Interaction forces between thermoresponsive surface and colloidal particle in aqueous solution studied using atomic force microscopy

The interaction forces between poly(N-isopropylacrylamide) (PNIPAAm)-grafted surfaces and colloidal particles in an aqueous solution were investigated using an atomic force microscope (AFM). Measurements were conducted between a smooth silicon wafer on which PNIPAAm was terminally grafted and silica particles hydrophobized with a silanating reagent in an aqueous electrolyte solution under controlled temperature. Below the lower critical solution temperature (LCST) of PNIPAAm, there were large repulsive forces between the surfaces, while attractive forces were observed above LCST. When surface hydrophobicity of the particles increased, the magnitude of attractive force tended to increase. The changes of hydration state of the grafted PNIPAAm chains depending on temperature is considered to greatly alter the interaction force properties. The role of the intermolecular interaction between the PNIPAAm chains and the hydrophobic particles in the interaction forces is discussed.