= Solution
Take a positive surface potential $\Phi_0>0$, so positive ions are <co-ions> and negative ions <counterions>. With $u_0=e\Phi_0/(k_BT)\ll1$,
$$
\boxed{n_{\rm co}(x)\simeq n_0[1-u_0e^{-x/\lambda_D}],\qquad
n_{\rm counter}(x)\simeq n_0[1+u_0e^{-x/\lambda_D}].}
$$
The <counterion> density is slightly enhanced at the surface, the <co-ion> density slightly depleted, and both approach $n_0$ over a <Debye length>. To first order their sum is $2n_0$, while their difference supplies the screening charge. Reverse the surface sign and the ionic roles interchange. These are number-density profiles, not trajectories of individual ions.
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