Alkali chloride thin films on Au(111) are investigated under ultrahigh vacuum conditions at room temperature by noncontact atomic force microscopy in combination with Kelvin probe force microscopy. Sample preparation is carried out in situ and optimized in order to achieve a sub-monolayer coverage showing extended alkali chloride island formation. The local surface potential for LiCl, NaCl, KCl and RbCl thin films on Au(111) was directly determined and compared to values of the bare Au(111) substrate. Hence, the absolute change in the local work function ΔΦ was evaluated. We observe a linear dependence between ΔΦ and the ionic radius of the mentioned alkali chloride thin films.