Partial molar volumes of organic solutes in water. XVIII: Selected polyethers(aq) and 3,6-dioxa-1-heptanol(aq) at T= (298 to 573) K and at pressures up to 30 MPa

Partial molar volumes of organic solutes in water. XXI: Cyclic ethers at temperatures T= (278 to 373) K and at low pressure

“…This is in accordance with data for aliphatic linear polyethers [1] which reveal that the contribution of oxygen atom becomes even negative at temperatures higher than 500 K (at elevated pressures, of course). As the group contribution is separable into two parts (cavity, interaction, see above) it is evident that the interaction part becomes more dominant as the temperature increases.…”

“…Then the values monotonously increase with increasing temperature as it is characteristic for hydrophobic groups. The values of the group contribution of the methylene group obtained in this work (V 0 CH 2 ¼ ð15:18AE 0:05Þ cm 3 Á mol À1 at T = 298.15 K) are slightly below the values for linear aliphatic chains {15.78 ± 0.11 cm 3 Á mol À1 ; average value at 298.15 K from [1][2][3]14,27,[30][31][32]}. On the other hand they are significantly higher than those for methylene group bonded in aliphatic carbon rings (homo-cycles, cycloalkanes) as shown in figure 3a where the dashed line represents the methylene group contribution evaluated from data for cyclopentanol(aq) through cyclooctanol(aq) [2,3] (the part of the curve below T = 298 K is a second-order polynomial extrapolation).…”

“…While the intrinsic volume (which determines the size of the cavity) of the oxygen atom remains unchanged, the hydration shell around it becomes more compressed due to strong attractive (hydrophilic) interactions and increased compressibility of water which lead to lower volume. The group contribution for the oxygen atom bonded in the cycle (V 0 O ¼ ð4:20AE 0:06Þ cm 3 Á mol À1 at T = 298.15 K) is lower than that for the oxygen atom in the linear aliphatic chain (5.54 cm 3 Á mol À1 [1] at the same temperature). Higher values are reported for an isolated oxygen atom in the aliphatic hydrocarbon chain (i.e., without any other correction for cyclization, vicinity, etc.)…”

“…Thus the structural contribution related to cyclization is already included in the value of the group contribution of the methylene group bonded in the cycle. In our previous study of linear aliphatic polyethers(aq) [1] we have found out that there is a small positive structural contribution (2.58 cm 3 Á mol À1 at 298.15 K) related to the close vicinity of oxygen atoms (the structure -O-CH 2 -O-), i.e., the structures -CH 2 -O-CH 2 -O-and -O-CH 2 -CH 2 -O-are not equivalent. Although this structural contribution is not substantially significant for cyclic ethers at ambient temperatures (at T = 298.15 K the difference between standard partial molar volume of 1,4-dioxane (80.95 cm 3 Á mol À1 ) and that of 1,3-dioxane (80.82 cm 3 Á mol À1 [20]) is small) we have incorporated a structural term V 0 ðOCOÞ into the proposed group additivity scheme since, as it is indicated by the present data, its importance increases with increasing temperature.…”

“…In one of our previous studies [1], experimental results for standard partial molar volumes, V 0 m;2 , of linear polyethers were presented. Recently data for monohydric alcohols comprising an aliphatic cycle in the molecule have been published [2,3].…”