Design, formulation and characterization of anhydrous highly concentrated surfactant mixtures

A microstructural and dynamical investigation of aqueous mixtures of a new zwitterionic surfactant, synthesized at University of Naples “Federico II”, is presented. Particularly, N,N-dimethyl-2-propyl-1-amine-N-oxide (C10-branched) is compared with the commercial linear analogue N,N-dimethyldecyl-1-amine-N-oxide (C10-linear). The final goal is rationalizing the effect of the branches on the self-aggregation process of a zwitterionic surfactant. Nowadays, home care formulations for solid surfaces mainly contain two types of surfactants: the trialkylamine oxides (usually referred to as N-oxides) and the sodium alkyl ethoxysulfates. Specifically, the N-oxides control the foaming and cleaning properties of the final product. The N,N dimethylalkylamine oxides (CH3(CH2)n-1N+(CH3)2O-) are the most common N-oxide surfactants. In aqueous mixtures, these surfactants present an equilibrium between the protonated and the non-protonated form. At pH lower than the surfactant pKa, these molecules are protonated and behave as cationic surfactants, while in mixtures with a pH value higher than the pKa, they are in the non-protonated form and behave as nonionic (more precisely, zwitterionic) surfactants. The most investigated term of the series is N,N-dimethyldodecyl-1-amine oxide (C12-linear) [1]. In Figure (1) we report the binary phase diagram of C12-linear in water.The phase diagram shows an extended micellar phase; however, liquid lyotropic crystalline phases, are stable above around 0.3 mole fraction. These phases, tend to limit the functionality of the detergent formulations. Consequently, during formulate production, a large amount of water must be included to assure mixture flow-ability. In this framework, we present the characterization of new N-oxides able to overcome these limitations. In principle C10-branched can enlarge the stability region of isotropic micellar aggregates. Thus, using branched species, it would be possible to formulate innovative surfactant mixture with higher active concentration, maintaining the feature to be ship-able, flow-able and stable. Their aqueous mixtures are characterized by surface tension, DLS, POM, QCMD, SANS, and rheology which allow the morphology of the aggregates to be deeply analyzed.