The surface charge characteristics of ferrous gluconate are closely related to its surrounding environment (e.g., solvent type, pH value), its own molecular structure, and whether aggregates are formed. A detailed analysis can be conducted from the following aspects:
First, from the perspective of inherent molecular structure, ferrous gluconate is formed by the ionic bonding of gluconate anions (C₆H₁₁O₇⁻) and ferrous cations (Fe²⁺). In the solid state, the positive and negative charges within the molecule are balanced, resulting in overall electrical neutrality. At this stage, there is no obvious "surface charge"—this is because the Fe²⁺ and the negative charges of gluconate within the molecule interact through chemical bonds, leaving no net charge sites exposed on the surface.
When ferrous gluconate dissolves in a polar solvent (e.g., water, which is also its most common application scenario, such as in nutritional supplements, food additives, or water treatment), the molecules dissociate: Fe²⁺ breaks free from the 束缚 of gluconate and exists as free cations in the solution, while gluconate disperses as anions. However, it should be noted that the discussion of "surface charge" at this point needs to focus on the charged state of the dissociated ions and the surface charge of any aggregates that may form. If Fe²⁺ in the solution forms tiny aggregates (e.g., oligomers or colloidal particles) through hydrolysis (e.g., combining with OH⁻) or partial coordination with gluconate, the surface charge of the aggregates will be determined by the groups exposed on their surface: if the surface adsorbs a large number of negatively charged gluconate anions, or if Fe²⁺ hydrolyzes to form positively charged hydroxyl complexes (e.g., Fe (OH)⁺), the aggregate surface will exhibit a corresponding net negative or positive charge.
Second, pH value is a key factor regulating the surface charge of the ferrous gluconate system (especially for aggregates or the ionic environment after dissociation). Under acidic conditions (e.g., in food matrices or aqueous solutions with low pH), the high concentration of H⁺ in the solution inhibits the hydrolysis of Fe²⁺ (preventing the formation of species such as Fe(OH)⁺ and Fe(OH)₂). At this point, the system mainly contains free Fe²⁺ (positively charged) and gluconate (negatively charged). If no aggregates are formed between them, the overall solution is electrically neutral; if weak aggregation occurs due to high concentration, H⁺ will compete with the hydroxyl groups (-OH) of gluconate for binding sites, reducing the adsorption of gluconate on the aggregate surface. As a result, the aggregate surface may exhibit a weak positive charge due to the exposure of a small amount of Fe²⁺.
Under neutral or weakly alkaline conditions, Fe²⁺ is prone to hydrolysis: On one hand, Fe²⁺ combines with OH⁻ to form positively charged hydrolyzed products (e.g., Fe(OH)⁺). If these hydrolyzed products are adsorbed on the aggregate surface, the surface positive charge density will increase. On the other hand, if the concentration of OH⁻ is too high, Fe²⁺ may undergo further hydrolysis to form Fe(OH)₂ precipitates. At this point, the charge on the precipitate surface changes with pH: when the pH is close to the isoelectric point of Fe(OH)₂, the surface net charge is zero; when the pH is higher than the isoelectric point, the surface will adsorb OH⁻ and carry a negative charge; when the pH is lower than the isoelectric point, it will carry a positive charge.
In addition, other ions in the solution (e.g., sodium chloride, citrate ions in food) also affect the surface charge characteristics of ferrous gluconate. These external ions may weaken the electrostatic interaction between Fe²⁺ and gluconate through the "charge screening effect," reducing the formation of aggregates and thereby altering the charge distribution of the system. If the external ions are oppositely charged ions (e.g., anionic additives), they may also combine with the positive charges on the aggregate surface, neutralizing part of the charge and reducing the surface net charge density.
In practical application scenarios, these surface charge characteristics are of great significance: For example, in the food industry, when ferrous gluconate is used as an iron supplement, its surface charge affects its dispersibility in food matrices—if the surface is negatively charged and matches the surface charge of food colloids (e.g., proteins, polysaccharides), it can reduce agglomeration and improve stability. In the field of water treatment, if Fe²⁺ (which can be converted to Fe³⁺ subsequently) is used to treat pollutants due to its oxidizing properties, the surface charge affects its ability to adsorb and bind to pollutants (e.g., negatively charged organic pollutants), thereby influencing treatment efficiency.
