Experimental heat capacity

Experimental data heat capacity should be the starting point to quantitative thermal analysis. Figure 1 shows the experimental heat capacity of a-D-glucose by adiabatic calorimetry and standard differential scanning calorimetry (DSC). The low-temperature experimental Cp of solid a-D-glucose, as collected from paper [38], was used to calculate the contribution of the group vibrations and the ATHAS Scheme allowed measurement of the contributions from the skeletal vibrations, as described below and the total vibrational heat capacity has been extended to high temperatures and serve as a baseline for the quantitative thermal analysis. Also Fig. 1 shows a comparison of the experimental heat capacity of amorphous with crystalline a-D-glucose by DSC for higher temperature regions [39]. Figure 2 shows the experimental specific heat capacity of dry amorphous starch and starch with different low concentration of water measured by adiabatic calorimetry and differential scanning calorimetry from 8 to 490 K. Details about all measured and recommended experimental Cp data can be found in the full paper [26].

u-D-Giucose

C (amorphous}

Cp(liquld)

^^ I

Cp(exp) -ad ¡a batic ca lo ry metry *

Cp(crystalline)

Temperature (K)

Fig. 1 Experimental heat capacities of a-D-glucose by adiabatic calorimetry (*data from Ref. [38]) and differential scanning calorimetry (DSC)

These experimental, macroscopic heat capacities presented in Fig. 1 and 2 were linked with molecular motion of a-D-glucose, starch and water in order to present quantitative thermal analysis of carbohydrate systems using two baselines as references: the solid heat capacity and liquid heat capacity. The low temperature solid heat capacity was linked with the vibrational motions of a-D-glucose, starch and water. The interpretation of a partial liquid heat capacity of dry and hydrated starch will be in the terms of vibrational, conformational and external (anharmonic) contributions.

0 100 200 300 400 500

Fig. 2 Experimental specific heat capacities of dry starch and starch with 11 and 17 wt.-% of water by adiabatic calorimetry and differential scanning calorimetry (DSC)

0 100 200 300 400 500

Temperature (K)

Fig. 2 Experimental specific heat capacities of dry starch and starch with 11 and 17 wt.-% of water by adiabatic calorimetry and differential scanning calorimetry (DSC)

SAMPLES

a-D-glucose was purchased from Aldrich Chem. Co. The amorphous starch was used as obtained from Blattmann AG, Switzerland (lot 51128), and after various pretreatments, as described in literature [40, 41]. In order to obtain dry starch, the sample was held in a vacuum oven at 353 K (80° C) for a minimum of 48 hours. To introduce known amounts of water either, the appropriate amounts were added to dry starch, or the dry starch was kept for 2-3 weeks in contact with the constant vapor pressure of different saturated inorganic salt solutions. Heat capacities of starch-water samples containing 53 mol-% (11 wt.-%) and 65 mol-% (17 wt.-%) of water were measured.

The concentrations are reported as weight fractions of water, WW, and starch, WS, or the corresponding mole fractions of water, XW, and starch, XS. Using the molar mass of water (MW = 18.0152 g mol-1) and the repeating unit of starch (MS = 162.142 g mol-1) the mole fractions of water and starch can easily be calculated from the weight fractions:

X _ Ww/ M w X _ Ws/ M s W Ww / M W + WJ Ms S Ww / M W + Ws/ Ms

The molar masses of the mixtures, M, were calculated as follows:

and the molar heat capacities of the mixtures are given by:

C Starch-Water _ ^Starch-Water (3)

where cpStarch Water is specific heat capacity of the starch-water mixture in J K-1g-1. The partial molar heat capacities of water and starch can be estimated from:

-f Starch -Water

C pWater (partial) = C pStarch "water + X S —p—--(4)

The samples were sealed in high-pressure stainless-steel pans (HPS) from the Perkin-Elmer Corp., Norwalk, CT, to prevent water loss during measurements by standard DSC.

Single-crystalline sapphire (Al2O3) was used for the calibration of the heat capacity at each temperature. Temperature calibrations were carried out at the phase transitions of indium (429.75 K), water (273.15 K), and tin (505.08 K).

INSTRUMENTATION AND MEASUREMENTS

For the measurement of the transition behavior and heat capacities two calorimeters were used: An adiabatic calorimeter for low-temperature measurements of heat capacity, and a heat-flux type MDSC 2920 of TA Instruments, Inc. for the DSC measurements at higher temperature. Heat capacities of starch and starch-water from 8 K to 330 K were measured with the adiabatic vacuum calorimeter which was completely automated and was fully described previously [42]. In short, the heat capacity of the sample was 60-70% of the total heat capacity of the calorimeter and the substance over the whole temperature range. The calorimetric ampule was a cylindrical vessel of platinum with a volume of ca. 15-10 6 m3. The heat capacity of an unloaded calorimetric ampule increased gradually from 0.0045 J K-1 to 1.440 J K-1 with increasing temperature from 5 K to 330 K. The temperature was measured with a platinum resistance thermometer. Liquid helium and nitrogen were used to obtain low temperature in the cooling system. Heat capacities were calibrated with benzoic acid standard before the measure -ments of starch and starch-water samples. The precision was estimated to be ±0.5% from 5 K to 330 K.

The measurement of heat capacity by standard DSC was carried out at a heating rate, q, of 10 K min-1. Three runs were carried out, one of the empty-empty pans, one with empty-sapphire for calibration, and one with empty-sample. After steady state was attained, the heat capacity was determined from the following equation [43]:

q where K is determined as a function of temperature from the sapphire calibration; HF is the heat-flow rate (proportional to the temperature difference between reference and sample, AT). The effects of calorimeter asymmetry and differences between empty reference and sample calorimeters are eliminated by using the empty pan run as baseline for the heat flow amplitudes. For measurements of heat capacity by standard DSC 10-30 mg of sample were employed. Three or more separate runs were made for each sample. The data of heat capacities were collected from second run. No water loss was detected after the measurements, as proven by the constant weight of the samples. The accuracy of the measurements is estimated to be ±3% or better.

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  • angelica
    What is the experimental heat capacity?
    3 years ago

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