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

Nitroxides: Versatile Reporters and Reactants

Stable nitroxyl radicals are important tools in chemistry, biophysics, biology, and materials science. Their stability and the sensitivity of their EPR spectra to the local environment make them valuable molecular probes. This review seeks to give an overview of the developments in the field of nitroxide spin probes and their various applications, with the main focus on the pH-sensitive imidazoline nitroxide family.

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

Fluoroalkyl esters of 3-carboxy pyrroline nitroxide, FPn (n=8 and 12), containing (n-2) CF(2) groups in the side-chain, were used as novel ESR spin probes of fluorinated micellar systems. The method was applied to study aqueous solutions of perfluoropolyether surfactants of the general formula Cl(C(3)F(6)O)(2)CF(2)COOX, consisting of two perfluoroisopropoxy units and the counterion X = Na(+) or NH(+)(4). By measuring the change of (14)N hyperfine splitting with surfactant concentration the critical micellar concentration of the ammonium salt was determined at temperatures of 297, 313 and 333 K. The ESR line shape was also examined as a function of surfactant concentration and of temperature in the range 120-360 K. The results are discussed in terms of solubilization and local environment of the probes in micelles of different size and shape, depending on the surfactant concentration and the kind of the counterion.

Small-Angle Neutron Scattering of Mixed Ionic Perfluoropolyether Micellar Solutions

Aqueous mixed micellar solutions of perfluoropolyether carboxylic salts with ammonium counterions have been studied by small-angle neutron scattering. Two surfactants differing in the tail length were mixed in proportions n2/n3 = 60/40 w/w, where n2 and n3 are the surfactants with two and three perfluoroisopropoxy units in the tail, respectively. The tails are chlorine-terminated. The mixed micellar solutions, in the concentration range 0.1-0.2 M and thermal interval 20-40 degrees C, show structural characteristics of the interfacial shell that are very similar to ammonium n2 micellar solutions previously investigated; thus, the physics of the interfacial region is dominated by the polar head and counterion. The shape and dimensions of the micelles are influenced by the presence of the n3 surfactant, whose chain length in the micelle is 2 A longer than that of the n2 surfactant. The n3 surfactant favors the ellipsoidal shape in the concentration range 0.1-0.2 M with a 1/2 ionization degree of n2 micelles. The very low surface charge of the mixed micelles is attributed to the increase in hydrophobic interactions between the surfactant tails, due to the longer n3 surfactant molecules in micelles. The closer packing of the tails decreases the micellar curvature and the repulsions between the polar heads, by surface charge neutralization of counterions migrating from the Gouy-Chapman diffuse layer, leading to micellar growth in ellipsoids with greater axial ratios.

Interactions of l-arginine with Langmuir monolayers of di-n-dodecyl hydrogen phosphate at the air–water interface

The surface phase behavior of di-n-dodecyl hydrogen phosphate (DDP) in Langmuir monolayer and its interactions with L-arginine (L-arg) have been investigated by measuring pi-A isotherms with a film balance and observing monolayer morphology with a Brewster angle microscopy (BAM). The DDP monolayers on pure water show a first-order liquid expanded-liquid condensed (LE-LC) phase transition and form fingering LC domains having uniform brightness at different temperatures. At 15 degrees C, the pi-A isotherms on pure water and on different concentration solutions of L-arg show a limiting molecular area at approximately 0.50 nm(2)/molecule. With increasing the subphase concentration of L-arg up to 4.0 x 10(-4)M, the LE and the LE-LC coexistence regions shift to larger molecular areas and higher surface pressures, respectively. With a further increase in the concentration of L-arg beyond this critical concentration, these isotherms show little or no more expansion. These results have been explained by considering the fact that the L-arg undergoes complexation with the DDP to form L-arg-DDP that remains in equilibrium with the components at the air-water interface. As the concentration of L-arg in the subphase increases, the equilibrium shifts towards the complex. At a concentration of L-arg > or =4.0 x 10(-4)M, the DDP monolayers get saturated and show the characteristics of the new amphiphile, L-arg-DDP. BAM is applied to confirm the above results. When the concentration of the L-arg is <4.0 x 10(-4)M, domains always start forming at an area of approximately 0.64 nm(2)/molecule, which is the critical molecular area for the phase transition in the DDP monolayers on pure water. In contrast, when the monolayers are formed on a solution containing > or =4.0 x 10(-4)M L-arg, comparatively smaller size domains are formed after the appearance of a new cusp point at approximately 0.55 nm(2)/molecule. With an increase in the concentration of L-arg in the subphase, the size of the domains decreases indicating that the fraction of the DDP gradually decreases, whereas the fraction of the complex gradually increases. In addition, a very simple procedure for determination of the stability constant, which is 2.6 x 10(4)M(-1) at 15 degrees C, has been suggested.

Small-Angle Neutron Scattering of Ionic Perfluoropolyether Micellar Solutions: Role of Counterions and Temperature

This paper reports a small-angle neutron scattering (SANS) characterization of perfluoropolyether (PFPE) aqueous micellar solutions with lithium, sodium, cesium and diethanol ammonium salts obtained from a chlorine terminated carboxylic acid and with two perfluoroisopropoxy units in the tail (n(2)). The counterion and temperature effects on the micelle formation and micellar growth extend our previous work on ammonium and potassium salts n(2) micellar solutions. Lithium, sodium, cesium and diethanol ammonium salts are studied at 0.1 and 0.2 M surfactant concentration in the temperature interval 28-67 degrees C. 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 multicomponent system reduced to a generalized one component macroions system (GOCM). At 28 degrees C, for all the salts, the micelles are ellipsoidal with an axial ratio that increases from 1.6 to 4.2 as the counterion volume increases. The micellar core short axis is 13 A and the shell thickness 4.0 A for the alkali micelles, and 14 and 5.1 A for the diethanol ammonium micelles. Therefore, the core short axis mainly depends on the surfactant tail length and the shell thickness on the carboxylate polar head. The bulky diethanol ammonium counterion solely influences the shell thickness. Micellar charge and average aggregation number depend on concentration, temperature and counterion. At 28 degrees C, the fractional ionization decreases vs the counterion volume (or molecular weight) increase at constant concentration for both C = 0.1 M and C = 0.2 M. The increase of the counterion volume leads also to more ellipsoidal shapes. At C = 0.2 M, at 67 degrees C, for sodium and cesium micelles the axial ratio changes significantly, leading to spherical micelles with a core radius of 15 A, lower average aggregation number, and larger fractional ionization.

Aggregation of Perfluoropolyether Carboxylic Salts in Aqueous Solutions. Fluorescence Probe Study

Aqueous solutions of anionic surfactants Cl(C3F6O)nCF2COOX, consisting of n = 2 and 3 perfluoroisopropoxy units and the counterion X = Na+ or NH4+, were studied by the method of fluorescence quenching with the use of (1-pyrenylbutyl)trimethylammonium bromide as a luminophore, and 1,1'-dimethyl-4,4'bipyridinium dichloride (methyl viologen) as a quencher. From the kinetics of fluorescence decay (time-resolved experiments) micellar aggregation numbers, N, and rate constants of the intramicellar quenching were determined for a wide range of surfactant concentrations, on the basis of the model developed by Infelta and Tachiya. The results are discussed in terms of the shape of the aggregates and the degree of counterion binding. The most important conclusions include: (i) a significant increase of N with increasing surfactant concentration suggests that spherical micelles formed at critical micellar concentration (CMC) transform into ellipsoidal aggregates, (ii) the degree of counterion binding to micelles is higher for NH4+ than for Na+, leading to higher N values in the case of the ammonium salt (n = 2), and (iii) at concentrations close to CMC the longer chain surfactant (n = 3) forms loose aggregates suggesting significant permeation with water molecules. An additional finding of this study is that the micelle-bound luminophore and quencher can form a ground-state complex, and for this reason the N values cannot be evaluated properly from the steady-state fluorescence intensity data using the equation proposed by Turro and Yekta.