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 6000. The conductivity was measured using a specially designed cell, which could be filled with the sample to be analysed. The results were analysed by the 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 of binary mixtures with p the concentration of PEG 200 including Mepyraminmaleat as a marker substance. The intrinsic dissolution rate was determined spectrophotometrically. The simultaneous determination of the critical exponent q and the percolation threshold p(c) of the conductivity experiment yielded 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. Thus 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 approximately mu=1. 94+/-0.045. R(2)=0.9991. This result is in excellent agreement with the theoretical prediction that the permeability of a porous network scales in the same way as the conductivity.