Dynamic Light Scattering Studies on the Effect of 1-Alkanols, Alkanediols and Alkoxyethanols on the Micelles of a Moderately Hydrophobic/Hydrophilic PEO-PPO-PEO Triblock Copolymer

摘要

The results on changes in the apparent hydro-dynamic diameter (D_h) of micelles in a solution containing 5% of a moderately hydrophobic/hydrophilic triblock PEO-PPO-PEO copolymer in the presence of several hydroxyl compounds at 23 °C from dynamic light scattering (DLS) are reported. Distribution plots show micelles with hydrodynamic diameter ~ 17 nm and low polydispersity (<0.1) except at low concentrations where a unimer peak (~ 4 nm) was also noticed. These additives increase/ decrease the micelle size and show micellar transition depending upon their hydrophilicity/hydrophobicity. The results are discussed in terms of the effect of the additives on altering water structure and their partitioning in micelle. Short chain alcohols (C1-C3) increase solvation of PEO and thus increase micelle hydrodynamic size while higher alcohols, initially reduce D_h due shrinkage of PEO followed by micellar growth at higher concentrations. Among αω-alkanediols, C2 and C4 diols increase micelle size by immobilizing water sphere around the micelles whereas higher diols form wicket like structures and reside in palisade layer. Isomeric hexanediols (1,2; 1,5; 2,5 and 1,6) alter micelle size in different ways depending on their hydrophobicity. In C6EO_m (m = 0, 1, 2), as the number of EO group increases, it becomes more hydrophilic and increases D_h at higher concentration. Addition of a hydrophobic triblock copolymer leads to unfavorable mixing with a moderately hydrophobic/hydrophilic triblock copolymer which results in increase in size, while the addition of a hydrophilic counterpart increases the average hydrodynamic size and follows appearance of unimer peak.