Structural thermodynamic characteristics and intermolecular interactions in aqueous solutions of hexamethylphosphorotriamide

Volumetric properties of (water+hexamethylphosphoric triamide) from (288.15 to 308.15)K

“…Herewith the ''structural pattern" of this interrelation on going from the water-like composition region to the HMPT-like one experience appreciable changes. As it can see from the figure, the correlation dependence between V E m and H E m for the (water + HMPT) at 0.3 6 x 2 6 1.0 and T = 298.15 K is a close-to-linear function that can be regarded as an argument in favor of the inferences [23,45,[50][51][52] that the state of HMPT molecules in this region of compositions is similar to their state in the pure amide considered. Spectroscopic data [51][52][53] reveal that here, water is mostly dispersed in separate molecules and enters the composition of mixed or heterocomponent associates where one H 2 O molecule interacts via H-bonding with one or two HMPT molecules.…”

“…The latter process (of increasing water structure) is accompanied by a decrease in V /,2 with increasing x 2 up to the solution concentration corresponding to maximum possible filling of the water cavities with HMPT molecules (V /,2,min , in figure 4b). At HMPT-richer compositions, the appearance of both HMPT-HMPT and heterocomponent contacts, and clusters consisting solely of water molecules as well, is observed [23,29,52].…”

“…The region of ''largest deviations from linearity" extending up to x 2 $ 0.1 and occupying a considerable part of the whole V E m ðx 2 Þ versus H E m ðx 2 Þ function (see in figure 3) is most likely to be related to a transformation of the water spatial hydrogen-bonding network from a slightly perturbed state to a percolation threshold near the above ''boundary" concentration [23]. According to conclusions [4,7,16,29], such a ''perturbation" can be caused by the effect of increasing water structure outside the first hydration sphere around a large-sized hydrophobic molecule of HMPT.…”

“…According to conclusions [4,7,16,29], such a ''perturbation" can be caused by the effect of increasing water structure outside the first hydration sphere around a large-sized hydrophobic molecule of HMPT. At mole fractions varying from x 2 $ 0.1 to x 2 $ 0.3, where the V E m À H E m function interval become substantially narrower (figure 3), the structural formations of both components along with ''heteroassociates" with different compositions are most likely to be simultaneously present [23].…”

“…The organic component molecules in these mixtures do not act as H-bonding donors, however, they are electron lone-pair donors capable of forming the strong hydrogen bonds with water molecules [7,10,[13][14][15][16]. Among such binary mixtures, a {water + hexamethylphosphoric triamide (HMPT)} is the liquid system, the component molar volumes of which differ each other one order of magnitude, and the organic component as a net solvent has a number of wideused distinctive characteristics [16][17][18][19][20][21][22][23]. In particular, noteworthy is the fact that the values of Gutmann's electron-donating number, DN SbCl 5 , and molecular dipole moment, l, for HMPT ($38.8 kcal Á mol À1 and 5.54 D, respectively) cannot be compared with those neither for any other aprotic dipolar solvent nor for water ($18.0 kcal Á mol À1 and 1.83 D, respectively) [16,17,24].…”