CF4 adsorbate uptake and spectra are simulated on various model icy surfaces and compared to experimental data measured for CR adsorbate on ice nanocrystals. Comparison suggests that surfaces of annealed ice nanocrystals are relatively smooth but laterally disordered, while surfaces of unannealed nanocrystals are disordered and rough. CF4 adsorbate structure and uptake display remarkable sensitivity to the structural features of the ice surface models and in particular to the extent of disorder. Asymmetric CF stretch spectroscopy of CF4 adsorbate is suggested as a useful probe of the surface structure of molecular solids and of large molecular clusters.