Properties of water in the interfacial region of a polyelectrolyte bilayer adsorbed onto a substrate studied by computer simulations (original) (raw)
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Macromolecules
Extensive all-atom molecular dynamics simulations have been performed to investigate the effect of surface features on the adsorption of poly(styrenesulfonate) (PSS) oligomers on top of a modified graphite substrate. In particular, we have investigated hydrophilic and hydrophobic model surfaces, accompanied by a variable surface charge density σ s in the range 0-0.164 C/m 2 . Our results demonstrate that short-range interactions originating from the adsorbing substrate play a significant role in the layer structure of the adsorbed PSS, and they alone are already sufficient to induce a stable PSS adsorption layer. The presence of additional hydrophilic hydroxyl groups and charges on the adsorbing surface can further enhance the adsorption of PSS sulfonate groups at lower σ s , whereas for the case of σ s = 0.164 C/m 2 , the influence of the surface hydroxyl groups becomes negligible compared to that of the surface charges. The adsorbed PSS chains show mostly conformations where the PSS backbones are approximately parallel to the adsorbing surface. In some case, however, also the PSS backbones stand on top of the surface. Both the obtained surface charge overcompensation and the surface coverage are in good agreement with a previous experimental work [Ahrens et al. Macromolecules 2001, 34, 4504-4512]. Between the first PSS adsorption layer and the adsorbing substrate, we always find a water-rich region. The orientation of the water molecules in this region depends crucially on the features of the adsorbing surface. Our simulations suggest that the water-involved hydrogen bondings play a dominant role in determining the orientation of the water molecules. We also observe a decrease of the dielectric constant of water in the region close to the adsorbing surface in all of the investigated systems that is more pronounced for the hydrophilically modified surface and moreover increases with rising surface charge density. We suggest that this effect could lead to an electrostatic stabilization of the monolayer surface.
In this work, we present the first molecular dynamics simulation on the formation of a polyelectrolyte bilayer resolved in atomistic detail, extending a previous study of the adsorption of poly(styrene sulfonate) [B. Qiao, J. J. Cerda`and C. Holm, Macromolecules, 2011, 44, 1707-1718 to the formation of a poly(styrene sulfonate)/poly(diallyldimethylammonium) bilayer. Extensive molecular dynamics simulations of the adsorption process on different substrates (hydrophilic/ hydrophobic, charged/neutral) were performed. Our results seem to indicate that a high enough surface charge density (0.164 C m À2 here) may be required to achieve a multilayer linear growth in the framework of the electrostatic driven mechanism for PEM growth. We furthermore demonstrate that the use of stiff hydroxyl groups for creating a hydrophilic surface from a hydrophilic one can lead to severe simulation artifacts, and we discuss a simple remedy for this problem.
Journal of Chemical Theory and Computation, 2006
The effect of solvent quality on the behavior of a polyelectrolyte chain near a charged surface is studied using molecular dynamics simulation with explicit solvent. The polyion adsorbs completely on the surface for a high enough surface charge density, and the surface charge required for complete adsorption becomes lower as the solvent quality is decreased. Several static and dynamic properties display a nonmonotonic dependence on surface charge density and solvent quality. For a given value of solvent quality the component of the radius of gyration (R g ) parallel to the surface is a nonmonotonic function of the surface charge density, and for a given surface charge density the component of R g perpendicular to the surface is a nonmonotonic function of the solvent quality. The center-of-mass diffusion coefficient and rotational relaxation time are nonmonotonic functions of the surface charge density. Translational diffusion coefficient increases, and the rotational relaxation time decreases as solvent quality is decreased for a fixed surface charge density.
Macromolecules, 2010
Explicit solvent all-atom molecular dynamics simulations of mixtures of poly(styrenesulfonate)-(PSS) and poly(diallyldimethylammonium) (PDADMA) polyelectrolytes at various salt (NaCl) concentrations are performed. We characterize the formed polyelectrolyte complexes (PECs) and relate the observed physical properties of PSS-PDADMA PECs to the properties found in polyelectrolyte multilayers (PEMs) made of the same compositions. Our results reveal a change in the way charges are compensated upon the addition of salt, namely from an intrinsic mechanism (polyanions pair with polycations) toward an extrinsic one (polyions pair with salt ions). The probability of the intrinsic compensating mechanism decreases from about 90% to about 60% when the salt concentration increases from 0.168 to 1 mol/L. The interaction energies of the ion-pairing follow the order of Na-Cl>PSS-Na>PDADMA-Cl=PSS-PDADMA. Furthermore, we investigate thoroughly the water distribution and study the hydration mechanisms in our system. Water is found to be homogeneously distributed inside our investigated systems, while we find a negligible difference between the hydration ability of (PDADMA þ Cl -) and (PSS þ Na þ ). This lack of asymmetric behavior demonstrates that the observed swelling-shrinking switch during the buildup of PEMs cannot be related to the hydration behavior, and we suggest that the presence of a substrate has to play a critical role. A further analysis of the water structure shows that the dielectric constant inside such mixtures is roughly 1 order of magnitude lower than in bulk water, and our determined values compare favorably with experimental measurements. Finally the diffusion of water molecules inside the PE mixtures is found to be 2 orders of magnitude slower than that in pure water.
NMR Studies of the Effect of Adsorbed Water on Polyelectrolyte Multilayer Films in the Solid State
Macromolecules, 2003
Fast-MAS 1 H NMR is used to probe the structure and dynamics of the adsorbed water and polymer components in polyelectrolyte multilayer (PEM) films and the bulk polyelectrolyte complex (PEC). The films, ranging in size from a single layer to 5 bilayers, consist of poly(diallyl dimethylammonium chloride), PDADMAC, and poly(sodium-4-styrenesulfonate), PSS, adsorbed onto colloidal silica. Relaxation and line width measurements show that the adsorbed water is less mobile in the films than in the analogous PEC. The environment of the water throughout the film is found to be affected by the nature of the outer layer with the water displaying a higher mobility and chemical shift when PDADMAC forms the outer layer. Relaxation measurements, together with 1 H double-quantum (DQ) NMR experiments, reveal that polymer dynamics in the PEMs are strongly influenced by the layer number and water content. 2D spin diffusion and DQ NMR are used to detect polymer-polymer and water-polymer association. The results support the diffuse interpenetrating model of the different layers and a partitioning of the water to the PSS component and to the surface layer.
Interaction of adsorbed polymers with supported cationic bilayers
2013
The interaction forces between bilayers of the cationic surfactant di(tallow ethyl ester)dimethyl ammonium chloride (DEEDMAC) were measured using a Surface Forces Apparatus (SFA) with and without an adsorbing polymer, polyacrylamide (PAM). In the absence of PAM, the forces measured between the bilayer surfaces were purely repulsive on approach and separation and is charge regulated. Addition of PAM induced structural changes to the bilayer interfaces, and resulted in the formation of bilayer-like patches of DEEDMAC decorated PAM (hydrated) on the mica surface. The interaction potential between these surfaces showed a modified DLVO interaction with an additional monotonic steric hydration repulsion on approach with an exponential force decay length of D steric $ 1 nm consistent with the measurements of hydration forces. On separating the surfaces, interdigitated polymers bridge between the two surfaces, resulting in a weak adhesion (adhesion energy, W 0 $ 0.1 mJ m À2 ). Our results provide a picture of the complex molecular structure and interactions between uncharged adsorbing water soluble polymers and supported charged bilayers, and highlight the effects of adsorbing polymers on the structure of bilayers. Implications for the stability of vesicles in dispersions have been also discussed.
Adsorbed monolayer of H-bonded water molecules with and without neutral polymer molecules
Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 1981
A statistical mechanical treatment of a monolayer consisting both of H-bonded solvent molecules adsorbed in an unspecified number of orientations and of polymeric molecules of a neutral solute is provided. The different size of solvent and solute molecules is accounted for using Flory-Huggins statistics, whereas local order within the monolayer is accounted for using the quasi-chemical approximation. The above treatment is applied to a hexagonal array of adsorbed water molecules oriented in such a way as to be in a condition to be singly or doubly H-bonded laterally in the monolayer; a further water orientation characterized by full alignment of the dipole moment in the direction away from the electrode and simulating chemisorbed water monomers is included in the molecular model treatment. An adsorption isotherm is derived upon generalizing the molecular model at hand so as to include the presence of polymeric neutral solute molecules adsorbed in a single orientation. The model accounts satisfactorily for a number of salient features of experimental capacity curves at metal-water interphases in the absence of adsorbed solute species, as well as for the adsorption behaviour of aliphatic compounds on mercury, provided that the doubly H-bonded water molecules are excluded from the molecular model. A justification for this exclusion, based on the existence of H-bonds between the first and second layer of water molecules, is provided.
The Journal of Physical Chemistry C, 2014
First principles molecular dynamics simulations are used to gain an atomistic-level insight into how the molecular behavior of interfacial water is influenced by specific surface adsorbates. Although the overall hydrophobic versus hydrophilic character of a given surface is widely recognized to be important in determining the behavior of interfacial water molecules, we show that subtle molecular details may also play a role in determining the dynamical behavior of water. By comparing water diffusivity at three different non-polar surfaces, we find that specific surface features can lead to a suppression of hydrogen bond network ring structures by enhancing hexagonal spatial distributions of water molecules near the surface. Such a distinct molecular dependent behavior of the interfacial water was found to persist well into the liquid, while the most structural properties are noticeably influenced in only the first water layer. Fig 8. Enumeration of rings in (a) region I and (b) region II at H-, CH3and CF3-Si surfaces
Effect of short-range interactions on polyelectrolyte adsorption at charged surfaces
2003
We have studied the effect of short-range interactions on polyelectrolyte adsorption at oppositely and similarly charged surfaces. The properties of the adsorbed layer, such as polymer surface coverage, layer thickness, and surface overcharging (for adsorption at oppositely charged surfaces), are calculated as a function of the surface charge density, the strength of the short-range interactions, and the ionic strength of the solution. The properties of dilute and semidilute two-dimensional adsorbed layers are calculated in the framework of the strongly correlated Wigner liquid model. In these regimes, the surface overcharging by adsorbed polyelectrolyte chains increases as a function of the square root of the salt concentration. At higher surface charge densities, when adsorbed polyelectrolytes form a three-dimensional adsorbed layer, we use the self-consistent meanfield theory to calculate the layer properties. Here, the polymer surface coverage shows nonmonotonic dependence on the salt concentration; it initially increases as the salt concentration increases at low ionic strengths and then decreases as the ionic strength becomes higher than some critical value. The decrease of the surface coverage at a higher salt concentration is due to additional screening of the surface charge by salt ions. We show that the adsorption of polyelectrolytes at similarly charged surfaces can only occur within the range of surface charge densities where the short-range interactions dominate the electrostatic repulsion between adsorbed chains and the charged surface. In these regimes, the salt dependence of the polymer surface coverage and layer thickness is similar to that for polyelectrolyte adsorption at oppositely charged surfaces that are dominated by short-range interactions. 2. Adsorption of a Polyelectrolyte Chain at a Neutral Surface Consider a flexible polyelectrolyte chain with a degree of polymerization N, a fraction of charged monomers f, and a bond † Part of the special issue "International Symposium on Polyelectrolytes".