Kōzō Shinoda

The effect of temperature on the phase equilibria and the types of dispersions of the ternary system composed of water, cyclohexane, and nonionic surfactant

Abstract The phase diagrams of water-cyclohexane systems containing 3 and 7 wt. % of polyoxyethylene (9.7) nonylphenylether have been determined as a function of temperature. In addition to solubilized regions in aqueous and in nonaqueous solutions, the importance of the three-phase realm (water, hydrocarbon, and surfactant phases) has been emphasized. The characteristic temperature at which the mutual solubility of oil and water increases markedly by the aid of nonionic surfactant is closely related to the phase inversion temperature in emulsions. The marked increase in mutual solubility may result from the sandwich-like structure of surfactant, water, and oil layers. The dispersion types of the system have been determined over a wide temperature range. Besides the usual W O type and O W type, W D , D W , D O , O D , and (W+O) D types have been observed, where D represents the surfactant phase. Except for the extreme volume fraction range, the water phase is continuous at low temperature, the oil phase is continuous at high temperature, and the surfactant phase is continuous at a medium temperature near the phase inversion temperature.

The Stability of O/W type emulsions as functions of temperature and the HLB of emulsifiers: The emulsification by PIT-method

The properties of emulsions containing 3 wt% of polyoxyethylene nonylphenylether per system were studied as functions of temperature, composition, and the hydrophile-lipophile balance (HLB) of emulsifiers. It has been found that (a) the size of emulsion droplets changes remarkably with temperature and HLB of emulsifiers, (b) the diameter of droplets is very small but less stable towards coalescence close to the phase inversion temperature (PIT), (c) relatively stable O/W type emulsions are obtained when the PITs of respective systems are about 20°–65°C higher than the storage temperature, (d) a stable and fine emulsion is obtained by rapid cooling of an emulsion emulsified at the PIT, which process we shall designate “emulsification by the PIT-method,” and (e) the optimum stability of an emulsion is relatively insensitive to the change of the HLB values or PITs of emulsifiers, but the instability of an emulsion is very sensitive to the PIT of the system. It seems difficult to determine the optimum HLB value of an emulsifier accurately from the stability vs. HLB value relation. On the other hand, since the change in the stability of an emulsion is sensitive to the temperature near the PIT, the selection of an emulsifier according to the PIT may be more accurate and reliable.

Conditions to produce so-called microemulsions: Factors to increase the mutual solubility of oil and water by solubilizer

Abstract It was confirmed from the studies of phase diagrams that Schulmans so-called micromulsion is not an emulsion, but a solubilized solution. Hence, a microemulsion is not an adequate term for such system, but a swollen micellar solution may be adequate. A dispersed system containing microdroplets, which is not thermodynamically stable, may be called a microemulsion. It is desirable to find conditions to produce so-called microemulsions in which the solution is stable and solubilization is so large that oil and water mix over wide composition range. As much as about 20–25 wt% of surfactant was necessary to produce Schulmans so-called microemulsions. The following conditions were found in order to produce microemulsions with a far less amount, about 5–10 wt%, of solubilizer: (a) Optimum hydrophile-lipophile balance (HLB) or phase inversion temperature (PIT) of a surfactant. (b) Optimum mixing ratio of surfactants (solubilizer), i.e., optimum HLB (or PIT) of the mixture. (c) Optimum temperature for a given nonionic solubilizer. (d) The closer the HLBs of two surfactants, the larger the solubilization. (e) The larger the size of solubilizer the more efficient the solubilization. (f) Mixtures of ionic and nonionic surfactants which are durable to temperature change.

The correlation between the dissolution state of nonionic surfactant and the type of dispersion stabilized with the surfactant

Abstract On the basis of such experimental facts as micellar dispersion, the clouding phenomena, and the temperature dependence of hydrocarbon (or water) solubilization in aqueous (or nonaqueous) solutions of nonionic surfactants, a correlation is shown to exist between the dissolution state of the nonionic surfactant, the curvature of the adsorbed monolayer of surfactant at the hydrocarbon-water interface, and the stability and type of dispersion as a function of temperature. The phase inversion temperature (PIT) in emulsions corresponds to the temperature at which the hydrophilic-lipophilic property of surfactant balances for a given hydrocarbon-water system. Thus, there is an interrelation between the phase inversion temperature and HLB-value. Thus, phase inversion temperature data are useful in interpreting the behavior of emulsions. The present discussion is similarly applicable to gas-water dispersions (foams) by considering the oil phase as replaced by a gas phase, i.e., a medium of almost zero intermolecular force.

The stability of W/O type emulsions as a function of temperature and of the hydrophilic chain length of the emulsifier

Abstract The stability of W/O type emulsions of the cyclohexane-water system stabilized with polyoxythylene nonylphenylether was studied as a function of temperature and of the hydrophilic chain length of the emulsifier. The interfacial tension of the same system was also studied as a function of temperature. It was found that: (1) the interfacial tension between the water and the oil phases changes markedly with temperature and becomes nearly zero (below 0.01 dyne/cm) close to the phase inversion temperature (PIT); (2) the mean droplet diameter is also small at the PIT but increases with temperature; and (3) the coalescence rate of the emulsion droplets is very fast close to the PIT. Relatively stable W/O type emulsions were obtained when the PITs of the emulsions are about 10°–40°C lower than the storage temperature.

The solubilization of hydrocarbons in aqueous solutions of nonionic surfactants

Abstract Solubilization of hydrocarbons in aqueous solutions of nonionic surfactants (mostly polyoxyethylene alkylphenyl ethers) has been investigated. The effects of the temperature, the nature and size of the solubilizates, and the oxyethylene chain length and the hydrocarbon chain length of the surfactants on the solubilization were elucidated. The effect of a small amount of added ionic surfactant (sodium dodecyl sulfate) on the solubilization has also been studied, and a comparison of the solubilizing power of ionic and nonionic surfactants has been made. The effects of the types of hydrocarbons (solubilizates) and the oxyethylene chain length of nonionic surface-active agents on the cloud points of nonionic agents in the presence of hydrocarbons were studied. These data are important for the selection of suitable nonionic surfactants for solubilization, emulsification, and detergent action.

Solution behavior and hydrophile-lipophile balance temperature in the aerosol OT-isooctane-brine system: Correlation between microemulsions and ultralow interfacial tensions

Abstract It becomes evident from studies of the solution behavior of Aerosol OT-oil-water containing salt that there exists a temperature at which the hydrophile-lipophile balance (HLB temperature) of adsorbed surfactant monolayer just balances toward a given oil-water system in a solution of a balanced ionic surfactant as well as nonionics. Interestingly enough, however, the effect of temperature reverses. From the studies of phase volumes of water (or oil) and surfactant as a function of temperature at various concentrations of Aerosol OT, it is clear that a phenomenon similar to a critical solution occurring between water and surfactant as well as oil and surfactant and ultralow interfacial tension in the HLB temperature range can be explained by this critical phenomenon. The HLB temperature is a function of the type of oil, the concentration of salts, etc.

The effect of added salts in water on the hydrophile-lipophile balance of nonionic surfactants: The effect of added salts on the phase inversion temperature of emulsions

Abstract Although an HLB-value is assigned to a definite surfactant, the real hydrophile-lipophile balance (HLB) of surfactant at the oil-water interface changes with the amount and kinds of added salts in water as well as with the types of oils. On the other hand, the phase inversion temperature (PIT) of emulsions accurately reflects that the real HLB of surfactant in a given system changes sensitively with the amount and kinds of added salts. The effect of added salts, acid, and alkali on the hydrophile-lipophile balance of nonionic surfactant has been studied from the measurement of the effect of these additives on the PIT of emulsions and on the cloud points of the surfactant solutions. It was found, for example, that both the PIT of the emulsions studied and the cloud point of an aqueous solution of polyoxyethylene (9.7) nonylphenylether (5 wt % per system) were depressed about 14°C in the presence of 6 wt % of sodium chloride in water. This means that the HLB value of the surfactant was depressed about 0.8–1.0 unit in the solution.

The solubilization of water in nonaqueous solutions of nonionic surfactants

Abstract Solubilization of water in nonaqueous solutions of polyoxyethylene nonylphenyl ethers has been studied. It was found that the solubilization of water in solutions of nonionic surfactants is remarkably large at the optimum temperature, and therefore the effect of temperature is highly significant in these studies. This characteristic phenomenon was observed above a relatively high surfactant concentration, i.e., 2% ∼ 15%. The correlation among the oxyethylene chain length of nonionic surfactants, the types of hydrocarbons or halogenocarbons, and the optimum temperature for the solubilization of water was obtained. This relation is very useful in the selection of a suitable solubilizer of water in respective solvents.

On the demonstration of bicontinuous structures in microemulsions

Abstract The problem of the demonstration of bicontinuous structures in microemulsions is critically examined, with an emphasis on systems with low concentrations of surfactant (s) and comparable volume fractions of oil and water. While the problem of microemulsion bicontinuity has been investigated by a large number of experimental approaches, it is found that to date, only a few can provide an unambiguous clarification of this point. Multicomponent self-diffusion data appear to provide the most convenient and reliable examination of the problem, and therefore the possibilities and limitations of this approach are analysed in some detail. From experimental studies it is found that microemulsions with approximately equal volume fractions of the two solvents are in general bicontinuous, also in the most narrow sense of the word. It is also demonstrated that surfactant molecules in these microemulsions are arranged in monomolecular layers, which are characterized by a spontaneous packing into surfaces of zero, or at least low, mean curvature. The relation between surfactant packing characteristics and microemulsion structure is discussed, in particular for the typically observed sequence of curvature towards oil ⪢ zero mean curvature ⪢ curvature towards water, on change of some parameter (e.g., salinity, cosurfactant, surfactant composition, temperature). As regards an understanding of the physical mechanisms behind alterations in surfactant packing (and thus also microstructure), the problem of nonionic oligo (ethylene oxide) surfactants appears to provide the most intriguing (and controversial) problem. This problem is therefore discussed in a slightly broader context.