Thermochimica Acta 457 (2007) 20–26 Thermodynamic investigation of several natural polyols (I): Heat capacities and thermodynamic properties of xylitol Bo Tong a,c , Zhi-Cheng Tan a,b,∗ , Quan Shi a,c , Yan-Sheng Li b , Dan-Ting Yue a , Shao-Xu Wang b a Thermochemistry Laboratory, Dalian Institute of Chemical physics, Chinese Academy of Sciences, Dalian 116023, China b College of Environmental Science and Engineering, Dalian Jiaotong University, Dalian 116028, China c Graduate School of the Chinese Academy of Sciences, Beijing 100049, China Received 7 July 2006; received in revised form 27 February 2007; accepted 27 February 2007 Available online 3 March 2007 Abstract The low-temperature heat capacity C 0 p,m of xylitol was precisely measured in the temperature range from 80 to 390 K by means of a small sample automated adiabatic calorimeter. A solid–liquid phase transition was found from the experimental C p –T curve in the temperature range 360–375 K with the peak heat capacity at 369.04 K. The dependence of heat capacity on the temperature was fitted to the following polynomial equations with least square method. In the temperature range of 80–360 K, C 0 p,m (J K -1 mol -1 ) = 165.87 + 105.19x + 1.8011x 2 - 41.445x 3 - 41.851x 4 + 65.152x 5 + 66.744x 6 ,x = [T (K) - 220]/140. In the temperature range of 370–390 K, C 0 p,m (J K -1 mol -1 ) = 426.19 + 5.6366x, x = [T (K) - 380]/10. The molar enthalpy and entropy of this transition were determined to be 33.26 ± 0.17 kJ mol -1 and 90.12 ± 0.45 J K -1 mol -1 , respectively. The standard thermodynamic functions (H 0 T - H 0 298.15 ) and (S 0 T - S 0 298.15 ), were derived from the heat capacity data in the temperature range of 80 to 390 K with an interval of 5 K. The standard molar enthalpy of combustion and the standard molar enthalpy of formation of the compound have been determined, Δ c H 0 m (C 5 H 12 O 5 , cr) = (-2463.2 ± 1.2) kJ mol -1 and Δ f H 0 m (C 5 H 12 O 5 , cr) = (-1219.3 ± 0.3) kJ mol -1 , by means of a precision oxygen bomb combustion calorimeter at T = 298.15 K. DSC and TG measurements were performed to study the thermal stability of the compound. The results were in agreement with those obtained from heat capacity measurements. © 2007 Elsevier B.V. All rights reserved. Keywords: Xylitol; Heat capacity; Phase transition; Thermodynamic properties; Standard molar enthalpy of combustion; Standard molar enthalpy of formation; Adiabatic calorimetry; TG–DTG; DSC 1. Introduction Xylitol [(CH 2 OH)(CHOH) 3 (CH 2 OH), CAS no. 87-99-0] is an important natural polyol in food and pharmaceutical applica- tions, as it is increasingly used to provide sweetness to various products or replaces sugar in confectionery. Its molecular for- mula is C 5 H 12 O 5 with molar mass of 152.15 g mol -1 and structural formula as follows: Compared with the sucrose, the xylitol has the characteristic sweet taste of sugars but the amount of energy (calories) in the ∗ Corresponding author. Tel.: +86 411 84379199; fax: +86 411 84691570. E-mail address: tzc@dicp.ac.cn (Z.-C. Tan). products is reduced [1–3]. Another important advantage is that it does not contribute to the development of dental caries. More- over, it is suitable for diabetics, because it does not require insulin of glucose in their metabolism [2,4]. In industrial applications, the state and phase transition of the xylitol affects its molecu- lar mobility and physicochemical properties [5]. However, the thermodynamic properties of xylitol were scarcely reported. For the application and theoretical research concerned with the sub- stance, the thermodynamic data of this compound are urgently needed. Heat capacity is one of the most fundamental thermody- namic properties of substances and it closely related to other physical and chemical properties. Heat capacity determina- tions of various compounds have attracted many researchers. Adiabatic calorimetry is one of the most accurate method for obtaining the heat capacity, melting point and enthalpy of fusion of substances. In the present paper, low-temperature 0040-6031/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2007.02.022