The Characteristic Curvature of Ionic Surfactants

Characterizing the hydrophilic-lipophilic nature of a surfactant molecule has been a challenge for colloid scientists and technologists. The hydrophilic-lipophilic balance (HLB), the packing factor, the phase inversion temperature (PIT) and the natural curvature of the surfactant are all terms that seek to address this issue. In this article we build on the hydrophilic–lipophilic difference concept (HLD) (Salager et al. Langmuir, 16, 5534–5539, 2000) to develop a methodology to determine a characteristic curvature (Cc) for ionic surfactants based on the phase behavior of mixed ionic surfactant microemulsions. In essence, the method consists of evaluating the shift in optimal electrolyte concentration as a function of the mole fraction of the test surfactant in a mixture with a reference surfactant, sodium dihexyl sulfosuccinate (SDHS) and applying the appropriate HLD equation for ionic surfactant mixtures to determine Cc. The values of Cc were determined for a range of surfactants, including sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), sodium naphthenate, and others. The method was also extrapolated to nonionic additives and hydrophilic linkers. It was observed that the calculated values of Cc were similar to those predicted by group contribution models, however the proposed method can be used even for complex surfactant mixtures. Finally, when Cc values were compared to apparent packing factor and HLB values, it was found that Cc is correlated with the apparent packing factor of ionic surfactants, and that Cc correlates with the HLB value for nonionic amphiphiles. The physical interpretation of Cc, and its potential application in the Net-Average Curvature equation of state for oil-surfactant-water systems is discussed.

Synthesis, Characterization, Biodegradation and Evaluation of the Surface Active Properties of Nonionic Surfactants Derived from Jatropha Oil

Four nonionic surface active agents were synthesized using the fatty acids obtained from the hydrolysis of Jatropha oil. The fatty acids obtained contained different fatty acids including: palmitic, stearic, oleic, linoleic and linolenic acids in different proportions. The chemical structures of the obtained surfactants were characterized using elemental analysis and FTIR spectroscopy. The surface activities of the different surfactants were determined using surface and interfacial tension measurements. The surfactants showed good surface and interfacial activities, which are dependent on their chemical structures. Thermodynamic parameters of adsorption and micellization confirmed these results. The biodegradation tests in river water showed that the surfactants are readily biodegradable, and reached the European standards after 24 days. Surfactants containing longer nonionic chains formed stable emulsions with paraffin oil, while shorter chains exhibit a lower emulsion stability performance.     

Synthesis and Characterization of Novel Surfactants based on 2-Hydroxy-4-(Methylthio)Butanoic Acid Part 3: Microemulsions from Nonionic Sulfoxide Ester Surfactants

In this work, ester sulfoxide (ESO) surfactants based on 2-hydroxy-4-(methylthio) butyric acid are shown to have temperature-sensitive microemulsion phase behaviors. Both C10 (C₁₀ESO) and C12 (C₁₂ESO) surfactants studied contained one sulfoxide unit in the structure. Phase inversion temperatures (PIT) and interfacial tensions (IFT) between water-rich and oil-rich phases have been measured for ternary systems of water, oil, and sulfoxide surfactants. Hydrophilic–lipophilic deviation (HLD) parameters of these surfactants were obtained by fitting the experimental data to a semiempirical HLD equation. The characteristic surfactant parameter and temperature sensitivity of C₁₀ESO and C₁₂ESO surfactants were obtained and compared with similar ethoxylated alcohol surfactants. By comparing the characteristic parameters of these surfactants with those of ethoxylated alcohol surfactants, it was shown that one sulfoxide ester moiety is equally hydrophilic as approximately 5 ethylene oxide groups. The temperature sensitivity of the ESO was roughly a factor of four less than ethoxylated surfactants based on the temperature coefficient of the HLD equation.     

Effect of Additives on the Cloud Point of the Octylphenol Ethoxylate (30EO) Nonionic Surfactant

The effect of different additives on the cloud point (CP) of an octylphenol ethoxylate (OPEO30) aqueous solution was investigated. The CP of OPEO30 is higher than 100 °C due to its approximately 30 oxyethylene units per molecule, which limits its use in cloud point extraction (CPE) procedures at room temperature. However, the presence of additives can promote a decrease in CP, allowing its use in CPE. Halide presence decreased the CP of OPEO30 in the following order F^? > Cl^? > Br^?. It was also observed that the addition of divalent anions resulted in a more pronounced salting‐out effect than monovalent anions. Blends of OPEO30 and its counterpart with 7.5 units of ethylene oxide (OPEO7.5) were also investigated, and it was verified that the CP of OPEO30 decreased linearly with increasing OPEO7.5 concentration, showing that mixing surfactants is a strategy to be explored. In the presence of small amounts of alcohols that are partially soluble in aqueous solution, such as isobutanol and pentanol, cloudiness was observed at temperatures below the CP of pure OPEO30 solution.