We have investigated the collapse-stretching transition of a surface-bound, brushlike copolymer, poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG), and the consequence of such transitions on the frictional properties of this coating. The frictional properties of the interface have been measured by colloidal-probe lateral force microscopy (LFM) in liquid environments on the nanoscale. The collapse-stretching transition has been induced through the systematic variation of the chemical composition of the binary solvent mixture comprised of an aqueous buffer solution and 2-propanol. The influence of solvent composition on the polymer conformation was monitored by comparing measurements conducted with optical waveguide lightmode spectroscopy (OWLS) and quartz crystal microbalance with dissipation monitoring (QCM-D). The combined approach employing QCM-D and OWLS has allowed the quantification of the mass of solvent molecules absorbed in the brushlike structure of PLL-g-PEG and has revealed a significant preferential solvation effect. This study has demonstrated preferential solvation of a surface-bound polymer and the role of such solvation in maintaining the favorable lubricating properties of a PEG brush when exposed to mixtures of good and poor solvents.