Sample Solution

methacrylate, methacrylic acid copolymer Type A, Type B and Type C [128].
Eudragit®S 100 is an anionic copolymer based on methacrylic acid and methyl methacrylate which is soluble at pH of 7 or higher [128, 129] , the ratio of the free carboxyl groups to the ester groups is approximately 1:2 and mean relative molecular mass of about 135,000. Its pH-dependent polymer so, it’s usually used as an enteric polymer for controlling drug release in GIT. It is practically insoluble in water, petroleum ether, ethyl acetate and dichloromethane while, it is freely soluble in acetone, alcohols (including ethanol 95%, methanol and propane-2-ole) and 1N NaOH solution [128].
Eudragit S100 is insoluble in acidic medium and dissolves above neutral pH. Dissolution occurs as a result of structural change of the polymer associated with ionization of the carboxylic functional group.
At acidic pH, Eudragit S100 particles posses low permeability because of hydrogen bonding between the hydroxyl group of carboxylic moiety and the carbonyl oxygen of the ester group in the polymer molecules. This bonding increase degree of compactness of the polymer and decrease its porosity and permeability [130], minimizing release of an encapsulating agent.

When the pH of aqueous medium is increased, Eudragit S100 start to dissolve as carboxylic functional groups ionizes. The reported theoretical dissolution threshold is pH 7.0 and pKa of polymer molecules is believed to be approximately 6 [131]. Moreover, swelling of Eudragit S100 matrix may accompany the dissolution process contributing to release. It is believed that Eudragit S swells at pH above 6.5 [132]. Therefore, release of active substance may be due to the combination of swelling and dissolution.

Hydroxyl propyl methyl cellulose phthalate (HPMCP):