Refined Flow Microcalorimetric Setup for Measurement of Mixing Enthalpies at High Dilutions: Determination of Infinite Dilution Dissolution Enthalpies of Some Alkanol and Ether Solutes in Water
Abstract:A refined flow microcalorimetric setup with a fully automatic control of the entire experimental sequence has
been designed for accurate measurement of mixing enthalpies of highly dilute aqueous solutions as a function of
composition and, in turn, for reliable determination of the solute's infinite dilution dissolution enthalpies
(Δsol
). The instrument and procedure were extensively tested, and their performance was verified using
various microcalorimetric standards and further test reactions that involved… Show more
“…4). The larger partial charges of PEG-like polyethers would also result in a larger enthalpy of hydration as compared to POM-like polyethers, and this could partly explain the larger negative enthalpy of solution 2,3 , and hence the better solubility. It should be noted that in general, solubility is determined not only by the hydration strength.…”
Section: Discussionmentioning
confidence: 99%
“…The same dichotomy exists for the enthalpies of solution: dissolving a mole of dimethoxyethane (DME, Fig. 1) releases >5 times as much heat as dissolving a mole of dimethoxymethane (DMM) 2,3 .Fig. 1Investigated polyethers and their solubilities 4 .…”
The solubilities of polyethers are surprisingly counter-intuitive. The best-known example is the difference between polyethylene glycol ([–CH
2
–CH
2
–O–]
n
) which is infinitely soluble, and polyoxymethylene ([–CH
2
–O–]
n
) which is completely insoluble in water, exactly the opposite of what one expects from the C/O ratios of these molecules. Similar anomalies exist for oligomeric and cyclic polyethers. To solve this apparent mystery, we use femtosecond vibrational and GHz dielectric spectroscopy with complementary
ab initio
calculations and molecular dynamics simulations. We find that the dynamics of water molecules solvating polyethers is fundamentally different depending on their C/O composition. The
ab initio
calculations and simulations show that this is not because of steric effects (as is commonly believed), but because the partial charge on the O atoms depends on the number of C atoms by which they are separated. Our results thus show that inductive effects can have a major impact on aqueous solubilities.
“…4). The larger partial charges of PEG-like polyethers would also result in a larger enthalpy of hydration as compared to POM-like polyethers, and this could partly explain the larger negative enthalpy of solution 2,3 , and hence the better solubility. It should be noted that in general, solubility is determined not only by the hydration strength.…”
Section: Discussionmentioning
confidence: 99%
“…The same dichotomy exists for the enthalpies of solution: dissolving a mole of dimethoxyethane (DME, Fig. 1) releases >5 times as much heat as dissolving a mole of dimethoxymethane (DMM) 2,3 .Fig. 1Investigated polyethers and their solubilities 4 .…”
The solubilities of polyethers are surprisingly counter-intuitive. The best-known example is the difference between polyethylene glycol ([–CH
2
–CH
2
–O–]
n
) which is infinitely soluble, and polyoxymethylene ([–CH
2
–O–]
n
) which is completely insoluble in water, exactly the opposite of what one expects from the C/O ratios of these molecules. Similar anomalies exist for oligomeric and cyclic polyethers. To solve this apparent mystery, we use femtosecond vibrational and GHz dielectric spectroscopy with complementary
ab initio
calculations and molecular dynamics simulations. We find that the dynamics of water molecules solvating polyethers is fundamentally different depending on their C/O composition. The
ab initio
calculations and simulations show that this is not because of steric effects (as is commonly believed), but because the partial charge on the O atoms depends on the number of C atoms by which they are separated. Our results thus show that inductive effects can have a major impact on aqueous solubilities.
“…This combined instrument integrates an isothermal mixing microcalorimeter [10] (a modified Model 4400, CSC, Provo, USA) and a vibrating-tube densimeter (Model DMA 5000, Anton Paar, Austria) into a tandem flow assembly with fully automatic control of the entire experimental sequence. The calorimeter is characterized by a thermal power sensitivity of 0.1 lW and the densimeter by a repeatability of 1Á10 À6 gÁcm À3 .…”
Section: Tandem Flow Mixing Calorimetry and Densimetry In Dilute Regionmentioning
“…This combined instrument integrates an isothermal mixing microcalorimeter [11] (a modified Model 4400, CSC, Provo, USA) and a vibrating-tube densimeter (Model DMA 5000, Anton Paar, Austria) into a tandem flow assembly with fully automatic control of the entire experimental sequence. The calorimeter is characterized by a thermal power sensitivity of 0.1 lW and the densimeter by a repeatability of 1 Á 10 À6 g Á cm À3 .…”
Section: Tandem Flow Mixing Calorimetry and Densimetry In Dilute Regionmentioning
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