Rheological evaluation of silicon/carbopol hydrophilic gel systems as a vehicle for delivery of water insoluble drugs.

The present study analyzed the thickening properties of Carbopol 974 and 971 in a 50:50 mixture of water/Silsense A-21, a new cationic silicon miscible in any proportion with water. Samples were prepared by simply dispersing different Carbopol amounts (0.5-4%) at room temperature or at 70 degrees C without neutralizing. Temperature sweep and time sweep analysis did not reveal significant structural changes at increasing temperature in the samples prepared following the first procedure. On the other hand systems obtained at 70 degrees C possessed higher elastic character particularly at polymer concentration higher than 2% (w/v). Analysis of the G' and G'' vs frequency curves by using different fitting equations (linear fitting, power law) gave information about the viscoelastic properties of the systems. The fitting of the frequency spectra and the calculation of the relaxation times from the master curves outlined the structural differences within the samples prepared with the two different procedures, confirming stronger gel-like behaviour for the samples prepared by the heating procedure. High preparation temperature promoted the polymer-solvent interactions, aiding the solvation of Carbopol. Heating facilitated polymer-solvent and polymer-polymer interaction, giving rise to a better organised structure typical of gel-like systems. Furthermore this preparation method provided good stability properties as shown by the stress sweeps tests performed during the three months of storage. The interpretation of the rheological results was supported by statistical analysis. A design methodology (screening and optimisation) was also applied in order evaluate the influence on dynamic rheological moduli of several parameters (polymer type and concentration, preparation method, temperature of the tests). This last method showed the relevance of the interaction of two main factors: polymer concentration and preparation procedure. Thus, statistical analysis confirmed that temperature increased the polymer-solvent interaction and improved the viscoelastic properties of the systems, particularly when Carbopols were present in considerable amounts.