Kinetic studies as a tool for the characterization of hydrocarbon and fluorocarbon surfactant mixtures

MARIA FLORENCIA TORRES; MARIANA ADELA FERNÁNDEZ; RITA HOYOS DE ROSSI

Riviera maya

Conferencia; 11th Latin American Conference on Physical Organic Chemistry; 2011

Fluorinated surfactants are characterized by larger surface activity and chemical stability than the corresponding hydrocarbon type compounds. Their unique properties make them very interesting for practical applications as well as for theoretical studies of micellar systems. The chemical and thermal stability of perfluorinated compounds and their extensive use has produced the accumulation of some derivatives in the environment. The search for an alternative to the fluorosurfactants currently employed in several formulations involves the development of new products. One possible solution is the use of a mixture combining a fluoro-surfactant with a hydrocarbon-based surfactant. Certain combinations of fluoro and hydrocarbon surfactants exhibit synergistic effects with respect to their ability to lower the surface tension of water. In this way, using mixtures of fluorocarbon and hydrocarbon surfactants, it would be possible to maintain the necessary properties for a good performance of the products with a reduction of the environmental impact and cost, in comparison with the use of pure perfluorinated systems. The mixtures of perfluorinated and hydrocarbon surfactants are non ideal in many cases and some sort of microscopic demixing takes place due to the mutual phobicity of both types of compounds. In some cases fluorocarbon rich and hydrocarbon rich micelles form and coexist in the same solution while in other cases demixing within the micelle occur resulting in micelles with hydrocarbon rich and fluorocarbon rich domains. We recently reported a study of mixtures of polyoxyethylene (23) lauryl ether (Brij-35) with perfluorononanoic acid (PFNA) and concluded that there are two events happening in these solutions which were attributed to a change in morphology of the micelle and not to the coexistence of two different types of micelles. Kinetic studies of reactions with well known mechanism can be used to get information of the surfactant system. We have studied the hydrolysis reaction of phenyl trifluoroacetate (1) in mixtures of perfluorononanoic acid (PFNA) with the non-ionic surfactant, Brij-35 and with an anionic surfactant, sodium dodecyl sulphate (SDS). The rate of the reaction was measured in mixtures of the fluorinated and hydrocarbon derived surfactants in molar fraction of the fluorinated surfactant (xF) ranging from 0.1 to 0.9 and at total surfactant concentration ranging from 1x10-6 to 0.05 M. The cmc of the mixtures was also determined by several techniques (conductivity, surface tension, kinetic and/or NMR). It is interesting to remark that in the system PFNA-Brij35 two cmc were determined while the system PFNA-SDS gives only one value with all the techniques used. Comparison of the experimentally determined values of cmc for the PFNA-SDS mixtures with the calculated value for ideal mixtures lead to the conclusion that the there is a repulsive interaction between the two surfactants with an interaction parameter â >2 which, according to the literature, is consistent with the formation of two types of coexisting micelles. The kinetic data, on the other hand, do not account for the formation of two types of coexisting micelles but rather indicate that the microscopic composition of the micelles changes with xF. The similarities and differences in the results obtained with the two mixed surfactants will be discussed on the bases of the microscopic behaviour of the systems. The similarities and differences in the results obtained with the two mixed surfactants will be discussed on the bases of the microscopic behaviour of the systems. 1) in mixtures of perfluorononanoic acid (PFNA) with the non-ionic surfactant, Brij-35 and with an anionic surfactant, sodium dodecyl sulphate (SDS). The rate of the reaction was measured in mixtures of the fluorinated and hydrocarbon derived surfactants in molar fraction of the fluorinated surfactant (xF) ranging from 0.1 to 0.9 and at total surfactant concentration ranging from 1x10-6 to 0.05 M. The cmc of the mixtures was also determined by several techniques (conductivity, surface tension, kinetic and/or NMR). It is interesting to remark that in the system PFNA-Brij35 two cmc were determined while the system PFNA-SDS gives only one value with all the techniques used. Comparison of the experimentally determined values of cmc for the PFNA-SDS mixtures with the calculated value for ideal mixtures lead to the conclusion that the there is a repulsive interaction between the two surfactants with an interaction parameter â >2 which, according to the literature, is consistent with the formation of two types of coexisting micelles. The kinetic data, on the other hand, do not account for the formation of two types of coexisting micelles but rather indicate that the microscopic composition of the micelles changes with xF. The similarities and differences in the results obtained with the two mixed surfactants will be discussed on the bases of the microscopic behaviour of the systems.