| Summary: | Surfactants are used in many industrial applications, such as flotation, flocculation, water treatment, enhanced oil recovery, emulsification, drug delivery, personal care, detergency, chemical mechanical polishing, etc. The design, development, and characterization of greener surfactants in their applications are required to meet the increasing requirements for environmental protection and an escalating demand from the society for sustainable development. It is a challenge currently to develop structure-property-performance relationships for surfactant mixtures containing greener reagents to obtain maximum efficiency with minimum environmental damages. Even though conventional surfactants and their mixtures are well studied, the fundamental studies of the structure, property and performance relationships for greener surfactants when used individually or in combination with others are just started as more and more attentions are focusing on this area. In this work, solution behaviors of surfactant systems containing several amino acid based greener surfactants have been systematically investigated using various techniques, including surface tensiometry, fluorescence spectroscopy, analytical ultracentrifugation, viscometry and computer simulation to obtain information of the structures of the aggregates formed by greener surfactants or their mixtures with conventional ones in solutions. This comprehensive study helps to elucidate the mechanism of micellization behaviors of the greener surfactant mixtures and understand the roles of molecular structures and interactions in determination of system properties and performance. The colloidal and interfacial properties of a series of lipopeptides synthesized by chemo-enzymatic reactions were studied. They were found very surface active, especially C12/oligo(L-Glu). The effects of chain length of hydrophobic moiety, composition of oligopeptide as well as aqueous chemical conditions, such as pH, on the interfacial properties of the lipopeptides were studied. Results showed the mechanism of how the interfacial and colloidal properties of amino acid based surfactants can be fine tuned by adjusting structures of the molecules. Two new surfactants, Surfactin and FA-Glu, synthesized by genetically engineered bacteria were also evaluated. Both of them showed exceptional surface activity. Genetic engineering also showed its advantages when it was found that Surfactin was limited by its solubility. Quick modifications were made in the bacteria and better option FA-Glu was produced with improved structure for better solubility. The results provide fundamental knowledge of those greener surfactants on their structure-property relationships. A number of amino acid based greener surfactants with systematic structural variations, including sodium lauroyl glutamate (C12Glu), sodium lauroyl alaninate (C12Ala), sodium lauroyl sarcosinate (C12Sar), and sodium lauroyl glycinate (C12Gly), were tested as triblend mixtures with dodecyl glucoside (C12G1) and lauramidoproply betaine (LAPB) to evaluate their performance as a function of their molecular structures. The C12Gly/C13G1/LAPB system was confirmed to have very high viscosity compared to the other systems. It was found the following micellar evolution processes happened in this system that delivered the high viscosity: small spherical micelles evolved into elongated rod-like micelles in the low concentration range, and networks of worm-like micelles formed in the high concentration range. The unique properties achieved by this system were attributed to the packing of the surfactant molecules in the system. Further results confirmed that C12Gly has a desired structure with a packing parameter of 0.4 which favors formation of worm-like micelles. Foaming performance of the selected greener surfactant was also evaluated to compare with commercial benchmark system. Even though sodium dodecyl glycinate itself is not a good foaming agent as the petroleum based non-green surfactant sodium lauryl ether sulfate, when it is mixed with the other two ingredients, significant improvement can be achieved for both foamability and foam stability due to strong synergistic interactions among them. The mixture of sodium dodecyl glycinate, dodecyl glucoside and lauramidopropyl betaine actually can deliver equal viscosity, foamability and foam stability as the benchmark system, which makes this system a leading option for the future formulations in personal care industry. Computer simulation as a powerful tool was used to understand the mechanism how greener surfactant molecules interacted with each other and how they aggregated into micelles from molecular level. Simulation found that when different surfactant molecules were mixed together, the synergistic interaction reduced the electrostatic repulsion between molecules, which led to reduction of the effective space occupied by head groups making the two head groups overlapping on each other partially. The overlapping led to a close packing of molecules. The results suggest that geometry of molecules and the interactions among them play equally important roles in determining the packing of surfactants and in turn the packing controls properties and performance of the whole system. Thus a new formula for effective packing parameter of surfactant mixtures was proposed, in which not only the geometries of molecules but also the interactions among them were included to calculate the effective packing parameter for surfactant mixtures for better predictions of the properties and performance of surfactant mixture systems. The fundamental and systematical work accomplished through this work will have a profound impact on the understanding of greener surfactant mixture systems, as structure, property and performance relationships developed herein will in turn direct the use of greener surfactants in future applications for efficiency and low chemical footprint
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