Detecting ion-specific forces between fatty acid colloids and salt crystals in brines using colloidal probe AFM

Hypothesis: Ion-specific forces in concentrated salt solutions play critical roles in many applications, ranging from biology to engineering, e.g., separating water-soluble minerals in brines by flotation using air bubbles. There should be some differences in colloidal forces between the surfactant precipitates in brines and NaCl crystals, and KCl crystals, making their selective aggregation and flotation separation possible.

Experiments: Micron-sized spheres of fatty acid colloids were successfully prepared using lead laurate, characterized, and used to fabricate the AFM probes. Using a special AFM cell design and procedure, interaction force spectroscopy and force mapping on NaCl and KCl crystal surfaces using the probes were performed in their brines (7 M) and quantified using numerical solutions of advanced van der Waals and electrical double-layer theories to reveal valuable distributions of attractive, repulsive, and adhesive colloidal forces between the surfactant colloids and salt crystals.

Findings: Attraction and adhesion between the lead laurate colloidal probe and the NaCl crystal surface were much stronger than those measured on the KCl crystal surface, explaining the selective separation between NaCl and KCl crystals by flotation in the brines. Theoretical analysis of the measured forces shows the potential role of ion-specific interactions in predicting selective aggregation and flotation separation. Our work provides an innovative approach to quantifying the intermolecular interactions between surfactant colloids and NaCl and KCl crystals, offering new theories on colloid and surface chemistry regarding ion-specific forces that underpin aggregation and separation in brines and beyond.