Counterion and Temperature Effects on Aqueous Ionic Perfluoropolyether Micellar Solutions by Small-Angle Neutron Scattering

Abstract

The effect of concentration and temperature on the microstructure of aqueous micelles of carboxylic perfluoropolyether surfactants, with two perfluoroisopropoxy units in the chain (n2) that is chlorine-terminated, are studied by SANS for the ammonium and potassium counterions. The SANS spectra have been analyzed by a two-shell model for the micellar form factor and a screened Coulombic plus steric repulsion potential for the structure factor in the frame of the mean spherical approximation of a multiion system reduced to an effective one component macroions system (OCM). At 28 °C, in the surfactant concentration range 0.05 to 0.12 M, the micelles display spherical shape with inner core radius of 15 Å for both counterions, and interfacial layer thickness of 4 Å. At higher concentration, both counterions provide ellipsoidal micelles, with axial ratio 2 and a limiting dimension of 13 Å. A sharp increase of temperature up to 80 °C enables the ammonium salt at 0.2 M to rearrange its ellipsoidal micelles into spherical ones, while the micelles of the potassium salt remain ellipsoidal. In all cases, the micellar size distribution is extremely narrow and the average aggregation numbers, as well as the surface charge, are found to slightly differ for the two counterions upon variation of concentration and temperature, driving ionization degrees globally spanning from 0.3 to 0.5. The interfacial hydration, the surface potential and the area per polar head at the micellar surface are discussed too.

ESR study of aqueous micellar solutions of perfluoropolyether surfactants with the use of fluorinated spin probes

“…The same conclusion can be drawn from 2a N data, keeping in mind that the value for the 500 mmol/dm 3 PFPE-Na system is overestimated due to contribution of the fast spectral component; this contribution increases at higher temperatures, as indicated by the spectrum recorded at 270 K for the sample recooled from 360 K. Under a reasonable assumption of the nitroxide group location close to the Stern layer, the hyperfine splitting data may be interpreted in terms of less ionic and less hydrated environment of the probe in large ellipsoidal micelles. An increase of the counterion binding degree as a function of surfactant concentration, in the range from ∼0.05 to ∼0.2 mol/dm 3 , was observed previously for PFPE-NH 4 [6,8,9], but PFPE-Na was examined only at two concentrations, ∼0.1 and 0.2 mol/dm 3 , and the results were the same within uncertainty [10]. No significant change in the micelle hydration number was deduced from SANS data for these systems [9,10].…”

“…An increase of the counterion binding degree as a function of surfactant concentration, in the range from ∼0.05 to ∼0.2 mol/dm 3 , was observed previously for PFPE-NH 4 [6,8,9], but PFPE-Na was examined only at two concentrations, ∼0.1 and 0.2 mol/dm 3 , and the results were the same within uncertainty [10]. No significant change in the micelle hydration number was deduced from SANS data for these systems [9,10]. Table 1 also shows the extreme line separations (2A zz ) in the slow motional spectra at 250 K. At a given surfactant concentration (rigid limit separation the same within experimental uncertainty), the 2A zz value is lower for the sodium salt system than for the ammonium salt system, reflecting the higher probe mobility in PFPE-Na micelles, in agreement with their smaller aggregation numbers [11].…”

“…Such spectra are characteristic for ordered systems [2]. The observed change of the ESR line-shape occurs in the concentration region where transformation of spherical aggregates into ellipsoidal aggregates was deduced from SANS studies [8][9][10]. This transformation likely involves ordering of surfactant chains in the micelle.…”

“…A counterion effect has been observed: at a given PFPE-X concentration, aggregation numbers, N , are higher for the ammonium salt than for the sodium salt [8][9][10][11].…”

“…Chart 1), consisting of two perfluoroisopropoxy units and the counterion X = Na + or NH + 4 . Micellization of the PFPE-X surfactants in water has been studied by surface tension [5], conductivity [6], 19 F NMR [7], SANS [8][9][10], and fluorescence quenching of a cationic pyrenebased probe [11]. It has been found that spherical micelles Chart 1.…”