2 research outputs found
Interactions of Hyaluronan Layers with Similarly Charged Surfaces: The Effect of Divalent Cations
We
used colloidal probe atomic force microscopy to measure the
normal forces between the surface of a silica colloidal particle and
a sparse layer of hyaluronan (hyaluronic acid, HA, MW ≈ 10<sup>6</sup> Da) covalently attached to a planar silica surface, both
across pure water and following the addition of 1 mM MgCl<sub>2</sub>. It was found that in the absence of salt the HA layer repelled
the colloidal silica surface during both approach and retraction.
The addition of the MgCl<sub>2</sub>, however, changes the net force
between the negatively charged HA layer and the opposing negatively
charged silica surface from repulsion to adhesion. This interaction
reversal is attributed to the bridging effect of the added Mg<sup>2+</sup> ions. Our results provide first direct force data to support
earlier simulation and predictions that such divalent cations could
bridge between negative charges on opposing surfaces, leading to an
overall reversal of force from repulsion to attraction
Temperature- and pH-Dependent Shattering: Insoluble Fatty Ammonium Phosphate Films at Water–Oil Interfaces
We
study the films formed by tetradecylamine (TDA) at the water–dodecane
interface in the presence of hydrogen phosphate ions. Using Fourier
transform infrared spectroscopy (FTIR), interfacial shear rheology,
confocal fluorescence microscopy, cryo-scanning electron microscopy
(cryo-SEM), and small-angle neutron scattering (SANS), we find that
between pH 5 and 8 tetradecylammonium cations bind to hydrogen phosphate
anions to form needle-shaped crystallites of tetradecylammonium hydrogen
phosphate (TAHP). These crystallites self-assemble into films with
a range of morphologies; below pH 7, they form brittle, continuous
sheets, and at pH 8, they form lace-like networks that deform plastically
under shear. They are also temperature-responsive: when the system
is heated, the film thins and its rheological moduli drop. We find
that the temperature response is caused by dissolution of the film
in to the bulk fluid phases. Finally, we show that these films can
be used to stabilize temperature-responsive water-in-oil emulsions
with potential applications in controlled release of active molecules