Hydration Repulsion between Carbohydrate Surfaces Mediated by Temperature and Specific Ions

Abstract

Stabilizing colloids or nanoparticles in solution involves a fine balance between surface charges, steric repulsion of coating molecules, and hydration forces against van der Waals attractions. At high temperature and electrolyte concentrations, the colloidal stability of suspensions usually decreases rapidly. Here, we report a new experimental and simulation discovery that the polysaccharide (dextran) coated nanoparticles show ion-specific colloidal stability at high temperature, where we observed enhanced colloidal stability of nanoparticles in CaCl2 solution but rapid nanoparticle-nanoparticle aggregation in MgCl2 solution. The microscopic mechanism was unveiled in atomistic simulations. The presence of surface bound Ca(2+) ions increases the carbohydrate hydration and induces strongly polarized repulsive water structures beyond at least three hydration shells which is farther-reaching than previously assumed. We believe leveraging the binding of strongly hydrated ions to macromolecular surfaces represents a new paradigm in achieving absolute hydration and colloidal stability for a variety of materials, particularly under extreme conditions.

Figure 1. Colloidal stability and swelling behavior of polysaccharide (dextran) coated nanoparticles.

Dynamic light scattering data for the hydrodynamic diameters of dextran coated nanoparticles in deionized water, 0.5 M MgCl₂, and 0.5 M CaCl₂ over a 40 day period at (a) T = 27 °C and (b) T = 90 °C.

Figure 2. Hydration pressure between carbohydrate surfaces calculated from molecular dynamics simulations.

(a) Snapshot of simulation setup. Note half of the water molecules in the simulation box are not shown for clarity. (b) Density distribution of carbohydrate monomers and waters along surface normal (z axis) within the column of x-y dimensions of the surfaces. The single (*), double (**), and triple stars (***) denote the inner, first, and second hydration layers on carbohydrates. (c) Water per carbohydrate monomer, n_w, and water number density between surfaces as a function of inter-surface distance r. I–V represent different water depolarization, dehydration, or direct interaction regimes (see Text). (d) Net pressure Π acting on carbohydrate surfaces as a function of r calculated from simulations plotted in a semi-logarithmic scale. The exponential decay length λ is fitted for Π(r) from 1 < r < 2 nm. (e) Total, direct, and indirect pressure (Π, Π_dir, and Π_ind) calculated from simulations. The errors were analyzed from the standard deviation of block averages of simulation time periods 40 to 60, 60 to 80, and 80 to 100 ns. (f) Total, direct, and indirect free energy (G, G_dir, and G_ind) calculated by integrating the pressures in (e) along r.

Figure 3. Free energies and enthalpy-entropy contributions for hydration repulsion at different temperatures in different solutions.

(a–c) Free energies, G, (d–f) enthalpic contributions, H, and (g–i) entropic contributions, -TS, for the hydration repulsion between carbohydrate surfaces in DI water (a,d,g), 0.5 M CaCl₂ (b,e,h), and 0.5 M MgCl₂ (c,f,i) at T = 27 or 90 °C.

Figure 4. Water polarization profile between carbohydrate surfaces.

Water polarization profile for surface distances (a,b), r = 2.8, (c,d) r = 2.0, and (e,f), r = 1.4 nm at T = 27 °C (a,c,e) or T = 90 °C (b,d,f). The single (*), double (**), and triple stars (***) denote the inner, first, and second hydration layers on carbohydrates which coincide with those in Fig. 2b.

Figure 5. Schematic of proposed mechanism for dextran-coated nanoparticles heated from room temperature to 90 °C in deionized water or electrolyte solutions.

Nanoparticle cores are shown as dark gray spheres; dextran coatings are shown as light gray strands; Ca²⁺ ions are shown as blue dots; Mg²⁺ ions are shown as orange dots.