Interfaces between water and apolar media (gases, liquids, or solids) have a high cost in free energy. Therefore they tend to recombine to reduce the total interfacial area: in water, oil drops coalesce and air bubbles recombine following collision. The metastability of emulsions, foams, and polymer dispersions is achieved through adsorption of amphiphilic molecules (ionic or non-ionic), macromolecules, or particles, which block the recombination. The mechanisms of this stabilization are well understood. [1] Yet very fine emulsions made of pure oil droplets in pure water have also been found to be metastable in the absence of any added stabilizers. [2][3][4] According to sum frequency generation (SFG) spectroscopy [5] and electrophoretic mobility measurements, [2,3] the droplets of these surfactant-free emulsions are ionized and carry a negative electrical charge. Similar results have been reported for the water/air interface. [6][7][8] Moreover, this negative charge increases rapidly with pH value and therefore with the bulk concentration of hydroxide ions. [2,4,9] The most frequent explanation given for these phenomena is that hydroxide ions adsorb at hydrophobic/water interfaces. While consistent with the pH signature of these phenomena, this explanation requires high adsorption energies, more than 20 times the thermal energy k B T (about 50 kJ mol À ), [2,6] and an outstanding selectivity of hydroxide ions over other simple anions [2,6,10,11] that do not adsorb at such interfaces. On the theoretical side, some models attempt to account for this unexpected adsorption, [12][13][14] while others find no accumulation of hydroxide ions at hydrophobic interfaces; [15] still, other models look for another origin of the surface charge. [16] At present, there is no clear and straightforward understanding of this intriguing phenomenon.The basic assumption of all previous experimental and theoretical studies has been that these systems have "pristine" oil/water interfaces, that is, oil molecules in contact with water molecules, although the possibility of contamination by anionic, surface-active impurities has been mentioned. [17] This assumption is supported by the use of pure components (99 %) with additional purification, thoroughly cleaned glassware and equipment, inert atmospheres, and good reprodu-cibility of the data. Moreover, the same variation with pH value of the surface charge has been found by different research groups with various experimental systems. However, the question remains whether these results leave no other choice but to accept a specific adsorption of hydroxide ions at hydrophobic interfaces? We propose that an alternative mechanism could be a reaction of hydroxide ions with species that are confined to the interface.We conducted systematic experiments to find out whether such alternative explanations could account for all the published experimental results. We used emulsions obtained through a solvent-shifting method. [18][19][20] Solutions of hexadecane, of different purities, were prepared a...

We have re-examined the phase inversion temperature (PIT) emulsification process. This is a low-energy method that uses a physicochemical drive to produce very fine oil/water emulsions in the absence of high shear flows. We used the polyoxyethylene 8 cetyl ether (C(16)E(8))/hexadecane/water system, which has a PIT of 76.2 degrees C. We find that successful emulsification depends on two conditions. First, the mixture must be stirred at low speed throughout the whole process: this makes it possible to produce emulsions at surfactant concentrations that are too low to form an equilibrium microemulsion. Second, the stirred mixtures must be heated above a threshold called the clearing boundary (CB) and then quenched to lower temperatures. The clearing boundary is determined experimentally by a minimum in the turbidity of the stirred mixture, which results from solubilization of all the oil into swollen micelles. This matches the emulsification failure boundary, and it is expressed mathematically by the condition R*C(0) = 1, where R* is the radius that results from the oil/surfactant composition for monodisperse spheres and C(0) is the spontaneous spherical curvature of the surfactant. Thus, we show that such cycles do not need to cross the PIT. In fact, sub-PIT cycles and cross-PIT cycles give exactly the same result. These conditions lead to emulsions that have a narrow size distribution and a mean diameter controlled by the oil/surfactant ratio. The typical range of those diameters is 20-100 nm. Moreover, these emulsions have an excellent metastability, in contrast with emulsions made with shorter oil and surfactant molecules.

Interfaces between water and apolar media (gases, liquids, or solids) have a high cost in free energy. Therefore they tend to recombine to reduce the total interfacial area: in water, oil drops coalesce and air bubbles recombine following collision. The metastability of emulsions, foams, and polymer dispersions is achieved through adsorption of amphiphilic molecules (ionic or non-ionic), macromolecules, or particles, which block the recombination. The mechanisms of this stabilization are well understood. [1] Yet very fine emulsions made of pure oil droplets in pure water have also been found to be metastable in the absence of any added stabilizers. [2][3][4] According to sum frequency generation (SFG) spectroscopy [5] and electrophoretic mobility measurements, [2,3] the droplets of these surfactant-free emulsions are ionized and carry a negative electrical charge. Similar results have been reported for the water/air interface. [6][7][8] Moreover, this negative charge increases rapidly with pH value and therefore with the bulk concentration of hydroxide ions. [2,4,9] The most frequent explanation given for these phenomena is that hydroxide ions adsorb at hydrophobic/water interfaces. While consistent with the pH signature of these phenomena, this explanation requires high adsorption energies, more than 20 times the thermal energy k B T (about 50 kJ mol À ), [2,6] and an outstanding selectivity of hydroxide ions over other simple anions [2,6,10,11] that do not adsorb at such interfaces. On the theoretical side, some models attempt to account for this unexpected adsorption, [12][13][14] while others find no accumulation of hydroxide ions at hydrophobic interfaces; [15] still, other models look for another origin of the surface charge. [16] At present, there is no clear and straightforward understanding of this intriguing phenomenon.The basic assumption of all previous experimental and theoretical studies has been that these systems have "pristine" oil/water interfaces, that is, oil molecules in contact with water molecules, although the possibility of contamination by anionic, surface-active impurities has been mentioned. [17] This assumption is supported by the use of pure components (99 %) with additional purification, thoroughly cleaned glassware and equipment, inert atmospheres, and good reprodu-cibility of the data. Moreover, the same variation with pH value of the surface charge has been found by different research groups with various experimental systems. However, the question remains whether these results leave no other choice but to accept a specific adsorption of hydroxide ions at hydrophobic interfaces? We propose that an alternative mechanism could be a reaction of hydroxide ions with species that are confined to the interface.We conducted systematic experiments to find out whether such alternative explanations could account for all the published experimental results. We used emulsions obtained through a solvent-shifting method. [18][19][20] Solutions of hexadecane, of different purities, were prepared a...