Ion-ion correlations, screening, and equilibrium bulk structure in various
concentrated electrolytes are investigated using synchrotron small angle X-ray
scattering (SAXS), theory, and molecular simulation. Utilizing SAXS
measurements we provide estimates of the Kirkwood Transition (KT) for a variety
of aqueous electrolytes (NaCl, CaCl2β, SrCl2β, and ErCl3β). The KT may be
defined as the concentration above which the ion-ion correlations cease to
decay exponentially with a single length scale given by the Debye length
Ξ»Dβ and develop an additional length scale, d=2Ο/Q0β that
reflects the formation of local domains of charge. Theoretical models of the KT
have been known for decades for highly idealized models of electrolytes, but
experimental verification of KT in real electrolytes has yet to be confirmed.
Herein, we provide consistent theoretical and experimental estimates of both
the inverse screening lengths a0β and inverse domain size, Q0β for the
aforementioned electrolyte systems. Taken together, a0β and Q0β are known
descriptors of the KT and provide a view into the complexity of ion-ion
interaction beyond the well-accepted Debye-H\"{u}ckel limit. Our findings
suggest a picture of interaction for real electrolytes that is more general
than that found in idealized models that is manifest in the precise form of the
non-local response function that we estimate through the interpretation of the
experimental SAXS signal. Importantly, the additional complexity of describing
ion-ion interaction of real electrolytes will implicate the short-range ion-ion
interactions that can only be computed via molecular simulation and provide a
quantitative approach to describe electrolyte phenomena beyond Debye-H\"{u}ckel
theory.Comment: 3