Fig. 4: Tuning the strength of the interparticle attraction in water-alcohol mixtures.

a Structure of colloidal particle suspensions for positively charged NH₂ (pink), and negatively charged SiO₂ (blue) and COOH particles (green), in water/IPA mixtures of increasing volume percentage of water (left to right). b Measured zeta potentials of the particles (\(\zeta\)) are shown in inset. NH₂ particles display attraction and cluster formation in pure IPA containing water at a concentration \(\lesssim 10\%\) (v/v). SiO₂ and COOH particle interactions are purely repulsive in pure IPA, but the onset of interparticle attraction and cluster formation occurs at around \(5\) and \(50\%\) (v/v) water, respectively (Supplementary Table 7 for experimental conditions). Scale bars \(20\) μm. c \(g(r)\) profiles as a function of water volume percent for the three particle surface types. d Inferred pair-interaction potentials, \(U(x)\), from BD simulations that match the experimental particle distributions (see Supplementary Table 27 for parameters). Error bars denote estimated uncertainties of ± 100 nm on particle diameter and ±1.5 \({{{{\rm{k}}}}}_{{{{\rm{B}}}}}T\) in \(w\). e Normalised pair-interaction potential well depths, \(w/{w}_{\max }\), as a function of water volume percent. f Profiles of interfacial water density, \(\rho\)(H₂O) as a function of distance \(z\) from the interface, situated at \({z}_{{{\mathrm{int}}}}\), calculated from MD simulations of a 2% (v/v) water/IPA mixture in contact with a silica (solid line) and model amine surface (dashed line) carrying surface group densities \(\varGamma\)≈ 4.7 and 4 nm⁻² respectively (see MD simulation methods). Simulations indicate significant water adsorption at the hydrophilic silica surface, in contrast with a model surface composed of amine groups. Inset: molecular dynamics simulation snapshot, with interfacial water molecules in a background medium of IPA (pink) adsorbed to silanol groups on the silica surface.