Direct imaging of flocculation/coalescence in surfactant-free oil/water emulsion was achieved by freeze-fracture electron microscopy (FFEM) on benzene oil droplets ultrasonically dispersed in water. Immediately after sonication, spherical droplets with diameters at 30−100 nm (S class) and aggregates consisting of S class droplets at 200−500 nm (M class) were observed. An hour later, we were able to obtain FFEM images, in which a number of S class droplets in flocculation coalesced into larger droplets. Additionally, two droplets of M class coalesced easily when these droplets were in contact with each other. These results support that M and L (≥1000 nm) class droplets are formed through flocculation of S class droplets and coalescence of M class droplets, respectively.
717The interaction between anionic and cationic surfactants was investigated by means of surface tension, conductivity, and nuclear magnetic resonance. It was found that a strong interaction exists between anionic and cationic surfactants and the mixed surfactant has a hydrophobic property. The phase diagram has been determined as a function of temperature for the water-steat~yltrimethylammonium chloride (STAC)-sodium laurate (NaL). The Krafft point rose remarkably in equimolar mixtures for this system. The phase diagram has been determined as a function of temperature for the STAC-sodium-N-lauroyl-N-methyl-/3-alanine (NaLMA) system. The liquid-liquid phase separation phenomenon was observed around an equimolar mixture. It can be identified with the cloud point, which is shown by nonionic surfactants. This phenomenon seemed to be caused by the decreased solute-solvent interaction (i.e., dehydration of the amido group contained in NaLMA molecule) as the temperature is raised. The decreasing effect of protein (ovalbumin) denaturation was observed in the high area of the mole fraction of cationic surfactant in the cationic-anionic system. We believe that this is due to the remarkable lowering of the monomer concentration by the formation of a hydrophobic complex. KEY WORDS: Cationic-anionic system, cloud point, ion pair.
An analytical description has been developed for micellization of multicomponent mixtures of nonionic surfactants partitioned between oil and water. The mixed micelle theory based on the assumptions of ideal mixing in the micelle and of a phase separation model for the micelle allows calculation of both micelle composition and monomer concentration in two-phase systems as a function of total surfactant concentration. The predictions of the theory are in good agreement with the observed results for the binary mixture of hexaoxyethylene nonyl phenyl ether and octaoxyethylene nonyl phenyl ether partitioned between cyclohexane and water. Moreover, it is shown that the theory reproduces the partition isotherm of multicomponent mixtures of polyoxyethylenated octylphenols, having a Poisson distribution of molecular weights, in the isooctane-water system. IntroductionThe description of physical behavior of mixed surfactants in equilibrated two-phase systems is very important in relation to the type of emulsion1 and interfacial tension m i r~i m a .~~~ There is currently much interest in the ultralow interfacial tension minimum observed in dilute petroleum sulfonate solution/oil systems because of the potential usefulness in tertiary oil It was found that the minimum in interfacial tension vs. surfactant concentration occurs when petroleum sulfonates having a broad distribution of molecular weights are used, and this was attributed to the partition behavior of surfactant molecules and the changes in monomer concentration and micelle composition with surfactant concentration as well as to the interfacial adsorption mechanism?+ Previous measurements of the partition isotherms of polyoxyethylenated nonylphenols in the cyclohexanewater system' have shown that the phase in which micelles are formed will be the continuous phase in the formation of emulsions, and the phase inversion temperature of an emulsion stabilized with a mixture of nonionic surfactants varies with composition of mixed micelles. It is also known that very low interfacial tensions are observed near the phase inversion temperature.8 In order to understand above-mentioned phenomena it is essential to have a comprehensive view of the behavior of mixed surfactants in two-phase systems.In the case of the one-phase system, the theoretical treatment for the critical micelle concentration (crnc) of mixed surfactants was developed independently by Langeg and by Shinoda.lo These early works were concerned mainly with the determination of the cmc of mixed systems. Mysels and Otter'l and Tokiwa et a1.12 calculated
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