Topological and thermodynamic investigations of molecular interactions in some lower amides + water mixtures

Abstract

Molar excess vulumes <em>V </em>^E, and molar excess enthalpies <em>H</em>^E<em>, </em>have been determined at 308.15 K for water (W) + formamide (FD) or dimethylformamide (NND) mixtures and the same have been rationalized by graph theoretical arguments to suggest that (i) while W exists as an equilibrium mixture of dimer and trimer, FD and NND exist mainly as dimers, (ii)compared to FD, NND forms stronger H-bond with W, (iii) though both NND and FD act as structure breakers towards W, FD is the stronger one, and (iv) FD has higher boiling point than NND. The <em>V</em>^E and <em>H</em>^E data have also been analysed in terms of the equilibrium W(A'₃) + FD or NND (B'₂) <img src='http://www.niscair.res.in/jinfo/harpoon.gif' border=0> A'B' + B' + A' by the ideal associated model approach and equilibrium constants for the A'₃ + B'₂ <img src='http://www.niscair.res.in/jinfo/harpoon.gif' border=0> A'B', A'₃ <img src='http://www.niscair.res.in/jinfo/harpoon.gif' border=0> 3A' and B'₂ <img src='http://www.niscair.res.in/jinfo/harpoon.gif' border=0> 2B' equilibria along with the enthalpy and volume changes accompanying the formation of A'B', B' and A' entities have been evaluated. The molar enthalpy change accompanying the formation of A'B' entities suggests that (i) compares to FD, NND interacts strongely with W and (ii) this interaction weakens the intermolecular attractive interactions in W and in NND molecules of the (W+ NND) mixture to a considerable greater extent than the corresponding interactions in W and in FD molecules of the (W+ FD) mixtures. The relevant <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0> <em>H</em>(A') and <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0> <em>H</em>(B') data support these conclusions. The <img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>H </em>(P) (P= A'B', B' or B') data combined with the corresponding appropriate equilibrium constant data yielded <em>T</em><img src='http://www.niscair.res.in/jinfo/delta.gif' border=0><em>S</em> data that suggest that both NND and FD act as structure breakers towards Wand that FD is a stronger structure breaker. The stronger structure breaking ability of FO has</p> been shown to favour W-Wand FD-FD interactions in (W+ FD) mixtures.