COOPERATIVITY OF SOLVENT-MACROMOLECULE INTERACTIONS IN AQUEOUS SOLUTIONS OF POLYETHYLENE GLYCOL AND POLYETHYLENE GLYCOL-UREA

Ultrasonic attenuation measurements have been carried out in aqueous polyethylene glycol and aqueous polyethylene glycol-urea solutions in order to investigate the cooperativity of polymer-solvent interactions. A single relaxation time is observed in aqueous polyethylene glycol solutions which can be attributed to solvent-poly-mer interactions involving hydrogen bonding of water to polymer, hydrogen-bonded solvent clusters around the hydrocarbon portion of the polymer, and hydrophobic interactions within the polymer. The relaxation time increases sharply with increasing molecular weight until a molecular weight of about 3400 is reached; in the molecular weight range 3400-20,000 the relaxation time remains essentially constant. This indicates an effective maximum in the size of the cooperative polymer-solvent unit occurs with a polymer of approximately molecular weight 3400. Addition of urea to the aqueous polymer solution causes a sharp decrease in the relaxation time at a urea concentration of around 2.5 m for polymers of molecular weight 3400-20,000. The sharpness of the change in relaxation time indicates a cooperative transition is occurring in the polymer-solvent structure. When urea is added to solutions of polymers with molecular weights below 3400, no decrease in the relaxation time occurs. Thus a minimum molecular size of the solvent-solute unit is necessary for a cooperative transition in the relaxation time to occur, namely a polymer of approximately molecular weight 3400 and its associated solvent molecules. The requirement of a minimum molecular size in order to obtain cooperative solvent-solute intreactions may be of relevance in un-derstanding the mode of action of biological macromolecules. © 1968, American Chemical Society. All rights reserved.