= Solution
For like-charged plates, <DLVO theory> combines an attractive dispersion contribution and a screened electrostatic repulsion. A useful far-field schematic model is
$$
W_{\rm tot}(d)=-\frac{A_H}{12\pi d^2}+B e^{-\kappa_Dd},\qquad B>0.
$$
At high salt concentration, $\lambda_D=\kappa_D^{-1}$ is short. The repulsion then disappears over a short distance, leaving attraction across much of the accessible range. \b[The usual high-salt sketch has a greatly reduced or absent repulsive energy barrier], so approach into a primary attractive minimum is easier. At very small separations, short-range repulsion cuts off the unphysical divergence of the simple $-d^{-2}$ continuum expression.
The large-distance tail of the stated model is attractive because an exponentially decaying repulsion eventually becomes smaller than the algebraic attraction. A barrier is not ruled out by the words “high salt” alone: its existence also depends on surface charge or potential, charge regulation, $A_H$ and the boundary conditions that determine $B$. The figure shows one declared barrier-free high-screening example rather than claiming a universal threshold concentration.
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