16 research outputs found
Solvation thermodynamics and heat capacity of polar and charged solutes in water
The solvation thermodynamics and in particular the solvation heat capacity of
polar and charged solutes in water is studied using atomistic molecular
dynamics simulations. As ionic solutes we consider a F− and a Na+ ion, as an
example for a polar molecule with vanishing net charge we take a SPC/E water
molecule. The partial charges of all three solutes are varied in a wide range
by a scaling factor. Using a recently introduced method for the accurate
determination of the solvation free energy of polar solutes, we determine the
free energy, entropy, enthalpy, and heat capacity of the three different
solutes as a function of temperature and partial solute charge. We find that
the sum of the solvation heat capacities of the Na+ and F− ions is negative,
in agreement with experimental observations, but our results uncover a
pronounced difference in the heat capacity between positively and negatively
charged groups. While the solvation heat capacity ΔCp stays positive and even
increases slightly upon charging the Na + ion, it decreases upon charging the
F− ion and becomes negative beyond an ion charge of q = −0.3e. On the other
hand, the heat capacity of the overall charge-neutral polar solute derived
from a SPC/E water molecule is positive for all charge scaling factors
considered by us. This means that the heat capacity of a wide class of polar
solutes with vanishing net charge is positive. The common ascription of
negative heat capacities to polar chemical groups might arise from the neglect
of non-additive interaction effects between polar and apolar groups. The
reason behind this non-additivity is suggested to be related to the second
solvation shell that significantly affects the solvation thermodynamics and
due to its large spatial extent induces quite long-ranged interactions between
solvated molecular parts and groups
The spontaneous curvature of the water-hydrophobe interface
The temperature-dependent solvation of hydrophobic solutes in water is
investigated by largescale molecular dynamics simulations. A simultaneous fit
of solvation free energies for spheres and cylinders with radii up to R = 2 nm
yields a negative Tolman length on the order of 1 Å at room temperature,
equivalent to a spontaneous curvature that favors water droplets over
cavities. Pronounced crossover effects of the surface free energy are analyzed
in terms of higherorder curvature corrections and water-discreteness effects
Charge/mass dynamic structure factors of water and applications to dielectric friction and electroacoustic conversion
We determine time correlation functions and dynamic structure factors of the
number and charge density of liquid water from molecular dynamics simulations.
Using these correlation functions we consider dielectric friction and electro
acoustic coupling effects via linear response theory. From charge-charge
correlations, the drag force on a moving point charge is derived and found to
be maximal at a velocity of around 300 m/s. Strong deviations in the resulting
friction coefficients from approximate theory employing a single Debye
relaxation mode are found that are due to non-Debyelike resonances at high
frequencies. From charge-mass cross-correlations the ultrasonic vibration
potential is derived, which characterizes the conversion of acoustic waves
into electric time-varying potentials. Along the dispersion relation for
normal sound waves in water, the ultrasonic vibration potential is shown to
strongly vary and to increase for larger wavelengths
Entropy and enthalpy convergence of hydrophobic solvation beyond the hard-sphere limit
The experimentally well-known convergence of solvation entropies and enthalpies of different small hydrophobic solutes at universal temperatures seems to indicate that hydrophobic solvation is dominated by universal water features and not so much by solute specifics. The reported convergence of the denaturing entropy of a group of different proteins at roughly the same temperature as hydrophobic solutes was consequently argued to indicate that the denaturing entropy of proteins is dominated by the hydrophobic effect and used to estimate the hydrophobic contribution to protein stability. However, this appealing picture was subsequently questioned since the initially claimed universal convergence of denaturing entropies holds only for a small subset of proteins; for a larger data collection no convergence is seen. We report extensive simulation results for the solvation of small spherical solutes in explicit water with varying solute-water potentials. We show that convergence of solvation properties for solutes of different radii exists but that the convergence temperatures depend sensitively on solute-water potential features such as stiffness of the repulsive part and attraction strength, not so much on the attraction range. Accordingly, convergence of solvation properties is only expected for solutes of a homologous series that differ in the number of one species of subunits (which attests to the additivity of solvation properties) or solutes that are characterized by similar solute–water interaction potentials. In contrast, for peptides that arguably consist of multiple groups with widely disperse interactions with water, it means that thermodynamic convergence at a universal temperature cannot be expected, in general, in agreement with experimental results
Ultralow Liquid/Solid Friction in Carbon Nanotubes: Comprehensive Theory for Alcohols, Alkanes, OMCTS, and Water
International audienceIn this work, we perform a theoretical study of liquid flow in graphitic nanopores of different sizes and geometries. Molecular dynamics flow simulations of different liquids (water, decane, ethanol, and OMCTS) in carbon nanotubes (CNT) are shown to exhibit flow velocities 1–3 orders of magnitude higher than those predicted from the continuum hydrodynamics framework and the no-slip boundary condition. These results support previous experimental findings obtained by several groups that reported exceptionally high liquid flow rates in CNT membranes. The liquid/graphite friction coefficient is identified as the crucial parameter for this fast mass transport in CNT. The friction coefficient is found to be very sensitive to wall curvature: friction is independent of confinement for liquids between flat graphene walls with zero curvature, whereas it decreases with increasing positive curvature (liquid inside CNT), and it increases with increasing negative curvature (liquid outside CNT). Furthermore, we present a theoretical approximate expression for the friction coefficient, which predicts qualitatively and semiquantitatively its curvature dependent behavior. The proposed theoretical description, which works well for different kinds of liquids (alcohols, alkanes, and water), sheds light on the physical mechanisms at the origin of the ultra low liquid/solid friction in CNT. In fact, it is due to their perfectly ordered molecular structure and their atomically smooth surface that carbon nanotubes are quasiperfect liquid conductors compared to other membrane pores like nanochannels in amorphous silica
Molecular Origin of Fast Water Transport in Carbon Nanotube Membranes: Superlubricity versus Curvature Dependent Friction
International audienceIn this paper, we study the interfacial friction of water at graphitic interfaces with various topologies, water between planar graphene sheets, inside and outside carbon nanotubes, with the goal to disentangle confinement and curvature effects on friction. We show that the friction coefficient exhibits a strong curvature dependence; while friction is independent of confinement for the graphene slab, it decreases with carbon nanotube radius for water inside, but increases for water outside. As a paradigm the friction coefficient is found to vanish below a threshold diameter for armchair nanotubes. Using a statistical description of the interfacial friction, we highlight here a structural origin of this curvature dependence, mainly associated with a curvature-induced incommensurability between the water and carbon structures. These results support the recent experiments reporting fast transport of water in nanometric carbon nanotube membranes
Sedentary behaviour in relation to ovarian cancer risk: a systematic review and meta-analysis
Sedentary behaviour is an emerging risk factor for several site-specific cancers. Ovarian cancers are often detected at late disease stages and the role of sedentary behaviour as a modifiable risk factor potentially contributing to ovarian cancer risk has not been extensively examined. We systematically searched relevant databases from inception to February 2020 for eligible publications dealing with sedentary behaviour in relation to ovarian cancer risk. We conducted a systematic review and meta-analysis, calculating summary relative risks (RR) and 95% confidence intervals (CI) using a random-effects model. We calculated the E-Value, a sensitivity analysis for unmeasured confounding. We tested for publication bias and heterogeneity. Seven studies (three prospective cohort studies and four case–control studies) including 2060 ovarian cancer cases were analysed. Comparing highest versus lowest levels of sedentary behaviour, the data indicated a statistically significant increase in the risk of ovarian cancer in relation to prolonged sitting time (RR = 1.29, 95% CI = 1.07–1.57). Sub-analyses of prospective cohort studies (RR = 1.33, 95% CI = 0.92–1.93) and case–control studies (RR = 1.28, 95% CI = 0.98–1.68) showed statistically non-significant results. Sensitivity analysis showed that an unmeasured confounder would need to be related to sedentary behaviour and ovarian cancer with a RR of 1.90 to fully explain away the observed RR of 1.29. Our analyses showed a statistically significant positive association between sedentary behaviour and ovarian cancer risk
Ultralow Liquid/Solid Friction in Carbon Nanotubes: Comprehensive Theory for Alcohols, Alkanes, OMCTS, and Water
In this work, we perform a theoretical study of liquid
flow in
graphitic nanopores of different sizes and geometries. Molecular dynamics
flow simulations of different liquids (water, decane, ethanol, and
OMCTS) in carbon nanotubes (CNT) are shown to exhibit flow velocities
1–3 orders of magnitude higher than those predicted from the
continuum hydrodynamics framework and the no-slip boundary condition.
These results support previous experimental findings obtained by several
groups that reported exceptionally high liquid flow rates in CNT membranes.
The liquid/graphite friction coefficient is identified as the crucial
parameter for this fast mass transport in CNT. The friction coefficient
is found to be very sensitive to wall curvature: friction is independent
of confinement for liquids between flat graphene walls with zero curvature,
whereas it decreases with increasing positive curvature (liquid inside
CNT), and it increases with increasing negative curvature (liquid
outside CNT). Furthermore, we present a theoretical approximate expression
for the friction coefficient, which predicts qualitatively and semiquantitatively
its curvature dependent behavior. The proposed theoretical description,
which works well for different kinds of liquids (alcohols, alkanes,
and water), sheds light on the physical mechanisms at the origin of
the ultra low liquid/solid friction in CNT. In fact, it is due to
their perfectly ordered molecular structure and their atomically smooth
surface that carbon nanotubes are quasiperfect liquid conductors compared
to other membrane pores like nanochannels in amorphous silica