J. Salcedo

A study of the electrophoretic properties of montmorillonite particles in aqueous electrolyte solutions

Abstract Experimental results on the electrophoretic velocity and mobility on Ca- and Na-montmorillonite in water and aqueous electrolyte solutions are reported. The effects of particle concentration, electric field strength and, particularly, pH and concentration and type of the electrolyte present in the dispersion medium are considered. The very weak variation of the mobility with the volume fraction of solids is discussed in the light of the predictions of three theoretical models of collective electrophoresis. The null effect of the variations of the applied field strength is considered on a qualitative basis assumming a simple double layer model. Finally, the general trends of the interesting behaviour of montmorillonite in different conditions of electrolyte concentration and valency are discussed taking into account the structural characteristics of the clay.

Electric and adsorption properties of pharmaceutical polymers. Part I: Electrokinetics of Aquacoat

The characterization of the electrical surface properties of Aquacoat, a polymer latex of great interest in pharmaceutical sciences, is described. The technique used is electrophoresis. Analysis was carried out of the effect of pH, electrolyte and surfactant concentration on the electrophoretic mobility of the latex particles. Increasing the pH of the dispersion medium provokes a monotonous increase in the value of the negative mobility. The electrolytes LiCl, KCl and NaCl give rise to larger mobilities when their concentration in solution is increased up to ca. 10−3 M, and a similar behavior is found in the presence of Na2SO4. The effect of raising the concentration of CaCl2 is to decrease the absolute value of the mobility as a consequence of double layer compression. Sodium dodecyl sulphate seems to adsorb on the particle surface increasing its negative charge, but when its concentration is close to 10−3 M saturation of the surface appears to take place, and an approximately constant mobility is suggested by data, whatever the pH of the medium. Finally, the mobility variations with LaCl3 concentration indicate adsorption of the La3+ cation when it is hydrolyzed (pH≥5), whereas non-hydrolyzed lanthanum has little effect on the particle charge.

Adsorption of hydrolysable cations at the aquacoat/water interface. An electrokinetic study

Abstract Some surface electrical properties of Aquacoat, a polymer latex of great interest in pharmaceutical technology, are characterised by means of electrophoretic mobility measurements. The effect of pH in the presence of CaCl2, BaCl2 MgSO4 and LaCl3 solutions of different concentrations is investigated. The two latter solutions can be considered as activating electrolytes for the polymer particles: two charge-reversal pH values are found associated with the surface precipitation of the corresponding hydroxide and with its isoelectric point respectively. The first charge reversal occurs at the same pH (lanthanum solutions) or at an even slightly higher pH (magnesium solutions) than that of bulk precipitation. This fact suggests that the charge reversal mechanisms proposed previously for inorganic colloids could be specific to the polymer/solution interface.

Electrokinetic study of omeprazole drug in aqueous suspensions

Abstract Omeprazole is a antisecretory drug used against gastric ulcers. In this work, we investigate the electric surface characteristics of omeprazole by means of determinations of its electrophoretic mobility, μ e . The effects of pH and the concentration of different electrolytes on the mobility of the drug particles will be analysed. pH seems to be the main variable governing the surface charge and stability of the drug. Specific adsorption of ions is also important.

Electrophoretic Study of Nitrofurantoin in Aqueous Suspensions. Effect of the Addition of a Polymeric Thickener

Abstract— The electrophoretic mobility of nitrofurantoin (based on the microelectrophoresis method) has been studied in dilute dispersions of the drug. Specifically, the effect of NaCl, CaCl2 and AlCl3, pH, and a thickener, Carbopol 934, was determined. The electrophoretic mobility (μ) increases in absolute terms when the pH is raised between 3 and 9, although μ remains negative in this pH range. The variation of absolute mobility (|μ|) with NaCl concentration shows a pronounced maximum for a concentration of about 10−4 M. However, when the concentration of CaCl2 in the medium is increased, |μ| decreases steadily. The effect of AlCl3 concentration on the mobility is markedly influenced by the pH of the dispersing medium although a general trend is observed for μ to become more positive with increasing concentration of the salt. Finally, Carbopol 934 appears to impart an extra negative charge to the nitrofurantoin surface, since higher negative mobilities were measured in the presence of the polymer.

Electrokinetics of cholesterol in the presence of bile salt and bile salt—lecithin mixtures

Abstract The effects of bile salts and bile salt + lecithin mixtures on the electrokinetic potential (ζ) of cholesterol are discussed in this paper. By comparison of the variation of ζ with pH, without the addition of any bile salt, two different mechanisms are proposed to describe the adsorption of sodium cholate and deoxycholate on the cholesterol surface at three different pH values. The important affinity of deoxycholate ions for the cholesterol surface is demonstrated, mainly when the pH is far from the isoelectric point of the cholesterol. Both salts, however, can change the sign of the surface charge of cholesterol from positive to negative at low pH. The effect of pH on ζ for four different sodium deoxycholate + lecithin mixtures is also considered, and the possibility of lecithin adsorption on the cholesterol surface is discussed.

A study of the primary electroviscous effect in pyrex glass suspensions

ABSTRACT An experimental investigation on the electrokinetic phenomenon known as primary electroviscous effect is described for suspensions of Pyrex glass, a highly charged and well known material. By means of an automatic method, the viscosity of the suspensions is measured for different volume fractions of solids, at various electrolyte concentrations and pH values. These measurements allow the estimation of the electroviscous coefficient, p. The electrophoretic mobility was determined for the same systems and zeta potential calculated from these experimental data in order to carry out the comparison between the measured values of p and different theoretical predictions. A qualitative agreement between theory and experiment was found in many cases, but the rigorous theories seem to systemmatically underestimate the experimental p values. The reasons for this disagreement are discussed in addition to the general trends of the electrokinetic behaviour of Pyrex glass.

An experimental study of the stability of cholesterol in inorganic electrolyte and bile salt solutions

Abstract The stability of cholesterol microcrystals in solutions of different pH values and different inorganic electrolyte and bile salt concentrations is experimentally analyzed using spectrophotometric techniques. The method employed allows a study of the aggregation kinetics of cholesterol suspensions. The zeta potentials characterizing the cholesterol/solution interface are computed from electrophoretic mobility data, a good correlation being found between changes in the electrical properties of the interface and variations in the stability of the suspensions. The latter are less stable the closer the pH to the isoelectric point, and the higher the concentrations of NaCl, CaCl 2 , or sodium deoxycholate. A very small effect is found on both mobility and stability of the suspensions when the concentration of sodium cholate is changed.

Effect of the preservatives antipyrin, benzoic acid and sodium metabisulfite on properties of the nitrofurantoin/solution interface

Abstract The possible usefulness of a group of pharmaceutical preservatives in aqueous suspensions of nitrofurantoin was analyzed in a previous study (Ruiz Martinez et al., 1987). The analysis of UV spectra and melting point data led to the conclusion that antipyrin (ANT), benzoic acid (BA) and sodium metabisulfite (NaMB) were among the best choices as photoprotector, antibacterial and antioxidant agents, respectively. Given the influence of the electrical properties of the interface on the physical stability of suspensions, we considered it worthwhile to study the effects of the three preservatives on these properties. Using photon correlation spectroscopy, we measured the electrophoretic mobility (μ) in nitrofurantoin suspensions containing different concentrations of ANT, BA and NaMB. Small changes in mobility were observed with concentrations of ANT ranging from 10 −5 to 10 −2 M, although μ tended to decrease slightly with concentration. The behaviour appeared to be different with BA and NaMB: mobility (consistently negative) increased initially up to a concentration of ≈ 10 −4 M, then decreased with rising concentration. These results are interpreted on the basis of the variations in conductivity and pH of the suspensions for the different concentrations of the three preservatives studied.

Electrokinetic detection of the phase transition of anhydrous cholesterol around human body temperature

Abstract Using experimental data on electrophoretic mobility, μ, the zeta potential (ζ) of cholesterol is calculated by means of two different models at a number of pH values and temperatures. The analysis of the temperature dependence of ζ shows that the well-established phase transition of cholesterol, which takes place at a temperature close to that of the human body, can be detected by electrokinetic techniques. The possible effect of the phase transition on the characteristics of the cholesterol/water interface is discussed.