Percolation theory, conductivity and dissolution of hydrophilic suppository bases (PEG systems)

We investigated the conductivity of binary mixtures of different volume to volume ratios of liquid polyethylene glycol 200 (PEG 200) and solid PEG 600 0. The conductivity was measured using a specially designed cell, which cou ld be filled with the sample to be analysed. The results were analysed by t he help of the percolation theory and its basic equation: X = S(p - p(c))(q ) with X = gamma the conductivity and X = k the dissolution rate constant o f binary mixtures with p the concentration of PEG 200 including Mepyraminma leat as a marker substance. The intrinsic dissolution rate was determined s pectrophotometrically. The simultaneous determination of the critical expon ent q and the percolation threshold p(c) of the conductivity experiment yie lded the following values q = 1.83 +/- 0.05 and p(c) = 11.6 +/- 1.1% with a squared correlation coefficient R-2 = 0.9986. For q = mu = 2.00 fixed the percolation threshold is equal to p(c) = 8.4 +/- 1.3% with R-2 = 0.9984. Th us if the same percolation threshold p(c) = 8.4 +/- 1.3% is adopted for the dependence of the dissolution process q becomes equal to q approximate to mu = 1.94 +/- 0.045. R-2 = 0.9991. This result is in excellent agreement wi th the theoretical prediction that the permeability of a porous network sca les in the same way as the conductivity. (C) 1999 Elsevier Science B.V. All rights reserved.