Abstract
Selective flotation of albite from potassium feldspar (microcline/orthoclase) with cationic collectors is controlled by monovalent salt, yet the surface forces responsible have been inferred from electrokinetic and thermodynamic data rather than measured directly. Here we probe these interactions by colloidal-probe atomic force microscopy (AFM), interpreted through classical and extended DLVO theory. Force curves between a silica microsphere and polished albite and orthoclase surfaces were acquired versus NaCl and at the separation condition. Constant-potential DLVO fits returned Debye lengths of the expected order, and effective potentials reproducing the electrokinetic ordering (albite more negative than orthoclase) and, at high ionic strength, resolving a mineral-distinguishing difference absent from the zeta potentials. Surface free-energy components from thin-layer wicking resolved the forces into Lifshitz–van der Waals, double-layer, and Lewis acid–base contributions. Added electrolyte shortened the force range through double-layer compression, while a non-DLVO short-range repulsion re-emerged at high ionic strength. At the separation condition, albite showed a stronger short-range repulsion than orthoclase; a rigorous van Oss decomposition attributes this to a repulsive acid–base (hydration-type) force more than twice as strong on albite, mirroring selectivity, whereas a simplified rule returns an attractive term contradicting the measurement. With the amine collector present, the force resolves its action directly: it weakens the albite repulsion, the direct-force counterpart of orthoclase’s faster, thermodynamically established hydrophobization. Short-range acid–base and hydration forces, rather than the double-layer force, emerge as the dominant contribution distinguishing the two feldspars; the continuum model captures the sign and ordering of this control but not its magnitude.