Liquids molecular motions

T. A. Litovitz and C. J. Montrose. Interaction Induced Light Scattering in Liquids Molecular Motion and Structural Relaxation. In J. van Kranendonk (ed.), Intermolecular Spectroscopy and Dynamical Properties of Dense Systems—Proceedings of the International School of Physics Enrico Fermi, Course LXXV, North-Holland, Amsterdam, 1980, pp. 307-324. 

The most important principles in NMR investigation of adsorption layers at solid/liquid interfaces are the averaging of interactions and the relaxation of spins as measures of segmental mobility, while only in a few systems spectral shifts were analysed. As mentioned before, the motional state of surface adsorbed molecules can range from solid to liquid type behaviour, determining the NMR methods applicable to the system. The difference between the solid or liquid state in NMR is established by anisotropic interactions which cause broad spectra for solids, whereas in liquids molecular motions are averaging this anisotropy in time and are causing narrow spectra. 

The principal dilTerence from liquid-state NMR is that the interactions which are averaged by molecular motion on the NMR timescale in liquids lead, because of their anisotropic nature, to much wider lines in solids. Extra infonnation is, in principle, available but is often masked by the lower resolution. Thus, many of the teclmiques developed for liquid-state NMR are not currently feasible in the solid state. Furthemiore, the increased linewidth and the methods used to achieve high resolution put more demands on the spectrometer. Nevertheless, the field of solid-state NMR is advancing rapidly, with a steady stream of new experiments forthcoming. 

Turning from chemical exchange to nuclear relaxation time measurements, the field of NMR offers many good examples of chemical information from T, measurements. Recall from Fig. 4-7 that Ti is reciprocally related to Tc, the correlation time, for high-frequency relaxation modes. For small- to medium-size molecules in the liquid phase, T, lies to the left side of the minimum in Fig. 4-7. A larger value of T, is, therefore, associated with a smaller Tc, hence, with a more rapid rate of molecular motion. It is possible to measure Ti for individual carbon atoms in a molecule, and such results provide detailed information on the local motion of atoms or groups of atoms. Levy and Nelson " have reviewed these observations. A few examples are shown here. T, values (in seconds) are noted for individual carbon atoms. 

An important experimental quantity for studying molecular interactions in gases and liquids is the scattering of laser light. When polarized light is scattered by a fluid, both polarized and depolarized components are produced. The depolarized spectrum is several orders of magnitude less intense than the polarized spectrum and much more difficult to observe. A great deal of information has been obtained about molecular motions from such spectral analyses. 

So far, there have been few published simulation studies of room-temperature ionic liquids, although a number of groups have started programs in this area. Simulations of molecular liquids have been common for thirty years and have proven important in clarifying our understanding of molecular motion, local stmcture and thermodynamics of neat liquids, solutions and more complex systems at the molecular level [1 ]. There have also been many simulations of molten salts with atomic ions [5]. Room-temperature ionic liquids have polyatomic ions and so combine properties of both molecular liquids and simple molten salts. 

Marabella L. J. Molecular motion and band shapes in liquids, Appl. Spectr. Rev., 7, 313-55 (1974). 

Breuillard C., Ouillon R. Infrared and Raman band shapes and dynamics of molecular motions for N20 in solutions v3 band in CCL and liquid SF6. Mol. Phys. 33, 747-57 (1977). 

Huong P., Couzi M., Perrot M. Molecular motions of hydrogen halides and deuterium halides in liquid sulfur hexafluoride, Chem. Phys. Lett. 7, 189-90 (1970). 

Surface tension accounts for a number of everyday phenomena. For example, a droplet of liquid suspended in air or on a waxy surface is spherical because the surface tension pulls the molecules into the most compact shape, a sphere (Fig. 5.14). The attractive forces between water molecules are greater than those between water and wax, which is largely hydrocarbon. Surface tension decreases as the temperature rises and the interactions between molecules are overcome by the increased molecular motion. 

According to free-volume interpretations, the rate of molecular motions is governed entirely by the available unoccupied space ( free volume ). Early studies of molecular liquids led to the Doolittle equation, relating the viscosity to the fractional free volume, / [23,24]... 

The dramatic slowing down of molecular motions is seen explicitly in a vast area of different probes of liquid local structures. Slow motion is evident in viscosity, dielectric relaxation, frequency-dependent ionic conductance, and in the speed of crystallization itself. In all cases, the temperature dependence of the generic relaxation time obeys to a reasonable, but not perfect, approximation the empirical Vogel-Fulcher law ... 

The diffusion of one liquid into another also demonstrates molecular motion. Figure 2 shows that if a drop of ink is added to a beaker of still water, the color slowly but surely spreads throughout the water. The water molecules and the molecules that give ink its color move continuously. As they slide by one another, the ink molecules eventually become distributed uniformly throughout the volume of liquid. 

Let us now turn our attention to liquid water. Just as in ice I, molecular motions may be divided into rapid vibrations and slower diffusional motions. In the liquid, however, vibrations are not centred on essentially fixed lattice sites, but around temporary equilibrium positions that are themselves subject to movement. Water at any instant may thus be considered to have an I-structure. An instant later, this I-structure will be modified as a result of vibrations, but not by any additional displacements of the molecules. This, together with the first I-structure, is one of the structures that may be averaged to allow for vibration, thereby contributing to the V-structure. Lastly, if we consider the structure around an individual water molecule over a long time-period, and realize that there is always some order in the arrangement of adjacent molecules in a liquid even over a reasonable duration, then we have the diffusionally averaged D-structure. 

The polymer at the gel point is in a critical state [3], and the name critical gel [4] is appropriate for distinguishing polymers at the gel point from the various materials which commonly are called gels. The critical gel has no intrinsic size scale except for the size of its oligomeric building block, and molecular motions are correlated over large distances. The combination of liquid and solid... 

For liquids, as the temperature increases, the degree of molecular motion increases, reducing the short-range attractive forces between molecules and lowering the viscosity. The viscosity of various liquids is shown as a function of temperature in Appendix A. For many liquids, this temperature dependence can be represented reasonably well by the Arrhenius equation ... 

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