A sustained drug delivery system is developed by using nonionic polymer to formulate drug release rate from silica capsules. To serve this purpose, silica capsules filled with poly(ethylene glycol) (PEG) were incorporated with a veterinary antibiotic drug enrofloxacin (ENF); as a model hydrophobic drug by using a general and facile sol–gel route. The physicochemical properties of the prepared drug-loaded composites were investigated by scanning electron microscope (SEM), nitrogen adsorption, Fourier transform infrared spectroscopy and thermal analysis (TGA). The impact of the media’s ionic strength on the drug release was evaluated over a range of 0–0.4 M to simulate the gastrointestinal feed in two physiological pH conditions. Sodium chloride was applied for ionic concentration adjustment due to its ability to salt out polymers in the midrange of the lyotropic series. Simultaneously, the drug release kinetics was evaluated by fitting experimental data to common empirical (zero-order, first order and Higuchi) and semi-empirical (Ritger–Peppas and Sahlin–Peppas) models. The drug release kinetics from capsules revealed a non-Fickian diffusion and pure relaxation-controlled release. Of these models, Sahlin–Peppas equation best fit the release data of ENF. To determine the best model, non-linear regressions were carried out.
