10,733 research outputs found
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
On the predictions and limitations of the BeckerDoring model for reaction kinetics in micellar surfactant solutions
We investigate the breakdown of a system of micellar aggregates in a surfactant solution following an order-one dilution. We derive a mathematical model based on the Becker–Döring system of equations, using realistic expressions for the reaction constants fit to Molecular Dynamics simulations. We exploit the largeness of typical aggregation numbers to derive a continuum model, substituting a large system of ordinary differential equations for a partial differential equation in two independent variables: time and aggregate size. Numerical solutions demonstrate that re-equilibration occurs in two distinct stages over well-separated time-scales, in agreement with experiment and with previous theories. We conclude by exposing a limitation in the Becker–Döring theory for re-equilibration and discuss potential resolutions
Desorption of hydrocarbon chains by association with ionic and nonionic surfactants under flow as a mechanism for enhanced oil recovery
The need to extract oil from wells where it is embedded on the surfaces of
rocks has led to the development of new and improved enhanced oil recovery
techniques. One of those is the injection of surfactants with water vapor,
which promotes desorption of oil that can then be extracted using pumps, as the
surfactants encapsulate the oil in foams. However, the mechanisms that lead to
the optimal desorption of oil and the best type of surfactants to carry out
desorption are not well known yet, which warrants the need to carry out basic
research on this topic. In this work, we report non equilibrium dissipative
particle dynamics simulations of model surfactants and oil molecules adsorbed
on surfaces, with the purpose of studying the efficiency of the surfactants to
desorb hydrocarbon chains, that are found adsorbed over flat surfaces. The
model surfactants studied correspond to nonionic and cationic surfactants, and
the hydrocarbon desorption is studied as a function of surfactant concentration
under increasing Poiseuille flow. We obtain various hydrocarbon desorption
isotherms for every model of surfactant proposed, under flow. Nonionic
surfactants are found to be the most effective to desorb oil and the mechanisms
that lead to this phenomenon are presented and discussed.Comment: 10 figures; to appear in Scientific Report
Phase-field modeling droplet dynamics with soluble surfactants
Using lattice Boltzmann approach, a phase-field model is proposed for simulating droplet motion with soluble surfactants. The model can recover the Langmuir and Frumkin adsorption isotherms in equilibrium. From the equilibrium equation of state, we can determine the interfacial tension lowering scale according to the interface surfactant concentration. The model is able to capture short-time and long-time adsorption dynamics of surfactants. We apply the model to examine the effect of soluble surfactants on droplet deformation, breakup and coalescence. The increase of surfactant concentration and attractive lateral interaction can enhance droplet deformation, promote droplet breakup, and inhibit droplet coalescence. We also demonstrate that the Marangoni stresses can reduce the interface mobility and slow down the film drainage process, thus acting as an additional repulsive force to prevent the droplet coalescence
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