We investigate the rheological properties of silica nanoparticle monolayers at the air-water interface by using two orthogonal Wilhelmy plates in the Langmuir trough and Brewster angle microscopy (BAM). Remarkable anisotropic effect of surface pressure is observed when the layers are fully covered by particles. The pressure anisotropy is the most prominent for the layer of particles with 34%SiOH on their surface. The elastic compression and the shear moduli present the maxima at intermediate hydrophobicity. The dependence of rheological properties on particle hydrophobicity is closely related to the foamability and the stability of the foams made from these particle dispersions. A shape memory effect is observed in the condensed layer of the most hydrophobic particle(20%SiOH), which may result from the irreversible organization of particles and the particle arrangement driven by the inner stress stored in the layer.