Microemulsions as cutting fluid concentrates: Structure and dispersion into hard water
Résumé
Microemulsions of O/W, of bicontinous or W/O type were formulated from a paraffinic oil, a lipophilic alcohol (n-decyl alcohol), an aqueous phase made of MonoEthanol Amine Borate (MEAB) and hard water, and a hydrophilic mixture of cationic and nonionic surfactants. In addition to their intrinsic properties (in particular stability), these microemulsions have some of the most important properties of cutting fluid concentrates, that is to say the easiness with which the emulsion forms when they are dispersed into hard water (a synthetic water that mimics mains water) and the production of very stable emulsions. Actually, their dispersion into hard water at the volume ratios 5/95, 20/80, and 60/40 led to the formation of oil-in-water emulsions with an average droplet diameter of 50-150 nm. Three mechanisms of emulsion formation have been identified (i) the dispersion of droplets already present in the system for O/W microemulsion; (ii) the bursting of swollen bilayer structures for bicontinuous microemulsions and; (iii) the formation-bursting sequence of transient swollen bilayer structures for W/O microemulsions. The shift towards highly hydrophilic conditions during the emulsification process plays a major role in achieving emulsions with these small droplet sizes. Their high stability is to a great extent the result of the small size of the droplets and also the fact that droplets are coated with a layer of charged surfactant complexes.