Extractant aggregation is quantified by using both geometric and thermodynamic descriptions. For the geometric description, a graph-theoretic cluster analysis is performed on the extractant molecules 59,61,100 using the position of the carbon connecting the two amide groups and a distance cutoff of 10 Å, taken from the first minimum in the radial distribution function (RDF) for pairs of that atom type (see Figure ). At low φ E (φ E = 0.08), we find that the cluster sizes are exponentially distributed. At φ E = 0.18, the distribution is a power law with exponent consistent with the percolation critical exponent for cluster size in three dimensions, τ = 2.19. 101 Then, at higher φ E , a spanning cluster exists in dynamic equilibrium with exponentially distributed clusters of finite size. Lastly, with φ E = 1.00, there is a single spanning cluster. Overall, these broad distributions of cluster sizes indicate that extractant self-association does not feature characteristic aggregate sizes in any concentration regime. Monodisperse particle models, such as the spherical tetramers or larger rod-like reverse micelles that have been proposed in the literature are, therefore, not necessarily appropriate for organic phases dominated by critical concentration fluctuations of the binary extractant/diluent mixture. The temperature dependence of scattering patterns or other measures of particle size, such as diffusivity, 36 may assist differentiation of structure factor and particle factor contributions to scattering. As we previously noted, geometric and thermodynamic descriptions of solution structure are not identical and the divergence of the mean cluster size and the maximum of the Ornstein-Zernike correlation length do not generally coincide. For a second approach to quantifying solution structure, next we directly measure the thermodynamic concentration fluctuations observed from the simulations. For the thermodynamic approach, we quantify the critical fluctuations using the same Ornstein-Zernike approach as applied to the experimental scattering data. While directly fitting the correlation length from simulation scattering patterns is not possible due to finite simulation sizes and the large Q-spacing of the Fourier transformed pair correlation functions at low Q, we can observe the concentration fluctuations in real space. The total densitydensity correlation function decays as r -1 e -r/ξ in classical Ornstein-Zernike theory for a unary system. For the binary mixture here, where self-association of the extractant drives concentration fluctuations, we consider the long-range decay of DMDBPMA head group correlations defined by the same RDF described above.