The standard molar enthalpy of formation, molar heat capacities, and thermal stability of anhydrous caffeine

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Abstract

The constant-volume energy of combustion of crystalline anhydrous caffeine (C8H10N4O2) in α (lower temperature steady) crystal form was measured by a bomb combustion calorimeter, the standard molar enthalpy of combustion of caffeine at T = 298.15 K was determined to be −(4255.08 ± 4.30) kJ · mol−1, and the standard molar enthalpy of formation was derived as −(322.15 ± 4.80) kJ · mol−1. The heat capacity of caffeine in the same crystal form was measured in the temperature range from (80 to 387) K by an adiabatic calorimeter. No phase transition or thermal anomaly was observed in the above temperature range. The thermal behavior of the compound was further examined by thermogravimetry (TG), differential thermal analysis (DTA) over the range from (300 to 700) K and by differential scanning calorimetry (DSC) over the range from (300 to 540) K, respectively. From the above thermal analysis a (solid–solid) and a (solid–liquid) phase transition of the compound were found at T = (413.39 and 509.00) K, respectively; and the corresponding molar enthalpies of these transitions were determined to be (3.43 ± 0.02) kJ · mol−1for the (solid–solid) transition, and (19.86 ± 0.03) kJ · mol−1 for the (solid–liquid) transition, respectively.

Introduction

Caffeine (molecular structure: figure 1.; formula: C8H10N4O2; CAS registry number: 58-08-2; molecular mass: 194.19 g · mol−1) is one of the most frequently consumed alkaloidal compounds and is omnipresent in many plants. The traditional significant sources of caffeine in our daily lives are coffee, black tea, and cocoa. It is also an ingredient of cola beverages and energy drinks. Many analgesics sold over the counter contain caffeine. Caffeine is currently gathering increasing attention because of its wide range of applications and the potential of new analytical tools [1], [2].

The thermochemical properties of a compound are very important for the quantum chemistry, computer-aided molecular designs of drugs and new materials, technological processes of synthesis and purification, and biochemical reactions if quantitative studies on the energetic are to be performed. As caffeine has been consumed by human beings for a long history, numerous researchers have studied it for near two hundred years. Some thermodynamic properties have been studied. Stern, Cesàro and Gill [3], [4], [5] studied the solution thermodynamics of caffeine using the solution calorimeters. They determined the enthalpy of solution of the compound and studied the self-association behavior of it in aqueous solution. Origlia-Luster [6] researched the self-association of caffeine in water by measuring the apparent molar volumes and apparent molar heat capacities of aqueous caffeine. The thermodynamic properties of anhydrous caffeine in solid state such as the polymorphic transition, fusion, and sublimation behaviors have been studied using the methods of DSC, TG, X-ray, and microcalorimeter [7], [8], [9], [10], [11]. Those researches were focused on solution thermodynamics and phase transition. Unfortunately, some basic thermochemical data such as the standard molar enthalpy of formation and the heat-capacities have not been well reported till now.

In the present work, the enthalpy of combustion of caffeine at T = 298.15 K was determined by a rotating-bomb combustion calorimeter, and the standard molar enthalpy of formation of caffeine in crystalline state at T = 298.15 K was derived. The heat capacities of the compound were measured from (80 to 387) K by an adiabatic calorimeter. The thermal stability of the compound was further examined in the range of (300 to 700) K through thermal analytic techniques.

Section snippets

Materials

Caffeine was purchased from Shanghai No. 2 Reagent Company. It was a long white needle crystalline and the purity mass fraction was above 0.999. Further purification was not performed. The TG–DTA and DSC tests showed it was in α (a lower temperature steady) crystal form. Nitrogen gas with a purity of 99.999% was used in the TG–DTA and DSC tests.

Thermal analysis

The TG–DTA test was performed in a thermal analyzer, NETZSCH STA 449C from NETZSCH Thermal Analysis. 4.688 mg caffeine sample was used. The heating rate

Thermal analysis

The TG–DTA measurements were carried out in N2 atmosphere. The TG–DTG–DTA results were presented in figure 2. It can be seen from the mass-loss curve that most of the activities occur in the temperature range form (454 to 564) K in a single step. No residue was founded in the crucible after the experiment was completed. Three obvious peaks were observed from the DTA curve. The first peak was associated with the phase transition of caffeine from the crystal form α to form β; the second one

Conclusion

In this paper the standard molar enthalpy of formation, the molar heat capacity from (80 to 387) K, and the phase transition behavior of anhydrous caffeine were reported.

Acknowledgements

The authors thank Prof. Xu-Wu Yang and Prof. Sheng-Li Gao, Department of Chemistry, Northwest University, Xi’an 710069, PR China, for their help with combustion calorimetric measurement. This work was financially supported by the National Natural Science Foundation of China (NSFC Nos. 20373072 and 30570015) and the Excellent Young Teacher’s Teaching and Scientific Research Award Program, Chinese Ministry of Education.

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