The Effect of pH and Salt on Surface Activity and Self-Assembly of Copolymers Containing a Weak Polybase

Neta Cohen, Department of Chemical Engineering , Ben-Gurion University, Beer Sheva, Israel
Lana Binyamin, Department Of Chemical Engineering , Ben-Gurion University, Beer Sheva, Israel
Yael Levi-Kalisman, The Center For Nanoscience And Nanotechnology, The Hebrew University Of Jerusalem, Jerusalem, Israel
Anthony Convertine, Department Of Bioengineering, University Of Washington, Seattle, Washington
Patrick Stayton, Department Of Bioengineering, University Of Washington, Seattle, Washington
Rachel Yerushalmi-Rozen, Department Of Chemical Engineering , Ben-gurion University, Beer Sheva, Israel

pH-sensitive blockcopolymers that assemble into desired structures may enable  efficient drug delivery and controlled release of hydrophobic molecules. Their utilization relies on improved understanding of the relations between molecular composition and the salt-and-pH responsiveness, as well as the size and shape of the copolymers assemblies. Assembly of amphiphilic copolymers that contains ionizable or charged residues at the water –air interface and in the bulk solution is driven by various interactions (hydrophobic, electrostatic, acid-base equilibrium, specific ion affects) with energies of the order of k_bT (the thermal energy).  Here we investigated the combined effect of pH and salt composition on the surface activity and self-assembly of copolymers containing poly(dimethylaminoethyl methacrylate) pDEAEMA residues in aqueous solutions at pH 7-7.4 and pH 5.5, via surface tension measurements and cryo-TEM. Our findings suggest that both pH and salt composition of the buffer solutions affect the surface activity, and the shape and size of the self-assembled aggregates that are formed in solutions of copolymers that contain DEAEMA group. In addition we find that covalently linked hydrophobic groups moderate the pH responsiveness of the DEAEMA residue as compared to non-ionic and hydrophilic groups. Cryo-TEM imaging of the bulk solutions reveal the co-existence of a variety of assemblies.