The electrostatic advantages of cross-linked polystyrene organogels swollen with limonene for selective adsorption and storage of hydrophobic drugs

The organogels poly(styrene-co-divinylbenzene), PS gel (1); poly(styrene-co-divinylbenzene-co-vinylpyridine), PS-VP gel (2); poly(styrene-co-divinylbenzene-co-vinylbenzoic acid), PS-VBA gel (3); and poly(styrene-co-divinylbenzene-co-styrenesulfonate), PS-SS gel (4) were prepared with limonene by varying the concentration of cross-linker and comonomer to control the properties of the gels. Limonene was used as the swelling solvent due to its low toxicity and environmental friendliness. Additionally, 4-vinylpyridine (VP), 4-vinylbenzoic acid, and 4-styrenesulfonate were introduced as cationic and anionic moieties to act as interaction units. An increase in the divinylbenzene ratio improved the mechanical strength, as evidenced by an increase in the G′ value; for example, the G′ of gel 3 improved from 15,000 Pa with 5% cross-linking to 93,000 Pa with 10% cross-linking. However, the increased cross-linking reduced the solubility in limonene, resulting in a decrease in the swelling ratio from 1.5 to 0.7. Cationic gel 2 can adsorb anionic compounds, while anionic gel 4 can adsorb cationic compounds. Gel 2 with 5 mol% VP was able to adsorb approximately 4.4 mg/g of an anionic dye, while gel 4 with 5 mol% SS was able to adsorb approximately 6.6 mg/g of a cationic dye. The limonene organogels were used for lipophilic drug storage and the controlled release of testosterone due to their dense polymer network.