Thermotropic behavior of sodium cholesteryl carbonate
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Timothy S. Wiedmann University of Minnesota, Department of Pharmaceutics, Minneapolis, Minnesota 55455
Teerapol Srichanaa) Department of Pharmaceutical Technology, Faculty of Pharmaceutical Sciences, Prince of Songkla University, Hat Yai, Songkla 90112, Thailand (Received 22 April 2008; accepted 25 September 2008)
Sodium cholesteryl carbonate ester (SCC) was synthesized, and its phase behavior was studied. The chemical structure was assessed by solid-state infrared spectroscopy based on vibration analysis. The wave number at 1705 and 1276 cm1 corresponds to a carbonyl carbonate and O–C–O stretching of SCC, respectively. Molecular structure of SCC was further investigated with 1H and 13C NMR spectroscopy. The chemical shift, for the carbonyl carbonate resonance appeared at 155.5 ppm. A molecular mass of SCC was at m/z of 452. Differential scanning calorimetry (DSC), video-enhanced microscopy (VEM) together with polarized light microscopy, and small-angle x-ray scattering (SAXS) were used to characterize the phase behavior as a function of temperature of SCC. Liquid crystalline phase was formed with SCC. Based on the thermal properties and x-ray diffraction, it appears that SCC forms a structure analogous to the type II monolayer structure observed with cholesterol esters. I. INTRODUCTION
There is continued interest in developing biocompatible, lipid-based systems, which exhibit liquid crystalline behavior.1–5 In particular, lipid systems that are amenable for drug delivery are particularly important due to the more common discovery of active pharmaceutical ingredients (API) that have low water solubility. For this use, not only must the components be biologically acceptable, but by-products of degradation must also be nontoxic. Another pharmaceutical requirement is chemical stability so that an extended shelf-life may be obtained, and the drug product can be stored for at least 2 years. A relatively large number of materials have been investigated, which are often naturally occurring. Perhaps the most commonly used species are phospholipids, with phosphatidylcholines (PC) being most frequent.6 PCs may also be used to prepare liposomes or stabilize dispersed products such as emulsions or suspensions. Triglycerides, cholesterol, cholesterol esters (CE), and fatty acids are also often used in oral or externally applied products. A shortcoming of PCs and CEs is that these species contain an ester functional group, which is chemically unstable in water. Thus, identification of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0027
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 1, Jan 2009 Downloaded: 17 Apr 2015
lipid excipients that have good chemical stability would be of value. Over the last decade, the oleyl carbonate ester of cholesterol (COC) has been studied as a possible material that can be used to prepare liquid crystalline based systems.7,8COC undergoes a reversible smectic–cholesteric transition near 18 C and a cholesteric to isotropic
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