Viscoelastic and structural properties of pharmaceutical hydrogels containing monocaprin

The purpose of this study was to characterize the rheological and structural properties of a pharmaceutical multicomponent hydrogel formulation. This formulation consists of a hydrogel-forming poly(acrylic acid) polymer (Carbopol® 974P), microbicide (monocaprin), non-ionic surfactant (Tween® 20 or Tween® 40), and preservatives. The effects of surfactant addition, monocaprin concentration, and pH on the formulation are investigated with the aid of various rheological methods and small angle neutron scattering (SANS). A change of pH from 4 (reduced electrostatic interactions) to higher pH values (prominent electrostatic forces) resulted in a marked impact on both the structural and rheological characteristics, with higher values of the dynamic moduli. At pH 4, the rheological features were strongly influenced by the addition of surfactant and monocaprin concentration, whereas at higher values of pH the effect of additives was modest. The picture that emerges from SANS and rheology is that enhanced association structures evolve at pH 4, while at higher pH the tendency to form associations is inhibited. At all the conditions, the rheological results suggest a viscoelastic solid behavior, which is typical for many gels.