Heterogeneous Nucleation and Growth of Barium Sulfate at Organic-Water Interfaces: Interplay between Surface Hydrophobicity and Ba²⁺ Adsorption

Barium sulfate (BaSO₄) is a common scale-forming mineral in natural and engineered systems, yet the rates and mechanisms of heterogeneous BaSO₄ nucleation are not understood. To address these, we created idealized interfaces on which to study heterogeneous nucleation rates and mechanisms, which also are good models for organic-water interfaces: self-assembled thin films terminated with different functional groups (i.e., -COOH, -SH, or mixed -SH & COOH) coated on glass slides. BaSO₄ precipitation on coatings from Barite-supersaturated solutions (saturation index, SI, = 1.1) was investigated using grazing-incidence small-angle X-ray scattering. After reaction for 1 h, a little amount of BaSO₄ formed on hydrophilic bare and -COOH coated glasses. Meanwhile, BaSO₄ nucleation was significantly promoted on hydrophobic -SH and mixed -SH & COOH coatings. This is because substrate hydrophobicity likely affected the interfacial energy and hence thermodynamic favorability of heterogeneous nucleation. The heterogeneous BaSO₄ nucleation and growth kinetics were found to be affected by the amount of Ba²⁺ adsorption onto the substrate and incipient BaSO₄ nuclei. The importance of Ba²⁺ adsorption was further corroborated by the finding that precipitation rate increased under [Ba²⁺]/[SO₄^2-] concentration ratios >1. These observations suggest that thermodynamic favorability for nucleation is governed by substrate-water interfacial energy, while given favorable thermodynamics, the rate is governed by ion attachment to substrates and incipient nuclei.