Bulk liquid-state studies

We present a preliminary study on the structural dynamics of photo-excited iodine in methanol. At early time delays after dissociation, 1 - 10 ns, the change in the diffracted intensity AS(q, t) is oscillatory and the high-q part 4 -8 A 1 is assigned to free iodine atoms. At later times, 10-100 ns, expansive motion is seen in the bulk liquid. The expansion is driven by energy released from the recombination of iodine atoms. The AS(q, t) curves between 0.1 and 5 (is coincide with the temperature differential dS/dT for static methanol with a temperature rise of 2.5 K. However, this temperature is five times greater than the temperature deduced from the energy of dissociated atoms at 1 ns. The discrepancy is ascribed to a short-lived state that recombines on the sub-nanosecond time scale. 

In a first approximation a pseudo-first order reaction rate is often assumed. This must be checked against what really happens in the reactor. In semi-batch or nonsteady state oxidation, the concentration of the pollutants as well as the oxidants can change over time. A common scenario initially a fast reaction of ozone with the pollutants occurs, the reaction is probably mass transfer limited, the direct reaction in the liquid film dominates, and no dissolved ozone is present in the bulk liquid. As the concentration of the pollutants decreases, the reaction rate decreases, less ozone is consumed, leading to an increase in the dissolved ozone concentration. Metabolites less reactive with ozone are usually produced. This combined with an increase in dissolved ozone, may also shift the removal mechanism from the direct to the indirect if radical chain processes are initiated and promoted (see Chapter A 2). These changes are often not observed in waste water studies, mostly because dissolved ozone is often not measured. 

The molecular theory of surface tension was dealt with by Laplace (1749-1827). But, as a result of the clarification of the nature, of intermolecular forces by quantum mechanics and of the more recent developments in the study of molecular distribution in liquids, the nature and value of surface tension have been better understood from a molecular viewpoint. Surface tension is closely associated with a sudden, but continuous change in the density from the value for bulk liquid to the value for die gaseous state in traversing the surface. See Fig. 2. As a result of this inhomogeneity, the stress across a strip parallel to the boundary—pu per unit area—is different from that across a strip perpendicular to die boundary—pr per unit area. This is in contrast with die case of homogeneous fluid in which the stress across any elementary plane has the same value regardless of the direction of die plane,... 

The surface of an ionic crystal is structurally different from that of the bulk solid. Weil 24> contends that the surface extends for ca. 10,000 atom layers this means that the surface of a crystal, as examined by attenuated reflection spectroscopy, should show signs of this difference. Cooney et al. 25> took this idea up, and used the p2-mode of N03 in LiN03 and NaN03 to detect this effect the surface depth found is far greater than that predicted theoretically, and is dependent upon temperature (up to the melting point). Molten nitrates are studied very much in order to determine what the nature of the liquid state is, e.g. Devlin, James and Freeh 26) and references therein, James and Leony 27>, and Brooker 2 . 

The pH jump technique with picosecond fluorescence measurements were used to study apomyoglobin and the anionic specific channel, porin of E. coli [151]. The results indicated that the water in the sites deviates markedly from the liquid state in the bulk, having a lower dielectric constant and smaller diffusivity of protons. 

The liquid-liquid interface formed between two immissible liquids is an extremely thin mixed-liquid state with about one nanometer thickness, in which the properties such as cohesive energy density, electrical potential, dielectric constant, and viscosity are drastically changing from those of bulk phases. Solute molecules adsorbed at the interface can behave like a 2D gas, liquid, or solid depending on the interfacial pressure, or interfacial concentration. But microscopically, the interfacial molecules exhibit local inhomogeneity. Therefore, various specific chemical phenomena, which are rarely observed in bulk liquid phases, can be observed at liquid-liquid interfaces [1-3]. However, the nature of the liquid-liquid interface and its chemical function are still less understood. These situations are mainly due to the lack of experimental methods required for the determination of the chemical species adsorbed at the interface and for the measurement of chemical reaction rates at the interface [4,5]. Recently, some new methods were invented in our laboratory [6], which brought a breakthrough in the study of interfacial reactions. 

One of the fundamental problems in chemistry is understanding at the molecular level the effect of the medium on the rate and the equilibrium of chemical reactions which occur in bulk liquids and at surfaces. Recent advances in experimental techniques[l], such as frequency and time-resolved spectroscopy, and in theoretical methods[2,3], such as statistical mechanics of the liquid state and computer simulations, have contributed significantly to our understanding of chemical reactivity in bulk liquids[4] and at solid interfaces. These techniques are also beginning to be applied to the study of equilibrium and dynamics at liquid interfaces[5]. The purpose of this chapter is to review the progress in the application of molecular dynamics computer simulations to understanding chemical reactions at the interface between two immiscible liquids and at the liquid/vapor interface. 

Although capillary condensation theory has devoted to the determination of pore size distribution of mesopores, adsorption studies on regular mesoporous silica such as MCM-41 [1,2] or FSM [3,4] pointed that classical capillary condensation theory cannot explain the dependence of the adsorption hysteresis on the pore width. Also we have assumed that condensed states in mesopores have the same as bulk liquid. In case of molecules adsorbed in... 

Much work has recently been carried out to quantify the three-body and many-body interactions in small clusters (mostly rare gases and water), which have implications on the liquid state properties, however here we consider some studies that have directly determined their influence on the bulk fluid properties. In reference87 the significant influence of three-body interactions on properties of rare gas fluids is discussed, and a recent manuscript by Szalewicz et al,107 thoroughly reviews the importance of many-body forces in general. Here we just summarise some important recent results. 

Relationships between the structure and reactivity or adsorptivity of organic compounds in the catalytic hydrogenation in the liquid state have been studied, with a few exceptions (73, 76), in systems with solvents. The solvent is used in a great excess as a rule, so that the properties of the bulk phase are practically determined by its own properties. 

Molecular dynamics (MD) computations of coalescence have been made for silicon nanoparticles ranging in size from. 30 to 480 atoms, corresponding to a maximum diameter smaller than. 3 nm (Zachariah and Cairier, 1999), The compulations were based on an interatomic potential developed for silicon atoms with covalent bonding. The particle structure was assumed to be amorphous. The MD simulations indicate that the transition between solid and liquid-state behavior occurs over a wide temperature range significantly lower than the melting point of bulk silicon (1740 K), a well-known effect for nanoparticics (Chapter 9), The broadest transition occurred for the smallest particles studied (30 atoms), probably becau.se the surface atoms make up a large fraction of the particle mass. 

Quite recently, Lee and Strandburg have studied the finite size scaling behavior of the bulk free energy barrier between the solid and liquid states in the hard disk system, and have obtained the first unambiguous evidence for a first-order melting transition [1]. 

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