The amount of albumin does not appear to be hugely relavent. This is really important with regard to local anaesthetics. Morphine is highly ionised, fentanyl is the opposite. Consequently the latter has a faster onset of action. The degree of ionisation depends on the pKa of the drug and the pH of the local environment.
Most drugs are either weak acids or weak bases.
For a weak acid, the more acidic the environment, the less ionised the drug, and the more easily it crosses lipid membranes. Weak bases have the opposite effect.