Intramolecular Diffusive Motion in Alkane Monolayers Studied by High-Resolution Quasielastic Neutron Scattering and Molecular Dynamics Simulations

Molecular dynamics simulations of a tetracosane ($n\mathrm{\text{−}}{\mathrm{C}}_{24}{\mathrm{H}}_{50}$) monolayer adsorbed on a graphite basal-plane surface show that there are diffusive motions associated with the creation and annihilation of gauche defects occurring on a time scale of $\sim 0.1–4\mathrm{n}\mathrm{s}$. We present evidence that these relatively slow motions are observable by high-energy-resolution quasielastic neutron scattering (QNS) thus demonstrating QNS as a technique, complementary to nuclear magnetic resonance, for studying conformational dynamics on a nanosecond time scale in molecular monolayers.

Figure 1

Top view showing configuration of C24 molecules in the MD simulation cell: “smectic” phase (left); fluid phase (right).

Figure 2

The intermediate self-scattering function for the rigid rotational and intramolecular motions in the C24 monolayer calculated from the MD simulation at various temperatures: (a) 260 K; (b) 330 K (which is 10 K below the monolayer melting transition); and (c) at 330 K but with stiffened chains to inhibit formation of gauche defects.

Figure 3

Results of fitting the quasielastic spectra at low temperatures: the energy width of the Lorentzian component (half width at half maximum) is plotted versus wave vector transfer $Q$.

Figure 4

Results of fitting the quasielastic spectra at higher temperatures: (a) the energy width of the Lorentzian component vs wave vector transfer $Q$; (b) the intensity of the Lorentzian component vs $Q$. In (a), the dashed line indicates values calculated from the MD simulations at 330 K.