When selecting a solvent, we must attend to its solubility characteristics as defined by
Hansen Solubility Parameters, to its cost, its green credentials, its odor and its toxicity. But even if we have all these factors right, the solvent might be too volatile or too involatile for our purposes. So, for every solvent, we need to know about its vapor phase characteristics.
Scientifically, we need to know only two:
− Its vapor pressure at any given temperature, and
− Its flash point - the temperature at which the vapor can, in principle, burn when ignited by a spark
The flash point is, in general, a fact about a solvent and has to be found by measurement. It is of significance to a formulator because the safety restrictions on a flammable liquid (flash point < 37.8°C) are much more stringent than those of a combustible liquid (37.8° < flash point < 93.3°C) which are more stringent than a solvent with a flash point greater than 93.3°C.
Calculating the Key Parameters
If we know how the vapor pressure of a solvent depends on temperature, then we can deduce three key parameters of significance to the formulator:
− Vapor Pressure at 25°C, VP25 – This helps to define Volatile Organic Compounds
− Boiling Point, BPt – The temperature near to and above which we cannot conveniently use the solvent as a liquid
− Evaporation Rate – How fast the solvent will evaporate in given air flow. More usually described as Relative Evaporation Rate (RER) with respect to a standard solvent such as n-butyl acetate, nBuAc.
For any solvent, the vapor pressure VP can be calculated at temperature T using the three Antoine Constants, AA, AB, AC via:
log10VP = AA-AB/(T+AC)
− From this equation we automatically get VP25.
− The BPt is found from the value of T that gives VP=1 atm.
− And the RER (defined at 25°C) is basically 100.
VP25(Solvent)/ VP25(nBuAc) (plus a correction for the ratio of their molar volumes) because the rate of evaporation is proportional to the vapor pressure.
If, therefore, you need to select between different solvent options:
− First, ensure that you have the required flash point
− Then, find whether your VP25 fits your VOC requirements, that the process is comfortably below the BPt, and that the RER is suitably large (if you need to remove solvent) or small (if you want it to stay around)
A table of solvents can provide all those data – with one big confusing issue which is the large variety of numbers used for RER. At SpecialChem the standard will be nBuAc = 100. In many cases, you will find nBuAc=1. So, acetone is 560 at SpecialChem and might be 5.6 on another site. Even worse is that some solvents are quoted in terms of evaporation of ether and with an inverted scale based on time to evaporate rather than the rate of evaporation.
If we use nBuAc=100 then diethyl ether is a super-high 2574. If we use the ether scale with ether=1 then nBuAc=25.74 because it will take that much longer to evaporate. Formulators need to know about this confusion because although most of the time it is not too hard to work out if a value is based on nBuAc=1 or nBuAc=100 if the figures are quoted in terms of ether it is hard (because the scale is inverted) to check that the values make sense.
» Continue reading to better understand the real-world evaporation rates!