Forces of interaction between poly(2-vinylpyridine) brushes as measured by optical tweezers

Forces of interaction within single pairs of poly(2-vinylpyridine) (P2VP) grafted colloids have been measured by optical tweezers (OT) with an extraordinary resolution of ±0.5 pN. Parameters to be varied are the concentration and type of salt (KCl, CaCl₂, and LaCl₃) of the surrounding medium as well as its pH. The observed force-distance relation is quantitatively described by the Jusufi model [Colloid Polym. Sci. 2004, 282, 910-917] for spherical polyelectrolyte brushes which takes into account the entropic effect of the counterions and enables one to estimate the ionic concentration inside the brush. The transition from an osmotic to the salted brush regime is analyzed in detail. For the scaling of the brush height a power law is found having an exponent of 0.24 ± 0.01, which ranges between the values expected for spherical and planar brushes. At pH 4 a strong transition from a brush to a pancake conformation takes place.