Factors affecting the complexation of polyacrylic ... - ResearchGate

2742 DUBOLAZOV ET AL.takes place, but in the latter case there is only adistribution of Me þ metal ions near the polymerchain and no real binding of monovalent metalions. 30 Attraction between PAA and Me þstrongly affects the electric field of polymer surface,which can lead to the rearrangement ofPAA structure. As a general rule the increase insalt concentration of the medium changes theconformation of the polymer chains to a morecompact form. 31 In our case this factor probablyleads to the enhancement of hydrophobicity aswell as stability of PAA/UO 2þ2complex.Recently Iida observed the similar effect ofKCl on activity of Ca 2þ in aqueous PAA solution,using a Ca 2þ ion sensitive electrode. The Ca 2þactivity in the PAA solution at the low degree ofneutralization was increased by the presence ofdilute KCl and decreased by the presence of concentratedKCl. 32The size effect seems to be obvious explanationfor the influence of Me þ nature on thestability of PAA/UO 2þ2polycomplex. Kim and coworkers31 found that in the solutions containingalkali metal ions, aggregation of PAA wasincreased in the order Li þ < Na þ < K þ < Cs þ ,which means that the hydrophobicity of thechain decreases with increasing solvated radiusof the counterion. As it can be seen from Figure4, the addition of KNO 3 has more pronouncedeffect on increasing of I/I 0 value than NaNO 3 aswell as on the critical concentration of complexprecipitation (see Fig. 5). Indeed, the radius ofhydrated Na þ (R H 4Å) is bigger than that ofK þ (R H 3Å). 33 Therefore in our case condensationforce is higher for K þ , which can stabilizeand favor the aggregation of PAA/UO 2þ2polycomplexat lower concentrations than Na þ . Consideringthe difference in the critical concentration ofcomplex aggregation at the onset of phase separation,we conclude that the PAA/UO 2þ2 /KNO 3complex is more hydrophobic and stable thanPAA/UO 2þ2 /NaNO 3.The effect of concentrations of H þ , NaNO 3 ,and KNO 3 on quenching the luminescent intensityof UO 2þ2complexed with PAA has been analyzedby constructing relevant Stern–Volmerplots, namely, I 0 /I versus quencher concentrations,using the following relation:Figure 5. Effect of inorganic salts NaNO 3 (1) andKNO 3 (2) on the turbidity of PAA/UO 2þ2aqueous solutions.k ¼ 400 nm, pH ¼ 3.13, PAA/UO 2þ2¼ 2.I 0 =I ¼ I þ k q hs 0 i½QŠwhere I 0 and I are luminescent intensities in theabsence and presence of quencher, k q quenchingrate constant, hs 0 i average lifetime of the uranylion in water and [Q] concentration of quencher.The data presented in Figures 3a and 4 weretherefore used to prepare Figure 6. The inset inthis figure is for H þ ions. Ascending parts of thecurves correspond to quenching of luminescenceby corresponding salts or acid. By taking the hs 0 ivalue as 5 ls, which is typical average lifetime ofUO 2þ2in aqueous solutions, 14 and using the slopeof ascending parts of the curves of Figure 6quenching rate constant k q was calculated andlisted in Table 1. By analogy to the effect ofquenching, the descending parts of the curveswere evaluated similarly to calculate the rateconstants for enhancement effect, where thecoefficient k q in above given equation is replacedby k e . Both results are listed in Table 1. Thequenching rate constant k q ¼ 1.7 10 8 M 1 s 1determined for H þ is in very good accordanceFigure 6.3a and 4Stern–Volmer plot of the data of Figures