Enthaplic Pairwise Self-associations of Nicotinamide and Isonicotinamide in Aqueous KCl Solutions by Microcalorimetry†

doi:10.7503/cjcu20130388

Abstract

Abstract:

Dilution enthalpies of nicotinamide and isonicotinamide in pure water and aqueous KCl solutions of various molality(m=0—0.3 mol/kg) were determined at 298.15 K by isothermal titration microcalorimetry(ITC). The corresponding values of enthalpic pairwise self-association coefficient(h_xx) were evaluated accor-ding to the McMillan-Mayer theory. From the point of view of solute-solute and solute-solvent interactions, the equilibrium between hydrophobic and hydrophilic interactions of the two pyridinecarboxamides was discussed. It is found that both of the h_xx values of the two isomers are all largely negtive, and decrease rapidly with increasing molalities of KCl at its higher concentrations. It is concluded that both of them are characterized by prevailing hydrophilic-hydrophilic interaction, and that the π-electronic conjugation between aromatic pyridyl ring and carbonyl of amide group makes resonance structure molecule of the para-isomer be more hydrophilic due to charge separation. The results also indicate that the contribution from π-π packing between pyridyl rings can be neglected nearly, and that the increase of ionic strength I favors hydrophilic-hydrophilic interaction greatly since both of the h_xx absolute values of the two amides increase gradually with the increasing mola-lity of KCl in solutions.

Key words: Nicotinamide, Isonicotinamide, Aqueous KCl solution, Dilution enthalpy, Enthalpic pairwise self-association, Isothermal titration microcalorimetry(ITC)

CLC Number:

O645

TrendMD:

JIA Zhaopeng, HU Xingen, FANG Guoyong. Enthaplic Pairwise Self-associations of Nicotinamide and Isonicotinamide in Aqueous KCl Solutions by Microcalorimetry^†[J]. Chem. J. Chinese Universities, 2014, 35(2): 384.

Figures/Tables 6

Fig.1 Structures of nicotinamide(NA) and isonicotinamide(INA)

Table 1 Enthalpic pairwise self-association coefficients of NA and INA in aqueous KCl solutions of various molalities at 298.15 K*

m_KCl/(mol·kg^-1)	h_xx/(J·kg·mol^-²)		m_KCl/(mol·kg^-1)	h_xx/(J·kg·mol^-²)
m_KCl/(mol·kg^-1)	NA		INA	NA	INA
0	-5296(±31)	-4763(±23)	2.0018	-7151(±28)	-6655(±48)
0.5007	-5169(±30)	-4702(±53)	2.5034	-8240(±36)	-7442(±52)
1.0021	-6067(±27)	-4702(±65)	2.8287	-8570(±56)	-8278(±27)
1.5013	-6530(±35)	-6212(±40)

Fig.2 Typical ITC curve of NA in 0.5 mol/kg aqueous KCl solutions at 298.15 K

Fig.3 Experimental values of ΔH(mN-1→mN) of NA in 0.5 mol/kg aqueous KCl solutions at 298.15 K vs. injection number NSD is the standard deviation of regression.

Fig.4 Enthalpic pairwise self-association coefficients(hxx) of NA(a) and INA(b) in aqueous KCl solutions of various molalities at 298.15 K

Scheme 1 Resonance structures of nicotinamide(top) and isonicotinamide(bottom)

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