Ion Specificity and the Theory of Stability of Colloidal Suspensions

Abstract

A theory is presented which allow us to accurately calculate the critical coagulation concentration (CCC) of hydrophobic colloidal suspensions. For positively charged particles the CCC's follow the Hofmeister (lyotropic) series. For negatively charged particles the series is reversed. We find that strongly polarizable chaotropic anions are driven towards the colloidal surface by electrostatic and hydrophobic forces. Within approximately one ionic radius from the surface, the chaotropic anions loose part of their hydration sheath and become strongly adsorbed. The kosmotropic anions, on the other hand, are repelled from the hydrophobic surface. The theory is quantitatively accurate without any adjustable parameters. We speculate that the same mechanism is responsible for the Hofmeister series that governs stability of protein solutions.PACS numbers: 61.20. Qg, 64.75.Xc, 64.70.pv, 82.45.Gj All of biology is specific. The ion channels which control the electrolyte concentration inside living cells are specific to the ions which they allow to pass. The tertiary structure of proteins is sensitive to both the pH and to specific ions inside the solution. It has been known for over a hundred years that while some ions stabilize protein solutions, often denaturing them in the process, others lead to protein precipitation. In the fields as diverse as biophysics, biochemistry, electrochemistry, and colloidal science, ionic specificity has been known -and puzzled over -for a very long time. It has become known as the "Hofmeister effect" or the "lyotropic" series of electrolytes, depending on the field of science. The traditional physical theories of electrolytes completely fail to account for the ion specificity. The Debye-Hückel (DH) theory of electrolytes and the Onsager-Samaras theory of surface tensions treat ions as hard spheres with a point charge located at the center [1]. The cornerstone of colloidal science, the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory of stability of lyophobic colloidal suspensions is based on an even simpler picture of hard sphere-like colloidal particles interacting with the pointlike ions through Coulomb potential. The DLVO theory showed that the primary minimum of colloid-colloid interaction potential -arising from the mutual van der Waals (dispersion) attraction -is not accessible at low electrolyte concentrations because of a large energy barrier. When the electrolyte concentration is raised above the critical coagulation concentration (CCC), the barrier height drops down to zero leading to colloidal flocculation and precipitation. The DLVO theory, however, predicts that the CCC should be the same for all monovalent electrolytes, which is clearly not the case [2][3][4][5]. In fact it has been known for a long time that the effectiveness of electrolyte at precipitating hydrophobic colloids follows the lyotropic series. For positively charged particles the CCC concentration of sodium thiocyanide is an order of magnitude lower than the CCC of sodium fluoride. Even more dramatic is...

Strong non-classical induction forces in ion-surface interactions: General origin of Hofmeister effects

“…Therefore, the observed strong non-classical induction force will generally be the origin of Hofmeister effects at interface surfaces. In current treatments of Hofmeister effects, however, this important force has been ignored 1,2 ; thus they may be correct only for weak electric fields.…”

“…New work in this area may break down barriers between physics and biology 9 . Ionic sizes, hydration, quantum fluctuations (or dispersion forces) 3,4,8 , and surface charges 1,2 are crucial in Hofmeister effects. Nano-scale surfaces and colloidal particles (e.g., DNA, proteins, cells, bacteria, metal oxides, and clay) are usually strongly charged in aqueous solution, and the sign of the charge or the charge number for biological macromolecules will be dependent on pH and ionic strength 20 .…”

“…According to Yan Levin et al 1,2 , the chaotropic Na 1 cation would be more likely adsorbed at particle surfaces, whereas the kosmotropic Ca 21 cation would be more likely present in solution. This would decrease the preference of Ca 21 over Na 1 in the exchange because of the stronger dispersion and electrostatic forces per charge for Na 1 than that for Ca 21 , and K E /K C , 1.…”

Microscopic Origin of the Hofmeister Effect in Gelation Kinetics of Colloidal Silica

“…Models have been made to describe ion-specific distribution of ions near surfaces, and their surface adsorption: an important factor is the ion diameter, and whether or not the ions are hydrated 11 . Chaotropic ions have smaller effective diameters (as they are not hydrated), and they can adsorb to the surface.…”

Weak and Strong Coupling Theories for Polarizable Colloids and Nanoparticles

“…What is more surprising is that even for monovalent counterions, stability of colloidal suspensions depend strongly on the precise nature of the counterions. Thus, as one goes along the halogen series, the critical coagulation concentrations of positive colloidal particles can decrease by as much as an order of magnitude, when anion is changed from fluoride to iodide [1]. Another interesting phenomenon found in suspensions with multivalent ions is the reversal of electrophoretic mobility [2,3], or equivalently charge reversal [4][5][6][7].…”