New ethylene oxide-based catanionic mixtures – investigations towards potential applications

The topic of this work was the investigation of new catanionic mixtures with the focus on potential application in foaming, spreading, washing and emulsions. To this purpose, a new group of catanionic mixtures consisting of newly synthesized anionic and cationic surfactants by BASF was introduced. These substances have a linear molecular structure with an alkyl chain of different lengths and number of ethylene oxide (EO) group(s) combined with an ionic headgroup, a sulfate group for the anionic species and a choline group for the cationic species. The intention of inserting EO groups in the catanionic surfactant structure was to increase the scope of potential applications by improving their solubility, which is known to be low, especially around equimolar ratio. For understanding the influences of different structural aspects on properties and performances, the experimental data were correlated to molecular properties to make predictions regarding their performance possible. In the first part of the thesis, the catanionic mixtures were investigated at different anion-cation wt%-ratios from 9-1 to 1-9, regarding different physicochemical properties like solubility behavior, interfacial properties, adsorption behavior, and toxicity. An improved solubility, which could be led back to the present EO groups, could be confirmed. This precondition enabled the investigation of further properties. Synergistic effects were observed for the properties concerning the interfacial activity. For the cmc, as well as the surface tension reduction, small amounts of the oppositely charged surfactant were sufficient to reach the highest efficiency. High dynamics were found in combination with the anion SDS. For the adsorption on a hydrophobic and a hydrophilic surface, synergistic effects became obvious for the mixtures compared to the pure ionic surfactants. For the catanionic mixtures, a large variety of different aggregation structures was found with cryoTEM. A decrease in the toxicity of the cationic surfactant with the addition of an anionic surfactant was found. With the help of statistical analysis, several relations between the molecular and the observed physicochemical properties could be quantified. Especially for the solubility, the presence of EO groups is advantageous. The chain length influences the interfacial properties. In the second part, a pre-screening over all catanionic mixtures regarding their foaming behavior was performed with a high-throughput robot. For a selection of catanionic mixtures, the foamability was investigated deeper considering the total foam height, the liquid volume of the foam and the average bubble size. The anion-cation ratio itself did not follow a clear trend. But for combinations having viscoelastic behavior, increasing foam stability was found. A lower solubility of the surfactant solution was also favorable. A stabilization according to Pickering was suggested. Both methods, high-throughput robot and dynamic foam analyzer, showed the complexity of foams. By statistical analysis, a low surface tension of the surfactant solution could be correlated to higher foamability. The bubble size can be kept small, which correlates to high foam stability, by a small number of EO groups in the cationic surfactant, a small HLB value and a long average chain length of the surfactants. In the third part, the fat removal behavior was investigated. In a first screening, the pure catanionic mixtures were tested with a quartz crystal microbalance (QCM). Four catanionic mixtures were further investigated in the in-house-built washing apparatus. The combination of SDS and C12EO1Ch gave the best results. The effect of a catanionic combination in a more complex washing system based on the three surfactants sodium dodecylbenzylsulfonate, Lutensol AO7, and Texapon N70 was examined in a next step. The addition of a cationic surfactant had positive effects, but only if EO groups were present in the cationic surfactant. A significant influence of the nonionic character in the washing solution was observed. But, the nonionic surfactant was not necessary for full extent if the cationic surfactant C12EO1Ch was added. It could compensate for the decrease of the nonionic surfactant. Further addition of an enzyme showed no positive effect, compared to approaches where only the cationic surfactant or only the enzyme was added to the washing solution. Both seemed to hinder each other. With the use of the cationic surfactant, the enzyme was not necessary. Consequently, the catanionic combination can be a substitute for the commonly used lipase. In the fourth part, the spreading on a hydrophobic polyethylene surface was investigated. A synergistic effect of the mixtures was found. Around equimolar ratio, the contact angle was lowest for most mixtures. The combination of SDS and C12EO1Ch showed the behavior of a “super-spreader”. Three examples for a good, an intermediate and a bad wetting were examined via high-speed camera and compared to the wetting agent Plurafac LF300. The combination SDS-C12EO1Ch, mass ratio 5-5, gave the same good result as the wetting agent. A good spreading behavior was also confirmed on real plant leaves. Moreover, the three catanionic combinations were used to find a correlation to their physicochemical properties. The importance of the dynamics in the surfactant solutions and its surface activity became obvious. The same conclusion could be obtained by statistical analysis. For a good spreading behavior, EO groups were necessary to guarantee high solubility. A shorter chain length promoted spreading as well as high dynamics in the surfactant solution. With the investigated catanionic mixtures, the problem of precipitation, as it is often described in the literature, can be overcome by the insertion of EO groups. Thus, the maximal efficiency of them can be used, and super-spreaders can be tuned. In the last part, the emulsification behavior was investigated via a screening of 10 different oils. Compared to the pure ionic surfactants, a lot of mixtures showed higher emulsion stability. But in general, the emulsification performance of the catanionic mixtures was lower than expected. Highest emulsion stability was obtained for the anion-cation mass ratio 5-5. Triolein was the oil which could be emulsified the best. A low interfacial tension was found to be a supporting factor for a stable emulsion. The combination of the anionic surfactant Texapon N70 and the cationic surfactant Dehyquart ACA, mass ratio 5-5, gave the best results on average. Thus, it was investigated further regarding the potential switchability of the catanionic emulsion system. The two parameters NaSCN content and temperature were found to influence the catanionic system. Their potential was investigated in an emulsion. An effect on the emulsion stability was found for the NaSCN content and the temperature, which could be used to destabilize the emulsion system. The principle concept could be confirmed, but further investigations are necessary to get a deeper insight into the influences and the emulsion capacity. Within this work, the great potential of the catanionic mixtures for various applications could be shown. Catanionic mixtures have high potential especially due to their high efficiency at low concentrations. Their synergistic effect can enhance the performance. They can be tuned by changing different parameters like anion-cation ratio, number of EO groups or chain length. They can significantly improve surfactant properties and efficiency for several possible applications in industry as well as in household applications