Anisometric Polyelectrolyte/Mixed Surfactant Nanoassemblies Formed by the Association of Poly(diallyldimethylammonium chloride) with Sodium Dodecyl Sulfate and Dodecyl Maltoside

The soluble complexes of oppositely charged macromolecules and amphiphiles, formed in the one-phase concentration range, are usually described on the basis of the beads on a string model assuming spherelike bound surfactant micelles. However, around and above the charge neutralization ionic surfactant to polyion ratio, a variety of ordered structures of the precipitates and large polyion/surfactant aggregates have been reported for the different systems which are difficult to connect to globular-like surfactant self-assembly units. In this article we have demonstrated through SAXS measurements that the structure of precipitates and those of the soluble polyion/mixed surfactant complexes of poly­(diallyldimethylammonium chloride) (PDADMAC), sodium dodecyl sulfate (SDS), and dodecyl-maltoside (DDM) are strongly correlated. Specifically, SDS binds to the PDADMAC molecules in the form of small cylindrical surfactant micelles even at very low SDS-to-PDADMAC ratios. In this way, these anisometric surfactant self-assemblies formed in excess polyelectrolyte mimic the basic building units of the hexagonal structure of the PDADMAC/SDS precipitate and/or suspensions formed at charge equivalence or at higher SDS-to-PDADMAC ratios. The presence of DDM reduces the cmc and cac for the system but does not alter significantly the structure of the complexes in either the one-phase or two-phase region. The only exception is for samples at SDS-to-PDADMAC ratios close to charge neutralization and a high concentration of DDM where the precipitate forms a multiphasic or distorted hexagonal structure

Transformation from Globular to Cylindrical Mixed Micelles through Molecular Exchange that Induces Micelle Fusion

Transformations between different micellar morphologies in solution induced by changes in composition, salt, or temperature are well-known phenomena; however, the understanding of the associated kinetic pathways is still limited. Especially for mixed surfactant systems, the micelles can take a very wide range of structures, depending on the surfactant packing parameter and other thermodynamic conditions. Synchrotron-based small-angle X-ray scattering (SAXS) in combination with fast mixing using a stopped-flow apparatus can give direct access to the structural kinetics on a millisecond time scale. Here, this approach is used to study the formation of cylindrical micelles after mixing two solutions with globular micelles of the nonionic surfactant dodecyl maltoside (DDM) and the anionic surfactant sodium dodecyl sulfate (SDS), respectively. Two separate processes were identified: (i) a transition in micellar shell structure, interpreted as exchange of surfactant molecules resulting in mixed globular micelles, and subsequently, (ii) fusion into larger, cylindrical structures

Self-Healing Mussel-Inspired Multi-pH-Responsive Hydrogels

Self-healing hydrogels can be made using either reversible covalent cross-links or coordination chemistry bonds. Here we present a multi-pH-responsive system inspired by the chemistry of blue mussel adhesive proteins. By attaching DOPA to an amine-functionalized polymer, a multiresponsive system is formed upon reaction with iron. The degree of polymer cross-linking is pH controlled through the pH-dependent DOPA/iron coordination chemistry. This leads to the formation of rapidly self-healing high-strength hydrogels when pH is raised from acidic toward basic values. Close to the p<i>K</i>_a value, or more precisely the pI value, of the polymer, the gel collapses due to reduced repulsion between polymer chains. Thereby a bistable gel-system is obtained. The present polymer system more closely resembles mussel adhesive proteins than those previously reported and thus also serves as a model system for mussel adhesive chemistry

Release of Solubilizate from Micelle upon Core Freezing

By combining NMR (yielding ¹H chemical shift, spin relaxation, and self-diffusion data) and small-angle X-ray scattering experiments, we investigate the complex temperature dependence of the molecular and aggregate states in aqueous solutions of the surfactant [CH₃(CH₂)₁₇(OCH₂CH₂)₂₀OH], abbreviated as C18E20, and hexamethyldisiloxane, HMDSO. The latter molecule serves as a model for hydrophobic solubilizates. Previously, the pure micellar solution was demonstrated to exhibit core freezing at approximately 7–8 °C. At room temperature, we find that HMDSO solubilizes at a volume fraction of approximately 10% in the core of the C18E20 micelles, which consists of molten and thereby highly mobile alkyl chains. Upon lowering the temperature, core freezing is found, just like in pure micelles, but at a temperature shifted significantly to 3 °C. The frozen cores contain immobile alkyl chains and exhibit a higher density but are essentially devoid (volume fraction below 1%) of the solubilizate. The latter molecules are released, first gradually and then rather steeply, from the core in the temperature range that is roughly delimited by the two core freezing temperatures, one for pure micelles and one for micelles with solubilizates. The release behavior of systems with different initial HMDSO loading follows the same master curve. This feature is rationalized in terms of loading capacity being strongly temperature dependent: upon lowering the temperature, release commences once the loading capacity descends below the actual solubilizate content. The sharp release curves and the actual release mechanism with its molecular features shown in rich detail have some bearing on a diverse class of possible applications

Core Freezing and Size Segregation in Surfactant Core–Shell Micelles

Nonionic surfactants containing poly­(ethylene oxide) are chemically simple and biocompatible and form core–shell micelles at a wide range of conditions. For those reasons, they and their aggregates have been widely investigated. Recently, irregularities that were observed in the low-temperature behavior of surfactants of the kind [CH₃(CH₂)_<i>n</i>O­(CH₂CH₂O)_<i>m</i>H], (abbreviated C<i>n</i>E<i>m</i>) were assigned to a freezing–melting phase transition in the micellar core. In this work we expand the focus from the case of single component systems to binary surfactant systems at temperatures between 1 and 15 °C. By applying small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), and density measurements in pure C18E20 and C18E100 solutions and their mixtures, we show that core freezing/melting is also present in mixtures. Additionally, comparing SAXS data obtained from the mixture with those from the single components, it was possible to demonstrate that the phase transition leads to a reversible segregation of the surfactants from mixed micelles to distinct kinds of micelles of the two components

Structures of PEP–PEO Block Copolymer Micelles: Effects of Changing Solvent and PEO Length and Comparison to a Thermodynamic Model

Structures of poly­(ethylene propylene)–poly­(ethylene oxide) (PEP–PEO) block copolymer micelles were determined from small-angle X-ray scattering and static light scattering and compared to predictions from a thermodynamic model. Both the corona block length and the solvent water–ethanol ratio were changed, leading to a thorough test of this model. With increasing ethanol fraction, the PEP core–solvent interfacial tension decreases, and the solvent quality for PEO changes. The weight-average block masses were 5.0 kDa for PEP and 2.8–49 kDa for PEO. For the lowest PEO molar mass and samples in pure water (except for the highest PEO molar mass), the micelles were cylindrical; for other conditions they were spherical. The structural parameters can be reasonably well described by the thermodynamic model by Zhulina et al. [<i>Macromolecules</i> <b>2005</b>, <i>38</i> (12), 5330–5351]; however, they have a stronger dependence on solvent composition and PEO molar mass than predicted

Direct Observation of the Formation of Surfactant Micelles under Nonisothermal Conditions by Synchrotron SAXS

Self-assembly of amphiphilic molecules into micelles occurs on very short times scales of typically some milliseconds, and the structural evolution is therefore very challenging to observe experimentally. While rate constants of surfactant micelle kinetics have been accessed by spectroscopic techniques for decades, so far no experiments providing detailed information on the structural evolution of surfactant micelles during their formation process have been reported. In this work we show that by applying synchrotron small-angle X-ray scattering (SAXS) in combination with the stopped-flow mixing technique, the entire micelle formation process from single surfactants to equilibrium micelles can be followed in situ. Using a sugar-based surfactant system of dodecyl maltoside (DDM) in dimethylformamide (DMF), micelle formation can be induced simply by adding water, and this can be followed in situ by SAXS. Mixing of water and DMF is an exothermic process where the micelle formation process occurs under nonisothermal conditions with a temperature gradient relaxing from about 40 to 20 °C. A kinetic nucleation and growth mechanism model describing micelle formation by insertion/expulsion of single molecules under nonisothermal conditions was developed and shown to describe the data very well

High Electrokinetic Energy Conversion Efficiency in Charged Nanoporous Nitrocellulose/Sulfonated Polystyrene Membranes

The synthesis, characterization, and electrokinetic energy conversion performance have been investigated experimentally in a charged polymeric membrane based on a blend of nitrocellulose and sulfonated polystyrene. The membrane is characterized by a moderate ion exchange capacity and a relatively porous structure with average pore diameter of 11 nm. With electrokinetic energy conversion, pressure can be converted directly into electric energy and vice versa. From the electrokinetic transport properties, a remarkably large intrinsic maximum efficiency of 46% is found. It is anticipated that the results are an experimental verification of theoretical models that predict high electrokinetic energy conversion efficiency in pores with high permselectivity and hydrodynamic slip flow. Furthermore, the result is a promising step for obtaining efficient low-cost electrokinetic generators and pumps for small or microscale applications

How Hollow Are Thermoresponsive Hollow Nanogels?

A main challenge in colloid science is the development of smart delivery systems that store and protect actives from degradation and allow release in response to an external stimulus like temperature. Hollow nanogel capsules made of temperature-sensitive polymers are particularly promising materials. The stimuli-sensitive void size, shell thickness, and permeability determine cargo storage and its release behavior. Thus, determination and control of these morphological parameters are of outmost relevance for the design of new, functional drug delivery vehicles. Here we investigate quantitatively void size and shell thickness of hollow nanogels at different states of swelling by means of small-angle neutron scattering (SANS) employing contrast variation. We demonstrate the structure-sensitivity dilemma: hollow nanogels with a slightly cross-linked shell reveal distinct temperature sensitivity but possess nearly no void (14% of the initial core volume) and are thus hardly “hollow”. Nanogels with a stiff shell are indeed hollow (albeit with smaller void as compared to the core size of the template) but less temperature sensitive

Glycolipid Biosurfactants Activate, Dimerize, and Stabilize <i>Thermomyces lanuginosus</i> Lipase in a pH-Dependent Fashion

We present a study of the interactions between the lipase from <i>Thermomyces lanuginosus</i> (TlL) and the two microbially produced biosurfactants (BSs), rhamnolipid (RL) and sophorolipid (SL). Both RL and SL are glycolipids; however, RL is anionic, while SL is a mixture of anionic and non-ionic species. We investigate the interactions of RL and SL with TlL at pH 6 and 8 and observe different effects at the two pH values. At pH 8, neither RL nor SL had any major effect on TlL stability or activity. At pH 6, in contrast, both surfactants increase TlL’s thermal stability and fluorescence and activity measurements indicate interfacial activation of TlL, resulting in 3- and 6-fold improved activity in SL and RL, respectively. Nevertheless, isothermal titration calorimetry reveals binding of only a few BS molecules per lipase. Size-exclusion chromatography and small-angle X-ray scattering suggest formation of TlL dimers with binding of small amounts of either RL or SL at the dimeric interface, forming an elongated complex. We conclude that RL and SL are compatible with TlL and constitute promising green alternatives to traditional surfactants