We have examined the structure of the lamellar phase (Lalpha) that coexists with a micellar solution (L1) for a commercial sodium alkyl benzene sulfonate (LAS) mixed with water. The surfactant is a mixture containing C10-C13 alkyl chains, having all positional isomers of the benzene sulfonate group present except the 1-isomer. Unusually for ionic surfactants, the difference in compositions between the coexisting L1 and Lalpha phases is large (L1 = approximately 20 wt % LAS; Lalpha = approximately 65 wt %). The main technique employed was X-ray diffraction, supplemented by optical microscopy and differential scanning calorimetry (DSC). At ambient temperatures, the lamellar phase gives a single diffraction pattern with the main reflection (d) at approximately 32.5 A, whatever the composition. However, above 40 degrees C, the diffraction peak becomes broader and moves to higher d values. At higher temperatures still, several distinct and different diffraction peaks are observed, differing in detail according to composition. The largest d values (approximately 42-4 A) are observed for the lowest LAS concentrations, while the largest number of separate reflections (five) occurs for samples with approximately 44-50% LAS, both at the highest temperatures. Although there are some differences in the data between heating and cooling cycles, the d values return to the original value at low temperature. There are no observable transitions in DSC, nor is there any heterogeneity in the lamellar phase observable by microscopy. The data clearly indicate that there is some lateral separation of the different LAS isomers within the bilayers, which results in the formation of local lamellar regions having different surfactant compositions. This lateral phase separation may arise from the presence of an (electrostatic) attractive interaction, which gives rise to an upper consolute loop within the lamellar phase region of a pure LAS isomer. Similar mechanisms may occur in biological membranes and could be responsible for the occurrence of membrane lipid patches.
The effects of sodium dodecyl sulfate (SDS) concentration (≤ 10 mass %), temperature (35 and 50 °C), and pressure (0.1 to 100 MPa) on the cooperative diffusion coefficient and static correlation length of micelles in solutions in 1 M NaCl were studied using static and dynamic light scattering. These data are interpreted in terms of models of dilute and semidilute solutions of rod-like polymers. The results indicate that the effects of pressure are most significant near the crossover concentration between dilute and semidilute solution behavior. This concentration is a function of temperature and pressure because both of these thermodynamic parameters affect the mean micellar size.
We have used dynamic and static light scattering to study rodlike micelles of C & in HzO. Nine surfactant concentrations, C, in the range 3 < C < 510 mg/mL and in the temperature, T, range 15 < T < 50 OC were studied. The mean diffusion coefficient, determined using dynamic light scattering, shows a well-defined minima as a function of concentration along each isotherm. The concentration at which the minimum occurs is temperature dependent and to a very good approximation marks the threshold crossover concentration from dilute to semidilute solution behavior. From these data, using the recent theory of Carale and Blankschtein (J. Phys. Chem. 1992, 96, 459), we determine the persistence length, a measure of micellar flexibility, as a function of temperature. It obtains a value of 15 nm near 20 OC and 6 nm near 50 OC. From static lightscattering measurements we determine the characteristic length which can be interpreted as a pore size in semidilute solutions. This quantity shows a maximum at concentrations near where the diffusion coefficient obtains a minimum. From this measurement the persistence length is also determined. By comparison of theoretical expressions with these measurements and the dynamic light-scattering measurements the persistence length was also deduced. This determination of the persistence length is essentially temperature independent with a value of approximately 2 nm.
We describe an apparatus for multiangle light scattering studies of samples that are under hydrostatic pressures. The mean hydrodynamic radius and mean radius of gyration can be determined using quasielastic and static light scattering at temperatures between 5 and 85 °C and pressures between 0.1 and 100 MPa.
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