Substances has capacity to produce dipoles in another molecule. Dielectric constantis a measure of this capacity and it is a physical property. It is effected by both the attractive forces that exists between atoms and also molecules.
It is denoted by E
How to calculate the dielectric constant of a solvent..?
Measure the conductance between the two plates keeping the test solution in between the plates. After that measure the conductance by maintaining the vaccum in between the plates. Now the ratio of first conductance to second conductance gives dielectric constant of the material.
Sice it is the ratio, it does not have any dimensions.
So what exactly dielectric constant is...??
Let us consider there are two charges and solvent is kept in between them. So lets us take x is the amount of energy that is required to separate the charges. Now provide vaccum in between the two charges. At this time consider y is the energy required to separate them. The ratio of x and y is dielectric constant. Here y is considered as 1.
Let us take water as an example. Its dielectric constant is 78.5. so what exactly this means...?? This means the amount of energy required to seperatdee two charges is 78.5 times more when compared to the energy required to separate same charges
under the same cinditions under vaccum
Coming to the effect on solubility of drugs... Water is a dipolar substance. Drugs may be polar or semipolar. The water by either attracting or repelling the electrons, will induce a small charge in the non polar drug molecules whenever ther are mixed with water. So this leads to
production of induced dipoles; because of which the solubility increases.
Hence we can conclude that as the dielectric constant value of a solvent increases, the solubility increases.
On the scale, the vaccum will have the value of 1 and is present at one end whereas the water, which has the value of 78.5 is present art the opposite end. And other solvents are placed in between. So the exact polarity required can be often obtained by taking solvent alone or combination of solvents.
All these values are applicable at 25oC.
There is particular requirement of dielecric constant for the solutes for maximum solubility. If the mixture of solvents or solvent attains the particular value required, then the solubility will be maximum.
If a mixture of solvents is used to achieve the solubility, then the dielectric constant of that mixture can be calculated, if we know the dielectric constant of each solvent and percentage of each solvent present in mixture by using the formula:
(Percent of solvent 1 * E1 ) + (percent of solvent 2 * E2 )
E1, E2 - dielectric constants values of solvent 1 and 2
For example, if we take a mixture containing 50% of water and 50%ethanol. Dielecric constant of water and ethanol are 78.5 and 24.3.
So dielectric constant of resultant is
(50 * 78.5 )+ (50 * 24.32 )
Which is equal to 51.4.
So a compound having 51.4 dielectric requirement value gets maximum solubility in this combination.
And finally its better to take weights rather than volume as measurement for above calculation to get best resultsw.
Essentials of physical pharmacy by CVS Subrahmanyam, 2009 reprint, vallabh prakashan publishers, page no 145 to 149.
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