An experimental investigation of the stability of ethylcellulose latex: correlation between zeta potential and sedimentation.
ABSTRACT 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.
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ABSTRACT: In this work, the colloidal stability of a pharmaceutical ethylcellulose latex, Aquacoat, is analyzed by means of turbidity measurements. The effects of both electrolyte (KCl, CaCl2and LaCl3) concentration and pH on the stability are considered, and it is found that at pH 4 (6), the critical coagulations concentrations (c.c.c.) are 70 (150) mMKCl, 12 (20) mMCaCl2, and 1 (1) mMLaCl3. The results are interpreted in terms of both classical and extended versions of the DLVO theory of the stability of colloids; the Hamaker constant,A, characteristic of the van der Waals interaction between the particles in suspension, is calculated from measurements of the surface free energy of the polymer, based on contact angle measurements of selected liquids on polymer pellets. Concerning the electrostatic contribution to the total interaction energy between the polymer spheres, it is calculated from zeta potential data deduced from electrophoretic mobility of the latex suspensions. The application of the so-called extended DLVO theory includes not only van der Waals and electrostatic interactions, but also acid–base or structural ones, estimated using a recent model of the surface free energy of solids. Using the well-known Fuchs equation, we have computed the stability ratioWof the suspensions, using both the classical and extended DLVO theories, for the different experimental conditions. The results show that, when the electrolyte is KCl, the classical model correctly predicts the c.c.c. values, although the slopes of the logWconcentration trends are different from the experimental ones. The extended theory underestimates the c.c.c.’s and gives a better approximation to the slopes. In the case of both CaCl2and LaCl3, the best agreement is always found with the extended theory. It is suggested that consideration of ionic strength effects on the acid–base characteristics of the solid could improve the extended DLVO results.Journal of Colloid and Interface Science - J COLLOID INTERFACE SCI. 01/1996; 177(2):553-560.
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ABSTRACT: An experimental investigation is described of the electrokinetic properties of nitrofurantoin. The experiments are based on measurements of electrophoretic mobility in suspensions of this drug and the calculation of its zeta potential (zeta) following two different procedures. A simple mechanism is proposed for explaining the sign of the surface charge of nitrofurantoin and its increase with the pH of the medium. The effect on zeta of changes in the concentration of NaCl, CaCl2, or AlCl3 is studied for different pH values of the suspensions. The behavior of zeta in the presence of NaCl shows evidence of Cl- adsorption on the particles, especially at low electrolyte concentrations. The AlCl3 salt was capable of reversing the sign of the surface charge, changing it to positive when a given, pH-dependent concentration was added to the suspension. The effect of this salt on zeta for different pH values was probably due to the hydrolysis of the cation Al3+. A significant increase of the negative surface charge was observed when a small amount (0.1%) of a synthetic polymer, Carbopol 934, was present in NaCl or AlCl3 systems. This appears to indicate that the large polymer molecules adsorb on nitrofurantoin particles as part of their stabilizing mechanism.Journal of Pharmaceutical Sciences 02/1990; 79(1):82-6. · 3.13 Impact Factor
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ABSTRACT: The stability and redispersion properties of nitrofurantoin dispersions are experimentally studied using simple but reproducible techniques. Solutions of different electrolyte concentrations and pH values are employed as dispersing media. The pH appears to be a determinant factor in the properties studied. Thus, with all the electrolytes considered (NaCl, CaCl2, and AlCl3, almost optimum redispersion was achieved when the pH was maintained close to neutrality. However, the important effect of AlCl3 as compared with the other electrolytes is also clearly demonstrated. Especially interesting are the results obtained when 0.1% Carbopol is added to the suspensions. The important effect of this polymer on the surface charge of nitrofurantoin, mainly at pH 7, manifests in excellent redispersion of the suspensions with either of the electrolytes employed. Using simple expressions that approximate the equations proposed in the DLVO theory of interaction between colloidal particles, the variation of the total interaction energy between nitrofurantoin particles is calculated as a function of interparticle distance. The results are compared with the experimental determinations of the sedimentation and redispersion properties of nitrofurantoin. Although some discrepancies between theory and experiments are found, the results indicate a reasonable agreement between the general features of both.Journal of Pharmaceutical Sciences 09/1990; 79(8):709-15. · 3.13 Impact Factor