| 1. |
- Balogh, Joakim, et al.
(author)
-
Effects of oil on the curvature elastic properties of nonionic surfactant films: Thermodynamics of balanced microemulsions
- 2006
-
In: Physical Review E (Statistical, Nonlinear, and Soft Matter Physics). - 1539-3755. ; 73:4: 041506
-
Journal article (peer-reviewed)abstract
- The free energy of nonionic balanced microemulsions based on nonionic surfactants are analyzed using experimental data from (i) phase behavior, (ii) osmotic compressibility of the balanced microemulsion structure, which is obtained from small angle neutron scattering (SANS) experiments, and (iii) data on interfacial tensions obtained by T. Sottmann and R. Strey [J. Chem. Phys. 106 8606 (1997)]. The balanced microemulsion, where the spontaneous curvature vanishes at equal volumes of water and oil, has a finite swelling with the solvent with a minimum surfactant volume fraction, Phi(*)(S). At higher surfactant concentrations the balanced microemulsion phase having the surfactant volume fraction Phi(S1) coexists with a lamellar phase of volume fraction Phi(S2). Under the constraint of Phi(W)=Phi(O), where Phi(W) and Phi(O) are the water and oil volume fractions, respectively, the free energy density can be written as an expansion in the surfactant concentration. While the phase equilibria only depend on relative values of the expansion coefficients, absolute values can be obtained from compressibility and interfacial tension data. The osmotic compressibility of the surfactant film was measured by SANS through contrast matching water and oil. The phase behavior of nonionic surfactant-water-oil systems depends strongly on the chain length of the oil, when comparing a homologous series from octane to hexadecane using the same surfactant, here being pentaethylene oxide dodecyl ether (C12E5). The three concentrations Phi(*)(S), Phi(S1), and Phi(S2) increase markedly as the chain length of the oil is increased. However, from the analysis of the surface tension data it is concluded that there are no major changes in the bending rigidities as the oil is changed. The data are analyzed within the model free energy densities G/V=a(3)Phi(3)(S)+a(5)Phi(5)(S) and G/V=a Phi(3)(S)(1+b ln Phi(S)). We find that within experimental accuracy, the first of these models provides a quantitatively consistent description of the data. For the second model there is a larger discrepancy between observed and calculated values.
|
|
| 2. |
|
|
| 3. |
- Bulut, Sanja, et al.
(author)
-
Fusion of Nonionic Vesicles
- 2010
-
In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 26:8, s. 5421-5427
-
Journal article (peer-reviewed)abstract
- We present an experimental study of vesicle fusion using light and neutron scattering to monitor fusion events. Vesicles are reproducibly formed with an extrusion procedure using an single amphiphile triethylene glycol mono-n-decyl ether in water. They show long-term stability for temperatures around 20 C, but at temperatures above 26 C we observe an increase in the scattered intensity due to fusion. The system is unusually well suited for the study of basic mechanisms of vesicle fusion. The vesicles are flexible with a bending rigidity of only a few k(H)T. The monolayer spontaneous curvature, Ho, depends strongly on temperature in a known way and is thus tunable. For temperatures where H-0 > 0 vesicles tyre long-term stable, while in the range H-0 < 0 the fusion rate increases the more negative the Spontaneous curvature Through a quantitative;analysis of the fusion rate we arrive tit a barrier to fusion changing from 15 k(B)T at T = 26 degrees C to 10k(H) T at T = 35 degrees C. These results are compatible with the theoretical predictions using the stalk model of vesicle fusion.
|
|
| 4. |
- Bulut, Sanja, et al.
(author)
-
Lamellar phase separation in a centrifugal field. A method for measuring interbilayer forces
- 2010
-
In: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-6848 .- 1744-683X. ; 6:18, s. 4520-4527
-
Journal article (peer-reviewed)abstract
- We present a novel method for measuring interbilayer forces in lamellar liquid crystals of amphiphile-water systems. In a centrifuge the gravitational effect is easily strong enough to produce clearly observable concentration gradients. During the experiment the concentration profile in the test-tube is monitored using NMR imaging of the deuterium quadrupole splitting in the lamellar phase, by temporarily transferring the sample into a NMR spectrometer. We also present a theoretical analysis of the experiment, where interactions dominate over entropy of mixing effects. For a system at sedimentation equilibrium one obtains a direct measurement of the interbilayer force, or equivalently chemical potential of the components over a substantial concentration range. It requires long times to obtain equilibrium in the centrifuge but very useful information about equilibrium and dynamic parameters is also obtained through an analysis of the sedimentation process. Experiments were performed on samples of a dilute lamellar phase of the non-ionic surfactant C10E3. After a few days of centrifugation a consistent concentration pattern was observed. At the bottom of the sample there appears a pure water-phase. The concentration profiles stabilize after a long centrifugation time. If they are related to the phase boundary the different profiles superimpose. This observation is consistent with the theory and the observation allows for a determination of how the chemical potentials vary with composition. The observed profiles are consistent with a dominating undulation force with a bilayer bending rigidity of 4.8-5.1 kT.
|
|
| 5. |
- Evilevitch, Alex, et al.
(author)
-
Kinetics of oil solubilization in microemulsion droplets. Mechanism of oil transport.
- 2001
-
In: Electroanalysis. - : American Chemical Society (ACS). - 1040-0397. ; 16:23, s. 8755-8762
-
Journal article (peer-reviewed)abstract
- We have studied the kinetics of the solubilization of oil through a temperature jump into a droplet microemulsion phase in the system water-pentaethylene oxide dodecyl ether-decane at 25 C. The initial state is formed by subjecting the equilibrium system at 25 C to a temperature quench to 22, 20, and 14 C, respectively. At this lower temperature, which at equilibrium corresponds to a two-phase system, oil droplets form and grow in size with increasing time. By varying the time between the quench and the T-jump, the size of the initial oil drops is varied in a systematic and known way in the relaxation study. The relaxation process is monitored by following the turbidity of the system. We find that for all the systems the relaxation back to equlibrium is much faster than the drop growth process observed after the temperature quench. This general observation is explained by realizing that the redissolution of the oil drops is analogous to the oil transfer phase, which in the quench experiment occurs prior to the Ostwald ripening phase. More significant is that we observed a qualitative transition in the relaxation behavior when the initial aggregate distribution is varied. In all cases we have the same initial temperature and overall composition and one population of many small droplets and fewer larger drops. The size of the larger drops only affects the relaxation in a quantitative way. If the small droplets are only slightly smaller than the equilibrium size, equilibration occurs through the diffusion of oil molecules in the bulk phase. When the initial droplets are sufficiently small, a new kinetic route is available where there is an efficient direct oil transfer between the small droplets and large drops. This allows for a fast relaxation of the oil distribution between the two populations of drops.
|
|
| 6. |
- Evilevitch, Alex, et al.
(author)
-
Molecular transport in a nonequilibrium droplet microemulsion system.
- 2001
-
In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 17:22, s. 6893-6904
-
Journal article (peer-reviewed)abstract
- In this paper, we consider the problem of oil molecular transport between nonequilibrium microemulsion oil droplets in water. In particular, we have investigated the kinetics of solubilization of big oil drops by smaller microemulsion droplets transforming a bimodal size distribution into an equilibrium microemulsion with a unimodal size distribution. The process involves the diffusion of oil monomers across the aqueous solvent. Solubilization experiments are presented on a well-characterized nonionic microemulsion system, where interfacial area and dispersed volume are conserved and where the excess oil chemical potential is dominated by interfacial curvature energy. An analysis of the experiments shows that the solubilization kinetics depend significantly on the concentrations and sizes of big and small droplets. We formulate a quantitative model for the solubilization kinetics where the effects of size and concentrations are treated within the framework of a cell model. A quantitative agreement between model and experiment is obtained, and the analysis also shows that the majority of oil monomers are captured by small droplets already in the vicinity of the big drop surface when the concentration of small droplets is high.
|
|
| 7. |
- Gotter, Martin, et al.
(author)
-
A comprehensive, time-resolved SANS investigation of temperature-change-induced sponge-to-lamellar and lamellar-to-sponge phase transformations in comparison with 2H -NMR results
- 2007
-
In: European Physical Journal E. Soft Matter. - : Springer Science and Business Media LLC. - 1292-8941 .- 1292-895X. ; 24:3, s. 277-295
-
Journal article (peer-reviewed)abstract
- Time-resolved small-angle neutron scattering (TR-SANS) was employed to observe temperature-induced phase transitions from the sponge (L-3 to the lamellar ( L (alpha) phase, and vice versa, in the water-oil (n-decane)-non-ionic surfactant (C12E5 system using both bulk and film contrast. Samples of different bilayer volume fractions phi and solvent viscosities eta were investigated applying various amplitudes of temperature jump Delta T . The findings of a previous H-2-NMR study could be confirmed, where the lamellar phase formation was determined to occur through a nucleation and growth process, while it was concluded that the L-3-phase develops in a mechanistically different and more rapid manner involving uncorrelated passage formation. Likewise, the kinetic trends of the nucleation and growth transition (decreased transition time with increase of phi and Delta T were witnessed once again. Additionally, NMR and SANS data that demonstrate a strong dependency of that process on solvent viscosity eta are presented. Contrariwise, it is made evident via both SANS and NMR results that the L-alpha-to-L-3 transition time is independent (within experimental sensitivity) of the varied parameters (phi, Delta T, eta) . Unusual scattering evolution in one experiment, originating from a highly ordered lamellar phase, intriguingly hints that a major rate determining factor is the disruption of long-range order. Furthermore, the bulk contrast investigations give insight into structure peak shifts/development during the transitions, while the film contrast experiments prove the bilayer thickness to be constant throughout the phase transitions and show that there is no evidence for a change in the short-range order of the bilayer structure. The latter was considered possible, due to the different topology of the L-3 and L-alpha phases. Lastly, an unexpected yet consistent appearance of anisotropic scattering is detected in the L-3-to-L-alpha transitions.
|
|
| 8. |
- Gotter, M, et al.
(author)
-
Fusion and fission of fluid amphiphilic bilayers
- 2005
-
In: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1364-5498. ; 129, s. 327-338
-
Journal article (peer-reviewed)abstract
- The system water-oil (n-decane)-nonionic surfactant (C12E5) forms bilayer phases in a large concentration region, but, for a given oil-to-surfactant ratio, only in a narrow temperature range. In addition to the anisotropic lamellar phase (L-alpha) there is also, at slightly higher temperature, a sponge or L-3-phase where the bilayers build up an isotropic structure extending macroscopically in three dimensions. In this phase the bilayer mid-surface has a mean curvature close to zero and a negative Euler characteristic. In this paper we study how the bilayers in the lamellar and the sponge phase respond dynamically to sudden temperature changes. The monolayer spontaneous curvature depends sensitively on temperature and a change of temperature thus provides a driving force for a change in bilayer topology. The equilibration therefore involves kinetic steps of fusion/fission of bilayers. Such dynamic processes have previously been monitored by temperature jump experiments using light scattering in the sponge phase. These experiments revealed an extraordinarily strong dependence of the relaxation time on the bilayer volume fraction phi. At phi<0.1 the relaxation times are so slow that experiments using deuterium nuclear magnetic resonance (H-2-NMR) appear feasible. We here report on the first experiments concerned with the dynamics of the macroscopic phase transition sponge-lamellae by H-2-NMR. We find that the sponge-to-lamellae transition occurs through a nucleation process followed by domain growth involving bilayer fission at domain boundaries. In contrast, the lamellae-to-sponge transformation apparently occurs through a succession of uncorrelated bilayer fusion events.
|
|
| 9. |
- Ishikawa, Kazuhiro, et al.
(author)
-
Microemulsions of Record Low Amphiphile Concentrations Are Affected by the Ambient Gravitational Field
- 2016
-
In: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 120:26, s. 6074-6079
-
Journal article (peer-reviewed)abstract
- It is shown that the ternary system heavy water-heptane-hexadecyl hexaethylene oxide (C16E6) has a stable bicontinuous microemulsion phase down to an exceptionally low concentration at the balanced temperature of 26.8 °C. It is further demonstrated that the ambient gravitational field has an influence on the observed phase equilibria for typical sample sizes (∼1 cm). Direct measurements using a nuclear magnetic resonance imaging technique demonstrate that sample compositions vary with the height in the vials. It is furthermore found that some samples show four phases at equilibrium in apparent violation of Gibbs' phase rule. It is pointed out that Gibbs' phase rule strictly applies only when effects of gravity are negligible. A further consequence of the ambient gravitational field is that, for the system studied, the microemulsion one-phase samples are not observed, when using standard size vials, that is, sample heights on the order of a centimeter. Quantitative determinations of concentration profiles can be used to determine parameters of the free-energy density for the system.
|
|
| 10. |
- Le, Thao, et al.
(author)
-
Relaxation kinetics of an L-3 (sponge) phase
- 2002
-
In: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:36, s. 9410-9417
-
Journal article (peer-reviewed)abstract
- The kinetic response of an L-3 (sponge) phase formed in the C12E5-n-decane-brine system is studied using the Joule-heating temperature jump (JHTJ) technique. The equilibrium state of the spongelike membrane is instantaneously perturbed, and the kinetic response is monitored using a multi-angle light scattering setup. These measurements yield a time-dependent scattering intensity as a function of temperature, scattering vector q, and concentration. We observed a single-exponential relaxation characteristic time, tau(m). The q dependence of the scattering amplitude shows an Omstein-Zernike behavior, but we can identify two concentration regimes with respect to the relaxation behavior. For volume fractions Phi(m) = 0.20, there is no detectable q dependence of the relaxation times, while for samples with Phi(m) > 0.30, the relaxation times display the q(-2) dependence typical of a diffusive process and with relaxation times consistent with those found in dynamic light scattering. At the intermediate concentrations, there is a transition from the q-independent to the q(-2) dependence behavior. Analysis from each concentration regime reveals distinct differences in the dependence of tau(m) on temperature and concentration, with an extraordinarily strong concentration dependence of tau(m) (tau(m) approximate to Phi(-9)) in the low concentration regime and a temperature dependence corresponding to a formal Arrhenius activation energy of 720 kJ/mol or 275 kT.
|
|
