3 research outputs found
Adsorption of a Polyelectrolyte Chain at Dielectric Surfaces: Effects of Surface Charge Distribution and Relative Dielectric Permittivity
The characteristics of a surface, such as surface charge
distribution
and permittivity, play significant roles in polyelectrolyte (PE) adsorption.
However, systematic studies of these effects are rare in the literature.
In this work, we employ a local lattice Monte Carlo algorithm to investigate
the effects of dielectric contrast, charge fraction, and surface charge
density on the adsorption behavior of a PE chain onto surfaces with
different charge distributions. We consider three surface charge distributions:
uniform (smeared), regular (periodic), and random. For the same total
surface charge, the random charge distribution results in the strongest
PE adsorption, while the uniform distribution shows the weakest. In
the absence of dielectric contrast, the adsorption behaviors of a
PE near the regularly charged surface are similar to those near the
uniformly charged surface. In the presence of dielectric contrast,
the image repulsion inhibits PE adsorption onto the uniformly charged
surface. Surprisingly, surfaces with discrete charge distributions
(regular and random) exhibit enhanced adsorption compared to that
of the case with no image charge. In addition, the competition between
image charge repulsion and electrostatic attraction from the surface
results in nonmonotonic variation of the adsorbed amount with the
PE charge fraction
Formulation-Controlled Positive and Negative First Normal Stress Differences in Waterborne Hydrophobically Modified Ethylene Oxide Urethane (HEUR)-Latex Suspensions
Hydrophobically modified ethylene
oxide urethane (HEUR) associative
thickeners are widely used to modify the rheology of waterborne paints.
Understanding the normal stress behavior of the HEUR-based paints
under high shear is critical for many applications such as brush drag
and spreading. We observed that the first normal stress difference, <i>N</i><sub>1</sub>, at high shear (large Weissenberg number)
can be positive or negative depending on the HEUR hydrophobe strength
and concentration. We propose that the algebraic sign of the <i>N</i><sub>1</sub> is primarily controlled by two factors: (a)
adsorption of HEURs on the latex surface and (b) the ability of HEURs
to form transient molecular bridges between latex particles. Such
transient bridges are favored for dispersions with small interparticle
distances and dense surface coverages; in these systems; HEUR-bridged
latex microstructures flow-align in high shear and exhibit positive <i>N</i><sub>1</sub>. In the absence of transient bridges (large
interparticle distances, low surface coverage), the dispersion rheology
is similar to that of weakly interacting spheres, exhibiting negative <i>N</i><sub>1</sub>. The results are summarized in a simplified
phase diagram connecting formulation, microstructure, and the <i>N</i><sub>1</sub> behavior
Modeling the Adsorption of Rheology Modifiers onto Latex Particles Using Coarse-Grained Molecular Dynamics (CG-MD) and Self-Consistent Field Theory (SCFT)
We model the adsorption of hydrophobically
ethoxylated urethane (HEUR) thickeners onto two hydrophobic surfaces
separated by a 50 nm gallery using coarse-grained molecular dynamics
(CG-MD) with implicit solvent and three-dimensional self-consistent
field theory (SCFT) with explicit solvent. The CG-MD simulations can
be readily extended to encompass very long HEUR chains (up to 540
EO groups) but cannot with current computer speed simulate adsorption
of HEURs with hydrophobes longer than 12 carbons (C12). The SCFT method
can readily simulate HEURs with longer, C16, hydrophobes but has a
greater challenge simulating very long EO chains. For HEURs with 180
EO units and C8 and C12 hydrophobes, both methods can be applied,
allowing a combination of the two methods to span much of the parameter
space of interest to experimentalists. It is demonstrated that depending
on the hydrophobe strength and the HEUR concentration, HEUR chains
can adsorb to the surfaces directly or indirectly (as adsorbed micelles
or admicelles). We show that for hydrophobes as large or larger than
C12 micellization and subsequent adsorption of the micelles play an
important role in accurate prediction of adsorption isotherms and
the structure of adsorbed layers and that micelles in solution form
nodes that allow two or more HEUR chains to bridge the gallery between
the two surfaces. The study suggests the need to investigate the influence
of admicelles on the effective steric interaction potential, which,
in turn, will influence both colloidal stability and rheology of HEUR
thickened latex paints