  The melting transition in solid monolayers of a series of short-chained
n-alkanes, n-octane (n-C8H18), n-decane (n-C10H22), and n-dodecane (n-C12H26)
physisorbed onto the graphite basal plane are studied through use of molecular
dynamics simulations. Utilizing previous experimental observations of the solid
phase behavior of these monolayers, this study investigates the temperature
dependence of the phases and phase transitions in these three monolayers during
the solid-fluid phase transition, and compares the observed melting behavior to
previous studies of hexane and butane monolayers. In particular, this study
seems to indicate a greater dependence of the melting transition on the
formation of gauche defects in the alkyl chains as the chain length is
increased. In light of the previously proposed footprint reduction mechanism
and variations where the formation of gauche defects are energetically negated,
simulations seem to suggest that decane and dodecane monolayers are generally
equally as dependent upon the formation of gauche defects for the melting
transition to take place, whereas octane monolayers seem to have less
dependence, but follow a trend that is established in previous studies of
melting in butane and hexane monolayers. Also, the phase transition from a
solid herringbone phase into an orientationally ordered intermediate phase is
found to exhibit some differences as compared to a recent study of hexane
monolayers, which may be interpreted as originating from the greater influence
of gauche defects. Comparison to experimental melting temperatures is provided
where possible, and applications involving thin film manipulation and
lubrication is discussed.
