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):
These are natural cellulose synthetically modified to produce partly methyl ethers, 2-hydroxy propyl ethers and phthalyl esters. HPMCP is manufactured by esterification of hypromellose with phthalic anhydride. The level of alkyloxy and carboxybenzoyl substitutions determines the properties of polymer and in particular the pH at which it dissolves in aqueous medium.
HPMCP (also known as Hypromellose phthalate) [133] is commonly used in oral pharmaceutical formulations as an enteric coating material for tablets or granules [134-138]. Hypromellose phthalate is insoluble in gastric fluid, but will swell and dissolve speedily in the upper intestine. These polymers can be used as coating agents because they do not necessitate the addition of plasticizer or other film formers to fabricate coatings for oral formulations [128].
Hydroxypropyl methylcellulose phthalate (HPMCP), available in the market since 1971, it is a monophthalic acid ester of hypromellose, containing methoxy (-OCH3), 2-hydroxypropoxy (- OCH 2CHOHCH3) and phthaloyl (o-carboxybenzoyl C8H5O3) groups so, HPMCP is a cellulose in which some of the hydroxyl groups are replaced with methyl ethers, 2-hydroxy propyl ethers, or phthalyl esters.
Numerous different types of hypromellose phthalate are commercially presented with molecular weights in the range 20 000–200 000. Typical average values are 80000–130000 [133]. These types are (HP50, HP55 and HP55S) where HP55 grade is usually used for enteric coating, HP55S grade, because of its higher degree of polymerization compared with HP-55, it have greater solution viscosity, greater mechanical strength of the fil
