Liquid-Liquid Phase Transition in Supercooled H2O and D2O: A Path-Integral Computer Simulation Study

We perform path-integral molecular dynamics (PIMD) and classical MD simulations of H2O and D2O using the q-TIP4P/F water model over a wide range of temperatures and pressures. The density ρ(T), isothermal compressibility κT(T), and self-diffusion coefficients D(T) of H2O and D2O are in excellent agreement with available experimental data; the isobaric heat capacity CP(T) obtained from PIMD and MD simulations agree qualitatively well with the experiments. Some of these thermodynamic properties exhibit anomalous maxima upon isobaric cooling, consistent with recent experiments and with the possibility that H2O and D2O exhibit a liquid-liquid critical point (LLCP) at low temperatures and positive pressures. The data from PIMD/MD for H2O and D2O can be fitted remarkably well using the Two-State-Equation-of-State (TSEOS). Using the TSEOS, we estimate that the LLCP for q-TIP4P/F H2O, from PIMD simulations, is located at Pc = 167±9 MPa, Tc = 159±6 K, and ρc = 1.02±0.01 g/cm3. Isotope substitution effects are important; the LLCP location in q-TIP4P/F D2O is estimated to be Pc = 176 ± 4 MPa, Tc = 177 ± 2 K, and ρc = 1.13±0.01 g/cm3. Interestingly, for the water model studied, differences in the LLCP location from PIMD and MD simulations suggest that nuclear quantum effects (i.e., atoms delocalization) play an important role in the thermodynamics of water around the LLCP (from the MD simulations of q-TIP4P/F water, Pc = 203 ± 4 MPa, Tc = 175 ± 2 K, and ρc = 1.03 ± 0.01 g/cm3). Overall, our results strongly support the LLPT scenario to explain water anomalous behavior, independently of the fundamental differences between classical MD and PIMD techniques. The reported values of Tc for D2O and, particularly, H2O suggest that improved water models are needed for the study of supercooled water.

Hormones Nanofiltration in Carbon Nanotubes and Boron Nitride Nanotubes Using Uniform External Electric Field Through Molecular Dynamics

Hormones are a dangerous group of molecules that can cause harm to humans. This study based on classical molecular dynamics proposes the nanofiltration of wastewater contaminated by hormones from a computer simulation study, in which the water and the hormone were filtered in two single-walled nanotube compositions. The calculations were carried out by changing the intensities of the electric field that acted as a force exerting pressure on the filtration along the nanotube, in the simulation time of 100 ps. The hormones studied were estrone, estradiol, estriol, progesterone, ethinylestradiol, diethylbestrol, and levonorgestrel in carbon nanotubes (CNTs) and boron nitride (BNNTs). The most efficient nanofiltrations were for fields with low intensities in the order of 10-8 au and 10-7 au. The studied nanotubes can be used in membranes for nanofiltration in water treatment plants due to the evanescent field potential caused by the action of the electric field inside. Our data showed that the action of EF in conjunction with the van der Walls forces of the nanotubes is sufficient to generate the attractive potential. Evaluating the transport of water molecules in CNTs and BNNTs, under the influence of the electric field, a sequence of simulations with the same boundary conditions was carried out, seeking to know the percentage of water molecules filtered in the nanotubes.

Biarticular Hamstring Muscles Can Act as Knee Extensors – A Computer Simulation Study

Purpose: It is unclear whether biarticular hamstring muscles (HAM) can act as knee extensors or not. The purpose of this study is to identify the conditions that HAM can act as a knee extensor by using a computational simulation approach.Methods: The modified Gait2392 musculoskeletal model was used in this study. The posture was determined with a hip flexion angle that ranged from -30° to 90° and a knee flexion angle that ranged from -10 ° to 90 °. The simulations were executed under two conditions: all segments are free to move, non-contact with the ground (nCG), and the foot is constrained on the ground, contact with the ground (CG). Induced acceleration analysis was applied to determine the contribution of the HAM to the knee angular acceleration.Results: Three key findings were discovered. 1) HAM can act as knee extensors that have CG condition as well as nCG condition. 2) The HAM function changes depending on the posture. 3) The range of the hip joint that HAM was able to act as a knee extensor was expanded for the CG condition from the nCG condition.Conclusions: We identified the situations in which HAM can act as knee extensors and demonstrated that the HAM function on the knee joint changes depending on the posture and the foot contact condition. Our findings suggest that HAM can be used as compensatory movement strategy for patients with a reduced capacity to generate knee extension if the patients have enough HAM strength.