Youhei Kawabata

SAXS, SANS and NSE studies on unbound state in DPPC/water/CaCl2 system

The temperature and CaCl 2 concentration dependence of the lamellar repeat distance of a dipalmitoylphosphatidylcholine (DPPC) aqueous solution was investigated by small-angle X-ray and neutron scattering, and neutron spin echo. At certain CaCl 2 concentrations, the repeat distance in the liquid-crystalline phase tends to increase up to infinity while that in the gel phase has an upper limit. This behavior is attributable to the fact that the steric repulsion of lipid bilayers due to the membrane undulation in the liquid-crystalline phase is larger than that in the gel phase. The free-energy calculation suggested that the cooperation of the short-range electrostatic interaction and the long-range steric interaction is the origin of the increase in the repeat distance in the liquid-crystalline phase, and it can be the origin of an “unbinding transition”.

Transition processes from the lamellar to the onion state with increasing temperature under shear flow in a nonionic surfactant/water system studied by Rheo-SAXS.

In a previous paper, we reported for the first time the lamellar-to-onion transition with increasing temperature at around 67 °C under a constant shear rate (0.3-10 s(-1)) in a nonionic surfactant C(16)E(7)/water system. In this study, the first temperature-shear rate diagram has been constructed in a wider range of shear rate (0.05-30 s(-1)) than in our previous study based on the temperature dependence of the shear stress at constant shear rate. The results suggest that the critical temperature above which the transition begins does not depend on the shear rate very much, although it takes a very shallow minimum. Then we have performed simultaneous measurements of small-angle X-ray scattering/shear stress (rheo-SAXS) with a stepwise increase in temperature of 0.1 K per 15 min at a constant shear rate of 3 s(-1) near the transition temperature. When the temperature exceeds 67 °C, just before the increase in the shear stress, the intensity of the Bragg peak for the velocity gradient direction (approximately proportional to the number of lamellae with their normal along this direction) is suddenly increased. As the temperature increases by 0.2 K, the shear stress begins to increase. At the same time, the peak intensity in the velocity gradient direction rapidly decreases and instead the intensity in the neutral direction increases. As the temperature increases further, the intensities in both the neutral and gradient directions decrease whereas the intensity in the flow direction increases, corresponding to the formation of onions. We have also performed rheo-SAXS experiments with a stepwise increase in shear rate at 72 °C. The sequence of the change in the intensity in each direction is almost the same in the temperature scan experiments at constant shear rate, suggesting that the transition mechanisms along these two paths are similar. The abrupt enhancement of the lamellar orientation with the layer normal along the velocity gradient direction just before the transition is the first finding and strongly supports the coherent buckling mechanism in the lamellar-to-onion transition proposed by Zilman and Granek (Zilman, A. G.; Granek, R. Eur. Phys. J. B 1999, 11, 593).

Re-entrant Lamellar/Onion Transition with Varying Temperature under Shear Flow

We have found for the first time the reentrant lamellar/onion (lamellar-onion-lamellar) transition with varying temperature under constant shear rate by using simultaneous measurements of shear stress and small-angle X-ray scattering (Rheo-SAXS) for a nonionic surfactant (C(14)E(5))/water system, which exhibits the lamellar phase in a wide temperature range from 15-75 °C. The onion state exists in a closed region in the temperature-concentration diagram at a constant shear rate. Temperature dependence of the lamellar repeat distance (d) at rest has also been measured at several concentrations. It is shown that the increase of d with increasing temperature is necessary for the existence of the lower transition. We have investigated the change in the lamellar orientation in the lamellar-to-onion and onion-to-lamellar transition processes near the upper and lower transition temperatures. For all four kinds of transition processes, the following change in the lamellar orientation is observed; lamellar state (oriented to the velocity gradient direction) ↔ further enhancement of the orientation to the velocity gradient direction ↔ enhancement of the orientation to the neutral direction ↔ onion state.

Formation process of bilayer gel structure in a nonionic surfactant solution.

We investigated the structure and formation process of lamellar domains below the Krafft temperature in two nonionic surfactants, the C(16)E(7)/water and C(16)E(6)/water systems, using an optical microscope and small-angle X-ray scattering. We found that vesicles and long leek-like lamellar domain structures are formed in the C(16)E(7) and C(16)E(6) systems, respectively. This large difference between the lamellar domain structures of the systems can be explained by the elastic properties of bilayers in the structural formation process.

Microscopic investigation on morphologies of bilayer gel structure in the mixed polyoxyethylene-type nonionic surfactant systems.

We investigated morphologies of lamellar domains below the Krafft temperature in the mixed polyoxyethylene-type nonionic surfactant, a C(16)E(6)/C(16)E(7)/water system, by using optical microscopy, confocal microscopy, small/wide-angle X-ray scattering, and small-angle neutron scattering. We have found that the morphology discontinuously changes from network structures of lamellar domains to spherical vesicles with increasing mole fraction of C(16)E(7), via the coexistence region of vesicles and network structures of lamellar domains.

Effects of surfactant concentration on formation of high-aspect-ratio gold nanorods.

The effects of surfactant concentration in a growth solution on the elongation of gold nanorods were examined. Gold nanorods were synthesized in solutions with different concentrations of hexadecyltrimethylammonium bromide (HTAB): 100, 200, 300, 400, 500, and 600 mM. The nanorods grown in a solution with higher surfactant concentrations were longer (aspect ratio ~30) than those grown in that with lower concentrations (aspect ratio <10). The self-assembled surfactant structures in the solutions were analyzed using viscosity measurement and small-angle X-ray scattering. These results showed a decrease in the inter-micellar distance with increasing surfactant concentration. Taking the chemical equilibrium for the complex formation between Au ions and HTAB micelles into account, we found that the free Au ion concentration decreases accompanied with the increase in the surfactant concentration. This decrease in the free Au ion concentration suppresses undesirable secondary nucleation of gold crystals in a growth solution, resulting in gold nanorod elongation.

3D structure of lamellar domains in a surfactant solution below the Krafft temperature.

We have studied the 3D structure of lamellar domains in aqueous solutions of nonionic surfactant C(16)E(6) and C(16)E(7) below the Krafft temperature by means of confocal microscopy. A new morphology of lamellar domains has been found in the C(16)E(6) system, which is the network of lamellar domains. In the C(16)E(7) system, we have confirmed that the spherical vesicles have a hollow including excess water. Furthermore, we have investigated the initial formation process of lamellar domains in those two systems. It has been found that initial lamellar domains of both systems are tiny and plate-like and that the domains are gradually curved in the C(16)E(7) system, while in the C(16)E(6) system, they spread to fuse together and form networks.

Shear-induced structural transition in a lyotropic lamellar phase studied using small angle neutron and light scattering

We study the effects of shear flow on structures of the lamellar phase formed in a nonionic surfactant C 16 E 7 (hepta(oxyethylene glycol)-n-hexadecylether)/D20 system by using small angle neutron scattering (SANS) and small angle light scattering (SALS) in the range of shear rate 10 -3 -30 s -1 . As the shear rate increases from 0.3 to 1 s -1 , the lamellar repeat distance for the 48 wt% sample is decreased significantly and discontinuously. With further increase in the shear rate, d increases through a sharp minimum at 1 s -1 . The minimum value does not depend on the surfactant concentration very much and is nearly equal to the thickness of the bilayers. At this shear rate (1 s -1 ), the intensity of the polarized SALS is strongly enhanced in the small angle region in the vorticity direction. These results suggest that water layers are excluded by shear flow and that large concentration fluctuations on the μm scale are induced, which corresponds to local segregation into two regions; one of these has concentrated lamellar structures and the other is a water-rich region. As the shear rate increases further (to 3 s -1 ), a broad diffraction peak appears in the vorticity direction in the polarized SALS whereas a four-lobe pattern is observed in the depolarized SALS. There results, together with the two-dimensional SANS pattern, suggest the formation of close-packed onions elongated along the flow direction at 3 s -1 . Comparison is made with the study of Nettesheim et al (2003 Langmuir 19 3618) who report the formation of multilamellar cylinders as intermediate structures, between lamellae and spherical onions.

Shear-induced structural transition in the lamellar phase of the C16E7/D2O system. Time evolution of small-angle neutron scattering at a constant shear rate

The time evolution of small-angle neutron scattering is measured for the lamellar phase of a nonionic surfactant C16H33(OC2H4)7OH (C16E7) in D2O at 48 wt% at 343 K under shear flow. At the shear rates of 0.3, 1 and 3 s−1, a new diffraction peak appears at higher q [where q = (4π/λ)sin θ, and λ and 2θ are the wavelength of the neutron beam and the scattering angle, respectively] about 1–2 h after applying shear flow and coexists with the initial diffraction peak. The coexistence of two peaks continues even after 5 h at 0.3 s−1 whereas at 1 and 3 s−1 the peak at lower q disappears after about 3 h. These results indicate that the repeat distance decreases discontinuously and so suggest some sort of transition. A plot of the repeat distance after 5 h versus shear rate shows a minimum at 1 s−1, which is in good agreement with our previous results obtained by increasing the shear rate stepwise.

Neutron Spin Echo Study on Slow Dynamics of Lipid Bilayers in the DPPC/D2O/CaCl2 System

In order to study slow dynamics of lipid bilayers, neutron spin echo (NSE) experiments were carried out on the dilute lamellar phase in the DPPC/D2O/CaCl2 system. From the NSE results, we estimated the bending modulus κ of the bilayer using the theory presented by Zilman and Granek (Phys. Rev. Letters 77 (1996) 4788). The estimated values of κ decrease monotonically with increasing temperature and with increasing the lamellar repeat distance d1. They depend strongly on d1 though d1 is longer than 500A. We discuss validity of the theory for the analysis of the NSE results.