Small angle neutron scattering (SANS) measurements are reported for DNA gels under near physiological conditions in which the concentration of monovalent and divalent counter-ions and the pH are varied. The scattering intensity () is described by a two-term equation, one due to osmotic concentration fluctuations and the other coming from static inhomogeneities frozen in by the cross-links. SANS in the low range indicates the presence of large clusters and the size of which exceeds the resolution of the experiment. In the intermediate -range, the intensity increases with the CaCl concentration and the slope approaches -1, corresponding to linear (rod-like) scatterers. In the highest region, the scattering response is governed by the local chain geometry. Screening of electrostatic interactions by sodium chloride causes a moderate increase in the SANS intensity that is accompanied by an increase in the mesh size of the network. Addition of calcium chloride, or a decrease in pH, produces similar trends, and ultimately leads to phase separation. The scattering intensity at = 0, estimated from independent measurements of the osmotic pressure , is in excellent agreement with (0) from the SANS measurements. Anomalous small angle X-ray scattering (ASAXS) measurements on the uncross-linked DNA show that the monovalent ion cloud is only weakly influenced by the addition of divalent ions. Conversely, the divalent counter-ion cloud tightly follows the contour of polymer chains.
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| http://dx.doi.org/10.1039/d3sm00666b | DOI Listing |
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