(151e) Effect of the Surface Charge Distribution on the Fluid Phase Separation of Charged Colloids and Proteins

Within the last decade, there has been an increasing interest in studying the phase behavior and self-assembly of molecules and particles with directional interactions such as proteins and “smart” colloids [1]. Significant progress has been achieved in the theoretical characterization of anisotropic molecules via “patchy” colloidal models with short-range, adhesive interactions [2]. Although such models successfully capture the general behavior of many colloidal systems, they fail to represent the behavior of other systems like proteins and para- or ferro-magnetic particles where long-range electrostatic interactions play a predominant role [3,4]. Here, a generic but simple colloidal model is presented to evaluate the effect of the heterogeneous surface charge distribution of proteins and zwitterionic nanoparticles on their rich thermodynamic phase behavior via Wang-Landau Transition Matrix Monte Carlo [5]. By considering surface charges as continuous “patches”, the rich set of surface patterns that is embedded in proteins and charged patchy particles can readily be described. This model is used to study the fluid phase separation of charged particles where the screening length is of the same order of magnitude than the molecular/particle size. In particular, two type of charged particles are studied: dipolar fluids and protein-like fluids. The former represents the simplest case of zwitterionic particles, whose charge distribution can be described by the dipole moment. The latter system corresponds to molecules/particles with complex surface charge arrangements such as those in biomolecules. The results for both systems suggest a relation between the critical temperature, the strength of interparticle interactions, and the dispersion of charged patches, where the critical temperature is strongly correlated to the magnitude of the dipole moment. Additionally, competition between attractive and repulsive charge–charge interactions seems to be related to the formation of transient clusters along the dilute phase in dipolar fluids, as well as to the broadening of the binodal curve in protein-like fluids. Finally, a variety of self-assembled architectures are detected for dipolar fluids upon small changes in the charge distribution, providing the groundwork for studying the self-assembly of charged patchy particles.

[1] S. Sacanna et al., Soft Matter, 2013, 9, 8096

[2] E. Zaccarelli, J. Phys.: Condens. Matter, 2007, 19, 323101

[3] F. Kogler et al., Soft Matter, 2015, 11, 7356

[4] D. Staneva and D. Frenkel, J. Chem. Phys., 2015, 143, 194511

[5] V. K. Shen and D. W. Siderius, J. Chem. Phys., 2014, 140, 244106