 Solution

At higher coverage, neighboring outward dipoles generate a local field opposing each other's polarization. This dipole depolarization reduces the incremental dipole per added molecule, so the work function becomes less sensitive to additional adsorption.

Crowding can also tilt the molecules, alter their coordination or make them occupy less strongly donating sites. Any of these changes can decrease the normal dipole increment. A reduced slope is consistent with coverage-dependent depolarization and geometry changes; it is not evidence that coverage has stopped increasing.