Physicochemical properties microemulsions

Part II starts with the possibilities of ACE for characterizing the relevant physicochemical properties of drugs such as lipophilicity/hydrophilicity as well as thermodynamic parameters such as enthalpy of solubilization. This part also characterizes interactions between pharmaceutical excipients such as amphiphilic substances (below CMC) and cyclodextrins, which are of interest for influencing the bioavailability of drugs from pharmaceutical formulations. The same holds for interactions of drugs with pharmaceutical vehicle systems such as micelles, microemulsions, and liposomes. 

Electrokinetic chromatography (EKC) using microemulsion is one of the most powerful tools for the rapid measurement of log P w with high reproducibility. Because it is relatively easy to manipulate the pseudostationary phases of EKC, a lot of phases have been reported for the measurement not only of physicochemical properties but also of the separation selectivity, such as polymer micelles (64) and double-chain surfactant vesicles (56-58,60,61). These phases are also interesting in terms of the correlation to bioactivity. 

Itoh, K., S. Matsui, Y. Tozuka, T. Oguchi, and K. Yamamoto. 2002. Improvement of physicochemical properties of N-4472 part II Characterization of N-4472 microemulsion and the enhanced oral absdrfttidn. 

The phenomenon of microemulsification is mainly governed by factors such as (1) nature and concentration of the oil, surfactant, co-surfactant and aqueous phase, (2) oil/surfactant and surfactant/co-surfactant ratio, (3) temperature, (4) pH of the environment and (5) physicochemical properties of the API such as hydrophilicity/lipophilicity, plformulating microemulsions. From a pharmaceutical perspective, one of the most important factors to be considered is acceptability of the oil, surfactant and co-surfactant for the desired route of administration. This factor is very important while developing micro emulsions for parenteral and ocular delivery as there is only limited number of excipients which are approved for the parenteral and ocular route. In Chapter 3 of this book a more general overview of formulating microemulsions is given and formulation considerations with respect to the components of microemulsions used in pharmaceutical applications are discussed below. 

Microemulsions are transparent systems of two immiscible fluids, stabilized by an interfacial film of surfactant or a mixture of surfactants, frequently in combination with a cosurfactant. These systems could be classified as water-in-oil, bicontinuous, or oil-in-water type depending on their microstructure, which is influenced by their physicochemical properties and the extent of their ingredients. - SMEDDSs form transparent microemulsions with a droplet size of less than 50 nm. Oil is the most important excipient in SMEDDSs because it can facilitate self-emulsification and increase the fraction of lipophilic drug transported through the intestinal lymphatic system, thereby increasing absorption from the gastrointestinal tract. Long-chain and medium-chain... 

Charged colloids in solution are ubiquitous in a wide variety of biological and technical systems. Some examples are proteins made by amino acids, micelles formed by charged surfactants or charged block copolymers, microemulsions formed by water, oil, and charged surfactants, silica particles made by silica oxide, and polystyrene based latex particles. In these systems, the physicochemical properties are to a large degree determined by electrostatic forces. Despite extensive studies of these forces for the last 50 years, the electrostatic interactions in such systems remain a central problem in colloidal science [1,2]. 

Gao et al. used [bmim][BFJ to prepare nonaqueous [bmim][BFJ-benzene-TX-100 [26] and [bmim][BFJ-cyclohexane-TX-100 [30] microemulsions. Tliey reported the phase behavior of IL-oil microemulsion and found physicochemical properties similar to those of water-oil microemulsions [26]. The microstructure was investigated by SANS [28], electron microscopy [30], DLS, UV-Vis, FTIR, and H-NMR spectroscopy [26]. An in-depth phase diagram study of EAN-n-alkane-CiEj surfactant systems was reported by Atkin and Warr, where the influence of the n-alkane chain length and of the surfactant structure on the efficiency was highlighted [29]. 

DLS was used to obtain the hydrodynamic diameter of dispersed droplets in the microemulsion [38], To investigate the effect of alkyl chain length of IL on the physicochemical properties of microemulsions, ternary microemulsion systems of [C2mim][C SO ]/TX-100/cyclohexane (n of C SO 4-6) were investigated by DLS at 298 K. DLS measurements showed that the hydrodynamic diameter of IL/O microemulsion increased linearly with the increasing molar ratio of water to the total surfactant and the hydrodynamic diameter also increased with alkyl chain length of the IL, as shown in Figure 18.5 [42]. 

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