Abstract
A computational study of a colloidal dispersion stabilized with grafted polymer layers is presented here as a model for white, water-based paints. The interaction model includes repulsive, three-body interactions and attractive van der Waals forces. The electrostatic interactions are also studied. Stability criteria can be established for the dispersion, such as the thickness of the adsorbed polymer layers, and the quality of the solvent. Using implicit solvent molecular dynamics calculations, the spatial distribution of the pigments is obtained through the calculation of the radial distribution functions. The results show that the solvent quality and the thickness of the grafted polymer layer are key variables in the stability of the dispersion. Additionally, a structural phase transition is predicted, which is driven by the pigment concentration in the dispersion. It is argued that the predictions of this work are useful guidelines in the design of paints and coatings of current industrial interest.
Highlights
Colloidal dispersions are complex fluids composed of one or more dispersed phases immersed in a continuous phase
Using typical values for the parameters entering into ΔF0θ for a good quality solvent (β = 1), at room temperature, I obtain a value for the ratio of the potential in (8) over kBT of about 104, which means that the colloidal dispersion is stable [2]
I carry out a molecular dynamics computer simulation study of a model colloidal dispersion, where the motion of the colloidal particles is obtained from the solution of Newton’s second law for the force derived from the full potential in (4)
Summary
Colloidal dispersions are complex fluids composed of one or more dispersed phases immersed in a continuous phase (solvent). When the particles come close to each other, the polymers that constitute the layers start to overlap, which in turn reduces the number of available spatial configurations for them and their entropy This process gives rise to an effective repulsion between colloidal particles, which can be enough to stabilize the dispersion. Zhulina and coworkers [4] have developed an effective mean-field theory for this scenario, assuming the area that a polymer chain occupies on a particle surface (σ) is larger than the square of a monomer’s size (a), performing a virial expansion of the thermodynamic potential in terms of the monomer concentration. The origin of the term proportional to γ in (4) is the attractive vdW interaction between colloidal particles, UA, which for planar surfaces (of equal radius R) and for relative distances smaller than the colloids size is given by [2]. This trend has been fully corroborated by experiments that measure the force between polymer-coated particles; see for example, [8]
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