With increasing density imposed by external osmotic pressure, DNA in univalent salt solutions (e.g., NaCl) is known to go through a set of ordered mesophases, eventually crystallizing into an orthorhombic crystal. While the transition from the cholesteric to the line hexatic (LH) phase has been observed before, it has remained unclear whether the transition is of second order or first order. We use the small but accurately measurable temperature dependence of the osmotic pressure of a PEG solution to fine-regulate the osmotic stress with which it acts on the DNA subphase. This allows us to set the osmotic pressure to an accuracy never achieved before. This advance in experimental methodology allows us then to detect small but nevertheless finite changes in the density of DNA as it goes through the cholesteric → LH transition. In this way, we first determine experimentally the small density change that occurs at the cholesteric → LH phase transition. Further, we establish that this small density discontinuity of Na-DNA is merely increased when polyvalent salt Co(NH3)6Cl3, i.e. CoHex, is added to the solution. Increasing CoHex concentration finally leads to a phase separation at zero imposed osmotic pressure. Establishing a continuity of thermodynamic states for the cholesteric → LH transition and DNA condensation, thought to be completely unrelated before, represents an important advance in our understanding of DNA polymorphism in electrolyte solutions.
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