Enthalpy of transfer of -CH2- between water and organic solvents or mixed aqueous organic solvent systems

Enthalpies of transfer of the CH2 moiety into aqueous acetonitrile mixtures; comparison of values from n-alcohols and tetraalkylammonium ions; effect of temperature variation

“…Discussion Figure 1 shows the chain length dependence of the D t H (CH 2 ) values at the lowest temperature for which data are available; the variations at the other temperatures are similar but less pronounced. The chain length dependence of D t H (CH 2 ) into water was noted previously [2,3] at T = 298.2 K. It is clear from figure 1 that there is a pronounced chain length dependence at intermediate solvent compositions, where D t H (CH 2 ) becoming increasingly exothermic as the solute alkyl chains become longer. This is the opposite to the variation observed in highly aqueous solvents, where D t H (CH 2 ) becomes less exothermic with increasing chain length.…”

“…For transfers from organic solvents to water the D t H (CH 2 ) values obtained from the n-alcohols are also close to zero but become more endothermic (or less exothermic) with increasing chain length [2,3]. The D t H (CH 2 ) values obtained from the tetraalkyl ammonium ions show the same endothermic shift with increasing chain length but are systematically more exothermic, by around 2 kJ AE mol À1 , than those obtained from the alcohol data [3].…”

“…The sharp rise in D t H (CH 2 ) at high water concentrations is common to all of the aqueous organic solvent systems studied [2][3][4]. In contrast, minima at intermediate compositions havenÕt been observed in other mixed solvent systems.…”

“…We have reported enthalpies of solution for n-alcohols at T = 298.2 K previously [2]; thus enthalpies of solution were measured at T = (283.2 and 288.2) K (±0.1 K) using an isoperibol calorimeter running under computer control, as described previously [6]. The solution enthalpies of the n-alcohols were measured at 10 concentrations, with maximum concentrations ranging from 0.1 to 0.3 mol AE dm À3 (n-octanol and ethanol, respectively) and extrapolated to infinite dilution.…”

“…These studies showed that at T = 298.2 K D t H (CH 2 ) between organic solvents were close to zero and were independent of the length of the solute alkyl chain [2,3]. Moreover, the results are the same for D t H (CH 2 ) obtained from data for the n-alcohols, where there is a single alkyl chain and from data for the tetraalkylammonium ions, where interactions between chains on a single solute molecule are possible [3].…”