Interaction Forces Between Thermoresponsive Surface and Colloidal Particle in Aqueous Solution Studied Using Atomic Forc
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0942-W08-08
Interaction Forces Between Thermoresponsive Surface and Colloidal Particle in Aqueous Solution Studied Using Atomic Force Microscopy Naoyuki Ishida, and Mikio Kobayashi Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki, 305-8569, Japan
ABSTRACT 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.
INTRODUCTION It is now well known that Poly(N-isopropylacrylamide) (PNIPAAm) exhibits thermoresponsive properties. PNIPAAm has a lower critical solution temperature (LCST) in water around 32ºC [1] and PNIPAAm chains show strong hydration and expand at temperatures lower than the LCST. On the other hand, the chains dehydrate and shrink to the compact form above the LCST. Recently, PNIPAAm layers have been formed on solid substrates using several methods, including chemical grafting [2], plasma [3], UV light [4], and electron beam irradiation [5]. The surfaces on which PNIPAAm is immobilized have increasingly attracted interest as new functional materials because the hydrophobicity of the grafted surface can be altered by environmental temperature [2–5]. Such a property of grafted surfaces attracts applications of PNIPAAm-immobilized surfaces to on-off (capture-release) attachment of the colloidal-sized particles on the surface, which is expected to be applicable for many processes including cell adsorption, fine particle separation, and wastewater treatment. In order to achieve effective controls of the on-off particle attachment, it is important to analyze the interaction force between the thermo-responsive surface and colloid particle in detail and to understand the nature the forces depending on the temperature. Therefore, in the present study, we investigated the interaction force between PNIPAAm-grafted surfaces and colloidal silica particles with various hydrophobicities using an atomic force microscope (AFM). We grafted PNIPAAm onto smooth silicon wafers and measured the interaction forces between the wafers and silica particles hydrophobized with a silane coupling agent.
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