Liquid argon structure

Fig. 13—In-plane structure illustrated by results from simulations of liquid argon charts from (a) through (f) show the probability of particle distribution in different layers across the film, from the place adjacent to the wall stretching to the middle of the film.

In later measurements, Tewari and Freeman (1968,1969) measured the ion mobilities from drift-time measurement and obtained k/u values from the current decay following a pulse of X-rays of 1 ms duration. The purpose was to find the dependence of Gfl on molecular structure. It was found that Gf. increased with the sphericity of the molecule. In liquid argon Gf. 5 was measured, which indicated that all ionized electrons in argon are free. However, this... 

Lack of steady flow of a liquid-bearing colloidal solution requires the existence of a space-filling, three-dimensional structure. As we might select a perfect crystal as a csuionical solid, or liquid argon as a prototypical liquid, we csui choose the covalently crosslinked network, without any entanglements, to represent the ideal gel state. Then an appropriate time scale for reversible gels would be the lifetime of a typical crosslink bond if subjected to conditions that would cause flow in a pure... 

Table 16.1 Comparison of the calculated structure of liquid Argon with the experimental data obtained by X-ray diffraction in the same thermodynamic condition (T = 91.8 K and P = 1.8 atm)...

Fig. 2.5 An example of an experimentally determined structure factor, namely, that for liquid argon near its triple point [4]. (Reproduced from Physical Review A, by permission.)...

We also adopt a similar description for the solvent. This type of model requires some comment, even when applied to the simple solvents such as dense liquid argon or other noble gases. Although the static structural properties of such fluids are represented quite well by taking into account only the strongly repulsive parts of the potential," the weak attractive forces do have noticeable effects on dynamic properties such as the velocity autocorrelation function.However, a model that includes only the repulsive forces is not unreasonable for a description of the solvent dynamics in dense liquids, and this expedient is adopted. We focus on general features that are not expected to be especially sensitive to this approximation. 

The second field in which atomistic simulation has provided an important tool is statistical mechanics. In this case classical trajectories can be used to generate quasi-experimental many-body dynamics from which statistical mechanical theories can be derived or tested. The first use of continuous potentials in this field was the pioneering work of Rahman, who used classical dynamics simulations to probe the structure and many-body dynamics of liquid argon. ... 

Here total means that g(r) pertains to all the diffracting atoms. Application of x-ray diffraction results in the structure of liquid water, in terms of g(Ow-Ow, r), since only the oxygen atoms, but not the hydrogen atoms, diffract x-rays. This function resembles to some extent that of liquid argon, a non-structured liquid by aU accounts (Fisenko et al. 2008), as demonstrated by Marcus (1996). There is, thus, more in the notion of the structure of water than what is measurable by g(r), which is dominated by the strong repulsion of molecules that are too closely packed together. 

In my work on aqueous solutions of inert gases, I have reformulated the iceberg-formation idea. Instead of claiming that argon forms icebergs, I started from the assumption that structure, any structure for that matter, is already there in the pure liquid. Argon does not form any new structure but rather only... 

Fig. 2. Wave number dependence of the empirical function x(k

The time-dependence of structure in the liquid further complicates our grasp of liquid structure. In the solid, the molecule at a given position at a particular time is likely to be the same molecule an hour later. In the liquid, however, the molecules responsible for a peak in G(R) are constantly being replaced by other molecules. The timescale for this replacement is roughly the period for one cycle of the intermolecular vibrations, on the order of a picosecond (10 s) in liquid argon. Therefore, even the short range structure in the liquid fluctuates rapidly. 

In considering the motion of electrons in non-polar liquids one has to take into account the influence of the liquid structure on the elec-tron/atom or electron/molecule scattering process. For liquid argon, Cohen and Lekner (1967) and Lekner (1967) related the cross section in the liquid to the gas-phase cross section for scattering via the structure factor S(0) of the liquid. The introduced two mean free paths, and A, for energy and momentum transfer, respectively. Both mean free paths are related by the structure factor... 

None of what we have discussed so far is specific to polymers. One can almost conclude that every liquid forms a glass on cooling, provided it is cooled fast enough. A cooling rate of 10 K/s vitrifies liquid argon in a computer simulation, and lO K/s produces glassy metallic alloys, whereas polymers can be cooled as slowly as 10 K/s and still do not crystallize. The frozen-in amorphous structure can be kept at (meta-)stable equilibrium even close to Tg and may be as close to the thermodynamic equilibrium... 

Stillinger F 1973 Structure in aqueous solutions from the standpoint of scaled particle theory J. Solution Chem. 2 141 Widom B 1967 Intermolecular forces and the nature of the liquid state Sc/e/ ce 375 157 Longuet-Higgins H C and Widom B 1964 A rigid sphere model for the melting of argon Mol. Phys. 8 549... 

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