Clay slime coating is known to have a negative impact on coal flotation. It is therefore of great importance to study interactions between clay and coal under flotation conditions. In this study, the effect of electrolytes on the interactions, of montmorillonite (MMT) clays with carbon or asphaltenes on sensor surfaces of quartz crystal microbalance with dissipation (QCM-D) as model coal was investigated. Ion beam sputtering deposition and spin coating methods were used to prepare carbon and asphaltene (a class of natural polyaromatic compounds) surfaces. The prepared surfaces were characterized by Raman spectroscopy. QCM-D was used to determine in situ interactions between MMT clays and model coal surfaces in electrolyte aqueous solutions. The MMT clays were shown to deposit significantly on the model coal surfaces only in the presence of NaCl or CaCl2 as a result of the compression of electrical double layers. Moreover the enhanced deposition by calcium ions is stronger than by sodium ions. The results from this fundamental study provide a better understanding of the critical role of water chemistry in modulating the interactions of clays with coal surfaces and scientific guidance to achieve better performance of fine coal flotation.