Surface pressure molecular dynamics calculations

The interface between the droplet and the gas is not discontinuous the average molecular density decreases over a narrow region from the liquid side to the vapor. When the size of the droplet becomes sufhctently small compared with the thickness of the transition layer, the use of classical thermodynamics and the bulk surface tension become inaccurate the Kelvin relation and Laplace formula no longer apply. This effect has been studied by molecular dynamics calculations of the behavior of liquid droplets composed of 41 to 2(X)4 molecules that interact through a Lennard-Jones (LI) intermolecular potential (Thomp.son et al., 1984). The results of this analysis are shown in Fig. 9.5, in which the nondimensional pressure difference between the drop interior and the surrounding vapor (Pd — p)rr / ij is... 

Figure 9.5 Molecular dynamics calculations of the nondimcnsional surface pressure (difference between pressure inside a drop and the gas) for a Lennard-Jones intermolecular potential. Classical liquid drop theory begins to break down fordroplei radii smallcrihan about 10 times the Lennard-Jones diameter CTij. Calculations for Ar/su = 0.71 and = 0,58. (After Thompson et al, 1984.)...

We think that judicious application of molecular simulation tools for the calculation of thermophysical and mechanical properties is a viable strategy for obtaining some of the information required as input to mesoscale equations of state. Given a validated potential-energy surface, simulations can serve as a complement to experimental data by extending intervals in pressure and temperature for which information is available. Furthermore, in many cases, simulations provide the only realistic means to obtain key properties e.g., for explosives that decompose upon melting, measurement of liquid-state properties is extremely difficult, if not impossible, due to extremely fast reaction rates, which nevertheless correspond to time scales that must be resolved in mesoscale simulations of explosive shock initiation. By contrast, molecular dynamics simulations can provide converged values for those properties on time scales below the chemical reaction induction times. Finally,... 

The purpose of this work is to study the structure and the surface pressure of compressed noncohesive mono-layers consisting of silica nanoparticles at the water-air interface. We employ molecular dynamics computer simulations to analyse the pressure-area (11- 4) isotherms and to show that the use of the hexagonal array of monodisperse particles model can lead to a significant overestimation of the particle-particle (p-p) distances calculated from the H-A isotherms. We also examine the relevance of the rate of compression in terms of the structure formation of the layer and the isotherms. Finally, we consider a collapse mechanism based on the different potential energy of the ejected particles. 

Selected theoretical values of y and a Broughton and Gilmer [ 14] used molecular dynamics to calculate the surface energy and surface stress for a unary system with a P-T diagram of the type in Figure 1.4 for which the atoms were assumed to interact according to a Lennard-Jones potential. The solid phase was assumed to take the fee structure. The results from this simulation for temperatures and pressures near... 

Indeed, nature is much more comphcated. Even systems that show the same Hofmeister series may have different interactions that nnderpin the behaviour. As ion behaviour depends on the environment, it is also difficult to interpret widely different experiments by the same type of interactions. That the same series is found in two experiments is not a proof that the same interactions govern both experiments. This is a frequent mistake. How subtle these interactions can be is illustrated in Chap. 11, where the potentials of mean force from molecular dynamics simulations are taken and inserted into Poisson-Boltzmann calculations. Figure 4 of this chapter demonstrates that the double layer pressure increases in the non-Hofrneister sequence NaCl < Nal < NaBr at large separations (> 1.0 nm). Frequent changes in this sequence occur as the surfaces approach closer to each other, including NaCl < NaBr < Nal and NaCl > NaBr > Nal. Even if the underlying approximations do not fully reflect the real world, this result is far too complicated to be interpreted by simple rules. Note that integral equations such as the hypernetted chain theory are a more powerful alternative to the Poisson-Boltzmann equation, see Chap. 10. 

---