section ignores for the most part how the structure of the inorganic precursor influences the structure of the deposited film. In this section we shall show that the size and extent of branching of the solution precursors prior to film deposition and the relative rates of evaporation and condensation during film deposition control the pore volume, pore size, and surface area of the final film. 

Solvent cements are generally produced by removing the milled elastomer from the mill and dissolving it in solvent. This cutting of the rubber into a solvent is accomplished in low-speed mixing equipment known as chums or in a variety of high-speed, heavy-duty mixers. For natural rubber and SBR, solvents such as toluene, hexane, or naphtha are commonly used. For nitrile, neoprene, and other polar polymers, polar solvents such as methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), or chlorinated solvents may be used, alone or in blends with the nonpolar solvents. The blend of solvents finally used by the adhesive producer will depend not only on solvent power, but on the relative rates of evaporation which control drying time. The dissolved rubber can then be blended with whatever additives are needed to obtain the desired adhesive performance. 

An important characteristic of solvents is rate of evaporation. Rates of solvent loss are controUed by the vapor pressure of the solvent(s) and temperature, partial pressure of the solvent over the surface, and thus the air-flow rate over the surface, and the ratio of surface area to volume. Tables of relative evaporation rates, in which -butyl acetate is the standard, are widely used in selecting solvents. These relative rates are deterrnined experimentally by comparing the times required to evaporate 90% of a weighed amount of solvent from filter paper under standard conditions as compared to the time for -butyl acetate. The rates are dependent on the standard conditions selected (6). Most tables of relative evaporation rates are said to be at 25°C. This, however, means that the air temperature was 25°C, not that the temperature of the evaporating solvent was 25°C. As solvents evaporate, temperature drops and the drop in temperature is greatest for solvents that evaporate most rapidly. 

Lyophilization is a similar technique and is, in fact, evaporation at reduced temperature under vacuum. In some cases, an aqueous sample can be frozen and the vapor pressure of the ice is sufficient to produce a relatively rapid rate of evaporation. It can also be used effectively where the substances of interest have vapor pressures that are sufficiently high at room temperature, to cause substance loss under normal evaporation procedures. As the vapor pressure of a substance is exponentially related to the temperature, a relatively small reduction in temperature can reduce the vapor pressure of the sample components sufficiently to render any loss during evaporation relatively insignificant. This technique is gentler than evaporation and,... 

The BET approach is essentially an extension of the Langmuir approach. Van der Waals forces are regarded as the dominant forces, and the adsorption of all layers is regarded as physical, not chemical. One sets the rates of adsorption and desorption equal to one another, as in the Langmuir case in addition, one requires that the rates of adsorption and desorption be identical for each and every molecular layer. That is, the rate of condensation on the bare surface is equal to the rate of evaporation of molecules in the first layer. The rate of evaporation from the second layer is equal to the rate of condensation on top of the first layer, etc. One then sums over the layers to determine the total amount of adsorbed material. The derivation also assumes that the heat of adsorption of each layer other than the first is equal to the heat of condensation of the bulk adsorbate material (i.e., van der Waals forces of the adsorbent are transmitted only to the first layer). If it is assumed that a very large or effectively infinite number of layers can be adsorbed, the following result is arrived at after a number of relatively elementary mathematical operations... 

Infer the relative strength of intermolecular forces of alcohols from rate of evaporation data. 

Carbonates deposited in freshwater lakes exhibit a wide range in isotopic composition, depending upon the isotopic composition of the rainfall in the catchment area, its amount and seasonality, the temperature, the rate of evaporation, the relative humidity, and the biological productivity. Lake carbonates typically consist of a matrix of discrete components, such as detrital components, authigenic precipitates, neritic, and benthic organisms. The separate analysis of such components has the potential to permit investigation of the entire water column. For example, the oxygen isotopic... 

The rates of evaporation of water through unimolecular fi has been examined by Hedestrom and by the writer. It is foi that the rate of evaporation of water through unimolecular fi of fatty acids on the surface of water is relatively slow in compari to the high rate of evaporation calculated with the aid of Herz-Knudsen equation. The rate is approximately inveri proportional to the surface film pressure F as is indicated fi the following figures. 

Thanks to the relatively high heat of evaporation of ammonia, many reactions can be carried out at its boiling point, -33 C. The use of a dry ice-acetone reflux condenser is absolutely unnecessary. Losses due to evaporation are easily compensated by adding ammonia from the cylinder. The rate of evaporation can be limited by insulating the flask with cotton wool the ice which forms on the outside of the flask as a result of condensation can, to some extent, have a similar function. 

If we assume a uniform homogeneous adsorbing surface, then, by writing down the equations for the rate of condensation and rate of evaporation of two gases which are simultaneously adsorbed, it may be shown that the relative amounts of each taken up by the adsorbent should be independent of the absolute pressure of the gases, and dependent only on the relative pressures of the two. Indications that this condition is not always fulfilled have been interpreted by assuming ... 

Since the rate of evaporation is determined by diffusion of vapor in the surrounding air, it depends strongly on air movement and the geometry of the exposed surface. Close to this surface there will be relatively no movement of air. The substance will be transported from the surface only by molecular diffusion. Further away from the surface there is movement of air which can carry away the vapor more rapidly than molecular diffusion. This air movement operates in two ways. It continuously replaces the air around the evaporating surface, which is the only important factor for a small object like an isolated drop of water. This mere replacement, however, would be of little help to evaporation from a large area such as a lake because the replacement air has already been saturated upwind. 

Another early theory, which also attracted a great deal of attention, was the ink-bottle theory this was originally put forward by Kraemer (1931) and subsequently developed by McBain (1935). Kraemer pointed out that the rate of evaporation of a liquid in a relatively large pore is likely to be retarded if the only exit is through a narrow channel. This argument led Brunauer (1945) to conclude that the liquid in the pore cannot be in true equilibrium with its vapour during the desorption process and therefore it is the adsorption branch of the loop which represents thermodynamic reversibility. 

---