The ultimate objective of the project was to investigate the relationship between microemulsion phase behavior and detergency for oily soils. In this study, surfactant phase behavior was evaluated for hexadecane and motor oil as model oily soils. Producing microemulsions with these oils is particularly challenging because of their large hydrophobic character. To produce the desired phase behavior we included three surfactants with a wide range of hydrophilic/lipophilic character: alkyl diphenyl oxide disulfonate (highly hydrophilic), dioctyl sodium sulfosuccinate (intermediate character), and sorbitan monooleate (highly hydrophobic). This mixed surfactant was able to bridge the hydrophilic/lipophilic gap between the water and the oil phases, producing microemulsions with substantial solubilization and ultralow interfacial tension. The effects of surfactant composition, temperature, and salinity on system performance were investigated. The transition of microemulsion phases could be observed for both systems with hexadecane and motor oil. In addition, the use of surfactant mixtures containing both anionic and nonionic surfactants leads to systems that are robust with respect to temperature compared to single-surfactant systems. Under conditions corresponding to "supersolubilization," the solubilization parameters and oil/microemulsion interfacial tensions are not substantially worse than at optimal condition for a middle-phase system, so a middle-phase microemulsion is not necessary to attain quite low interfacial tensions. A potential drawback of the formulations developed here is the fairly high salinity (e.g., 5 wt% NaCl) needed to attain optimal middle-phase systems. The correlation between interfacial tension and solubilization follows the trend predicted by the Chun-Huh equation.

mixed surfactant system of sodium dioctyl sulfosuccinate (AOT), alkyl diphenyl oxide disulfonate (ADPODS) and sorbitan monooleate (Span 80) was shown to form Winsor type I and type III microemulsions with hexadecane and motor oil. In addition, high solubilization and low interfacial tension were obtained between the oils and surfactant solutions both in the supersolubilization region (Winsor type I system close to type III system) and at optimal conditions in a type III system. In the present study, this mixed surfactant system was applied to remove oily soil from fabric (a polyester/cotton blend), and detergency results were correlated to phase behavior. Dynamic interfacial tensions were also measured between the oils and washing solutions. In the supersolubilization and in the middlephase regions (type III), much better detergency performance was found for both hexadecane and motor oil removal than that with a commercial liquid detergent product. In addition, the detergency performance of our system at low temperature (25°C) was close to that obtained at high temperature (55°C), consistent with the temperature robustness of the microemulsion phase behavior of this system. Paper no. S1362 in JSD 6, 205-214 (July 2003).KEY WORDS: Detergency, dynamic interfacial tension, microemulsion, mixed surfactants, supersolubilization.Detergency, by definition, is the removal of unwanted substances, so-called soils, from a solid surface by contacting them with liquid (1). In this complex process, soil removal is dependent on several factors, such as the nature and composition of the washing solution, type of soil, hydrodynamic conditions, water hardness, temperature, and electrolyte level, as well as the nature of the solid surface. Interactions at the air-water, liquid-liquid and liquid-solid interfaces are involved (2). FIG. 4. Detergency performance for hexadecane (A) and motor oil (B) removal for our formulation (3% AOT, 2% ADPODS and 2% Span 80) at 25°C, 0.112% active surfactant concentration, and various salt concentrations compared with deionized water and commercial liquid detergent (CP), SPS is our formulation in the supersolubilization region, and S* is the optimum salinity at equilibrium, from phase studies. For other abbreviations see Figure 1. FIG. 5. Hexadecane removal with our formulation compared to CP and to the IFT between the oil and washing solution at various salt concentrations.FIG. 8. Comparison of detergency performance of our microemulsion-based formulation of 3% AOT, 2% AD-PODS, and 2% Span 80 at supersolubilization [5 and 12% NaCl with hexadecane (A) and motor oil (B), respectively] with liquid detergent (CP) at different active concentrations (wt% of total surfactants in solution).

The objective of this study was to investigate the correlation between oily soil removal efficiency and low oil-water interfacial tension (IFT) generated by microemulsion formation. A mixture of sodium dioctyl sulfosuccinate, alkyl diphenyl oxide disulfonate, and sorbitan monooleate was selected as a detergent formulation to evaluate detergency performance for two highly hydrophobic oils: hexadecane and motor oil. The maximum detergency corresponds to formation of a Winsor Type III microemulsion as well as to the supersolubilization region, which is a Winsor Type I microemulsion close to the Winsor Type III region. In addition, the oil removal in the rinse step is almost as high as that in the wash step for both regions. We propose the following mechanism to explain these results: During the wash step, the contact angle of the oil on the fabric surface is progressively increased, resulting in the detachment of the oil droplets. However, owing to the very low IFT, the spreading effect is dominant, thereby causing incomplete oil removal. During the subsequent rinse step, the IFT increases, passing through a composition at which the rollup mechanism causes additional oil removal. These results demonstrate that microemulsion formation and the resulting IFT reduction are important mechanisms in oily soil detergency.Paper no. S1448 in JSD 8, 147-156 (April 2005). KEY WORDS:Alkyl diphenyl oxide disulfonate, detergency, microemulsion, oil removal mechanism, spreading effect.Microemulsions are thermodynamically stable surfactantoil-water systems that can produce very low oil-water interfacial tensions (IFT) and very high oil solubilization. To achieve a microemulsion, a system parameter is progressively changed to decrease the hydrophile-lipophile balance (HLB) of the system. The sequence of systems is known as a "scan," and the typical transition is from Winsor Type I to Type III and to Type II microemulsions. The transitions are well correlated with the IFT of the system, as shown in Figure 1, and are closely associated with the microstructure of the microemulsion. The region on the left side of Figure 1 is a Winsor Type I system, where an oil-in-water (O/W) microemulsion exists along with an excess oil phase. The Winsor Type I microemulsion can be transformed to a Winsor Type III microemulsion by decreasing the system HLB (e.g., increasing salinity for ionic surfactant systems). The middle phase of a Winsor Type III system has a bicontinuous structure in equilibrium with excess oil and excess water phases. When the HLB value decreases further, the system tranforms from a Winsor Type III microemulsion to a Winsor Type II microemulsion, as shown on the right side of Figure 1. In a Winsor Type II microemulsion, a water-in-oil (W/O) microemulsion exists in equilibrium with an excess water phase. Several points within this phase scan are of special interest to this research. The point in the middle phase where the IFT between the excess oil phase and the middle phase (IFT O/M ) equals the IFT between the middle phase and th...

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