We've synthesized several new drug-based ionic liquids and want to measure their precise solubility in water, but no one in our lab has an experience in such measurements. I'd appreciate suggestions from experts in the field. Thanks a lot!
From a simple perspective, you can treat your ionic liquids as "NaCl". Think about how we measure the solubility of NaCl in water. If your samples are liquid at room temperature, you can still perform this by adding little by little with full shaking to make it mix with water well (e.g. without distinct phases appearing).
If you prefer solid, then you might perform this experiment at lower temperature, e.g. cool the system to 3 oC. Some ionic liquids may become solid at temperatures lower than room temperature.
I think this problem is the same as normal liquid-liquid miscibility measurement. For refrigerants with oil we use DIN 51514. In http://www.r744.com/assets/link/FUCHS_Puhl_VDA%20Winter%20Meeting%202009.pdf at page 14 you see four ampoules filled with a mixture of two liquids in the right upper corner. The ampoules will be heated and cooled in a thermostat and observed visually. The clouding by changing from one phase to two phases is the miscibility temperature. This is what you can do. For liquids it is better to go from one phase to two phase, instead from two phase to one phase like suggested for solids.
A second method to measure the solubility is measuring the vapor pressure of the solutions. Solutions existing of solvent and dissolved substances with lower vapor pressure (also lower humidity) show better solubility of the dissolved substances than such with higher vapor pressure. If you measure the temperature-pressure dependency of a defined mixture you will observe a change in the slope lg(p) ~ 1/T going from one phase into two phase.
It may be also possible to see a thermal effect of demixing at DSC by heating and cooling the mixtures.
Kind Regards
Steffen.
I totally agree with Steffen.
In detail, there may be 3 situation when one mix one ionic liquid with water.
1) Totally miscible at various compositions: form homogeneous solutions
2) Partially miscible: thus a phase diagram need to be drawn.
3) Immiscible: e.g. hydrophobic ionic liquids
4) A special case: at the nano range, there might be phase separation (agglomeration), which needs to be verified by MS or light scattering.
May I give few suggestions from my experience?
(1) mixing two liquids forning two phases at a constant temperature,
(2) from time to time, separating the phases and checking the composition of two phases; it will give you the saturation kinetics at a given temperature; with taking care regarding formation of microemulsions
(3) most probably, simple UV absorbance measurements will be efective to control the saturation of water by an ionic liquid (or...measuring electric conductivity?).
(4) perhaps, simultaneous determination of the solubility of water in the ionic liquids could be of significance, by considering that aqueous solubility when used in further applications (e.g., for correlating with organic compound properties, water-octanol/water-membrane/water-natural organic matter partitioning etc) may need to be linked to the properties of "pure" water-equilibrated organic phase.
Mixing two liquids at a constant temperature, those are completely miscible or partially miscible doesn't matter. By using HNMR or HPLC analysis, you can find the solubility of IL in water.
or please go through this article for more information
http://www.sciencedirect.com/science/article/pii/S0378381215001326
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