Distribution functions of binary solutions (exact analytic solution)

We show that the general solution of the Ornstein–Zernike system of equations for multicomponent solutions has the form $h_{\alpha\beta}= \sum A_{\alpha\beta}^j\exp(-\lambda_jr)/r$, where $\lambda_j$ are the roots of the transcendental equation $1-\rho\Delta(\lambda_j)=0$ and the amplitudes $A_{\alpha\beta}^j$ can be calculated if the direct correlation functions are given. We investigate the properties of this solution including the behavior of the roots $\lambda_j$ and amplitudes $A_{\alpha\beta}^j$ in both the low-density limit and the vicinity of the critical point. Several relations on $A_{\alpha\beta}^j$ and $C_{\alpha\beta}$ are found. In the vicinity of the critical point, we find the state equation for a liquid, which confirms the Van der Waals similarity hypothesis. The expansion under consideration is asymptotic because we expand functions in series in eigenfunctions of the asymptotic Ornstein–Zernike equation valid at $r\to\infty$.