Self-diffusion Coefficient Measurements Research Articles

Structure of SDS Micelles with Propylene Carbonate as Cosolvent: a PGSE−NMR and SAXS Study

The effect of propylene carbonate on SDS micelles was investigated by means of pulsed gradient spin-echo (PGSE) NMR, small-angle X-ray scattering (SAXS), conductivity and ion-selective electrode (ISE) measurements. The knowledge of the cosolvent partition between continuous phase and micelles (obtained by means of PGSE-NMR) allowed the identification of relevant dilution paths. Along these paths the system is composed of identical micelles that become more and more diluted. The extrapolation of measured self-diffusion coefficient to infinite dilution (where direct and hydrodynamic interactions are negligible) permits the determination of hydrodynamic size of the micelles. Moreover, the micelle ionization degree (measured by means of ISE) combined with PGSE-NMR and conductivity data furnishes an estimate of the aggregation number without any assumptions on micellar shape. On the other hand, troublesome hydrodynamic interactions are irrelevant to SAXS, and scattering data collected at fixed composition can be analyzed according to a reasonable model by exploiting the insight on the propylene carbonate partition gained through PGSE-NMR. By means of these approaches, we have found that propylene carbonate acts mainly as cosurfactant for the SDS micelles, decreasing their size and aggregation number by increasing the mean headgroup area of SDS.

Determination of the physico-chemical parameters and aggregation number of surfactant in micelles in binary alcohol–water mixtures

Abstract The effect of ethanol on the micellization of the anionic surfactant, sodium dodecyl sulfate (SDS), was investigated using potentiometric and pulsed field gradient–NMR spectroscopic techniques. Potentiometry studies showed that the critical micelle concentration (CMC) of surfactant decreases to a minimum value at around 10% ethanol (v/v) because of the co-surfactant effect. The mean values of the hydrodynamic radius, Rh, and aggregation number, NA, of micelles were determined by a combination of viscosity and self-diffusion coefficient measurements. The viscosity of the micellar solutions was approximately independent of ethanol concentration, indicating that the presence of the alcohol does not induce substantial changes in the micellar structure. The number of attachment of ethanol in the micellar core and the degree of dissociation of the counterion were evaluated using a potentiometric technique for alcohol concentrations between 5% and 40% v/v. In addition, an explicit expression is developed for the average size of micelles formed in aqueous solvents containing alcohol.

Partition Equilibria for Alcohols in Reverse Micellar AOT-Oil-Water Systems Studied by PGSE-FT NMR. A Comparison Between AOT-containing and the Corresponding AOT-free Systems

Microemulsions of the reverse micellar type were investigated by determining the self-diffusion coefficients of the components using the Pulsed Field Gradient Spin Echo – Fourier Transform NMR method (PGSE-FT NMR). The microemulsions were composed of the surfactant AOT, water and an oil (either benzene or cyclohexane), forming a water core in an oil continuum. The primary alcohols ranging from methanol to 1-decanol were added to the microemulsions as a fourth component. The degree of binding, p, of the alcohol to the micelles was determined from the measured self-diffusion coefficients for this component. Partition equilibrium constants were calculated from the values of p. Thermodynamic partition equilibrium constants, Kc, calculated from the partition coefficients are presented and compared with values based on literature data for AOT-free systems. Similarities and differences between these cases are revealed and interpreted.

The fragile-to-strong dynamic crossover transition in confined water: nuclear magnetic resonance results

By means of a nuclear magnetic resonance experiment, we give evidence of the existence of a fragile-to-strong dynamic crossover transition (FST) in confined water at a temperature T(L)=223+/-2 K. We have studied the dynamics of water contained in 1D cylindrical nanoporous matrices (MCM-41-S) in the temperature range 190-280 K, where experiments on bulk water were so far hampered by crystallization. The FST is clearly inferred from the T dependence of the inverse of the self-diffusion coefficient of water (1D) as a crossover point from a non-Arrhenius to an Arrhenius behavior. The combination of the measured self-diffusion coefficient D and the average translational relaxation time tau(T), as measured by neutron scattering, shows the predicted breakdown of Stokes-Einstein relation in deeply supercooled water.

Self-diffusion measurements by a mobile single-sided NMR sensor with improved magnetic field gradient

A simple and fast method of measuring self-diffusion coefficients of protonated systems with a mobile single-sided NMR sensor is discussed. The NMR sensor uses a magnet geometry that generates a highly flat sensitive volume where a strong and highly uniform static magnetic field gradient is defined. Self-diffusion coefficients were measured by Hahn- and stimulated echoes detected in the presence of the uniform magnetic field gradient of the static field. To improve the sensitivity of these experiments, a Carr–Purcell–Meiboom–Gill pulse sequence was applied after the main diffusion-encoding period. By adding the echo train the experimental time was strongly shortened, allowing the measurement of complete diffusion curves in less than 1 min. This method has been tested by measuring the self-diffusion coefficients D of various organic solvents and poly(dimethylsiloxane) samples with different molar masses. Diffusion coefficients were also measured for n-hexane absorbed at saturation in natural rubber with different cross-link densities. The results show a dependence on the concentration that is in good agreement with the theoretical prediction. Moreover, the stimulated-echo sequence was successfully used to measure the diffusion coefficient as a function of the evolution time in systems with restricted diffusion. This type of experiment proves the pore geometry and gives access to the surface-to-volume ratio. It was applied to measure the diffusion of water in sandstones and sheep Achilles tendon. Thanks to the strong static gradient G 0, all diffusion coefficients could be measured without having to account for relaxation during the pulse sequence.

Isothermal desorption measurements of self-diffusion in supercooled o-terphenyl

Isothermal desorption of o-terphenyl thin-film bilayers was used to measure self-diffusion coefficients of supercooled o-terphenyl near the glass transition temperature (Tg=243 K). Diffusion coefficients from 10(-15.5) to 10(-12) cm2 s(-1) were obtained between 246 and 265 K. Protio and deuterio o-terphenyl were sequentially vapor deposited, then annealed to simultaneously diffuse and desorb the sample in a vacuum chamber. During the desorption of the bilayer, the concentration of each isotope was detected by a mass spectrometer, which revealed the extent of interfacial broadening. In these experiments, isotopic interdiffusion is indistinguishable from self-diffusion and the measured interfacial broadening is consistent with Fickian diffusion. The samples prepared under several different deposition conditions yielded the same self-diffusion coefficients, indicating that the experiments were conducted in the equilibrium supercooled liquid state.

Mechanisms and dynamics of crystal growth, viscous flow, and self-diffusion in silica glass

We analyzed extensive literature data on crystal growth rate and viscosity in the temperature range between $1.1\phantom{\rule{0.3em}{0ex}}{T}_{g}$ (glass transition temperature) and the melting point of silica, ${T}_{m}$. We selected data for one silica glass type, having similar impurity contents, and confirmed that the normal growth model describes quite well the experimental growth rate data in this wide undercooling range. We then calculated effective diffusion coefficients from crystal growth rates, ${D}_{u}$, and from viscosity, ${D}_{\ensuremath{\eta}}$ (through the Eyring equation), and compared these two independent diffusivities with directly measured self-diffusion coefficients of silicon and oxygen in the same silica glass for which viscosity was measured. Our results show that silicon (not oxygen) controls the diffusion dynamics involved in both crystal growth and viscous flow in undercooled silica. This study not only unveils the transport mechanism in this important glass-forming material, but also validates the use of (easily measured) viscosity to account for the unknown transport term of the crystal growth expression in a wide range of undercoolings.

Influence of Ingredients on the Self-Diffusion of Aroma Compounds in a Model Fruit Preparation: An Nuclear Magnetic Resonance−Diffusion-Ordered Spectroscopy Investigation

Diffusion-ordered spectroscopy (DOSY)-pulsed field gradient (PGF) nuclear magnetic resonance (NMR) spectroscopy was used to measure self-diffusion coefficients of aroma molecules in model fruit preparations. The impact of the sucrose content on aroma diffusion was specifically investigated, and the relationship with viscosity, water activity, and dry matter parameters was evidenced. DOSY-PGF NMR spectroscopy was found to be a relevant and accurate technique to follow self-diffusion of aroma compounds at low concentrations in a complex food matrix and to obtain information on diffusion of the sucrose and of the water molecules. We showed that aroma self-diffusion was strongly decreased in fruit preparation because of the high sucrose content, which induces the formation of a network through hydrogen bonds with water. Self-diffusion coefficients were determined for aroma molecules of different natures, and values are related to the physicochemical properties of the molecule.

Measurement of Self-Diffusion Coefficients in Li Ionic Conductors by Using the Short-Lived Radiotracer of <SUP>8</SUP>Li

Measurement of Self-Diffusion Coefficients in Li Ionic Conductors by Using the Short-Lived Radiotracer of <SUP>8</SUP>Li

Measurement of self-diffusion coefficients in Li ionic conductors by using the short-lived radiotracer of 8Li

For the effective use of short-lived radioactive beams, soon to be available at the Tokai Radioactive Ion Accelerator Complex, the authors have developed a radiotracer method for diffusion studies in solids. The experimental test was performed by the measurement of the diffusion coefficients of Li in a sample of the compound βLiAl using an α-emitting radiotracer of 8Li (T1/2=0.84 s). It was found that the time-dependent yields of the α particles from the diffusing 8Li that was initially implanted in the sample could be used as a measure of the diffusivity of the tracer in a nondestructive way. The method was applied to measure the self-diffusion coefficients of Li in βLiGa, and for investigating how the Li diffusion in the Li ionic conductors is affected by the concentration of atomic defects (i.e., the existence of the atomic vacancies of Li and the defects in Ga sites that are replaced by Li).

NMR investigation of water and methanol transport in sulfonated polyareylenethioethersulfones for fuel cell applications

We report an investigation of water and methanol transport in polymer electrolyte membranes based on highly sulfonated polyarelenethioethersulfones (SPTES) for direct methanol fuel cell (DMFC) applications. Measurements of both water and methanol self-diffusion coefficients of SPTES polymer as well as in a reference sample of Nafion-117 equilibrated in 2 M methanol solution have been carried out, using the pulsed gradient spin echo technique, over a temperature range of 20–140 °C. The selectivity of the membrane, defined as ( D OH/ D CH3), decreased from 6 to 2.4 as temperature increased from 20 to 140 °C in SPTES sample while in Nafion, the value decreased from 3.2 to 1.4 as temperature increased from 20 to 100 °C. These results indicate significantly lower fuel molecular permeability in SPTES compared to that of Nafion. All results suggest high-temperature stability in these materials, offering the possibility of fuel cell operation at temperatures >120 °C. High pressure NMR diffusion measurements were also carried out for three different water contents (between 20 and 55 wt.%) in a static field gradient in order to get supplemental information regarding water transport in SPTES materials. The calculated activation volume increased from 1.54 to 8.40 cm 3/mol as the water content decreased from 55 to 20%. This behavior is qualitatively similar to previously reported results for Nafion-117.

PFG NMR measurements of tortuosity factors for diffusion in meso- and macropores of FCC catalysts

Pulsed field gradient (PFG) NMR is applied to measure self-diffusion coefficients of 1,3,5 tri-isopropyl-benzene in the particles of several industrial FCC catalysts having different systems of meso- and macropores. Relating these diffusivities to the self-diffusion coefficient of 1,3,5 tri-isopropyl-benzene in the liquid phase, the tortuosity factors for diffusion in the meso- and macropores of the particles were obtained. The values of the tortuosity factors were found to increase with a increasing volumetric fraction of the small (< 20 nm) meso- and macropores in the total intraparticle volume.

Polyphosphate composite: conductivity and NMR studies

A polyphosphate composite [NH 4PO 3] 6[(NH 4) 2SiP 4O 13] with high ionic conductivity was prepared and characterized by chemical analysis, X-ray diffraction, thermal gravimetry, impedance spectroscopy, and different NMR techniques. Experiments were carried out in dry and humid atmospheres and in temperatures ranging between 20 °C and 300 °C. During initial heating (activation) of the composite to 300 °C, a weight loss of 7% due to release of ammonia occurs resulting in a material of the formal composition of [HPO 3] 3[NH 4PO 3] 3[(NH 4) 2SiP 4O 13]. This material is thermally stable between room temperature and 300 °C. The conductivity was found to be remarkably high (0.1 S cm −1) in a water-rich environment. Mainly ammonium species could be found in the 1H MAS NMR spectra of the non-activated composite, whereas during the activation process another signal due to bridging hydrogen increases. Temperature-dependent 2D and 1D exchange NMR spectroscopy, PFG (pulsed field gradient) NMR, and SFG (stray field gradient) NMR measurements were performed in order to compare diffusion coefficients and exchange rates with dc conductivities. The small differences between experimentally obtained conductivities and those determined from the measured self-diffusion coefficients by means of the Nernst–Einstein equation hints to an ammonium vehicle as charge carrier, but can be also explained by H + conductivity. The slow NMR exchange rates between hydroxyl groups and ammonium ions exclude proton conductivity via hydroxyl groups.

Dynamical dimer structure and liquid structure of fatty acids in their binary liquid mixture: decanoic/octadecanoic acid and decanoic/dodecanoic acid systems

Dimer structure and liquid structure of fatty acids in their binary mixtures such as decanoic acid (DA)/octadecanoic acid (SA) and DA/dodecanoic acid (LA) were studied through the measurements of self-diffusion coefficient ( D), differential scanning calorimetry (DSC), density and viscosity. The obtained phase diagrams showed that DA and SA form a eutectic in the solid state but partly a solid solution in the SA-rich region; DA and LA form an incongruent-melting compound which forms a eutectic with DA. In the liquid mixture of DA and SA, the D of DA is larger than that of SA over the entire range of compositions and tends to approach the D of SA with increasing SA-mole fraction; the D of DA in the DA/LA system is also larger than that of LA especially in the LA-poor region and steeply approaches that of LA with increasing LA-mole fraction. These D values and phase diagrams were compared with those for the binary mixtures of n-alkanes (C 14/C 20, C 19/C 20 and C 20/C 24); it is concluded that the two kinds of fatty acids always form their individual homodimers in their liquid mixtures regardless of their compositions and temperatures.

Atomic diffusion in liquid Ni, NiP, PdNiP, and PdNiCuP alloys

We investigated the self-diffusion of Ni in liquid Ni, Ni80P20, Pd40Ni40P20, and Pd43Ni10Cu27P20 at temperatures up to 1795K with incoherent, quasielastic neutron scattering. Values of measured self-diffusion coefficients vary over the accessible temperature ranges as a function of composition only within 10%. Although mixing has a drastic effect on the liquidus temperature and the undercooling capabilities, a relation between these properties and the atomic diffusion in the liquid is not observed. Apparently, diffusive motion is governed by the packing fraction of the atoms, that is very similar in these dense liquids.

Aggregation behavior of acrylamide/2-phenoxyethyl acrylate and its interaction with sodium dodecyl sulfate in aqueous solution studied by proton 1D and 2D NMR

H-1 NMR self-diffusion coefficient, spin-lattice relaxation time, spin-spin relaxation time, and two-dimensional nuclear Overhauser enhancement (2D NOESY) measurements have been used to study the association behavior of a novel hydrophobically associating copolymer composed of acrylamide ( AM) and a small amount of 2-phenoxyethyl acrylate ( POEA), and its interaction with the anionic surfactant sodium dodecyl sulfate (SDS). Three sets of copolymers with approximately the same composition but with different hydrophobic POEA contents were investigated. The POEA contents for these copolymers were about 1.41, 1.03, and 0.56 mol% respectively, as validated by H-1 NMR spectra. Self-diffusion coefficient measurements show that the aggregation process occurs in a relative narrow concentration range and the critical association concentrations (cacs), of these copolymers are within this narrow concentration range, which are in agreement with those measured by viscosity measurements (6 g L-1). Above this concentration, the hydrophobic POEA moieties are found to associate and possibly build a transitory three-dimensional network along the polyacrylamide (PAM) backbones, which induces a strong decrease in NMR parameters including self-diffusion coefficients and relaxation times. The surfactant SDS showed a significant interaction with the copolymer in the dilute solution. Addition of SDS resulted in the binding of SDS on copolymer POEA-PAM segments and reinforced the interchain transient network formation of copolymer at a concentration below its cac. In the SDS/POEA-PAM mixed systems, the hydrophobic methylene groups of the SDS molecules were preferentially located in the vicinity of the phenoxy groups of the POEA hydrophobes.

Size Polydispersity Determination in Emulsion Systems by Free Diffusion Measurements via PFG-NMR

A new method to determine particle size distribution from NMR self-diffusion data is presented. The method is not subjected to the usual limitations of standard techniques, like an a priori hypothesis on the form of the distribution, or the prerequisite that suitable experimental conditions exist to enclose the whole volume fraction distribution. The new procedure, based on the measurements of free self-diffusion coefficients, was applied first by way of demonstration, to a water-in-oil emulsion system. Here, the experimental conditions were such that the data had enough information to describe the distribution function completely. Very good agreement was found for the size distribution obtained via NMR and optical microscopy data analysis. Remarkably, using the same approach in the case of a more complex oil-in-water emulsion system, for which experiments did not cover the whole range of size distribution, very good agreement was also obtained.

Kinetics and mechanisms of crystal growth and diffusion in a glass-forming liquid

Extensive data on the viscosity, covering 15 orders of magnitude, and crystal growth rate, covering seven orders of magnitude, of liquid diopside (CaO.MgO.2SiO(2)) were collected in a wide range of undercoolings from 1.10T(g) to 0.99T(m) (T(g) is the glass transition temperature and T(m) the melting point). The raw growth rate data were corrected for the increased interfacial temperature produced by the heat released during crystallization. A detailed analysis confirms that growth mediated by screw dislocations reasonably explain the experimental data in these wide ranges of temperatures and growth rates. Effective diffusion coefficients were calculated from crystal growth rates and from viscosity, and were then compared with measured self-diffusion coefficients of silicon and oxygen in diopside melt. The results show that oxygen and silicon control the diffusion dynamics involved in crystal growth and viscous flow. This study not only unveils the transport mechanism in this complex liquid, but also validates the use of viscosity (through the Stokes-Einstein or the Eyring equations) to account for the kinetic term of the crystal growth expression in a wide range of temperatures.

Mixed self-assembly of poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers and sodium dodecyl sulfate in aqueous solution.

Interaction of amphiphilic poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers with anionic sodium dodecyl sulfate (SDS) has been investigated in aqueous solution. Formation of mixed micelles has been confirmed by surface tension measurements, whereas the influence of the surfactant on the copolymer self-assembling has been studied by measurement of the 1H NMR self-diffusion coefficients and by small-angle neutron scattering. As a rule, the surfactant decreases the heterogeneity of the micellar structures formed by the copolymer in water. Moreover, increasing the content of SDS results in the increasingly more important extension of the poly(ethylene oxide) (PEO) corona chains and the copolymer micelle deaggregation. The stability of the micelles against SDS increases with the length of the hydrophobic block. Preliminary two-dimensional NMR measurements with nuclear Overhauser enhancement have confirmed the spatial vicinity between SDS and the constitutive blocks of the copolymer.

Study of the electrostatic and steric contributions to the free energy of ionic/nonionic mixed micellization

We have investigated mixed micelle formation in mixtures of ionic and nonionic surfactants. Three types of ionic surfactant were used: CTAB (hexadecyltrimethylammonium bromide), CPC (hexadecylpyridinium chloride) and CPB (hexadecylpyridinium bromide). These surfactants all have a hexadecyl chain but contain different head groups. The use of CPC and CPB, which differ only in counter ion, allowed investigation of counter ion effects. TritonX-100 ( p-(1,1,3,3-tetramethylbutyl) polyoxyethylene) was used as the nonionic surfactant in all experiments. PFG-NMR was used to measure the self-diffusion coefficients of the mixed micelles as a function of solution composition and total surfactant concentration. The aggregation number and hydrodynamic radius of each type of mixed micelle were determined by combining viscosity and self-diffusion coefficient measurements. The electrostatic and steric contributions to the free energy of mixed micellization, which are considered to be the most important contributions for mixtures of ionic and nonionic surfactants with different head groups, were determined. The electrostatic free energy was determined by solving an analytical approximation to the Poisson–Boltzmann equation. The electrostatic free energy varied dramatically with increasing mole fraction of ionic surfactant in the CTAB/TritonX-100 system but increased more gradually in the CPC/TritonX-100 and CPB/TritonX-100 systems. The more pronounced change for CTAB/TritonX-100 system can be attributed to the trimethylammonium headgroup of CTAB, which confers a high surface charge density and thus a high electrostatic free energy.