Cloud point transition in nonionic micellar solutions

Abstract

309 Figure 9. Shematic representations for possible structures of a bound monomer (a), a bound racemic aggregate (b), and a bound enantiomeric aggregate (c). The ligands in Cop2+ are represented by two ovals around a small open circle. The shaded and unshaded parts denote the portion of a ligand directed downward and upward from the paper, respectively. below the possible structures of the bound states on the basis of the calculations.For a bound monomer state, the experimental values of pI and pIr (+1.10 and -0.40 in Table I, respectively) were reproduced from the curves in Figure 8, if a and @(E) were chosen as 0' and 0.4, respectively. Under this conformation, it was possible for each of the naphthalene rings in Cop2+ to penetrate between the two neighboring styrenesulfonate residues. Accordingly the bound chelate would be stabilized due to the van der Waals interactions between the naphthalene of COP,+ and the benzene rings of PSS-. The hydrophobicity of PSS-as seen here was already noted by other investigator^.'^ For a racemic aggregate state, the experimental pI and. pII (+0.43 and +O. 10, respectively) corresponded to the theoretical values at a = 35O and @(E) = 0.4. If (+)-Cop2+ and (-)-Cop,+ (13) Tondre, C.; Kale, K. M.; Zana, R. Eur. Polymn. J. 1978, 14, 139.were placed alternatively along the Y axis under this conformation as shown in Figure 9b, the two facing ligands belonging to the neighboring chelates stacked preferably with each other. As a result, the racemic aggregate established a rigid rodlike structure. This explains why the racemic mixture attained a stereoregular racemic aggregate as the most thermodynamically stable state.For an enantiomeric aggregate state, the experimental pI and prI (+0.21 and +0.25, respectively) were reproduced by choosing a = 45' and @(E) = 0.4. When (+)-Cop,+ ions were placed along the Y axis with a = 45' in a sequence, no notable stacking appeared between the neighboring ligands. If, however, one chelate was placed at a = 45' and the other at a = -45' instead, the two ligands in the neighboring chelates stacked to a certain degree (Figure 9c).I4 The relative orientations of the stacking ligands were not so favored that the enantiomeric aggregate was much looser than the racemic aggregate.The stopped-flow experiments revealed the presence of an intermediate state during the course of formation of a racemic aggregate. The intermediate aggregate was formed about 4 ms after mixing when the initial rapid increase of absorbance was completed. It is suspected that, in such an intermediate state, the (+) and (-) isomers take a random distribution on the PSSchain. Accordingly, the sequence could not attain a close stacking as a whole. If this was the case, the succeeding slow step in the time range of 1-lo2 s might represent the process that the bound chelates rearranged on each polymer chain until the alternative sequence of (-)-Cop2+ and (+)-Cop2+ was realized. At that stage, the loosely bound aggregate changed into a rigid stereoregular racemic aggregate. This mechanism ...

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