L18- DRUG SOLUBILITY AND DISSOLUTION RATES 2

What happens to weak bases /weak acids in the GI tract (think about the varying ph in the GI tract and how they’ll react to it )

Weak bases have a high dissolution rate in the’e stomach but as the ph increases the dissolution decreases

Whereas weak acids have minimal dissolution in the stomach but as the pH in the tract increases the dissolution of the acid also increases also

The dissolution of salts better are typically better than the free form acid

Explain why salts of weak acids (e.g. sodium salt of a weak acid ) have increased solubility

This is because the when in the the body the salt will dissolve into its ions the sodium ions and the conjugate base (A-)

The conjugate base with react with the h+ ions in the diffusion layers and remove them

Making the fluid more basic so increasing the ph in the diffusion layer

This increases the PH as increased ph makes weak acids shift towards ionised

Explain why salts of the weak base like sodium salt increases dissolution rate

As the the salt will dissociate into its strong acid ( conjugate acid) and base ion

The dissociation will increase the concentration of protons in the diffusion layer so the ph will decrease

This increases dissolution in weak acids as it causes equilibrium will shift to the side which is ionised

Explain what cosolvency is

When you add another solvent to water to increase its drug solubility this can be used on drugs that don’t work well in water as they act as weak/non electrolytes (uncharged )

The solvents added have the specific property to decrease the polarity of water as water is very polar

Adding somethings that is less polar decreases the solubility

Usually a solute is more soluble in a mixture of solvents compared to just one alone

• What is the 3 purposes of cosolvency

• Increase drug solubility in water

• Prepare more concentrated drug solutions

If the drug dissolves better more of it can be put into the same volume or water/liquid

• Improves stability

Give some examples of solvents that can be used as cosolvents

Exam question:how do cosolvents work

1. Cosolvents weaken the hydrogen bond density of the aqueous system-they break up some of the hydrogen bonds within water molecules

2. This reduces the cohesive interactions within water molecules

3. Doing this reduces the polarity of the solution as it’s now less polar than pure water

4. This causes less surface tension,lower dielectric constant and lower solubility parameter

5. Therefore the solution becomes less effective than water at excluding non- polar molecules