Mixtures of Nonionic and Ionic Surfactants. The Effect of Counterion Binding in Mixtures of Tetradecyldimethylamine Oxide and Tetradecyltrimethylammonium Bromide

Abstract

Surfactant mixtures of the nonionic tetradecyldimethylamine oxide (TDMAO) and the cationic tetradecyltrimethylammonium bromide ((TTA)Br) have been investigated by means of small-angle neutron scattering (SANS), static light scattering, and conductivity measurements. Upon increasing the content of (TTA)Br in these mixtures, a transition from rodlike to spherical micelles is observed. The SANS curves were analyzed with models for the structure factor of charged colloids (random phase approximation and mean spherical approximation), and from this analysis the effective charge z of the aggregates was derived as a fit parameter. With rising ionic surfactant content, z at first increases linearly up to a threshold of 8-10 mol %. For higher ionic substitution it rises only more slowly and then remains practically constant for mixtures with more than 30 mol % (TTA)Br; i.e., for higher ionic content the additional counterions become effectively bound to the micellar surface. In contrast to that, the electric conductivity still increases continuously in that range, however, much more slowly than in the range before the onset of the counterion binding. The bound counterions still contribute to the electrical conductance to some degree, since other effects like the rise of monomer concentration and the decrease of the micellar size with rising (TTA)Br content of the mixtures are not sufficient to explain the increase of conductivity observed experimentally.

Anomalous small-angle X-ray scattering from charged soft matter

“…Nevertheless, about 72% of the bromide ions are directly bound to the surfactant head groups. This result is consistent with the previous estimates of counterion association from neutron scattering [50] and electrochemical methods [44]. It should be also mentioned that due to strong shape fluctuations of the micelles the assumption of a spherical core double-shell model is only an approximation and the system could be more realistically modelled by polydisperse ellipsoids.…”

Phase Behavior, Structure, and Physical Properties of the Quaternary System Tetradecyldimethylamine Oxide, HCl, 1-Hexanol, and Water

“…This value is somewhat lower than those observed in a similar matter for spherical micelles (24,28). As mentioned above, the model of small unilamellar vesicles describes the scattering curves very well except for the very low q range, where systematic deviations toward higher scattering intensity are observed.…”

“…As the vesicles are charged and will interact via a shielded Coulomb potential, we employed a random phase approximation for charged particles (23) in a fashion we have successfully used before for the description of scattering curves of charged micelles (24). For this S(q) the screening length is basically given by the concentration of the dissociated counterions, the effective hard-sphere radius was fixed to R m + D/2 + 2Å (the 2Å shall account for the hydration shell of the polar head groups), and therefore the only free parameter is the effective charge of the aggregates (z/e 0 ).…”

A Small-Angle Neutron Scattering Investigation of the TDMAO Micelle System at High Hydrostatic Pressure

“…In the dilute concentration range, just above the critical micelle concentration CMC -0.14 mmol l-1 at 298 K (Oetter & Hoffmann, 1988~89), the system forms globular micelles up to about l0 mmol l-1 (Hoffmann, Oetter & Schwandner, 1987). Rod-like aggregates with values of 1.94 + 0.03 nm for the radius R and 14.0 + 0.4 nm for the length L were found at a concentration C= 55 mmol l-1 and a temperature of 293 K (Gorski, Gradzielski & Hoffmann, 1994). The upper solid line in Fig.…”