13 research outputs found
Two-chamber lattice model for thermodiffusion in polymer solutions
When a temperature gradient is applied to a polymer solution, the polymer
typically migrates to the colder regions of the fluid as a result of thermal
diffusion (Soret effect). However, in recent thermodiffusion experiments on
poly(ethylene-oxide) (PEO) in a mixed ethanol/water solvent it is observed that
for some solvent compositions the polymer migrates to the cold side, while for
other compositions it migrates to the warm side. In order to understand this
behavior, we have developed a two-chamber lattice model approach to investigate
thermodiffusion in dilute polymer solutions. For a short polymer chain in an
incompressible, one-component solvent we obtain exact results for the
partitioning of the polymer between a warm and a cold chamber. In order to
describe mixtures of PEO, ethanol, and water, we have extended this simple
model to account for compressibility and hydrogen bonding between PEO and water
molecules. For this complex system, we obtain approximate results for the
composition in the warmer and cooler chambers that allow us to calculate Soret
coefficients for given temperature, pressure, and solvent composition. The sign
of the Soret coefficient is found to change from negative (polymer enriched in
warmer region) to positive (polymer enriched in cooler region) as the water
content of the solution is increased, in agreement with experimental data. We
also investigate the temperature dependence of the Soret effect and find that a
change in temperature can induce a change in the sign of the Soret coefficient.
We note a close relationship between the solvent quality and the partitioning
of the polymer between the two chambers, which may explain why negative Soret
coefficients for polymers are so rarely observed.Comment: 12 pages, 8 figure
Local and chain dynamics in miscible polymer blends: A Monte Carlo simulation study
Local chain structure and local environment play an important role in the
dynamics of polymer chains in miscible blends. In general, the friction
coefficients that describe the segmental dynamics of the two components in a
blend differ from each other and from those of the pure melts. In this work, we
investigate polymer blend dynamics with Monte Carlo simulations of a
generalized bond-fluctuation model, where differences in the interaction
energies between non-bonded nearest neighbors distinguish the two components of
a blend. Simulations employing only local moves and respecting a non-bond
crossing condition were carried out for blends with a range of compositions,
densities, and chain lengths. The blends investigated here have long-chain
dynamics in the crossover region between Rouse and entangled behavior. In order
to investigate the scaling of the self-diffusion coefficients, characteristic
chain lengths are calculated from the packing length of the
chains. These are combined with a local mobility determined from the
acceptance rate and the effective bond length to yield characteristic
self-diffusion coefficients . We find that the
data for both melts and blends collapse onto a common line in a graph of
reduced diffusion coefficients as a function of reduced chain
length . The composition dependence of dynamic properties is
investigated in detail for melts and blends with chains of length twenty at
three different densities. For these blends, we calculate friction coefficients
from the local mobilities and consider their composition and pressure
dependence. The friction coefficients determined in this way show many of the
characteristics observed in experiments on miscible blends.Comment: 12 pages, 13 figures, editorial change
Transitions of tethered polymer chains: A simulation study with the bond fluctuation lattice model
A polymer chain tethered to a surface may be compact or extended, adsorbed or
desorbed, depending on interactions with the surface and the surrounding
solvent. This leads to a rich phase diagram with a variety of transitions. To
investigate these transitions we have performed Monte Carlo simulations of a
bond-fluctuation model with Wang-Landau and umbrella sampling algorithms in a
two-dimensional state space. The simulations' density of states results have
been evaluated for interaction parameters spanning the range from good to poor
solvent conditions and from repulsive to strongly attractive surfaces. In this
work, we describe the simulation method and present results for the overall
phase behavior and for some of the transitions. For adsorption in good solvent,
we compare with Metropolis Monte Carlo data for the same model and find good
agreement between the results. For the collapse transition, which occurs when
the solvent quality changes from good to poor, we consider two situations
corresponding to three-dimensional (hard surface) and two-dimensional (very
attractive surface) chain conformations, respectively. For the hard surface, we
compare tethered chains with free chains and find very similar behavior for
both types of chains. For the very attractive surface, we find the
two-dimensional chain collapse to be a two-step transition with the same
sequence of transitions that is observed for three-dimensional chains: a
coil-globule transition that changes the overall chain size is followed by a
local rearrangement of chain segments.Comment: 17 pages, 12 figures, to appear in J. Chem. Phy
Configurational contribution to the Soret effect of a protein ligand system
Many of the biological functions of proteins are closely associated with their ability to bind ligands and change conformations in response to changing conditions. Since binding state and conformation of a protein affect its response to a temperature gradient, they may be probed with thermophoresis. In recent years, thermophoretic techniques to investigate biomolecular interactions, quantify ligand binding, and probe conformational changes have become established. To develop a better understanding of the mechanisms underlying the thermophoretic behavior of proteins and ligands, we employ a simple, off-lattice model for a protein and ligand in explicit solvent. To investigate the partitioning of the particles in a temperature gradient, we perform Wang-Landau-type simulations in a divided simulation box and construct the density of states over a two-dimensional state space. This method gives us access to the entropy and energy of the divided system and allows us to estimate the configurational contribution to the Soret coefficient. In this work, we focus on dilute solutions of hydrophobic proteins and investigate the effect of ligand binding on their thermophoretic behavior. We find that our simple model captures important aspects of protein-ligand interactions and allows us to relate the binding energy to the change in Soret coefficient upon ligand binding
Transitions of Tethered Chain Molecules under Tension
An applied tension force changes the equilibrium conformations of a polymer chain tethered to a planar substrate and thus affects the adsorption transition as well as the coil-globule and crystallization transitions. Conversely, solvent quality and surface attraction are reflected in equilibrium force-extension curves that can be measured in experiments. To investigate these effects theoretically, we study tethered chains under tension with Wang-Landau simulations of a bond-fluctuation lattice model. Applying our model to pulling experiments on biological molecules we obtain a good description of experimental data in the intermediate force range, where universal features dominate and finite size effects are small. For tethered chains in poor solvent, we observe the predicted two-phase coexistence at transitions from the globule to stretched conformations and also discover direct transitions from crystalline to stretched conformations. A phase portrait for finite chains constructed by evaluating the density of states for a broad range of solvent conditions and tensions shows how increasing tension leads to a disappearance of the globular phase. For chains in good solvents tethered to hard and attractive surfaces we find the predicted scaling with the chain length in the low-force regime and show that our results are well described by an analytical, independent-bond approximation for the bond-fluctuation model for the highest tensions. Finally, for a hard or slightly attractive surface the stretching of a tethered chain is a conformational change that does not correspond to a phase transition. However, when the surface attraction is sufficient to adsorb a chain it will undergo a desorption transition at a critical value of the applied force. Our results for force-induced desorption show the transition to be discontinuous with partially desorbed conformations in the coexistence region
Partition Function Zeros and Finite Size Scaling for Polymer Adsorption
The zeros of the canonical partition functions for a flexible polymer chain tethered to an attractive flat surface are computed for chains up to length N = 1536. We use a bond-fluctuation model for the polymer and obtain the density of states for the tethered chain by Wang-Landau sampling. The partition function zeros in the complex e(β)-plane are symmetric about the real axis and densest in a boundary region that has the shape of a nearly closed circle, centered at the origin, terminated by two flaring tails. This structure defines a root-free zone about the positive real axis and follows Yang-Lee theory. As the chain length increases, the base of each tail moves toward the real axis, converging on the phase-transition point in the thermodynamic limit. We apply finite-size scaling theory of partition-function zeros and show that the crossover exponent defined through the leading zero is identical to the standard polymer adsorption crossover exponent ϕ. Scaling analysis of the leading zeros locates the polymer adsorption transition in the thermodynamic (N → ∞) limit at reduced temperature Tc (*)=1.027(3) [βc=1/Tc (*)=0.974(3)] with crossover exponent ϕ = 0.515(25). Critical exponents for the order parameter and specific heat are determined to be β̃=0.97(5) and α = 0.03(4), respectively. A universal scaling function for the average number of surface contacts is also constructed
Thermodiffusion of aqueous solutions of various potassium salts
Thermophoresis or thermodiffusion has become an important tool to monitor protein-ligand binding as it is very sensitive to the nature of solute-water interactions. However, the microscopic mechanisms underlying thermodiffusion in protein systems are poorly understood at this time. One reason is the difficulty to separate the effects of the protein system of interest from the effects of buffers that are added to stabilize the proteins. Due to the buffers, typical protein solutions form multicomponent mixtures with several kinds of salt. To achieve a more fundamental understanding of thermodiffusion of proteins, it is therefore necessary to investigate solutions of buffer salts. For this work, the thermodiffusion of aqueous potassium salt solutions has been studied systematically. We use thermal diffusion forced Rayleigh scattering experiments in a temperature range from 15 degrees C to 45 degrees C to investigate the thermodiffusive properties of aqueous solutions of five potassium salts: potassium chloride, potassium bromide, potassium thiocyanate, potassium acetate, and potassium carbonate in a molality range between 1 mol/kg and 5 mol/kg. We compare the thermophoretic results with those obtained for non-ionic solutes and discuss the thermophoresis of the salts in the context of ion-specific solvation according to the Hofmeister series
Sign change of the Soret coefficient of poly(ethylene oxide) in water/ethanol mixtures observed by thermal diffusion forced Rayleigh scattering
Soret coefficients of the ternary system of poly(ethylene oxide) in mixed water/ethanol solvent were measured over a wide solvent composition range by means of thermal diffusion forced Rayleigh scattering. The Soret coefficient S(T) of the polymer was found to change sign as the water content of the solvent increases with the sign change taking place at a water mass fraction of 0.83 at a temperature of 22 degrees C. For high water concentrations, the value of S(T) of poly(ethylene oxide) is positive, i.e., the polymer migrates to the cooler regions of the fluid, as is typical for polymers in good solvents. For low water content, on the other hand, the Soret coefficient of the polymer is negative, i.e., the polymer migrates to the warmer regions of the fluid. Measurements for two different polymer concentrations showed a larger magnitude of the Soret coefficient for the smaller polymer concentration. The temperature dependence of the Soret coefficient was investigated for water-rich polymer solutions and revealed a sign change from negative to positive as the temperature is increased. Thermodiffusion experiments were also performed on the binary mixture water/ethanol. For the binary mixtures, the Soret coefficient of water was observed to change sign at a water mass fraction of 0.71. This is in agreement with experimental results from the literature. Our results show that specific interactions (hydrogen bonds) between solvent molecules and between polymer and solvent molecules play an important role in thermodiffusion for this system