Ionization, Hydrophobicity, and Solubility

ionization

occurs when a chemical interacts with an aqueous solution and dissociates

ionizable molecules

more water soluble than unionized

HA + H₂O = H₃O⁺ + A^-

HA is unionized, A^- is ionized

percentage ionization of a drug

acids: 100/(1+10^(pKa-pH))

bases: 100/(1+10^(pH-pKa))

drug distribution

-polar drug dissolves readily in an aqueous solution but not well in nonpolar solvents

-nonpolar does not dissolve well in aqueous, but does in organic/nonpolar solvents

partition coefficient (P)

relative water or lipid solubility of a series of compounds

P = [drug]_{lipid environment}/[drug]_{aqueous environment}

log P

measures how lipid soluble a drug is

-additive property (individual components can be added together)

higher the log P

more lipid soluble