Fig. 8: Surfactants suppress cluster formation.
From: Chemical control of colloidal self-assembly driven by the electrosolvation force
a Images of colloidal suspension structure for negatively charged SiO2 particles suspended in an aqueous solution containing increasing concentrations, \({c}_{{{{\rm{b}}}}},\) of Tween 20 and glycerol. Scale bars \(20\) μm. b \(g(r)\) profiles (left axis) and measured zeta potentials (\(\zeta\)) (right axis) for Tween 20 and glycerol experiments. c Inferred pair-interaction potentials \(U\left(x\right)\) as a function of Tween 20 or glycerol concentration (see Supplementary Table 30 for parameters). Control experiment in water with pH and conductivity identical to 5% Tween (grey). Error bars denote estimated uncertainties of ± 100 nm on particle diameter and ±1.5 \({{{{\rm{k}}}}}_{{{{\rm{B}}}}}T\) in \(w\). d Normalised pair-interaction potential well depths, \(w/{w}_{\max }\) as a function of additive concentration yield \({c}_{1/2}=0.015\) M for Tween 20 and \(1.9\) M for glycerol. Schematic representation of possible mechanism by which Tween 20 disrupts cluster formation, depicting adsorption of surfactant to the silica surface (inset). e Schematic view of how microscopic interfacial structuring drives long-range interparticle forces. Electrosolvation-governed attraction can occur in solvent mixtures when, e.g., trace amounts of surface-associated water generate a thin aqueous interfacial layer (dark blue spheres – case 1). Strongly adsorbing additives (orange) such as amino acids and surfactants may disrupt interfacial water structure, thus suppressing the interparticle attraction even at low concentrations \({c}_{{{{\rm{b}}}}}\) (case 2). Additives that potentially entail weaker disruptive effects on interfacial structure, such as glycerol, may have no significant impact on cluster formation up to high concentrations, \({c}_{{{{\rm{b}}}}} \, \lesssim \, 1\) M. Surface-adsorbing alkylated zwitterions may reinforce interfacial water orientation (hatched region – case 3), offsetting disruptive effects due to adsorption, thereby sustaining cluster formation up to molar \({c}_{{{{\rm{b}}}}}\) values.
