The energetics of aqueous solutions of the ionic liquid (IL) 1-ethyl-3-methylimidazolium ethylsulfate in the concentration range m < 165 mol·kg(-1) was analyzed on the basis of the enthalpies of solution of the IL in water (Δ(sol)H(m)) and the enthalpies of dilution (Δ(dil)H(m)) of solutions with different IL concentrations. The data were both obtained experimentally, by calorimetry, and theoretically, by using Molecular Dynamics (MD) simulations. Particular attention was given to the low-concentration range (m < 5 mol·kg(-1)), which had not been covered in previous experimental studies of this system. The dependence of Δ(sol)H(m) from the molality of the IL observed within this m < 5 mol·kg(-1) range could be fitted to a fourth-order polynomial with an average relative deviation of ∼0.13%. This polynomial function shows a minimum of Δ(sol)H(m) at m ≈ 0.6 mol·kg(-1) (or a molar fraction x(IL) ≈ 0.01) that could be approximately captured by the MD simulations performed in this work but not through extrapolations based on previously reported experimental or simulation data. The decomposition of our MD simulation Δ(sol)H(m) results in contributions from different types of interaction (IL-IL, H(2)O-H(2)O, and IL-H(2)O), indicated that the minimum essentially results from two opposite effects: the differences between the IL-IL and H(2)O-H(2)O interactions in the solution and in the pure liquids are both positive and increase with the dilution of the IL, while the contribution of the IL-H(2)O interactions (that is only present in the solution) is negative and decreases with the IL dilution. It was also found that the observed trends in Δ(sol)H(m) are dominated by electrostatic rather than dispersion interactions.
An electrically calibrated and computer-controlled apparatus based on the LKB 10700-1 flow microcalorimeter was set up and tested by determining the enthalpies of dilution of aqueous solutions of sucrose and HCl at 298.15 K. The obtained results are in good agreement with the corresponding reference values recommended in the literature. The quality of the calorimetric signal in terms of noise, linearity and baseline drift, and the instrument time constant were analyzed. Also investigated were the influences of the flow rate and of the viscosity of the calorimetric liquid on the position of the baseline. The new calibration system enables powers ∼25 times smaller than those possible with the original one to be released inside the calorimetric cells. Electrical calibrations carried out with water as the sole calorimetric liquid indicated that the precision of the calibration constant, ε, quickly improved from ∼9% to ∼0.4% with the increase of the input power, P, from ∼2 µW to ∼50 µW, and stabilized at ∼0.1% for P ⩾ 100 µW. The calibration constant was also found to linearly increase with the flow rate of the calorimetric liquid and to vary by less than 1% over a period of eight months under similar experimental conditions. Finally a new experimental procedure for heat of dilution measurements, which combines calibration and experiment in a single run, was evaluated. This method allows a significant economy in experiment time with apparently no accuracy loss relative to the conventional procedure recommended in the literature.
The energetics of the reaction between Ca(NO3)2(aq) and (NH4)2HPO4(aq) leading to the formation of calcium phosphate nanoparticles was investigated by flow calorimetry. The relationship between the observed enthalpy change, the pH of the (NH4)2HPO4(aq) solution, and the elemental composition and morphology of the obtained compounds was studied. Results of elemental analysis, combined thermogravimetry−infrared spectroscopy, X-ray powder diffraction, and scanning electron microscopy showed that the change of the pH during the precipitation reaction through the addition of controlled amounts of NaOH(s) to the (NH4)2HPO4(aq) solution leads to significant and reproducible changes of the chemical composition, morphology, and amorphous character of the obtained materials. These changes are reflected by the corresponding enthalpy of reaction, which seems to be predominantly determined by the differences in chemical composition.
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