微量热法研究烟酰胺和异烟酰胺在KCl水溶液中的焓对自缔合作用

利用等温滴定微量热法(ITC)分别测定了298.15 K时烟酰胺(NA)和异烟酰胺(INA)在纯水及不同浓度KCl(m=0~0.3 mol/kg)水溶液中的稀释焓,根据McMillan-Mayer理论计算得到相应的焓对自缔合作用系数(h xx),发现两者的h xx都是很大的负值,且在较高KCl浓度时都随KCl浓度的增大而减小.从溶质-溶质、溶质-溶剂相互作用的观点出发,对这2种吡啶羧酸酰胺异构体的疏水和亲水作用平衡进行了讨论:NA和INA都是亲水-亲水作用占优势,芳香性的吡啶环与酰胺基团羰基的π电子共轭使得对位异构体的共振结构分子电荷分离而更具亲水性;吡啶环之间的π-π堆叠对焓对自缔合作用的贡献几乎可忽略;溶液离子强度(I)的增大有利于加强亲水-亲水作用,从而使h xx的绝对值都随溶液中KCl浓度的增大而逐渐增大.

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

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(hxx) 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 hxx 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 hxx absolute values of the two amides increase gradually with the increasing mola-lity of KCl in solutions.