These results demonstrate that
the nature of Br is the main reason determining its specific influence.
Effects of KCl, KH2PO4 and K2SO4 are not dependent on the
nature of these anions; therefore, they are nonspecific and mainly
caused by the change of the ionic strength.
Impact of KBr is determined by the ability of Br anions to
penetrate into macrocyclic cavity. Preferable inclusion of Br in
comparison with Cl, H2PO4
and SO42 has been shown in our recent
publication [20] as well as in the literature [21–24]. According to
the NMR results [20,21], bromide is inserted into macrocyclic
cavity and located near the inner protons H-5 of CDs. Inclusion
complexes of CDs with Br are rather weak [20]. In accordance
with this fact, two inclusion equilibria coexist in KBr solution
and they can influence each other. In particular, insertion of Br
into CD cavity leads to the weakening of the strength of pABA/CD
complex formation (Table 2). It should be mentioned that the effect
of Br does not depend on the CD cavity size. As one can see from
table 2, similar tendency of K variation was observed for complex
formation of pABA with a-CD, b-CD and c-CD in KBr solutions
(Table 2). In all these systems formation of ternary complexes
CD/acid/Br characterized by increasing stability was not observed.
Effect of Br is also reflected in the thermodynamic parameters
of complex formation. Values of DcG, DcH and TDcS for binding of
pABA with CDs in water and in solutions of KCl and KBr
(0.2 mol/kg) were also derived (Table 3). In this case, KCl and KBr
were considered as electrolytes displaying nonspecific and specific
salt effect, respectively. First of all, a good agreement between DcH
obtained for complex formation in water using van’t Hoff approach
and calorimetry [10] should be stressed. In particular, previously
determined by calorimetry of solution DcH values were equal to