619 research outputs found
Coupling hydrophobic, dispersion, and electrostatic contributions in continuum solvent models
Recent studies of the hydration of micro- and nanoscale solutes have
demonstrated a strong {\it coupling} between hydrophobic, dispersion and
electrostatic contributions, a fact not accounted for in current implicit
solvent models. We present a theoretical formalism which accounts for coupling
by minimizing the Gibbs free energy with respect to a solvent volume exclusion
function. The solvent accessible surface is output of our theory. Our method is
illustrated with the hydration of alkane-assembled solutes on different length
scales, and captures the strong sensitivity to the particular form of the
solute-solvent interactions in agreement with recent computer simulations.Comment: 11 pages, 2 figure
Polyglyceryl Ester based surfactants – an attractive green alternative to the current non- ionic surfactants.
The consumer demand in personal care for “natural” non-ionic surfactants is growing. The current offerings are mainly based on petrol derived non-ionics and the alkyl polyglucoside based surfactants are one of the few exceptions. In this paper we will present results on relatively new and little known surfactants entirely based on natural renewable sources – polyglyceryl esters. The lack of enough data on their functionality in the literature and their full characterisation has delayed their penetration in a crowded cosmetics market and this presentation will try address this gap. The provided data will manifest the surfactants’ ability to reduce the surface/interfacial tension to very low values. Their mildness combined with an excellent foamability profile make them the surfactant of choice for formulations aimed at sensitive skin consumers
Analytical modeling of micelle growth. 2. Molecular thermodynamics of mixed aggregates and scission energy in wormlike micelles
Hypotheses: Quantitative molecular-thermodynamic theory of the growth of
giant wormlike micelles in mixed nonionic surfactant solutions can be developed
on the basis of a generalized model, which includes the classical phase
separation and mass action models as special cases. The generalized model
describes spherocylindrical micelles, which are simultaneously multicomponent
and polydisperse in size. Theory: The model is based on explicit analytical
expressions for the four components of the free energy of mixed nonionic
micelles: interfacial-tension, headgroup-steric, chain-conformation components
and free energy of mixing. The radii of the cylindrical part and the spherical
endcaps, as well as the chemical composition of the endcaps, are determined by
minimization of the free energy. Findings: In the case of multicomponent
micelles, an additional term appears in the expression for the micelle growth
parameter (scission free energy), which takes into account the fact that the
micelle endcaps and cylindrical part have different compositions. The model
accurately predicts the mean mass aggregation number of wormlike micelles in
mixed nonionic surfactant solutions without using any adjustable parameters.
The endcaps are enriched in the surfactant with smaller packing parameter that
is better accommodated in regions of higher mean surface curvature. The model
can be further extended to mixed solutions of nonionic, ionic and zwitterionic
surfactants used in personal-care and house-hold detergency
Effective capillary interaction of spherical particles at fluid interfaces
We present a detailed analysis of the effective force between two smooth
spherical colloids floating at a fluid interface due to deformations of the
interface. The results hold in general and are applicable independently of the
source of the deformation provided the capillary deformations are small so that
a superposition approximation for the deformations is valid. We conclude that
an effective long--ranged attraction is possible if the net force on the system
does not vanish. Otherwise, the interaction is short--ranged and cannot be
computed reliably based on the superposition approximation. As an application,
we consider the case of like--charged, smooth nanoparticles and
electrostatically induced capillary deformation. The resulting long--ranged
capillary attraction can be easily tuned by a relatively small external
electrostatic field, but it cannot explain recent experimental observations of
attraction if these experimental systems were indeed isolated.Comment: 23 page
A General Theory of Non-equilibrium Dynamics of Lipid-protein Fluid Membranes
We present a general and systematic theory of non-equilibrium dynamics of
multi-component fluid membranes, in general, and membranes containing
transmembrane proteins, in particular. Developed based on a minimal number of
principles of statistical physics and designed to be a meso/macroscopic-scale
effective description, the theory is formulated in terms of a set of equations
of hydrodynamics and linear constitutive relations. As a particular emphasis of
the theory, the equations and the constitutive relations address both the
thermodynamic and the hydrodynamic consequences of the unconventional material
characteristics of lipid-protein membranes and contain proposals as well as
predictions which have not yet been made in already existed work on membrane
hydrodynamics and which may have experimental relevance. The framework
structure of the theory makes possible its applications to a range of
non-equilibrium phenomena in a range of membrane systems, as discussions in the
paper of a few limit cases demonstrate.Comment: 22 pages, 2 figures, minor changes and addition
A mesoscopic model for microscale hydrodynamics and interfacial phenomena: Slip, films, and contact angle hysteresis
We present a model based on the lattice Boltzmann equation that is suitable
for the simulation of dynamic wetting. The model is capable of exhibiting
fundamental interfacial phenomena such as weak adsorption of fluid on the solid
substrate and the presence of a thin surface film within which a disjoining
pressure acts. Dynamics in this surface film, tightly coupled with
hydrodynamics in the fluid bulk, determine macroscopic properties of primary
interest: the hydrodynamic slip; the equilibrium contact angle; and the static
and dynamic hysteresis of the contact angles. The pseudo- potentials employed
for fluid-solid interactions are composed of a repulsive core and an attractive
tail that can be independently adjusted. This enables effective modification of
the functional form of the disjoining pressure so that one can vary the static
and dynamic hysteresis on surfaces that exhibit the same equilibrium contact
angle. The modeled solid-fluid interface is diffuse, represented by a wall
probability function which ultimately controls the momentum exchange between
solid and fluid phases. This approach allows us to effectively vary the slip
length for a given wettability (i.e. the static contact angle) of the solid
substrate
Coupling nonpolar and polar solvation free energies in implicit solvent models
Recent studies on the solvation of atomistic and nanoscale solutes indicate
that a strong coupling exists between the hydrophobic, dispersion, and
electrostatic contributions to the solvation free energy, a facet not
considered in current implicit solvent models. We suggest a theoretical
formalism which accounts for coupling by minimizing the Gibbs free energy of
the solvent with respect to a solvent volume exclusion function. The resulting
differential equation is similar to the Laplace-Young equation for the
geometrical description of capillary interfaces, but is extended to microscopic
scales by explicitly considering curvature corrections as well as dispersion
and electrostatic contributions. Unlike existing implicit solvent approaches,
the solvent accessible surface is an output of our model. The presented
formalism is illustrated on spherically or cylindrically symmetrical systems of
neutral or charged solutes on different length scales. The results are in
agreement with computer simulations and, most importantly, demonstrate that our
method captures the strong sensitivity of solvent expulsion and dewetting to
the particular form of the solvent-solute interactions.Comment: accpted in J. Chem. Phy
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