Limitation observed in the application of the three dimensional solubility parameters to the coating formulation of poly (3-hydroxybutyrate-hydroxyvalerate) systems

Florence Eichie¹*, Roland Sydney Okor¹ and Rudiger Groning²

¹Department of pharmaceutics and Pharmaceutical Technology, Faculty of Pharmacy, University of Benin, Benin City, Nigeria.

² Institute for Pharmaceutical Technology, University of Munster, Munster, Germany.

 

Tropical Journal of Pharmaceutical Research 2005; 4(1): 355-362

ISSN: 1596-5996

 

Abstract

Purpose: Poly (3-hydroxybutyrate-hydroxyvalerate) displayed high dipole-dipole interaction, a high hydrogen bonding but low polar interaction, and was therefore expected to be miscible with solvents/plasticizers that exhibit similar pattern of cohesive interaction. To determine the applicability, or otherwise of the theory of the three dimensional solubility parameters to the formulation of poly (3-hydroxybutyrate-hydroxyvalerate) polymeric coating system, and hence identify any limitation in the application of the theory. This aspect was investigated in the study.

Method: The structural group contribution method was employed to compute the partial and total solubility parameters of the compounds – the biopol polymer, a series of organic solvents and plasticizers.  The computed partial solubility parameters included: dipole-dipole (d_d), polar (d_p) and hydrogen bonding (d_h).  Following a standard procedure in the literatures, the d_d and d_p values were combined to form a composite solubility parameter, d_v: where d_v = .  A plot of d_h versus d_v gave the energy maps, which depicted the energy levels of the various compounds and from which the miscibility of the compounds were predicted.

The closer the position of the solvent or plasticizer to the polymer in the map, the greater, the probability of mixing.  Cast films of the various polymeric formulations were made and examined for homogeneity by scanning electron microscopy.

Results: It was possible to select suitable plasticizers that were miscible with the polymer by applying theory of solubility parameters. The prediction for the solvents was, however, erroneous and this may be attributable to the inability of the d_v parameter to clearly reflect the differences between the d_d and d_p interactions of the polymer on the one hand and those of the various solvents in all situations on the other hand. This means that in certain instances, the d_v   values of the polymer and the solvents were similar even though their d_d and d_p interactions were dissimilar.

Conclusion: The analysis of the data showed that the composite solubility parameter d_v of compounds could be similar even though the actual energies of d_d and d_p interactions are different. This is a limitation in the application of the theory of the three dimensional solubility parameters. 

Key words:  Solubility parameters, cohesive energy, miscibility of compounds, poly (3-hydroxybutyrate-hydroxyvalerate) system.

To whom correspondence should be addressed: E-mail: eichiefe@yahoo.com