= 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. \b[A reduced slope is consistent with coverage-dependent depolarization and geometry changes; it is not evidence that coverage has stopped increasing.]
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