Microemulsion-based media as novel drug delivery systems

Purpose This review highlights the state-of-the-art in pharmaceutical microemulsions with emphasis on self-emulsifying systems, from both a physical and biopharmaceutical perspective Although these systems have several pharmaceutical applications, this review is primarily focused on their potential for oral drug delivery and intestinal absorption improvement

Lipid microemulsions for improving drug dissolution and oral absorption: physical and biopharmaceutical aspects

Structure, dynamics and transport properties of microemulsions

The typical phase behavior of microemulsion systems undergoing phase inversion is briefly reviewed. As a model system H2O-n-octane-C12E5 is studied with various experimental techniques. The occurring microstructures are visualized by freeze fracture electron microscopy and the corresponding domain sizes are quantified by small-angle neutron scattering. From the variations of the domain sizes the mean and Gaussian curvatures of the interfacial film with temperature are determined. It is found that the mean interfacial curvatureH changes gradually and nearly linearly with temperature from positive (Winsor I) to negative (Winsor II), passing through zero for bicontinuous microemulsions where these contain exactly equal volume fractions of water and oil. There the interfacial tension between bulk water-and oil-rich phases passes through an extreme minimum. Quantitative knowledge of the curvatures permits the measurements of interfacial tensions between the bulk phases to be discussed in terms of the relative contributions of bending energy and entropy of dispersion.

Microemulsion microstructure and interfacial curvature

From simple micelles in water, nearly spherical aggregates of amphiphilic molecules, to bicontinuous microemulsions, oil and water microheterogeneous mixtures stabilised by a surfactant film with both local and large-scale disordered structures, the world of surfactant-containing systems is fascinating. Depending on a subtle balance of attractive and repulsive interactions between molecules at interfaces, an extraordinarily rich polymorphism of aggregated structures can be observed. After summarising the basic general constraints controlling surfactant aggregation and the formation of interfaces, different structures of micelles and microemulsions are reviewed and related to interfacial film properties and the intermolecular interactions inside them.

https://iopscience.iop.org/article/10.1088/0034-4885/53/3/002/pdf

The structure of micelles and microemulsions

How to study microemulsions

Conformational rearrangements of membrane-active peptide and depsipeptide antibiotics such as enniatin B, valinomycin, and valine-gramicidin A have been studied as a function of solvent polarity employing ultrasonic absorption methods. The results provide information about the number of occupied conformational states and their respective rates of interconversion. The interpretation of the results from kinetic measurements was confirmed by spectroscopic studies.



The remarkable differences in the stabilities of the various cation complexes of depsipeptide antibiotics bound to lecithin vesicles as well as in homogeneous solution were related to different conformations of the ligands in these complexes as characterized by spectroscopic techniques. Kinetic studies by relaxation methods led to the elucidation of the mechanism of complex formation. Complexation of cations follows a multistep reaction. For valinomycin the rate-limiting step of cation complexation is a ligand conformational change which occurs during the stepwise substitution of solvent molecules from the cation.

Dynamic properties and membrane activity of ion specific antibiotics.

Ultrasonic techniques have been widely applied for material testing and also in the field of medical diagnostics, substituting X-rays for the visualization of tissues and organs. However, in fewer cases ultrasonic waves have been used to determine chemical properties of substances; ultrasonic measurements have contributed the earliest results in fast chemical kinetics in liquids with relaxation times r< l /gs , i.e., the time range of elementary processes such as protonation, solvation, and hydrogen bonding. Following the fundamental work of Nernst, Einstein, Herzfeld, Kneser, Bazulin, Meixner, Tamm, Eigen, Lamb, and others, the theoretical and experimental aspects of ultrasonic relaxation techniques have been treated in a number of comprehensive publications [1-16]. Therefore this short paper only intends to give a brief report on ultrasonic technical developments in our laboratory during the last years, including some illustrative kinetic examples.

Ultrasonic relaxation spectroscopy in liquids

LCAO-MO-description of the different association types of formic acid

A new acoustic resonator with nonplanar quartz transducers for liquids (sample volume about 20 ml) in the 0.2− to 2−MHz range is described. The application of static overpressure onto the liquid cavity causes concave bending of both quartz transducers. This reduces loss contributions from diffraction and ’’side−wall effects’’ considerably, resulting in higher Q values for this resonator; moreover, air inclusions in the cavity are suppressed.Subject Classification: 35.24, 35.80.

Th. Funck

Papers

How to study microemulsions

Dynamic properties and membrane activity of ion specific antibiotics.

Ultrasonic relaxation spectroscopy in liquids

LCAO-MO-description of the different association types of formic acid

New acoustic resonator for liquids in the 0.2− to 2−MHz range