Worm-like Aggregates Research Articles

Water Diffusion in Polymer-like Reverse Micelles. 2. Composition Dependence

A pulsed field gradient FT 1H NMR study on the system soybean lecithin/water/perdeuterated cyclohexane, covering a wide range of compositions, is presented. The water diffusion was found to be Gaussian with a well-defined, time-independent, single diffusion coefficient. After the contribution from the diffusion of water dissolved in the cyclohexane has been properly taken into account, all the data follow the same master curve as a function of the water to lecithin molar ratio (W0). The dominating mechanism for the water diffusion was found to be the motion inside giant wormlike reverse aggregates mediated by an interaggregates exchange with a characteristic time on the order of microseconds. For all the lecithin concentrations investigated, increasing the value of W0 increases the water diffusion up to W0 = 15, where a structural transition toward spherical aggregates causes a decrease in the water diffusion.

Surfactant Curvilinear Diffusion in Giant Wormlike Micelles

We present the first experimental measurements of surfactant curvilinear diffusion in giant wormlike micelles. The surfactant diffusion was monitored by pulsed field gradient NMR for various observation times $t$ ranging from 0.020 to 1.5 s. The surfactant mean-square displacement was found to scale as ${t}^{1/2}$. A model of lateral diffusion along wormlike micellar aggregates with Gaussian statistics is found to describe the echo attenuation well. This type of diffusion is analogous to polymer segment diffusion in the tube-reptation model.

Influence of Alcohol on the Behavior of Sodium Dodecylsulfate Micelles

The effect of medium chain alcohol molecules on the size and shape of sodium dodecylsulfate micelles, and on the self-diffusion coefficient of the surfactant and alcohol, has been investigated by means of small angle neutron scattering (SANS), and Fourier transform pulsed field gradient spin echo (FT-PGSE) nuclear magnetic resonance measurements. All measurements were done in D2O containing a sodium chloride concentration of 0.4 mol/kg, and a surfactant concentration of 0.04 mol/kg. The alcohols used were 1-propanol, 1-butanol, and 1-pentanol. The data obtained from the different techniques agrees qualitatively. The results show that propanol successively breaks down the micelles while pentanol brings about a structural change toward large wormlike aggregates. Butanol shows a highly complex behavior on the structure of the micelles and can decrease and increase the size of the aggregates, depending on the added alcohol concentration range. All analyzed solutions show a distribution of the alcohol between the aqueous bulk solution and the palisade layer of the micelles, resulting in an increased “hydration” of D2O in the micellar surface. Moreover, the structural changes of the micelles indicate that butanol and pentanol solubilize in the micellar core in the high molality range, near the end of the solubility limit. Propanol is apparently too hydrophilic to do so.

Fibers and Other Aggregates of ω-Substituted Surfactants

The aggregation in dilute aqueous solutions of several ω-substituted single-chain quaternary ammonium surfactants has been studied by Krafft temperature and critical aggregation/micelle concentration measurements and by cryo-transmission electron microscopy: (16-carboxyhexadecyl)trimethylammonium bromide (1a); (16-methoxycarbonylhexadecyl)trimethylammonium bromide (1b); (17,18-dihydroxyoctadecyl)trimethylammonium bromide (1c). Octadecyltrimethylammonium bromide (2) was included for comparison. The aggregation of 1a was also studied by Raman spectroscopy. Surfactant 1a at pH 6.8 forms ribbonlike fibers with lengths of up to 1.2 μm and cross sections of 3−4 nm × 12−16 nm; at pH 2.2, ribbon to rodlike fibers; and at pH 11.5, wormlike aggregates. Surfactants 1b and 1c at pH 6.8 form rod-shaped aggregates. The Raman spectroscopy results indicated that 1a is not in a fully extended conformation within its fibers at pH 6.8. Tentative structural models have been proposed for the aggregates of surfactants 1.

Living polymers and polymer solutions: a misleading analogy

Experimental results from colloidal suspensions of worm-like micelles are currently interpreted in terms of close analogies between this kind of systems and polymeric solutions. In particular, it was hypothesized that the viscoelastic properties of dense systems of giant flexible cylindrical micelles can be rationalized in terms of an entangled network of worm-like aggregates, very similar to a neutral random polymeric network. Such an idea is strongly supported by theoretical results that, in a mean-field approximation, suggests for an unlimited growth process of the micellar contour length with concentration. The mean-field theory indicates for an exponentially shaped length distribution function, with mean 〈L〉 depending on concentration, φ, in agreement with a scaling law 〈L〉 ∝ ϕα (α=0.5 in the simpler approach). A number of experimental results seem to be successfully interpretable within this framework.

Measurement of depletion interaction in semi-dilute solutions of worm-like surfactant aggregates

The force as a function of separation is measured between two mica surfaces coated with adsorbed bilayers of cetyltrimethylammonium bromide (CTAB) and immersed in aqueous solutions of elongated and semi-flexible micelles of CTAB. For the range of concentrations studied, where the ionic strength is kept constant, the worm-like micelles form a transient network similar to an entangled polymer solution (semi-dilute regime). The electrical double-layer repulsion between the adsorbed bilayers is preceded at larger separations by an attraction with two distinct regimes. The first one is the strongest and is attributed to a complete depletion of micelles. As predicted by the theory, the range of the interaction is reduced while the strength is increased as the volume fraction is increased. The second regime appears at larger separations where an oscillation-like shape is superimposed on a weak attractive background. It is suggested that the attraction results from an orientation depletion of the anisotropic micelles aligning along the surfaces. The oscillatory behavior is further evidenced with measurement of the structure factor by SANS.

Characterization of viscoelastic surfactant mixtures part II: Rheological studies

The temperature dependence of the rheological behavior of worm-like aggregates formed in 0.3 M C 16TAB/0.05 M SXS mixtures is examined. Three distinct regions are observed for all of the rheological parameters studied (η 0, G′, G″, Δ, τ s). Region I, belo the critical micellization temperature ( T< 18°C) is characterized by highly rigid elastic coagels. In Region II (18≤ T≤40°C) Maxwell body behavior is observed, with a rapid decrease in the structural relaxation time, and a change in the phase angle from ≈ 3 to 90°. In Region III ( T40°C), the aggregates exhibit terminal region behavior, yielding viscoelastic properties only at very high oscillation frequencies. The data can be interpreted in terms of a repeating chain model for worm-like aggregates above the overlap concentration. The increase in interior fluidity of the aggregates observed by FT-IR spectroscopy (Part I) causes a continuous change in the overall flexibility and morphology of these micelles, and a consequent drop in the relaxation time with increasing temperature. The concept of temperature- and compositional-dependent flexibility for worm-like surfactant aggregates serves as a link between the viscoelastic behavior of these systems and ordinary concentrated polymer solutions, and can qualitatively explain the large variations in the relaxation time of surfactant aggregates with small changes in temperature, methylene chain length, or cationic surfactant head group size.