  Hydrocarbon films confined between smooth mica surfaces have long provided an
experimental playground for model studies of structure and dynamics of confined
liquids. However fundamental questions regarding the phase behavior and shear
properties in this simple system remain unsolved. With ultra-sensitive
resolution in film thickness and shear stress, and control over the
crystallographic alignment of the confining surfaces, we here investigate the
shear forces transmitted across nanoscale films of dodecane down to a single
molecular layer. We resolve the conditions under which liquid-solid phase
transitions occur and explain friction coefficients spanning several orders of
magnitude. We find that commensurate surface alignment and presence of water at
the interfaces each lead to moderate or high friction, whereas friction
coefficients down to 0.001 are observed for a single molecular layer of
dodecane trapped between crystallographically misaligned dry surfaces. This
ultralow friction is attributed to sliding at the incommensurate interface
between one of the mica surfaces and the laterally ordered solid molecular
film, reconciling previous interpretations.
