Collisions of Ne, CH₄, NH₃, and D₂0 with a perfluoropolyether (PFPE) liquid are explored by directing molecular beams at the surface of PFPE in vacuum and monitoring the velocity of the scattered products. The protic and aprotic gases undergo similar impulsive energy transfer and thermal accommodation when striking the surface of PFPE. When compared with glycerol and squalane, PFPE absorbs less energy and exhibits smaller trapping-desorption fractions than the alcohol or hydrocarbon. We correlate these results with the energy lost in single and multiple hard spherelike collisions between the incoming particle and the protruding functional groups of the liquid phase molecules. Lacking the lighter CH and OH groups of glycerol and squalane and the capacity to hydrogen bond, PFPE exposes heavy CF_x groups which provide a hard and weakly attractive surface that resists energy transfer and impedes thermal accommodation.