Path-Dependent Preparation of Complex Micelle Packings of a Hydrated Diblock Oligomer

Small-angle X-ray scattering analyses reveal that the hydrated diblock oligomer n-C₁₆H₂₃(OCH₂CH₂)₂₀-OH (C₁₆E₂₀ or Brij 58) forms lyotropic liquid crystals (LLCs) exhibiting face-centered cubic (FCC), body-centered cubic (BCC), Frank-Kasper (FK) A15, and cylindrical (H_I) morphologies over the concentration range 30-65 wt % amphiphile. Heating LLCs comprising 54-59 wt % C₁₆E₂₀ drives the temperature-dependent phase transition sequence: A15 → BCC → H_I. However, rapidly quenching the resulting H_I phase from 70 to 25 °C initially forms a BCC phase that isothermally transforms into a complex, tetragonal FK σ phase comprising 30 quasispherical micelles. The metastability of this micellar σ phase is shown to depend on the sample cooling rate, thermal quench depth, and isothermal annealing temperature. We rationalize the preference for the A15 structure at 25 °C in terms of minimizing unfavorable water/hydrophobic contacts, while maximizing local particle sphericity. The symmetry breaking transition kinetics in these micellar LLCs apparently stem from the temperature-dependent activation barriers for phase nucleation and growth, which are intimately coupled to the time scales for micelle reconfiguration by amphiphile chain exchange and their spatial rearrangement. These findings highlight how thermal processing influences nucleation and growth of the self-assembled morphologies of intrinsically reconfigurable, soft spherical particles.

Molecular conformation and bilayer pores in a nonionic surfactant lamellar phase studied with 1H-13C solid-state NMR and molecular dynamics simulations

The structure of the lamellar phase of aqueous pentaethylene glycol mono-n-dodecyl ether (C12E5) surfactant at various temperatures and molar fractions is studied by using united atom molecular dynamics simulations and nuclear magnetic resonance measurements. Namely, the simulation model is used to interpret the magnitude and temperature dependence of experimental C-H order parameter profiles in terms of the molecular conformation and orientation. Our simulations suggest that the low order parameters that are generally measured in poly(ethylene oxide) surfactant bilayers are due to the presence of bilayer pores throughout the entire lamellar phase region.

Solubilization of Hydrophobic Dyes in Surfactant Solutions

In this paper, the use of surfactants for solubilization of hydrophobic organic dyes (mainly solvent and disperse dyes) has been reviewed. The effect of parameters such as the chemical structures of the surfactant and the dye, addition of salt and of polyelectrolytes, pH, and temperature on dye solubilization has been discussed. Surfactant self-assemble into micelles in aqueous solution and below the concentration where this occurs-the critical micelle concentration (CMC)-there is no solubilization. Above the CMC, the amount of solubilized dye increases linearly with the increase in surfactant concentration. It is demonstrated that different surfactants work best for different dyes. In general, nonionic surfactants have higher solubilization power than anionic and cationic surfactants. It is likely that the reason for the good performance of nonionic surfactants is that they allow dyes to be accommodated not only in the inner, hydrocarbon part of the micelle but also in the headgroup shell. It is demonstrated that the location of a dye in a surfactant micelle can be assessed from the absorption spectrum of the dye-containing micellar solution.

How protein surfaces induce anomalous dynamics of hydration water

Water around biomolecules slows down with respect to pure water, and both rotation and translation exhibit anomalous time dependence in the hydration shell. The origin of such behavior remains elusive. We use molecular dynamics simulations of water dynamics around several designed protein models to establish the connection between the appearance of the anomalous dynamics and water-protein interactions. For the first time we quantify the separate effect of protein topological and energetic disorder on the hydration water dynamics. When a static protein structure is simulated, we show that both types of disorder contribute to slow down water diffusion, and that allowing for protein motion, increasing the spatial dimensionality of the interface, reduces the anomalous character of hydration water. The rotation of water is, instead, altered by the energetic disorder only; indeed, when electrostatic interactions between the protein and water are switched off, water reorients even faster than in the bulk. The dynamics of water is also related to the collective structure--à voir the hydrogen bond (H-bond) network--formed by the solvent enclosing the protein surface. We show that, as expected for a full hydrated protein, when the protein surface offers pinning sites (charged or polar sites), the superficial water-water H-bond network percolates throughout the whole surface, hindering the water diffusion, whereas it does not when the protein surface lacks electrostatic interactions with water and the water diffusion is enhanced.

Lower consolute boundaries of the nonionic surfactant C8E5 in aqueous alkali halide solutions: An approach to reproduce the effects of alkali halides on the cloud-point temperature

In the temperature-composition phase diagram of the nonionic surfactant n-octyl-hydroxypenta(oxyethylene), C(8)E(5), there are three principal curves; the one for the critical micelle concentration (cmc), the one delineating the existence of the hexagonal phase, and then the lower consolute boundary (lcb). In this work it is clarified how the presence of the alkali halides NaF, LiCl, NaCl, NaBr and NaI in the aqueous solutions, up to high molalities, change the lcb temperature-position and shape. The lcbs are obtained from measurements of cloud-point temperatures. Rather marked anion-controlled shifts are observed in the boundary temperature-position, and the order of the anions is in accordance with the prediction of the Hofmeister series. Also the shape of the boundary is affected in an anion-specific way, so that the largest changes are found with the strongest salting-out agent. The separation point varies in distinctly non-linear manners with the molality of the studied alkali halides. An approach is presented that can reproduce the effects of the alkali halides on the cloud-point temperature of C(8)E(5) and a poly(ethylene oxide) polymer, at low amounts of the macroentities. In this approach use is made of the known behaviour of the electrolytes at the air/water surface and the virial expansion, to account for the initial salting-out/-in effect and the variation of the effect with electrolyte molality.

Structure and Dynamics of Hydrogen Bonds in the Interface of a C12E6 Spherical Micelle in Water Solution: A MD Study at Various Temperatures

The temperature dehydration of a C(12)E(6) spherical micelle is characterized through the study of the structure and dynamics of the hydrogen bonds formed by water within the micellar interface. Water molecules in proximity of the hydrophilic fragment of the C(12)E(6) surfactants form strong H-bonds with the oxyethilene units E and with the polar alcoholic heads. The activation energies of such H-bonds fall in the range 2-3 Kcal mol(-1). On the exposed oil core, the number of water-water H-bonds decreases as an effect of dehydration. The dynamics of such bonds exhibits a slow relaxation with respect to the bulk, and two time scales can be discerned: the first one, tau approximately 3-6 ps, is typical of water-water H-bonds around small hydrophobic molecules, whereas the second one, tau approximately 40-80 ps, is probably due to the confining effect of the long hydrophilic fragments which reduces the probability of a water molecule to leave the hydration layer of the exposed oil core. Water molecules around the core form H-bond clusters whose size and distribution change with temperature. From a cluster analysis, the system appears to be below the percolation threshold, suggesting that the exposed oily surface is formed by disconnected patches of size around 1 nm(2), close to the estimate of the solvated hydrophobic patches on protein surfaces. The network connectivity is also considered for concentric hydration shells along the interface: it turns out that near the oil core, the cluster size is larger than elsewhere in the interface demonstrating a strong structural effect induced by the exposed hydrocarbon tails. Temperature affects the cluster size only in the innermost shell.

Tetrabutylammonium alkyl carboxylate surfactants in aqueous solution: self-association behavior, solution nanostructure, and comparison with tetrabutylammonium alkyl sulfate surfactants

A series of long and ultralong chain tetrabutylammonium alkyl carboxylate (TBACm, TBA = tetrabutylammonium ion; Cm = carboxylate ion C(m-1)H(2)(m-1)CO(2)(-) of total carbon number m) surfactants have been obtained by direct neutralization of the fatty acids with m = 12, 14, 18, 22, and 24 by tetrabutylammonium hydroxide. Time-resolved fluorescence quenching has been used to determine the micelle aggregation number (N) of the surfactants with m = 12, 14, and 18 in the temperature range 10-50 degrees C and of the surfactants with m = 22 and 24 in the temperature range 25-60 degrees C. In all instances the values of N were well below those that can be calculated for the maximum spherical micelle formed by surfactants with the same alkyl chain as the investigated surfactants on the basis of the oil drop model for the micelle core. The microstructure of selected solutions of TBAC22 was examined using transmission electron microscopy at cryogenic temperature and compared to the microstructure of solutions of TBA dodecyl and tetradecyl sulfates. These observations generally confirmed the findings of TRFQ. The self-association behavior of these anionic surfactants with TBA counterions is explained on the basis of the large size and the hydrophobicity of the tetrabutylammonium ions. The important differences in behavior that have been evidenced between tetrabutylammonium alkyl carboxylates and alkyl sulfates are discussed in terms of differences in distribution of the surfactant electrical charge on the headgroup and alkyl chain predicted by quantum chemical calculations (Langmuir 1999, 15, 7546).

Fluorescence probing investigation of the mechanism of formation of MSU-type mesoporous silica prepared in fluoride medium

The mechanism of formation of a MSU-type siliceous material from tetraethyl orthosilicate (TEOS) in the presence of the nonionic surfactant tergitol T-15-S-12, sulfuric acid, and sodium fluoride has been investigated using mainly fluorescence probing techniques and, to a lesser extent, dynamic light scattering (DLS) and 29Si NMR spectroscopy. The tergitol micelles present in the systems obtained by progressively generating the reaction mixture giving rise to the mesostructured material by adding to an appropriate tergitol solution sulfuric acid, TEOS, and NaF were characterized by fluorescence probing (micelle aggregation number, micropolarity, and microviscosity) and also by dynamic light scattering (apparent micelle diameter). 29Si NMR experiments were also performed on selected systems after hydrolysis of the TEOS. The fluorescence probing techniques were also used to follow the changes of micelle characteristics with time during the evolution of the full reaction mixture from a limpid solution to a system containing a minor amount of condensed siliceous material. The synthesized solid material was characterized by X-ray diffraction and nitrogen adsorption-desorption analyses. The micelle aggregation number N was found to change only little, and the micropolarity remained constant when going from the tergitol solution to the full reaction mixture. The results of DLS measurements agree with this finding. Besides, while the condensation of silica took place after addition of NaF, the N value increased only very little with time up to the point where a small amount of mesostructured material precipitated out. These results indicate that the interaction between tergitol micelles and the siliceous species formed in the system by the hydrolysis of TEOS and also between micelles and the growing siliceous species must be very weak. As in our previous studies of the mechanism of formation of MCM41-type material from sodium silicate in the presence of cetyltrimethylammonium bromide, it appears that the locus of formation of the mesostructured material is not the micelle surface but the bulk phase. Micelles only act as reservoirs of surfactant providing surfactant monomer that binds to the growing siliceous species.

Raman spectroscopic study of the hydration of short-chain poly(oxyethylene)s C 1EnC 1 (n=1−4)

The hypothesis that the degree of hydration of poly(oxyethylene) (POE) in aqueous solution depends on the mole ratio of water molecules to ether oxygen atoms in the molecule has been verified by studying the isotropic Raman spectra in the O−H stretching region for four short-chain POEs (C 1EnC 1 withn=1−4). Excellent coincidence of the O−H stretching Raman band for all four POEs studied in the range of mole ratio H2O/Oether from 25 to 0.6 was observed, thus confirming the assumption stated above. A conclusion that all ether oxygen atoms in the POE molecule participate in hydrogen bonding with water molecules has been made.

Temperature and concentration effects on supramolecular aggregation and phase behavior for poly(propylene oxide)-b-poly(ethylene oxide)- b-poly(propylene oxide) copolymers of different composition in aqueous mixtures, 1

The phase behavior (temperature vs composition) and microstructure for the two binary systems Pluronic 25R4 [(PO)19(EO)33(PO)19]-water and Pluronic 25R2 [(PO)21(EO)14(PO)21]-water have been studied by a combined experimental approach in the whole concentration range and from 5 to 80 degrees C. The general phase behavior has been identified by inspection under polarized light. Precise phase boundaries have been determined by analyzing 2H NMR line shape. The identification and microstructural characterization of the liquid crystalline phases have been achieved using small-angle X-ray scattering (SAXS). The isotropic liquid solution phases have been investigated by self-diffusion measurements (PGSE-NMR method). 25R2 does not form liquid crystals and is miscible with water in the whole concentration range; with increasing temperature, the mixtures split into water-rich and a copolymer-rich solutions in equilibrium. 25R4 shows rich phase behavior, passing, with increasing copolymer concentration, from a water-rich solution to a lamellar and copolymer-rich solution. A small hexagonal phase, completely encircled in the stability region of the water-rich solution, is also present. In water-rich solutions, at low temperatures and low copolymer concentrations, the copolymers are dissolved as independent macromolecules. With increasing copolymer concentrations an interconnected network of micelles is formed in which micellar cores of hydrophobic poly(propylene oxide) are interconnected by poly(ethylene oxide) strands. In copolymer-rich solutions water is molecularly dissolved in the copolymer. The factors influencing the self-aggregation of Pluronic R copolymers (PPO-PEO-PPO sequence) are discussed, and their behavior in water is compared to that of Pluronic copolymers (PEO-PPO-PEO sequence).

Molecular dynamics study of temperature dehydration of a C12E6 spherical micelle

Hydration of a spherical micelles of C12E6 in solution is studied by molecular dynamics simulation. The interface is found to be separated in an inner part composed of water and hydrophobic and hydrophilic moieties and an outer part with hydrophilic moiety and water only. Hydration numbers in the inner and in the outer parts are in excellent agreement with experimental data from various different methods. Temperature dehydration occurs in the inner region only and is related to the presence of water molecules directly in contact with the hydrophobic core at low temperature.

Aggregation Behavior of Tyloxapol, a Nonionic Surfactant Oligomer, in Aqueous Solution

The aggregation behavior of Tyloxapol, a nonionic surfactant oligomer with a repeating unit close to Triton X-100 (TX100), and a maximum degree of polymerization of about 7, has been investigated in aqueous solution by means of fluorescence probing, time-resolved fluorescence quenching (TRFQ) and transmission electron microscopy at cryogenic temperature (cryo-TEM). The plot of the pyrene fluorescence intensity ratio I1/I3 against the Tyloxapol concentration shows no clear evidence of a critical micelle concentration contrary to TX100. Nevertheless, the fitting of these data, assuming a partition of pyrene between Tyloxapol aggregates and water, yields cmc values in the micromolar range, i.e., about a hundred times lower than for the "monomer" TX100. The values of I1/I3 at high surfactant concentrations indicate that Tyloxapol micelles provide pyrene a less polar environment than TX100 micelles. The use of the viscosity-sensitive probe 1,3-dipyrenylpropane indicates that the microviscosity of Tyloxapol micelles is quite high, three to four times larger than that for TX100 micelles, and decreases rapidly with increasing temperature. Also the microviscosities of both TX100 and Tyloxapol micelles are larger than those for the micelles of the nonionic ethoxylated surfactant C12E9. The aggregation numbers of Tyloxapol and of TX100 micelles measured using TRFQ increase with temperature, with the Tyloxapol micelles being smaller than the TX100 micelles. Cryo-TEM shows that the Tyloxapol micelles remain spheroidal up to a concentration of about 10 wt%. At 15 wt%, some regions of ordered elongated micelles are also observed which may be the precursors of the hexagonal phase known to occur at about 35 wt%. Copyright 1999 Academic Press.