Rupture of Pluronic Micelles by Di-Methylated β-Cyclodextrin Is Not Due to Polypseudorotaxane Formation

Spectroscopic measurements (uv/vis absorbance and fluorescence) and time-resolved small-angle neutron scattering experiments (TR-SANS) were used to follow the breakdown of Pluronic micelles by heptakis(2,6-di-<i>O</i>-methyl)-β-cyclodextrin (DIMEB) over time in order to elucidate the mechanism of micellar rupture, generally attributed to polypseudotorotaxane (PR) formation between the cyclodextrin and the central hydrophobic PPO block. The spectroscopic measurements with two different probes (methyl orange and nile red) suggest that very rapid changes (on the order of seconds) take place when mixing DIMEB with F127 Pluronic and that no displacement of the probe from the cyclodextrin cavity occurs, which is in disagreement with PR formation. TR-SANS measurements demonstrate for the first time that the micelles are broken down in less than 100 ms, which categorically rules out PR formation as the mechanism of rupture. In addition, the same mechanism is demonstrated with other Pluronics, P85 and P123. In the latter case, after micellar rupture, lamellar structures are seen to form over a longer period of time, thus suggesting that after the instantaneous micellar disruption, further, longer-scale rearrangements are not excluded

From Crab Shells to Smart Systems: Chitosan–Alkylethoxy Carboxylate Complexes

In this work, self-assembly of alkyl ethylene oxide carboxylates and the biopolymer chitosan into supramolecular structures with various shapes is presented. Our investigations were done at pH 4.0, where the chitosan is almost fully charged and the surfactants are partially deprotonated. By changing the alkyl chain length and the number of ethylenoxide units very different water-soluble complexes can be obtained, ranging from globular micelles incorporated in a chitosan network to formation of ordered multiwalled vesicles. The structural characteristics of these complexes can be finely controlled by the mixing ratio of chitosan and surfactant, i.e., simply by the solutions composition. For instance, the vesicle wall thickness can be varied between 5 and 50 nm just by varying the mixing ratio. Accordingly, we expect this system to be an outstanding carrier for hydrophilic compounds with tunable release time option. Moreover, an easy route for preparation of chitosan-based complexes in the solid state with controlled mesoscopic order is presented. This work opens the way to prepare biofriendly materials on the basis of chitosan and mild anionic surfactants which are rather versatile with respect to their structure and properties, allowing for preparation of complexes with highly variable structures in both aqueous and solid phase. Formation of such different structures can be exploited for preparation of carriers, which are able to transport hydrophilic as well as hydrophobic molecules. Furthermore, as chitosan is well known to exhibit antibacterial and anti-inflammatory properties, different applications of these complexes can be indicated, i.e., as drug delivery systems or as coatings for medical implants

Chitosan/Alkylethoxy Carboxylates: A Surprising Variety of Structures

In this work, we present a comprehensive structural characterization of long-term stable complexes formed by biopolycation chitosan and oppositely charged nonaoxyethylene oleylether carboxylate. These two components are attractive for many potential applications, with chitosan being a bioderived polymer and the surfactant being ecologically benign and mild. Experiments were performed at different mixing ratios <i>Z</i> (ratio of the nominal charges of surfactant/polyelectrolyte) and different pH values such that the degree of ionization of the surfactant is largely changed whereas that of chitosan is only slightly affected. The structural characterization was performed by combining static and dynamic light scattering (SLS and DLS) and small-angle neutron scattering (SANS) to cover a large structural range. Highly complex behavior is observed, with three generic structures formed that depend on pH and the mixing ratio, namely, (i) a micelle-decorated network at low <i>Z</i> and pH, (ii) rodlike complexes with the presence of aligned micelles at medium <i>Z</i> and pH, and (iii) compacted micellar aggregates forming a supraaggregate surrounded by a chitosan shell at high <i>Z</i> and pH. Accordingly, the state of aggregation in these mixtures can be tuned structurally over quite a range only by rather small changes in pH

Structural and Spectroscopic Characterization of TPGS Micelles: Disruptive Role of Cyclodextrins and Kinetic Pathways

The aggregation and structure of d-α-tocopheryl polyethylene glycol succinate micelles, TPGS-1000, an amphiphilic derivative of vitamin E, were characterized using scattering and spectroscopic methods, and the impact of different cyclodextrins (CDs) on the self-assembly was investigated, with the view of combining these two versatile pharmaceutical excipients in drug formulations. Combined small-angle neutron scattering (SANS), dynamic light scattering, and time-resolved and steady-state fluorescence emission experiments revealed a core–shell architecture with a high aggregation number (<i>N</i>_agg ≈ 100) and a highly hydrated poly­(ethylene oxide) corona (∼11 molecules of solvent per ethylene oxide unit). Micelles form gradually, with no sharp onset. Structural parameters and hydration of the aggregates were surprisingly stable with both temperature and concentration, which is a critical advantage for their use in pharmaceutical formulations. CDs were shown to affect the self-assembly of TPGS in different ways. Whereas native CDs induced the precipitation of a solid complex (pseudopolyrotaxane), methylated β-CDs led to different outcomes: constructive (micellar expansion), destructive (micellar rupture), or no effect, depending on the number of substituents and whether the substitution pattern was regular or random on the rims of the macrocycle. Time-resolved SANS studies on mixtures of TPGS with regularly dimethylated β-CD (DIMEB), which ruptures the micelles, revealed an almost instantaneous demicellization (<100 ms) and showed that the process involved the formation of large aggregates whose size evolved over time. Micellar rupture is caused by the formation of a TPGS–DIMEB inclusion complex, involving the incorporation of up to three macrocycles on the tocopherol, as shown by proton nuclear magnetic resonance (NMR) and ROESY NMR. Analysis of NMR data using Hill’s equation revealed that the binding is rather cooperative, with the threading of the CD favoring the subsequent inclusion of additional CDs on the aliphatic moiety

Self-Assembly in Mixtures of an Anionic and a Cationic Surfactant: A Comparison between Small-Angle Neutron Scattering and Cryo-Transmission Electron Microscopy

The self-assembly in SOS-rich mixtures of the anionic surfactant sodium octyl sulfate (SOS) and the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) has been investigated with the complementary techniques small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). Both techniques confirm the simultaneous presence of open and closed bilayer structures in highly diluted samples as well as the existence of small globular and large elongated micelles at higher concentrations. However, the two techniques sometimes differ with respect to which type of aggregates is present in a particular sample. In particular, globular or wormlike micelles are sometimes observed with cryo-TEM in the vicinity of the micelle-to-bilayer transition, although only bilayers are present according to SANS and the samples appear bluish to the eye. A similar discrepancy has previously been reported but could not be satisfactorily rationalized. On the basis of our comparison between in situ (SANS) and ex situ (cryo-TEM) experimental techniques, we suggest that this discrepancy appears mainly as a result of the non-negligible amount of surfactant adsorbed at interfaces of the thin sample film created during the cryo-TEM specimen preparation. Moreover, from our detailed SANS data analysis, we are able to observe the unusually high amount of free surfactant monomers present in SOS-rich mixtures of SOS and CTAB, and the experimental results give excellent agreement with model calculations based on the Poisson–Boltzmann mean field theory. Our careful comparison between model calculations and experiments has enabled us to rationalize the dramatic microstructural transformations frequently observed upon simply diluting mixtures of an anionic and a cationic surfactant

Spontaneous Transformations between Surfactant Bilayers of Different Topologies Observed in Mixtures of Sodium Octyl Sulfate and Hexadecyltrimethylammonium Bromide

The influence of adding salt on the self-assembly in sodium octyl sulfate (SOS)-rich mixtures of the anionic surfactant SOS and the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) have been investigated with the two complementary techniques, small-angle neutron scattering (SANS) and cryo-transmission electron microscopy. We are able to conclude that addition of a substantial amount of inert salt, NaBr, mainly has three effects on the structural behaviors: (i) the micelles become much larger at the transition from micelles to bilayers, (ii) the fraction of bilayer disks increases at the expense of vesicles, and (iii) bilayer aggregates perforated with holes are formed in the most diluted samples. A novel form factor valid for perforated bilayer vesicles and disks is introduced for the first time and, as a result, we are able to directly observe the presence of perforated bilayers by means of fitting SANS data with an appropriate model. Moreover, we are able to conclude that the morphology of bilayer aggregates changes according to the following sequence of different bilayer topologies, vesicles → disks → perforated bilayers, as the electrolyte concentration is increased and surfactant mole fraction in the bilayer aggregates approaches equimolarity. We are able to rationalize this sequence of transitions as a result of a monotonous increase of the bilayer saddle-splay constant (<i>k̅</i>_<i>c</i>^bi) with decreasing influence from electrostatics, in agreement with theoretical predictions as deduced from the Poisson–Boltzmann theory

Growth of Mesoporous Silica Nanoparticles Monitored by Time-Resolved Small-Angle Neutron Scattering

Since the first development of surfactant-templated mesoporous silicas, the underlying mechanisms behind the formation of their structures have been under debate. Here, for the first time, time-resolved small-angle neutron scattering (tr-SANS) is applied to study the complete formation of mesoporous silica nanoparticles. A distinct advantage of this technique is the ability to detect contributions from the whole system, enabling the visualization not only of particle genesis and growth but also the concurrent changes to the coexistent micelle population. In addition, using contrast-matching tr-SANS, it is possible to highlight the individual contributions from the silica and surfactant. An analysis of the data agrees well with the previously proposed “current bun” model describing particle growth: Condensing silica oligomers adsorb to micelles, reducing intermicellar repulsion and resulting in aggregation to form initial particle nuclei. From this point, the growth occurs in a cooperative manner, with condensing silica filling the gaps between further aggregating micelles. The mechanistic results are discussed with respect to different reaction conditions by changing either the concentration of the silica precursor or the temperature. In doing so the importance of in situ techniques is highlighted, in particular, tr-SANS, for mechanism elucidation in the broad field of materials science

Acclimation responses to temperature vary with vertical stratification: implications for vulnerability of soil-dwelling species to extreme temperature events

The occurrence of summer heat waves is predicted to increase in amplitude and frequency in the near future, but the consequences of such extreme events are largely unknown, especially for belowground organisms. Soil organisms usually exhibit strong vertical stratification, resulting in more frequent exposure to extreme temperatures for surface-dwelling species than for soil-dwelling species. Therefore soil-dwelling species are expected to have poor acclimation responses to cope with temperature changes. We used five species of surface-dwelling and four species of soil-dwelling Collembola that habituate different depths in the soil. We tested for differences in tolerance to extreme temperatures after acclimation to warm and cold conditions. We also tested for differences in acclimation of the underlying physiology by looking at changes in membrane lipid composition. Chill coma recovery time, heat knockdown time and fatty acid profiles were determined after 1 week of acclimation to either 5 or 20 °C. Our results showed that surface-dwelling Collembola better maintained increased heat tolerance across acclimation temperatures, but no such response was found for cold tolerance. Concordantly, four of the five surface-dwelling Collembola showed up to fourfold changes in relative abundance of fatty acids after 1 week of acclimation, whereas none of the soil-dwelling species showed a significant adjustment in fatty acid composition. Strong physiological responses to temperature fluctuations may have become redundant in soil-dwelling species due to the relative thermal stability of their subterranean habitat. Based on the results of the four species studied, we expect that unless soil-dwelling species can temporarily retreat to avoid extreme temperatures, the predicted increase in heat waves under climatic change renders these soil-dwelling species more vulnerable to extinction than species with better physiological capabilities. Being able to act under a larger thermal range is probably costly and could reduce maximum performance at the optimal temperatur

Surfactant (Bi)Layers on Gold Nanorods

Gold nanorods in aqueous solution are generally surrounded by surfactants or capping agents. This is crucial for anisotropic growth during synthesis and for their final stability in solution. When CTAB is used, a bilayer has been evidenced from analytical methods even though no direct morphological characterization of the precise thickness and compactness has been reported. The type of surfactant layer is also relevant to understand the marked difference in further self-assembling properties of gold nanorods as experienced using 16-EO₁-16 gemini surfactant instead of CTAB. To obtain a direct measure of the thickness of the surfactant layer on gold nanorods synthesized by the seeded growth method, we coupled TEM, SAXS, and SANS experiments for the two different cases, CTAB and gemini 16-EO₁-16. Despite the strong residual signal from micelles in excess, it can be concluded that the thickness is imposed by the chain length of the surfactant and corresponds to a bilayer with partial interdigitation

Effects of Structure Variation on Solution Properties of Hydrotropes: Phenyl versus Cyclohexyl Chain Tips

The physicochemical behavior of the phenyl-<i>n</i>-alkanoate (PhenCx) and cyclohexyl-<i>n</i>-alkanoate (CyclohexCx) series has been investigated, supporting previous work on the understanding of hydrotropes (Hopkins Hatzopoulos, M.; Eastoe, J.; Dowding, P.J.; Rogers, S. E.; Heenan, R.; Dyer, R. <i>Langmuir</i> <b>2011</b>, <i>27</i>, 12346–12353). Electrical conductivity, surface tension, ¹H NMR, and small-angle neutron scattering (SANS) were used to study adsorption and aggregation in terms of critical aggregation concentration (cac). The PhenCx series exhibited very similar d log­(cac)/d<i>n</i> to <i>n</i>-alkylbenzoates (CnBenz), exhibiting two branches of behavior, with a common inflection point at four linear carbons, whereas the CyclohexCx series showed no break point. Electrical conductivity and ¹H NMR concentration scans indicate a difference in physicochemical behavior between higher and lower homologues in both the PhenCx and CyclohexCx series. Surface tension measurements with compounds belonging to either group gave typical Gibbs adsorption profiles, having d log­(cac)/d<i>n</i> curves consistent with limiting headgroup areas in the region of (35–55 Ų) indicating monolayer formation. SANS profiles showed no evidence for aggregates below the electrical conductivity determined cac values, inferring an “on–off” mode of aggregation. Analyses of SANS profiles was consistent with charged ellipsoidal aggregates, persisting from lower through to higher homologues in both the PhenCx and CyclohexCx series