Fig. 7: Highly disparate influence of zwitterions on interparticle attraction.
From: Chemical control of colloidal self-assembly driven by the electrosolvation force
a Schematic depiction of average TMG and TMAO molecular orientation at a silica surface compared to glycine and water, inferred from MD simulations. b Digitised experimental images of colloidal suspension structure for negatively charged SiO2 particles in aqueous solutions of increasing concentrations, \({c}_{{{{\rm{b}}}}}\), of TMG, TMAO and glycine. Scale bars \(20\) μm. c Inferred pair-interaction potentials \(U(x)\) (see Supplementary Table 29 for parameters) for \({c}_{{{{\rm{b}}}}} \,=\, {10}^{-5}\) M and higher concentrations of zwitterion. 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 zwitterion concentration in solution, indicate \({c}_{1/2}=\) \(3\) M (TMG in water), \(1\) M (TMG solution with pH and conductivity matched to TMAO solution – TMG*), 50 mM (TMAO in water), and \(0.3\) mM (glycine in water). e Contribution of water alone to the total excess dipole moment density at the interface, \({\mu }_{{{{\rm{av}}}},{{{\rm{w}}}}}(0)\), in MD simulations of a silica surface with a group density of 4.7 OH nm−2 immersed in water containing varying zwitterion concentration (see MD simulation methods). Zwitterions can be grouped into two categories depending on their qualitative effect on the sign and magnitude of \({\mu }_{{{{\rm{av}}}},{{{\rm{w}}}}}(0)\). TMG, TMAO, and proline zwitterions enhance the value of \({\mu }_{{{{\rm{av}}}},{{{\rm{w}}}}}(0)\) significantly above that of pure water at a silica surface (blue horizontal dashed line). Data corresponding to two TMAO models (Shea(m) and Netz) are presented (see MD simulation methods). f Schematic illustration of excess dipole moment density, \({\mu }_{{{{\rm{av}}}},{{{\rm{w}}}}}\), at a silica surface in pure water, and a solution containing \({c}_{{{{\rm{b}}}}}\approx 3\) M TMAO (see Supplementary Note 6 for detail). g Spatial profiles of the water dipole moment density at the interface for TMAO- and TMG-containing aqueous media (solid blue lines) are consistently more positive than the pure water case (blue dashed line), which yields \({\mu }_{{{{\rm{av}}}},{{{\rm{w}}}}}\left({c}_{{{{\rm{b}}}}}=\, 3{{{\rm{M}}}}\right)\, > \, {\mu }_{{{{\rm{av}}}},{{{\rm{w}}}}}\left({c}_{{{{\rm{b}}}}}=0\right)\).
