Investigation of the melting behavior of the reference materials biphenyl and phenyl salicylate by a new type adiabatic scanning calorimeter
Graphical abstract
Melting transition of biphenyl by pASC.
Introduction
Many fields of research and applications require detailed information on thermal properties of condensed matter systems. In particular, the temperature dependence of the energy or enthalpy H(T) of a sample of a given substance is important. Although, as we will see further, the direct measurement of H(T) is possible, in many cases one only has direct experimental access to the heat capacity Cp = (∂H/∂T)p, the slope of the enthalpy curve as a function of temperature. The enthalpy curve itself requires then an additional integration. This approach cannot work properly when there is a discontinuity in the enthalpy curve (latent heat) at a first-order phase transition, where the essential information on the latent heat cannot be obtained from Cp data alone and a direct measurement (in steps or by scanning) of the enthalpy is necessary.
Several types of calorimetric methodologies have been developed in the past with different degrees of uncertainty and resolution depending on the application envisioned. For low-uncertainty data, one usually employs a classical adiabatic (heat-step) calorimeter [1], [2], [3], [4]. After the commercial introduction in the 1960s of differential scanning calorimeters (DSC), the use of these types of instruments has become widespread in thermal analysis [5], [6], [7], [8]. Other heat capacity measuring techniques have been developed, such as ac-calorimetry [9], [10], the 3ω method [11], and photoacoustic and photopyroelectric techniques [12], [13].
Another technique, adiabatic scanning calorimetry (ASC) has only received limited attention so far. It was introduced in the late 1970s by a group at KU Leuven (Belgium) for high-resolution measurements of the heat capacity and enthalpy near phase transitions and critical points in liquid mixtures and liquid crystals [14], [15]. The innovation introduced was to apply under adiabatic conditions a constant, known electrical power to the sample cell instead of imposing a constant rate (as done in DSC-type calorimeters). With the power and temperature evolution known, the heat capacity and enthalpy of the sample can be calculated with high resolution [16]. However, ASCs remained largely research instruments not the least because of complicated sample cell mounting and elaborate construction to impose adiabatic conditions. These problems have been eliminated in a new design of ASCs by incorporating Peltier elements between the sample cell and the adiabatic shield [17], [18]. Measurements, over large temperature ranges on mg-sized samples, are now possible with high resolution in temperature (sub-mk), enthalpy and heat capacity. Accurate data can be obtained by a one-time calibration of the incorporated thermometers and of the background heat capacity of the sample cell and addenda.
In this work, we illustrate the possibilities of the novel Peltier-element-based adiabatic scanning calorimeter (pASC) by a series of measurements on biphenyl and phenyl salicylate (salol) near their melting point. In a first part, the ASC concept and the novel implementation will be discussed. Subsequently, high-resolution enthalpy and heat capacity data will be presented and discussed for both compounds. Attention will also be paid to impurity determination. Biphenyl and phenyl salicylate were chosen because high-purity samples of these compounds are being used as calibration substances for DSC instruments [19]. The DSC technique is a dynamic technique and the quality of DSC measurements is influenced by parameters related to the instrument, the scanning rate, the sample and the operator, and depends critically on the quality of the frequent temperature and caloric calibration of the instrument [19], [20], [21]. For the calibration of a DSC, one is strongly dependent on high-purity reference materials accurately characterized by absolute calorimetric techniques as adiabatic heat-step calorimetry [22], [23], [24], [25].
Section snippets
Methodology of adiabatic scanning calorimetry
Adiabatic scanning calorimetry (ASC) is a calorimetric technique aimed at the simultaneous measurement of the temperature dependence of the enthalpy and the heat capacity of condensed matter materials. The basic concept of ASC resides in applying a constant heating or cooling power to a sample holder containing a sample. This is opposite to what is done in DSC where a constant heating rate is imposed and the changing power needed to maintain the constant rate is measured in a differential
Measurements on biphenyl
In Fig. 3 the time evolution of the direct measurement data for the temperature T(t) (panel a) and the specific enthalpy (per unit mass) h(t) (panel b) are given for a constant power run on a 72.6 mg biphenyl sample in a 120 μl stainless steel medium pressure Mettler-Toledo crucible. In the solid phase, the temperature increases nearly linearly, while during melting the temperature stays constant until all the latent heat is delivered. In the liquid phase, the temperature increase with time
Summary and conclusions
High-resolution specific enthalpy and specific heat capacity results have been obtained for the reference materials biphenyl and phenyl salicylate by means of a novel type of Peltier-element-based adiabatic scanning calorimeter (pASC). The pASC operates by delivering constant power (to sample and sample holder) and measures the changing rate, which is opposite to what is done in differential scanning calorimetry (DSC) where a constant rate is imposed and changing power measured. The pASC
Acknowledgements
This research is supported by FWO research project G.0492.10, FWO research Project G.0360.09 and KU Leuven research project OT/11/064.
References (33)
- et al.
NIST and standards for calorimetry
Thermochim. Acta
(2000) - et al.
Critical review of small sample calorimetry: improvement by auto-adaptive thermal shield control
Thermochim. Acta
(2002) High-resolution ac calorimetry and critical behavior at phase transitions
Thermochim. Acta
(1985)- et al.
Temperature, heat and heat flow rate calibration of differential scanning calorimeters
Thermochim. Acta
(2000) - et al.
Critical assessment of the enthalpy of fusion of metals used as enthalpy standards at moderate to high temperatures
Thermochim. Acta
(1999) - et al.
The thermodynamic properties of biphenyl
J. Chem. Thermodyn.
(1989) - et al.
Low-temperature adiabatic calorimetry of salol and benzophenone and microscopic observation of their crystallization: finding of homogeneous-nucleation-based crystallization
J. Chem. Thermodyn.
(2002) Chemical Thermodynamics
(1950)- et al.
High-precision adiabatic calorimetry and the specific heat of cyclopentane at low temperature
J. Therm. Anal.
(1993) - et al.
A differential scanning calorimeter for quantitative differential thermal analysis
Anal. Chem.
(1964)
The analysis of a temperature-controlled scanning calorimeter
Anal. Chem.
Thermal Analysis
Steady-state, ac-temperature calorimetry
Phys. Rev.
Specific-heat spectroscopy of the glass transition
Phys. Rev. Lett.
Photothermal techniques for heat capacities
Cited by (22)
Phase behavior of medium-length hydrophobically associating PEO-PPO multiblock copolymers in aqueous media
2023, Journal of Colloid and Interface ScienceHigh resolution study of the n = 7–9 p,p’-n-alkylazobenzenes phase transitions by photopyroelectric and adiabatic scanning calorimetries
2021, Thermochimica ActaCitation Excerpt :As discussed in depth in Ref. [37,38], the ASC technique allows the absolute determination of both the specific heat and enthalpy of transition within an accuracy of 2%, provided that uncertainty on the sample mass does not exceed this value. The validity of this assumption has been experimentally confirmed [38] by a statistical analysis of repeated measurements carried out on reference samples (biphenyl and phenyl salicylate), being the results in agreement within 2% with literature values. Unlike ASC, only the specific heat variations with respect to a reference value can be obtained by means of the PPE back configuration.
Adiabatic scanning calorimetry investigation of the melting and order–disorder phase transitions in the linear alkanes heptadecane and nonadecane and some of their binary mixtures
2021, Journal of Chemical ThermodynamicsCitation Excerpt :Detailed descriptions of this kind of implementations and results can be found in [25–27] and references therein. However, this approach has in the last decade been superseded by the development of the novel Peltier-element-based adiabatic scanning calorimeter (pASC), providing greater user-friendliness and allowing also much smaller sample quantity [28–31]. In a pASC the uncertainty on the final results, cp(T) and h(T), depends on the uncertainties on the measured quantities T(t), P(t), the sample mass m and the heat capacity of the addenda Cadd.
High-resolution investigation by Peltier-element-based adiabatic scanning calorimetry of binary liquid crystal mixtures with enhanced nematic ranges
2021, Journal of Molecular LiquidsCitation Excerpt :Detailed descriptions of this kind of implementations can be found in [40–42] and references therein. However, in the past decade, this approach has been superseded by the development of the novel Peltier-element-based adiabatic scanning calorimeter (pASC), providing greater user-friendliness and requiring essentially smaller amounts of samples [31,43–47]. In Fig. 1 a schematic representation of the central part of the pASC is given.
Supercooling of phase change: A new modeling formulation using apparent specific heat capacity
2020, International Journal of Thermal SciencesCitation Excerpt :Also, further studies on the DSC sample dynamics [28,29], the identification of particular substances by inversion [30] or the comparison of different equipment have been achieved [31]. The accuracy of the method can be improved using a highly insulated device referred to as adiabatic scanning calorimeter [32]. Some authors [33,34] have focused on supercooling using the T-history methods.
- 1
Present address: Institute for Materials Research IMO, Hasselt University, Belgium.