Rheological properties of topical fluoride gels.

The rheological properties of several commercial topical fluoride gels were studied. For that purpose, we investigated hysteresis loops under standard conditions, equilibrium values, apparent viscosities as a function of shear rate, rate of thixotropic recovery, and the influence of temperature. The rheological equilibrium values, treated by the Power Law and the Cross Equation, show very important differences. Five gels show pseudoplastic behavior; seven have thixotropic properties. For the risks of fluoride gel toxicity to be lowered, high pseudoplasticity seems most desirable for the first group; for the thixotropic gels, a combination of yield value (very high viscosity at rest), the presence of a static yield value (very fast decrease in viscosity at very low shear rates), and both fast and high thixotropic recovery after destruction seems ideal. Positive and negative, as well as practically no, influence on rheological properties can be noted when the temperature changes.

Interaction of povidone with aromatic compounds III: Thermodynamics of the binding equilibria and interaction forces in buffer solutions at varying pH values and varying dielectric constant.

The complex formation of a series of aromatic compounds with povidone was studied in buffer solutions and organic solvent mixtures by equilibrium dialysis. For all the ligand molecules studied, a linear relationship was found between r, the number of moles of bound ligand per mole of povidone, and the free ligand concentration. The binding constants and the free energies of binding (-delta F), were greater for compounds in the nonionic state and increased with the number of hydroxyl groups which were capable of forming hydrogen bonds. They decreased with temperature elevation. The thermodynamic data showed entropy gains during the binding process accompanied by small negative enthalpy values. The increased ability to form hydrogen bonds and the increase in ionization of the ligand molecule was reflected in more negative delta H and decreasing delta S values. (The thermodynamic values were interpreted on the basis of the "iceberg" concept of water structure.) From these entropy and enthalpy changes, hydrogen and hydrophobic bondings appeared to be the most important types of binding. In organic solvent mixtures, the association constants lowered with increasing ethanol or propylene glycol concentration; a line relationship between the free energy and the dielectric constant of the solvent mixtures was observed.

Interaction of povidone with aromatic compounds II: Evaluation of ionic strength, buffer concentration, temperature, and pH by factorial analysis.

The interaction of a series of ligand molecules, all consisting of substituted benzoic and nicotinic acid derivatives, and povidone was studied. The influence of ionic strength, buffer concentration, and temperature was evaluated using factorial analysis. Complex formation was not affected at low ionic strength, but increased considerably at higher values due to dehydration of the macromolecule. Complex formation was enhanced in phosphate solutions, particularly in the presence of dibasic phosphate ions. A linear relationship was found between the logarithm of the percentage of bound ligand and ionic strength and buffer capacity. Increasing the temperature lowered complex formation. Although dehydration of the macromolecule also occurred, the decrease in complex formation could be attributed to the solubility increase of the ligand molecules. The influence of the degree of dissociation of the ligand molecules was investigated by factorial analysis. The compounds mainly interacted to a lesser extent in the dissociated than in the nondissociated state. In addition, a negative effect of a pyridine ring with respect to a phenyl ring was observed. The binding tendency was markedly increased by substituting the aromatic ring structure with hydroxyl functions and by esterification of the carboxyl function attached to the ring. The results suggested that lipophilicity and hydrogen bonding played a predominant role in povidone complexation.

Interaction of povidone with aromatic compounds I: Evaluation of complex formation by factorial analysis.

In a study of complex formation between macromolecules and small ligands such as drugs, it appeared that the association constants must be calculated with more care (i.e., after a thorough investigation of the influencing parameters such as buffer composition, ionic strength, and temperature) to allow meaningful interpretations of the phenomena. For this purpose, factorial analysis seems to be the method of choice; it offers the advantage of evaluating the influence of several variables and their interactions at the same time with a minimum of experiments. The method was applied to the association of povidone with two ligands, salicylic acid and benzoic acid. Parameters such as buffer composition and ionic strength, which affect binding, could be distinguished. Especially at pH 7.00, a great positive influence of buffer ions (phosphate buffer) and a relative positive interaction between temperature and ionic strength were noted. Knowledge of the influences of these parameters allowed comparison of the effects of the functional groups attached to the ligand molecules, as well as their degree of dissociation, on adsorption to permit more meaningful interpretation of thermodynamic constants.

Fluorescence assay of hydroxynaphthaoquinones.

Three procedures for fluorescence assay of hydroxynaphthoquinones are reported. The first procedure, based on prior sodium dithionite reduction and determination of the fluorophore in butyl acetate, is applicable to all of the hydroxynaphthoquinones investigated (detection limit: +/- 0.020 microgram of quinone/g of butyl acetate). The second method, specific for 5-hydroxy-1,4-naphthoquinones (juglone series), involves warm reduction with stannous chloride in an acid medium and determination of the fluorophore in chloroform (detection limit: +/- 0.020 microgram of quinone/g of chloroform). The third method, based on the reaction of Guilbault and Kramer, is applicable to 5-hydroxy- and 5,8-dihydroxy-1,4-naphthoquinones having both free quinonoid positions (detection limit: +/- 0.020 microgram of juglone/g of dimethyl sulfoxide and 0.070 microgram of p-naphthazarin/g of dimethyl sulfoxide). The nature of fluorophores also was investigated.