Waterlike anomalies in hard core–soft shell nanoparticles using an effective potential approach: Pinned vs adsorbed polymers

Abstract

In this work, a two dimensional system of polymer grafted nanoparticles is analyzed using largescale Langevin Dymanics simulations. Effective core-softened potentials were obtained for two cases: one where the polymers are free to rotate around the nanoparticle core and a second where the polymers are fixed, with a 45 • angle between them. The use of effective core-softened potentials allow us to explore the complete system phase space. In this way, the P T , T ρ and P ρ phase diagrams for each potential were obtained, with all fluid and solid phases. The phase boundaries were defined analyzing the specific heat at constant pressure, the system mean square displacement, the radial distribution function and the discontinuities in the density-pressure phase diagram. Also, due the competition in the system we have observed the presence of waterlike anomalies, such as the temperature of maximum density -in addition with a tendency of the TMD to move to lower temperatures (negative slope)-and the diffusion anomaly. It was observed different morphologies (stripes, honeycomb, amorphous) for each nanoparticle. We observed that for the fixed polymers case the waterlike anomalies are originated by the competition between the potential characteristic length scales, while for the free to rotate case the anomalies arises due a smaller region of stability in the phase diagram and no competition between the scales was observed.

Hard core-soft shell particles near repulsive interfaces: interplay between adsorption, aggregation and diffusion

“…The fluid consists of spherical nanoparticles (NP) with a hard core and a soft corona, as schematically depicted in the figure 1(a). The effective core-softened interaction potential, obtained in a recent work by Marques and co-authors, 31 is composed by a short-range attractive Lennard Jones potential and three Gaussian terms, each one centered in c j , with depth h j and width w j :…”

“…for the pressure and temperature, respectively, where σ = 1.4 nm, ǫ core /k B = 10179 K, with k B the Boltzmann constant, are the distance and energy parameters as previously works. 31,32 m is the mass of a single NP. Since all physical quantities are defined in reduced LJ units, the * will be omitted, in order to simplify the discussion.…”

“…[22][23][24] The short range attraction is caused by van der Walls forces or solvent effects, 25 while the long range repulsion can be generated by many factors. For instance, it can be caused by electrostatic repulsion in charged colloids and molecules, 26,27 or by soft shells ad in the case of spherical colloids obtained by PEG aggregates [28][29][30] or even by a polymeric brush, as metallic NP [31][32][33][34][35] decorated with polymers and star polymers. 36 From experimental 28,29 and computational 31,32 works, is well known that the SALR NP and colloids effective interaction can be depicted by core-softened potentials.…”

“…For instance, it can be caused by electrostatic repulsion in charged colloids and molecules, 26,27 or by soft shells ad in the case of spherical colloids obtained by PEG aggregates [28][29][30] or even by a polymeric brush, as metallic NP [31][32][33][34][35] decorated with polymers and star polymers. 36 From experimental 28,29 and computational 31,32 works, is well known that the SALR NP and colloids effective interaction can be depicted by core-softened potentials. In this way, many works have been devoted to study the behavior of competitive colloidal systems in bulk solutions.…”

“…IV. 31 The inset is the product of the pair force F ij = −dU(r ij )/dr ij and the distance r ij as function of the distance.…”