Is the combination of two particles with different degrees of hydrophobicity an alternative method for tuning the average particle hydrophobicity?

Abstract

A systematic investigation into the equivalence between two methods of tuning average particle hydrophobicity: varying the extent of chemical anchoring of alkyl chains onto the surface of a single particle (chemical method) and mixing particles of different hydrophobicity (mechanical method), in stabilization of highly concentrated water-in-oil emulsions with an overcooled dispersed phase, in the presence of low-molecular-weight conventional surfactant (Sorbitan Monooleate - SMO), has been made. Two single particles with different degrees of surface treatments, expressed in terms of hydrophobicity index (HI), HI = 0.97 and HI = 1.34 were used (chemical method). Equivalent twin mixtures were produced by using specific ratios between fumed silica nanoparticles of HI = 0.60, 1.34 and 3 (mechanical method): HI = 0.97: (HI = 1.34)/(HI = 0.60) = 50/50 or (HI = 3)/(HI = 0.60) = 15/85 and HI = 1.34: (HI = 3)/(HI = 0.60) = 31/69. Each single or twin mixture was combined with different amounts of SMO in the range of 0–10 wt%. Rheology of oil dispersions with no surfactant demonstrated that the mechanical method is quantitatively equivalent to the chemical method, in agreement with previous works where different techniques were used. Conversely to the case without surfactant, it has been found that the influence of such factors as surfactant-toparticle ratio or average particle HI on stability and rheology of fresh, as well as aged, oil dispersions on emulsion refinement kinetics, emulsion stability against the crystallization of the inner phase is strongly dependant on the method used to tune the particle hydrophobicity. Regardless of the SMO/particle ratio, a drastic change in the rigidity of flocs in aged oil dispersions and emulsion properties occurs when single particles are replaced by their equivalent twin mixtures in the presence of a surfactant. Interestingly, homogeneous nucleation within the droplets is the dominant mechanism of crystallization of nitrate salts in the super-cooled droplets stabilized with single particles (chemical method). On the other hand, heterogeneous nucleation catalysed at the surface of droplets was found to be the dominating mechanism of crystallization of nitrate salts in the super-cooled droplets for equivalent binary mixtures (mechanical method).