Introduction Dialkyl phthalate esters are a group of contaminants commonly detected in sediment, soil and water [1]. Al- though beneficial to society as plasticisers, they have been linked with a variety of toxic effects including their ability to mimic oestrogen [2]. Phthalate esters are still used extensively in the processing of polyvinyl chloride (PVC) resins to produce a range of flexible products from the rigid polymeric material [3]. These compounds impart flexibility and are particularly use- ful because of their stability, fluidity and low volatility. PVC resins have been used for more than forty years and are currently used in building materials, clothing, transportation, medical products [4], perfumes, cosme- tics [5], food packaging and pharmaceuticals. The widespread production and use of phthalate esters, combined with the fact that phthalates are not chemically bound to the polymeric matrix and are able to migrate from the plastic, makes their environmental fate and subsequent effects relevant. Phthalate esters enter the environment during use and disposal causing disruption of lipid bilayers and non-specific toxicity in organisms [6]. Phthalate esters consist of a benzene ring with two adjoining ester groups, permitting alkyl chain extension thus creating a series of compounds – dialkyl phthalate esters. The first three in the series are dimethyl phthalate ester (DMP), diethyl phthalate ester (DEP) and dipropyl phthalate ester (DPP). Controlled transport of molecules and ions across biological membranes is the key to a number of cellular processes. A lipid bilayer provides a barrier that controls the movement of molecules or ions into or out of the cell. The thermodynamic tendency to transport a species through the bilayer is partially de- termined by an activity gradient across the mem- brane [7]. It should be noted that hydrophobic com- pounds, such as dialkyl phthalate esters, can move across biological membranes in two distinct ways – either through passive or active movement with the former being the focus of this work. An important physicochemical parameter for de- fining the hydrophobicity of a chemical, which in turn influences toxicity, is the partition coefficient (P). The simplest method for in vitro determination of a partition coefficient is the ‘slow stir’ method, allow- ing equilibrium for a compound to be reached be- tween octanol and an aqueous phase whilst minimis- ing the formation of emulsions that can confound ex- perimental results. However, there is limited reliabil- ity concerning the octanol-water partition coefficient data available for dialkyl phthalate esters [8]. This is because of the hydrophobic nature of the compounds that introduces practical difficulties with measure- ment. In addition to the physical constraints upon measurement, octanol may not truly represent biolog- ical systems as there are significant structural differ- ences. For example, it has been previously found that 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2007 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands Journal of Thermal Analysis and Calorimetry, Vol. 90 (2007) 1, 283–288 DETERMINATION OF MICELLE/WATER PARTITION COEFFICIENTS AND ASSOCIATED THERMODYNAMIC DATA FOR DIALKYL PHTHALATE ESTERS Laura J. Waters 1* , S. A. Leharne 2 , J. C. Mitchell 2 and J. P. Hanrahan 3 1 Department of Chemical and Biological Sciences, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK 2 School of Science, University of Greenwich, Chatham Maritime, Kent, ME4 4TB, UK 3 Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork, Cork, Ireland Micelle/water partition coefficients for three dialkyl phthalate esters – dimethyl phthalate ester (DMP), diethyl phthalate ester (DEP) and dipropyl phthalate ester (DPP) were obtained by micellar liquid chromatography (MLC). Experiments were conducted over a temperature range which led to calculation of a Gibbs free energy, enthalpy and entropy of transfer for the phthalate esters. In addition, small angle neutron scattering (SANS) experiments were conducted with no substantial change observed in micelle size before and after phthalate ester incorporation. Overall, a novel method for obtaining thermodynamic information, based on partitioning data, has been developed for dialkyl phthalate esters using micellar liquid chromatography. Keywords: micellar liquid chromatography (MLC), micelle size, partitioning, phthalate esters, thermodynamics * Author for correspondence: l.waters@hud.ac.uk