Investigation on the intrinsic wetting thresholds of liquids by measuring the interaction forces of self-assembled monolayers

90 degrees is the limitation of lyophilicity and lyophobicity for ideal surface for centuries, but it has been proved to be contradictory on some occasions. The symmetrical surfaces with different surface tensions can attract or repel each other in water. Therefore, at the molecular level, the lyophilicity or lyophobicity is the results of interactions between the liquids and substrates. Here, using atomic force microscope (AFM) to measure interaction forces between symmetrical self-assembled monolayers (SAMs) in different liquids, we found that the SAMs repel each other when the surfaces are hydrophilic whereas attract when hydrophobic in water. The contact angle corresponding to the transition of attraction to repulsion is approximate to 65 degrees, defined as the intrinsic wetting threshold (IWT) of water. For ethylene glycol (EG), dimethyl sulfoxide (DMSO), N,N-dimethyl formamide (DMF), the IWTs could be determined by changes of adhesion forces between SAMs. This research redefined the IWTs for liquids, which is the essential guide to both basic theory and applications of wettability.