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Polyethyleneimine (PEI) functionalized with hydrophobic toluoyl groups (T-PEI) was used to control the viscosity of silicon carbide (SiC) nanoparticle aqueous suspensions. The effects of the toluoyl group ratio and molecular weight (MW) of the dispersant on the suspension viscosity were examined. For T-PEIs (MW = 1800), the suspension viscosities decreased as the toluoyl group ratios increased, whereas for T-PEIs (MW = 10000), the suspension viscosities were remarkably reduced when the toluoyl group ratios were 15% and 30%. To discuss these changes, the adsorption ratios of the dispersants on particles and the interaction forces between SiC nanoparticles were characterized using thermogravimetric-differential thermal analysis and colloidal probe atomic force microscopy, respectively. The dispersant adsorption ratio increased dramatically with the introduction of toluoyl groups on PEI. For T-PEIs (MW = 1800), a steric repulsive force between particles began to appear, and its strength increased as the toluoyl group ratio increased. However, for T-PEIs (MW = 10 000), whereas a large repulsive force was generated when the toluoyl group ratios were 15% and 30%, a weak repulsive force was observed when the ratio was 40%. The adsorption structure of the dispersant was suggested based on these changes in particle interaction behaviors.