Salt concentration dependency of the hydrated swollen structure of cholinephosphate-type polyzwitterion brushes

The swollen thicknesses of zwitterionic polymer brushes with phosphorylcholine (PC), cholinephosphate (CP), sulfobetaine, and carboxybetaine groups were measured in aqueous sodium chloride (NaCl) or calcium chloride (CaCl₂) solutions via force curve measurements by scanning probe microscopy (SPM) and neutron reflectivity measurements. PC and CP have different charge positions in betaine units consisting of ammonium and phosphate. The PC-type polymer brush did not distinctly reduce the swollen thickness in either the NaCl or CaCl₂ solution, even at 1000 mM. However, the swollen thickness of the CP-type polymer brushes was clearly reduced in the aqueous CaCl₂ solution. Unlike the phosphate of PC, which is located inside the betaine unit, the phosphate of CP tends to form insoluble calcium phosphate, resulting in the collapse of the brush structure.