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Abstract. This paper describes the development of a stable, controlled‐release formulation of metronidazole for use in the treatment of periodontal disease. It is formulated as a suspension, which undergoes transformation to a release‐controlling, semi‐solid on contact with gingival fluid. The system is based on the ability of mixtures of monoglycerides and triglycerides to form liquid crystals, i.e., reversed hexagonals, in contact with water. The reversed hexagonal form was found to have the most favourable sustained release properties, compared with those from the cubic form. The source of metronidazole is the prodrug, metronidazole benzoate, which further helps to slow down the release rate. Product characteristics are assessed by differential scanning calorimetry and viscometry. The release data derive from the results of in vitro dissolution tests. X‐ray diffraction, phase diagrams, and polarized light microscopy were used to elucidate the structure of the liquid crystalline phases.

Some cellulosic derivatives are soluble in cold water, but above a clouding temperature they form a polymer-rich phase which progressively expels water as the temperature is raised. This phase separation may be modified by adding surfactants. Over a certain range of compositions and temperatures, some of these systems form thermoreversible gels. Small angle neutron scattering experiments are reported for the gels made of ethyl hydroxyethyl cellulose (type CP-30) and cationic surfactants. The results show that the surfactants cause a dissociation of the polymer-rich phase into smaller lumps, with sizes on the order of 500 Å. Each lump is formed by the loose association of polymer strands belonging to many macromolecules and is covered by surfactant. The mechanical rigidity of the gel originates from the association of the macromolecules through these lumps.

Monolayers of palmitic (C16:0) and lignoceric acid (C24:0) and their equimolar mixture were transferred to a hydrophilic mica substrate at various surface pressures and investigated by means of atomic force microscopy (AFM) in contact and lateral force modes. The first-order transition of lignoceric acid gives a plateau region, representing a liquid expanded to liquid condensed phase transition in the pressure-area isotherm. This was visualized by AFM as stripes of a condensed phase within the expanded phase, exhibiting a small height difference but a significant difference in friction. The corresponding phase transition of the palmitic acid was continuous, and no changes of the Langmuir-Blodgett films with respect to pressure were observed with AFM. Both the surface pressure-area isotherms and the direct observations of domains of irregular size and shape using the AFM showed that lignoceric and palmitic acid were immiscible. The height difference between the domains was 1.1 nm corresponding to the difference in hydrocarbon chain length of the two fatty acids.

In this study the effect of cholesterol in Langmuir-Blodgett monolayers of fatty acids of varying chain lengths was investigated by atomic force microscopy (AFM). Domain formation due to lateral phase separation was studied at different lipid compositions and surface pressures. A small amount of cholesterol is miscible with palmitic acid (C16:0) and forms a flat monolayer while excess cholesterol forms a rougher cholesterolrich phase. No miscibility was observed in monolayers of lignoceric acid (C24:0) and cholesterol. For the ternary mixed monolayer (palmitic acid, lignoceric acid, and cholesterol) the two fatty acids formed separate domains and the miscibility of cholesterol in the two phases showed behavior corresponding to that of the binary fatty acid-cholesterol systems. From the shape, size, and height differences of the domains one can conclude that the driving force to minimize the interfacial length between different phases is reduced in the presence of cholesterol. This can be attributed to line active properties of cholesterol.