Cross sections of three different -alkane solid phases as viewed down the molecular axis with the and axes as shown. Colored lines show the orientation of the molecules (the average projection of the C–C bonds onto the plane). (a) shows an orthorhombic herringbone crystal. (b) shows an orthorhombic RI phase. The light lines show molecules that are turned by 90° from (a). (c) shows the hexagonal RII phase. Molecules remain in layers but are oriented in various directions.
Results for constant temperature simulations using the FW force field (Table II). Vertical lines show experimental X-RI (solid), RI-RII (dashed), and RII-melt (dotted) temperatures for pure C23 (Table I). (a) Area per molecule in the plane. (b) Ratio of lattice parameters .
Histograms of orientations of molecules, each taken from 250 ps of data. Orientation angle is measured from the lattice axis. (a) Herringbone crystal phase, taken from the FW simulation at 290 K. (b) RII phase, taken from the FW simulation at 335 K. The molecules do not rotate completely freely; six orientations are slightly preferred. (c) RI phase, taken from the FW simulation at 315 K. The rotating molecules tend to take four distinct orientations (±43° and ±137°) separated by approximately 90°.
Representative snapshots of portions of single layers of the simulated systems in crystal phases, showing molecule orientation (arrows) and position (circles) colored as explained in the text. (a) shows the herringbone crystal, taken from the FW simulation at 290 K. (b) shows the monoclinic crystal, taken from the FW simulation at 305 K. A domain boundary spans the system near . Single unit cells are marked for both phases; is in the x direction.
Representative snapshots of portions of single layers of the simulated systems in rotator phases, showing molecule orientation (arrows) and position (circles) colored as explained in the text. (a) shows the RII phase, taken from the FW simulation at 335 K. (b) shows the RI phase, taken from the FW simulation at 315 K.
Order parameter and enthalpy results for C23 heated from 150 to 350 K at a rate of 20 K/ns using the FW force field. All data were smoothed using a 25 point moving average. Experimental transitions are marked as in Fig. 2. (a) Order parameters (cyan), (yellow), and (magenta). Data points were taken every 2 ps. (b) Enthalpy per molecule , showing fits to the orthorhombic and RII phases. Data points were taken every 0.1 ps.
Experimental transition temperatures. Transition temperatures for pure and a 50-50 mixture of and in [K], as found by experiment (Refs. 4 and 5).
Results for . and are the average dimensions of a unit cell of two molecules and is the average thickness of a layer, found by dividing the simulation box height by the number of simulated layers. A is the area per molecule in the plane. and . In all phases, the average unit cell angles and are 90°.
Nearest neighbor state fractions. Typical results for , , and showing the fraction of molecules for which all six nearest neighbors have the same sing, mono, or Potts state. Values for ideal phases are discussed in Sec. II B. The four phases were found using the Flexible Williams force field at the indicated temperatures.
Results for mixed . Same as Table II, but for mixed and .
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