Organic phase structure across the phase diagram is quantified to understand the extent of critical fluctuations in these systems. For aggregation driven by such fluctuations, we expect increased lengthscales associated with compositions near the critical point. Scattering data for the DMDBPMA systems are plotted in Figure , and data for DMDBTDMA in Figure . Using the Ornstein-Zernike equation, become negative and the fitted background scattering, B, increases with φ E compared to the weighted average of the pure components. To address this, we instead fit I(Q) sub to the Ornstein-Zernike equation, , is defined as the locus of correlation length maxima extending from the critical point into the single phase region. Along the critical isotherm, T = T C , the correlation length diverges at the critical composition; for T > T C , it instead reaches a maximum at the Widom line with the strength of this maximum increasing as T approaches T C (see discussion of temperature scaling in the following section). The Widom line is often conceptualized as demarcating the transition between low-and high-density-like mixtures, such as liquid-like and gas-like regions above T C for a liquid-vapor critical point, 89,92 or lowand high-density-like water mixtures above the liquid-liquid critical point in supercooled water. For the liquid-liquid critical point associated with third phase formation, the low-density compositions corresponds to extractant clustering within the nonpolar alkane diluent, while the high-density corresponds to the inverse case. While there is no thermodynamic phase transition separating extractant-in-diluent mixtures from diluent-in-extractant, the location of the Widom line at a given T represents the composition at which the two components are most strongly interspersed.