In this article, the influence of electrolytes on the viscosity of sterilized hydrogels containing 60 g/L of hydroxypropyl methylcellulose (HPMC 4000) was investigated. The electrolytes mostly used in ophthalmics were sodium chloride, pilocarpine hydrochloride, atropine sulphate, and scopolamine hydrobromide. The hydrogels were prepared by using stationary method with a long period of storage in the refrigerator following the steam sterilization. Cone and plate rotational viscometry readings were modeled by using the three-parameter power law, which was validated by regression coefficients in the range of 0.9980-0.9999. In comparison to the hydrogel without additives, the significant decrease in viscosity was observed in the presence of pilocarpine hydrochloride in concentrations of 10 g/L and 30 g/L. On the other hand, the addition of sodium chloride, atropine sulphate, and scopolamine hydrobromide resulted in increasing of viscosity of hydrogels. The differences were nonsignificant when the single-point rheological characteristic, the apparent viscosity eta100, was evaluated, in contrast to using the area under rheogram (AUR) data when the significant effect of sodium chloride could be proved. Therefore, in conclusion, the multipoint rheological AUR data could be recommended. In the investigation of gentle plastic rheograms, nevertheless, the allocation of the lower limit of the integration to yield value is emphasized, which prevents the integration of a negative portion of flow curve and, consequently, the lower values of AUR to be obtained.

Department of Pharmaceutical Technology Charles University Prague Faculty of Pharmacy Hradec Králové Czech Republic

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