A study of the electrokinetic and stability properties of nitrofurantoin suspensions. II: Flocculation and redispersion properties as compared with theoretical interaction energy curves

An experimental investigation of the stability of ethylcellulose latex

This paper aims at explaining the experimental observations of the stability and redispersibility of an aqueous ethylcellulose latex through the electrokinetic characterization of the particles. The surface charge and the electrical double layer thickness play an essential role in the stability of the system, hence the need for a full characterization of the polymeric particles. The effect of both pH and ionic strength of the dispersion medium were investigated. It was found that at acid pH values the latex displays "delayed" or "hindered" sedimentation: in such conditions, the electrophoretic mobility and zeta potential are rather low, indicating a small electrokinetic charge on the particles. At alkaline pH, when the dissociation of ionizable surface groups must be complete, the zeta potential is high and negative. The electrostatic repulsion between polymer particles is responsible for the low sedimentation volume and poor redispersibility of the latex. The effect of NaCl and CaCl(2) concentration on both the zeta potential and stability of the latexes was also investigated: it was found that CaCl(2) has the greatest influence, yielding flocculated, easily re-dispersible systems when its concentration in the dispersion medium is high enough. There qualitative observations were ascertained by means of calculations of the potential energy of interaction between particles. In the case of NaCl solutions, a high and relatively wide potential energy barrier was predicted, that may prevent the particle aggregation. Above 5mM NaCl a shallow minimum in the potential energy curves must lead to the formation of aggregates. Similar results were found with CaCl(2) solutions, although in this case the secondary minima are deeper and appear at lower concentrations.

Electrophoretic properties of acrylic latex suspensions (Kollicoat MAE 30 D) and ibuprofen

We have compared the electrophoretic properties (measured on the electrical surface) of the commercial latex Kollicoat MAE 30 D and the non-steroidal anti-inflammatory drug (NSAID) ibuprofen in preparation for attempts to develop a suitable vehicle for the NSAID to obtain a modified release formulation. Electrophoretic mobility of the latex and the active principle was measured in solutions containing different concentrations of inorganic electrolytes (NaCl, CaCl2 and AlCl3) at different pH values. This was considered an indispensable first step for further characterization of the substance's electrical properties. Suspensions of both the latex and the drug had negative mobility values throughout the range of pH values studied here. Of the electrolytes, neither NaCl nor CaCl2 led to positive mobility, and no isoelectric point could be determined. However, AlCl3 at a concentration of 10(-3) M led to the greatest reduction in mobility. We therefore found that trivalent cations were more effective than divalent cations, which in turn were more effective than monovalent cations, in reducing mobility.

Interfacial properties of barium sulfate suspensions. Implications in their stability

A surface characterization of barium sulfate particles in aqueous suspensions was carried out in this work. With the aim of predicting the stability conditions of these widely used suspensions, the electrical surface properties of the particles were first studied by electrophoretic mobility determinations. It was found that both H(+) and OH(-) ions can be considered as potential-determining ions for the barium sulfate/water interface. The same conclusion was reached concerning the lattice ions, Ba(2+) (mainly) and SO(4)(2-). It was also found that increasing the concentration of sodium chloride in the dispersion medium can even change the sign of the zeta (zeta) potential: it is suggested that this behavior is an indirect effect provoked by changes in the solubility of barium sulfate with the ionic strength. To compute the van der Waals (LW) attraction between the particles as well as the acid-base contribution to the total energy of interaction, a thermodynamic characterization of the interface was also carried out by measuring the rate of penetration of selected liquids through plugs of the particles. It was found that barium sulfate particles are essentially monopolar in nature; that is, they show electron-donor character as demonstrated by the essentially zero value of the electron-acceptor component of their surface free energy. Pretreatment of the particles with 10(-2) M solutions of BaCl2 and CaCl2 significantly reduced the electron-donor component, whereas both NaCl and Na(2)SO(4) provoked the opposite effect. This result is explained in terms of the acid-base character of the ions added. These data were used to calculate the interaction energy between the particles. The effect of electrolyte concentration on the stability of the suspensions was analyzed on the basis of the dependence with distance of the interaction energy between the particles. Our results suggest that more stable suspensions of barium sulfate are predicted if moderate amounts of SO(4)(2-) ions are added to the dispersion medium.

Effect of amino acids on the stability properties of nitrofurantoin suspensions. Flocculation and redispersion compared with interaction energy curves

The influence of the addition of amino acids, lysine, alanine and glutamic acid on the properties of nitrofurantoin suspensions was investigated. Flocculation and redispersability was investigated, taking into account the electrical properties of the nitrofurantoin/solution interface. We conclude that lysine and glutamic acid, but not alanine, would be suitable amino acids for control of the stability of nitrofurantoin suspensions.