Anticaking Activity of Ferrocyanide on Sodium Chloride Explained by Charge Mismatch

Abstract

Sodium chloride crystals have a strong tendency to cake, which can be prevented by treating them with the anticaking agent ferrocyanide. Using surface X-ray diffraction, we show how the ferrocyanide ion sorbs onto the {100} face of the sodium chloride crystal where it replaces a sodium ion and five surrounding chloride ions. The coverage is about 50%. On the basis of the determined atomic structure, we propose the following anticaking mechanism. Because of the charge of the ferrocyanide ions sorbed on the surface, the crystal can only continue growing by leaving an energetically unfavorable sodium vacancy, or by desorbing the ferrocyanide ion. Therefore, the ferrocyanide effectively blocks further growth of sodium chloride crystals, thereby preventing them from agglomerating and caking.

Crystallization Inhibitors and Their Remediation Potential on Brine‐Contaminated Soils

“…Some crystallization inhibitors, such as hexacyanoferrate, are known to promote noncrusting salt efflorescence rather than the destructive crusting efflorescence and subflorescence (i.e., salt nucleation and crystallization within pores) in ridged porous media such as building materials and industrial infrastructures (Gupta et al, 2012; Nachshon et al, 2011; Fujimaki et al, 2006; Selwitz and Doehne, 2002). The hexacyanoferrate ion geometry and size is similar to NaCl salt clusters that allow ion replacement and block salt crystallization via differences in ion charges (Bode et al, 2012). This leads to a supersaturated state that is diffusion limited as compared with the convective water flux supplying evaporation.…”

“…In contrast, subflorescence without a crystallization inhibitor occurs as massive cubic crystals, forming a hard crust within the medium's pores (Selwitz and Doehne, 2002). When a crystallization inhibitor limits salts from forming a cemented crust (i.e., blocking pores), but instead causes noncrusting efflorescence, the medium's convective water and salt transport maintained as a higher rate (Nachshon and Weisbrod, 2015; Shokri, 2014; Bode et al, 2012; Gupta et al, 2014; Nachshon et al, 2011; Fujimaki et al, 2006). Therefore, the efflorescence yields large amounts of salt in a period of just hours or days.…”

“…These crystallizing salts exert pressure on pore walls until the shear forces cause failure. Crystallization inhibitors change salt crystal formation by ion blocking, thus limiting their development within pores (Bode et al, 2012). However, some crystallization inhibitors such as hexacyanoferrate consequently cause salts to effloresce as soft, hollow hopper crystals in dendritic formations on the porous medium's outer surfaces rather than within the medium's pores (i.e., subflorescence) (Gupta et al, 2012; Rivas et al, 2010; Cassar et al, 2008; Lubelli and van Hees, 2007; Rodriguez‐Navarro et al, 2002; Selwitz and Doehne, 2002).…”

The role of surface and interface structure in crystal growth

“…In order to answer this question, we need to know the structure of FCN at the NaCl(100) surface. A carefully prepared (and flat) (100) surface with approximately 0.5 monolayer of FCN was subjected to an interface X-ray diffraction investigation [21]. The final model from this research is shown in Fig.…”

Dual-Barrel Conductance Micropipet as a New Approach to the Study of Ionic Crystal Dissolution Kinetics

“…39 NaCl dissolution is also of practical importance in relation to developing anticaking technologies in the food industry. 6,40,41 Since conductance measurements 42 have been widely used for dissolution studies on a macro-scale in suspensions, we envisage that the dual-barreled conductance micropipet described herein could be widely adopted for the study of dissolution (and growth) processes. More generally, the work in this paper further enhances a growing family of quantitative pipet-based imagining techniques.…”