Self-diffusion and collective diffusion of charged colloids studied by dynamic light scattering

A microemulsion of decane droplets stabilized by a nonionic surfactant film is progressively charged by substitution of a nonionic surfactant molecule by a cationic surfactant. We check that the microemulsion droplets remain identical within the explored range of volume fraction (0.02-0.18) and of the number of charges per droplet (0-40). We probe the dynamics of these microemulsions by dynamic light scattering. Despite the similar structures of the uncharged and charged microemulsions, the dynamics are very different. In the neutral microemulsion, the fluctuations of polarization relax, as is well-known, via the collective diffusion of the droplets. In the charged microemulsions, two modes of relaxation are observed. The fast one is ascribed classically to the collective diffusion of the charged droplets coupled to the diffusion of the counterions. The slow one has, to our knowledge, not been observed previously neither in similar microemulsions nor in charged spherical colloids. We show that the slow mode is also diffusive and suggest that its possible origin is the relaxation of local charge fluctuations via the local exchange of droplets bearing different numbers of charges. The diffusion coefficient associated with this mode is then the self-diffusion coefficient of the droplets.     

Observation of two diffusive relaxation modes in microemulsions by dynamic light scattering

Water-in-oil microemulsions stabilized by AOT and dispersed in n-alkane oils with a constant molar water-to-surfactant ratio were studied by dynamic light scattering. A dilution series (in the range of volume fraction of water plus surfactant, phi approximately 0.02-0.52) was used, which allowed us to extract information about droplet sizes, diffusion coefficients, interactions, and polydispersity from experimental data. We report the observation of two diffusive relaxation modes in a concentrated microemulsion (0.20 < phi < 0.5) due to density (collective diffusion) and concentration or polydispersity (self-diffusion) fluctuations. Below this concentration it was difficult to resolve two exponentials unambiguously, and in this case one apparent relaxation mode was observed. It was found that for a given composition self-diffusion is more pronounced in apparent relaxation mode for a shorter chain length alkane. The concentration dependence of these diffusion coefficients reflects the effect of hard sphere and the supplementary attractive interactions. It was observed that the attractive part becomes more pronounced in the case of a large alkane chain oil at a given temperature. This explains the shift of the region of microemulsion stability to lower temperatures for higher chain length alkanes. Increase in hydrodynamic radius, Rh, obtained from the diffusion coefficient extrapolated to infinite dilution was observed with increase of alkane chain length. The polydispersity in microemulsion systems is dynamic in origin. Results indicate that the time scale for local polydispersity fluctuations is at least 3 orders of magnitude longer than the estimated time between droplet collisions.     

Growth of silica nanoparticles in methylmethacrylate-based water-in-oil microemulsions

The mechanism of silica particle formation in monomer microemulsions is studied using dynamic light scattering (DLS), atomic force microscopy, small-angle X-ray scattering (SAXS), and conductivity measurements. The hydrolysis of tetraethylorthosilicate (TEOS) in methylmethacrylate (MMA) microemulsions (MMA = methylmethacrylate) is compared with the formation of SiO₂ particles in heptane microemulsions. Stable microemulsions without cosurfactant were found for MMA, the nonionic surfactant Marlophen NP10, and aqueous ammonia (0.75 wt%). In the one-phase region of the ternary phase diagram, the water/surfactant ratio (R _w) could be varied from 6 to 18. The DLS and SAXS measurements show that reverse micelles form in these water-in-oil (w/o) microemulsions. The minimum water-to-surfactant molar ratio required for micelle formation was determined. Particle formation is achieved from the base-catalyzed hydrolysis of TEOS. According to atomic force microscopy measurements of particles isolated from the emulsion, the particle size can be effectively tailored in between 20 and 60 nm by varying R _w from 2–6 in heptane w/o microemulsions. For MMA-based microemulsions, the particle diameter ranges from 25 to 50 nm, but the polydispersity is higher. Tailoring of the particle size is not achieved with R _w, but adjusting the particle growth period produces particles between 10 and 70 nm.     

Interfacial composition, structural parameters and thermodynamic properties of water-in-oil microemulsions

The formation and structural characteristics of water-in-oil microemulsions comprising hexadecylpyridinium chloride (CPC), alkanols (C₄–C₆) and alkanes (C₅, C₈–C₁₀) have been investigated by the method of dilution. The compositions of the surfactant and the cosurfactant in the interfacial region (interphase) of the microemulsion droplets have been determined. The thermodynamics of transfer of the cosurfactants (alkanols) from the continuous oil (alkane) phase to the interface have been evaluated from dilution measurements at different temperatures. The structural parameters, radii of the droplet and the waterpool, aggregation numbers of CPC and the alkanols in the interphase of a droplet, and the nanoparticle density of solution have been estimated assuming monodispersity of the droplets. The thermodynamics and structural parameters have been examined in terms of the chain lengths of the alkanols and alkanes. Received: 12 September 2000 Accepted: 27 October 2000     

Water droplet charging process in water-in-oil microemulsions: an electrical conductivity study

 The electrical conductivity σ of water-in-oil microemulsions formulated with AOT in pentane, heptane, decane and carbon tetrachloride was investigated in a wide range of composition and temperature. Assuming an Arrhenius-type temperature dependence of the conductivity, from the slopes of the linear portions of the plots ln(σ) vs. 1/T, the equilibrium constant K of the overall kinetic reaction yielding the droplet ionization has been evaluated as a function of the water-to-surfactant molar ratio, W. The activation energies of the whole conductivity process lie in the range 25–40 kJ mol^-1, depending on the characteristics of the oil phase. These values suggest that the ionization of the water droplets, responsible of the observed conductivity, is caused by the exchange of the droplet content (Na⁺ ions in aqueous solution, deriving from surfactant molecule dissociation), when droplets coalesce and then redisperse. Received: 12 December 1997 Accepted: 16 June 1998     

Microemulsions with Didodecyldimethylammonium Bromide Studied by Neutron Contrast Variation

The structure of water-in-oil microemulsion droplets, stabilized by didodecyldimethylammonium bromide (DDAB), has been investigated by small-angle neutron scattering (SANS). Detailed information about the curved surfactant film has been obtained by selectively deuterating the water, DDAB, and cyclohexane components. For each surfactanth-DDAB andd-DDAB and concentration, three sets of complementary neutron contrast data were analyzed together in terms of a Schultz distribution of core–shell particles. The modeling was consistent with a simple liquid-like surfactant layer, of density 0.80 g cm⁻³, with no evidence for any solvent penetration. This film thickness was found to be 11–12 Å, about 70% of an all-transC₁₂chain length. At the water interface the area per head was 56–61 Ų, while for the alkyl chains at the outer surface it was 90–125 Ų(15–30% lower than that for a truncated cone molecular configuration). The cyclohexane–water interfacial tensions γ_o/w, measured by surface light scattering, were used along with the droplet polydispersities to find that the rigidity of the DDAB film, 2K+ is close to 1.0k_BT. This means that rather than acting as an effective parameter in the SANS analysis, the polydispersity is a natural consequence of the film rigidity. These results show that the film bending energy model accounts well for the behavior of such DDAB microemulsions.     

Dependence of the equilibrium thickness and double layer potential of foam films on the surfactant concentration

Summary The effect of the surfactant concentrationc _s on the equilibrium thicknessh _r , of foam films and the potential ₀ of the diffuse electric layer is studied in a wide concentration range (including cmc). The cell for investigation of thin liquid films is improved and a good reproducibility of the measuredh _r is obtained. The possibilities for determination of the capillary pressureP in.the cell for investigation of microscopic thin liquid films are discussed. The values ofP can be measured with sufficient accuracy by the capillary-rise method, using a tube, identical with the tube of the cell for thin liquid films. The dependenceh _r (c _s ) is studied for nonionic surfactants (decylmethyl sulphoxide, nonyl-phenol 20-glycol ether and dodecyl 11-ethylene oxide) and the ionic sodium dodecyl sulphate. After reaching a steady value (a plateau in theh _r (c _s ) curve) at high concentrations (about cmc) a decrease in the equilibrium thickness of nonionic surfactants is found. The possible reasons for this decrease are discussed. It is shown that this effect probably is due to the electrostatic disjoining pressure. The values of the _o-potential at the plateau of theh _r (c _s ) curves are determined by the DLVO-theory using theh _r , values. The so determined ₀-values are more correct than the obtained earlier by this method because of the more precise measurement of the equilibrium thickness and the direct experimental determining of the capillary pressure. A qualitative interpreting of the ₀(c _s ) dependence is made on the basis of the surfactant adsorption at the solution/air interface and it is found that for nonionic surfactants the surface charge saturation is very near to the saturation in the adsorption layer, and for the ionic surfactants the adsorption layer saturation is at higher concentration.

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Zusammenfassung Es wurde der Einfluß der Tensidkonzentrationc _s auf die Gleichgewichtsdickeb _r von Schaumfilmen und das Potential ₀ der diffusen elektrischen Doppelschicht in einem weiten Konzentrationsbereich (einschließlich cmc) für nichtionische Tenside (Decylmethylsulfoxide, Nonylphenol-20-Glycolether und Dodecyl-11-Ethylenoxid) und ionogene Tenside (Natriumlaurylsulfat) studiert. Nach dem Erreichen eines konstanten Wertes (= Plateau in derb _r (c _s )-Kurve) bei höheren Konzentrationen (etwa crnc) wird eine Abnahme in der Gleichgewichtsdicke der nichtionogenen Tenside gefunden. Dies hängt wahrscheinlich mit dem elektrostatischen disjoining pressure zusammen. Aus den Überlegungen über das ₀-Potential wird gefolgert, daß für nichtionogene Tenside die Oberflächen-ladungssättigung nahe bei der Sättigung in der Adsorptionsschicht liegt. Bei ionogenen Tensiden liegt die Sättigung in der Adsorptionsschicht bei höheren Konzentrationen.

     

Diffusion Coefficients for Binary, Ternary, and Polydisperse Solutions from Peak-Width Analysis of Taylor Dispersion Profiles

Binary mutual diffusion coefficients D can be estimated from the width at half height W _1/2 of Taylor dispersion profiles using D=(ln 2)r ² t _R/(3W ² h) and values of the retention time t _R and dispersion tube radius r. The generalized expression D _h=−(ln h)r ² t _R/(3W ² _h ) is derived to evaluate diffusion coefficients from peak widths W _h measured at other fractional heights (e.g., (h = 0.1, 0.2,…,0.9). Tests show that averaging the D _h values from binary profiles gives mutual diffusion coefficients that are as accurate and precise as those obtained by more elaborate nonlinear least-squares analysis. Dispersion profiles for ternary solutions usually consist of two superimposed pseudo-binary profiles. Consequently, D _h values for ternary profiles generally vary with the fractional peak height h. Ternary profiles with constant D _h values can however be constructed by taking appropriate linear combinations of profiles generated using different initial concentration differences. The invariant D _h values and corresponding initial concentration differences give the eigenvalues and eigenvectors for the evaluation of the ternary diffusion coefficient matrix. Dispersion profiles for polymer samples of N i-mers consist of N superimposed pseudo-binary profiles. The edges of these profiles are enriched in the heavier polymers owing to the decrease in polymer diffusion coefficients with increasing polymer molecular weight. The resulting drop in D _h with decreasing fractional peak height provides a signature of the polymer molecular weight distribution. These features are illustrated by measuring the dispersion of mixed polyethylene glycols.     

A study of dimethylferrocene diffusion and electrode kinetics using microelectrode voltammetry in poly(ethylene glycol) solvent

The heterogeneous electron transfer rate constant (k _s) of dimethylferrocene (DMFc) was estimated using cyclic voltammetric peak potential separations taken typically in a mixed diffusion geometry regime in a polyelectrolyte, and the diffusion coefficient (D) of DMFc was obtained using a steady-state voltammogram. The heterogeneous electron transfer rate constant and diffusion coefficient are both smaller by about 100-fold in the polymeric solvent than in the monomeric solvent. The results are in agreement with the difference of longitudinal dielectric relaxation time (τ_L) in the two kinds of solvents, poly(ethylene glycol) (PEG) and CH₃CN, indicating that k _s varies inversely with τ_L; k _s is proportional to D of DMFc. Both D and k _s of DMFc in PEG containing different supporting electrolytes and at different temperatures have been estimated. These results show that D and k _s of DMFc increase with increasing temperature in the polyelectrolyte, whereas they vary only slightly with changing the supporting electrolyte. Received: 5 February 1998 / Accepted: 23 July 1998     

Interfacial Composition,Thermodynamic Properties,and Structural Parameters of [bmim][BF4] + Cetyltrimethylammonium Bromide + Alkanol+Toluene Microemulsions

The interfacial composition, thermodynamic properties and structural characteristics of [bmim][BF₄] + CTAB + alkanol + toluene microemulsions were studied by the method of dilution under varied temperatures (298 K, 308 K, 318 K). The compositions of surfactant and cosurfactant at the interfacial region, the distribution of cosurfactant between the interfacial region and the continuous oil phase have been estimated. The thermodynamics of transfer of cosurfactant from the continuous oil phase to the interface have been evaluated. The structural parameters viz. radii of the droplet (R _e) and the ionic liquid pool (R _IL), the thickness of the interfacial layer (l), the number of droplets (N _d) have also been estimated assuming monodispersity of the spherical droplets.     

Scaling trend in diffusion coefficients of low generation G0–G3 PAMAM dendrimers in aqueous solution at high and neutral pH

Aqueous solution diffusion coefficients for G0–G3 PAMAM dendrimers were determined from DOSY-NMR spectroscopy at high and neutral pH. The study was performed in a dilute regime and diffusion coefficients at infinite dilution (D ₀) were estimated from the variation of diffusion coefficients with dendrimer concentration. Hydrodynamic radii (R _h) for each dendrimer were estimated from D ₀ using the Stoke–Einstein relationship at both pH. According to D ₀ and R _h values, the structure of G0–G1 PAMAM dendrimers is almost insensitive to pH variations, whereas G2–G3 PAMAM dendrimers undergo swelling at neutral pH, due to surface amino groups protonation. Experimental diffusion coefficients show a scaling trend with the number of dendrimer atoms (N), with scaling laws of the type D₀ μ N^a D_{0} \propto N^{\alpha } , where α takes values of −0.39 and −0.50 at pH 12 and 7, respectively. For the first time, experimental data accounts for the scaling behavior of aqueous diffusion coefficients for low generation PAMAM dendrimers, as previously reported from molecular dynamics simulations.     

Synthesis of polypyrrole nanoparticles by oil-in-water microemulsion polymerization with narrow size distribution

Conductive spherical polypyrrole nanoparticles were obtained by polymerization in oil-in-water (o/w) microemulsions using sodium dodecyl sulfate (SDS) as anionic surfactant, ethanol as co-surfactant, and potassium persulfate (KPS, 0.017 wt.%) as oxidizing agent. The average particle diameter (Dp) of the nanoparticles was between 38 and 45 nm with narrow particle size distributions (D _w/D _n < 1.2). Dp increases with the ethanol concentration due to the intercalation between the polar heads of SDS, promoting instability of the nanoparticles and some coagulation. In this work, low surfactant concentration was used, and the molar ratio of the oxidizing agent to monomer was 8.5 × 10⁻³, a value much lower compared with others reported in the literature. Increasing the ethanol concentration in the recipes enhanced the conductive properties of the polymers due to the high π-conjugation length obtained.     

Molecular reorientation of liquid benzene. Anisotropic Raman scattering of the CH and CD stretching modes in the deuterated molecules

The anisotropic Raman spectra of the CH and CD stretching modes in seven deuterated benzenes of D_6h, D_3h, D_2h and C_2h symmetry are reported. The reorientational linewidths are interpreted within the model of anisotropic rotational diffusion. The data are consistent with NMR relaxation studies. The study covers the temperature range between T/T_c = 0.49 and T/T_c = 0.97.     

Surfactant Diffusion Near Critical Micelle Concentrations

Taylor dispersion and differential refractometry are used to measure mutual diffusion coefficients (D) for binary aqueous solutions of octylglucopyranoside, dodecylsulfobetaine, and sodium dodecyl sulfate (nonionic, zwitterionic and ionic surfactants, respectively). Aggregation causes a sharp drop in D as the concentration of each surfactant is raised through the critical micelle concentration (cmc). Differential mutual diffusion coefficients are determined in this composition region by using small initial concentration differences (3 mmol-dm^–3) and by extrapolating the measured D values to zero initial concentration difference relative to the carrier stream. The drop in D for each surfactant is more gradual than the concentration dependence predicted by the chemical equilibrium model of surfactant diffusion. Micelle polydispersity and nonideal solution behavior are discussed as possible explanations for this discrepancy. Intradiffusion coefficients (D*) for aqueous octylglucopyranoside and dodecylsulfobetaine are evaluated by integrating the relation d(cD*) = Ddc previously derived for dilute solutions of self-associating nonelectrolyte solutes.     

Structure formation of surfactants in concentrated sulphuric acid: a light scattering study

A common cationic surfactant,n-hexadecylammonium hydrogensulphate, dissolved in concentrated sulphuric acid, has been studied by static and dynamic light scattering. Micelle formation has been observed even in this unusual solvent. An apparent molar mass of 45 500±4.5% was found for the aggregates. A translational diffusion coefficientD ₀=5.5×10^–9 cm²/s was measured which gave a hydrodynamically effective radius ofR _h=17.7 nm. The geometric radius of gyration wasR _g=76.2 nm. The ratioR _g/R _h=4.33 is indicative for rodlike structures. Assuming a polydispersity ofL _w/L _n=2 this corresponds to a cylinder ofL _w=152 nm. An axial ratiop _w=(L _w/d)=60.4 nm was estimated which leads to a cylinder diameter of 2.53 nm. At surfactant concentrations higher than 5% (w/vol) the rod-like micelles aggregate to form more globular structures. The time correlation function, recorded by dynamic light scattering, exhibited a two-step decay which indicates a bimodal distribution of particle sizes. The fast motion coincides with that of the micelles at low concentrations while the other is slower than the fast one by three orders of magnitude and corresponds to the translational motion of large clusters.     

Effect of viscosity on termination rate constant in radical polymerization at low conversion

By using the expression, k_t = A₁D_s for the chain termination rate constant (where A₁ is a constant and D_s is the diffusion constant of radical chain end), a familiar chain termination rate constant, k_t = A₂/η_s (where A₂ is a constant and η_s is solvent viscosity) was examined with variation of conversion x. It was found that the proportionality of chain termination rate constant and solution viscosity is a valid relation at conversion 0 but is approximate at conversion x_c ≥ x > 0. Here x_c denotes a critical conversion under the average distance around spherical polymers formed in polymerization solution is zero. At conversions above x_c, the inverse relation between chain termination rate constant and solution viscosity is not correct.     

Formation of Concentrated Nanoemulsions by Extreme Shear

Abstract

We have systematically investigated the production of “nanoemulsions,” droplets of one liquid phase in another immiscible liquid phase that have diameters less than 100 nm. Our approach relies on a combination of extreme shear due to multipass, high‐pressure microfluidic injection and systematic control of the emulsion's composition. By repeatedly shearing a silicone oil‐in‐water emulsion in an inhomogeneous extensional shear flow, the multipass approach enables us to reduce the droplet polydispersity and average radius. Using dynamic light scattering, we study the changes in the average radius, ?a?, as a function of the number of passes, driving injection pressure (i.e., shear rate), droplet volume fraction, surfactant concentration, and droplet oil viscosity. The smallest nanoemulsion that we obtain has ?a?=18 nm. At large droplet volume fractions φ≥0.65, we observe phase inversion, rather than a reduction in the droplet size. This provides evidence that droplet coalescence can occur during extreme shear, even when a significant excess of a strongly stabilizing surfactant is present.     

Oil-Induced Structural Change in Nonionic Microemulsions

Effect of added oil (heptane or squalane) on the microemulsion structures in polyoxyethylene dodecyl ether (C₁₂EO_n) systems was investigated by means of phase behavior and NMR diffusion experiments. In the binary water-C₁₂EO_n systems, an isotropic fluid, D₂ (or L₃), and an aqueous micellar solution, Wm, phases are successively formed with increasing the EO-chain length. Upon addition of heptane, D₂ and Wm phases are merged and a microemulsion of large solubilization is produced at a low surfactant concentration. With squalane, the solubilization of oil in D₂ phase is very low or almost zero, whereas the oil solubilization in Wm phase is relatively large. These structural changes in microemulsions are discussed based on the self-diffusion coefficients of water, oil, and surfactant measured by the PGSE-NMR method. The difference in the phase behavior may be attributed to the difference in the penetration tendency of oil in the surfactant palisade layer.     

Determination of the electrochemical diffusion coefficient of methylene blue in SDS/n-C5H11OH/H2O system by non-probe microelectrode voltammetry

The diffusion coefficient of methylene blue (MB) is determined by the method of non-probe microelectrode voltammetry in sodium dodecyl sulfate (SDS)/n-C₅H₁₁OH/H₂O lyotropic liquid crystal system. The results obtained show that the diffusion coefficient of MB increases with water and n-pentanol contents in the microemulsions and the lyotropic liquid crystal but decreases with SDS content. The diffusion coefficient of SDS droplet in the microemulsions and the diffusion coefficient of SDS molecule in the lyotropic liquid crystal with MB all are less than those without MB. The magnitude order of the diffusion coefficient of MB is as follows: the coefficient in the oil-in-water (O/W) microemulsion is greater than the coefficient in the water-in-oil (W/O) microemulsion which is greater than the coefficient in the lamellar liquid crystal (LLC), which is also greater than the coefficient in the Hex.     

Coalescence in quiescent polymer blends with a high content of the dispersed phase

Coalescence in molten quiescent polymer blends induced by van der Waals forces is studied theoretically. Interaction between a droplet and its nearest neighbor keeping spherical shape during drainage of the matrix trapped between them is considered. It is assumed that droplets with time dependent radius R or effective droplets with radius R + h_c/2, where h_c is the critical inter-droplet distance for breakup of the matrix trapped between them, are randomly distributed in the blend through the whole course of the coalescence. Various approaches to calculation of the average time of coalescence, t_c (calculation with preaveraged distance between a droplet and its nearest neighbor at the start of coalescence, h₀, direct averaging of t_c and averaging coalescence frequency, 1/t_c) are compared. Calculated dependence of R on the time of coalescence, t, is compared with experimental results for polypropylene/ethylene–propylene rubber (PP/EPR) blends with EPR content from 15 to 30 wt.%. Calculations using average h₀ and direct averaging of t_c lead to more-less linear dependence of R³ on t. Bare averaging of 1/t_c leads to a steeper than linear dependence of R³ on t and to unreasonably high rate of the coalescence; averaging of 1/t_c with exclusion of pairs with elapsed coalescence time leads to decreasing rate of R³ growth but unreasonably low rate of coalescence. Theories based on the concept of effective droplet radius give smaller differences among various methods of calculations of t_c than the theories assuming random distribution of the droplets. Experimental results show decreasing slope of R³ vs. t dependence, especially for a higher content of dispersed EPR. Theories using average h₀ or direct averaging of t_c predict somewhat smaller rate of coalescence than that experimentally determined for PP/EPR blends. Reasons of these differences are discussed.