Premicellar interaction of PEO-PPO-PEO triblock copolymers with partially hydrophobic alcohols: NMR study

The interactions of three alcohols, namely, 2-butanol (BuOH), 3-methyl-2-butanol (MeBuOH), and 3,3-dimethyl-2-butanol (Me2BuOH) with propylene oxide octamer (PO8) and the copolymers (EO)8(PO)13(EO)8(L35) and (EO)13(PO)30(EO)13(L64) in D2O were studied using (13)C NMR spectra and relaxations and (1)H PFG NMR diffusion measurements. For L64, it was shown that the temperature at which the PO chain starts to change its conformation under dehydration decreases by 6 K for each additional methyl group in the alcohol molecule (i.e. with increasing its hydrophobicity), and the analogous conformation states are attained at temperatures approximately 10 K lower compared using ketonic analogs of the alcohols under the same conditions. Also, the first signs of L64 aggregation, according to the normalized diffusion coefficients, are at temperatures 7, 10, and 13 K lower for BuOH, MeBuOH, and Me2BuOH, respectively. These effects are much weaker for (PO)13 in L35 or nonexistent for (PO)8 in PO8, thus showing the role of cooperativity in dehydration and aggregation processes. According to diffusion measurements, the molar fraction of the alcohol hydrogen bonded to L64 increases with its hydrophobicity and, in an apparent conflict with thermodynamics, with increasing temperature at which also higher NOE can be observed. Strong hydrogen bond interaction, which is in cooperation with hydrophobic interaction, does not preclude the exchange between bound and free states of the alcohol, however. Using (13)C transverse relaxation, its correlation time is shown to be of the order of 10 ms.