Structure and state of water in reversed micelles. 3

Dynamics of water nanodroplets and aqueous protons in non-ionic reverse micelles

We present a study of the microscopic dynamics of water trapped in reverse non-ionic micelles by means of a series of molecular dynamics simulations. The analysis of the effects of micellar confinement on spectroscopical properties of an excess proton has also been considered. Our micelles were microemulsions made with the neutral surfactant diethylene glycol monodecyl ether [CH(3)(CH(2))(11)(OC(2)H(4))(2)OH]. Simulation experiments including the proton species were performed using a multistate empirical valence bond Hamiltonian model. Diffusion of water in the micelle is markedly slower than that in the bulk liquid, in the same fashion as happens with reorientational dynamics. Spectral densities of hydrogens revealed a blue-shift of the OH-stretching vibration together with a split of the main band into two components. Absorption lineshapes of the solvated proton in the vicinity of the internal surface of the micelle indicate the coexistence of Eigen-like and Zundel-like structures and a tendency to red-shifting (compared to the aqueous unconstrained excess proton case) of the two relevant spectral bands (around 2000 and 2500 wavenumbers) mainly due to the slower dynamics of proton vibrations in water near interfaces.

Circular dichroism of polynucleotides: Interactions of NiCl2 with poly(dA-dT).poly(dA-dT) and poly(dG-dC).poly(dG-dC) in a water-in-oil microemulsion

The thermal behavior of the synthetic, high molecular weight, double stranded polynucleotides poly(dA-dT).poly(dA-dT) [polyAT] and poly(dG-dC).poly(dG-dC) [polyGC] solubilized in the aqueous core of the quaternary water-in-oil cationic microemulsion CTAB|n-pentanol|n-hexane|water in the presence of increasing amounts of NiCl(2) at several constant ionic strength values (NaCl) has been studied by means of circular dichroism and electronic absorption spectroscopies. In the microemulsive medium, both polynucleotides show temperature-induced modifications that markedly vary with both Ni(II) concentration and ionic strength. An increase of temperature causes denaturation of the polyAT duplex at low nickel concentrations, while more complex CD spectral modifications are observed at higher nickel concentrations and ionic strengths. By contrast, thermal denaturation is never observed for polyGC. At low Ni(II) concentrations, the increase of temperature induces conformational transitions from B-DNA to Z-DNA form, or, more precisely, to left-handed helical structures. In some cases, at higher nickel concentrations, the CD spectra suggest the presence of Z'-type forms of the polynucleotide.

Isn't self-assembly a misnomer? Multi-disciplinary arguments in favor of co-assembly

Self-assembly is a ubiquitous physicochemical phenomenon. It is inherent to molecular recognition effects in the biological domain, and thus presents a basis for understanding the constitution and dynamic organization of living beings. However, it is argued in this paper that the very notion of self-assembly presents an incorrect and misleading one for the physical effects that it stands for. Self-assembly presents a misnomer as each self-assembly process not only depends on, but also includes a certain degree of ordering in the immediate surrounding of the 'self-assembling' entities. Claims are provided to support the idea that every self-assembly process is, in fact, a co-assembly event, as it implies mutual changes and structuring of both the 'self-assembled' system and its nearest surrounding. This point of view is related to the systemic observation that there could be no physical qualities independent on the context of their existence. In order to illustrate that the concept of co-assembly presents only an instance of a more profound and systemic philosophy of life, the examples of the interplay between internal and external factors of development are provided from the fields of quantum and classical physics, biology, evolutionary theory and psychology.

1H NMR studies of aerosol-OT reverse micelles with alkali and magnesium counterions: preparation and analysis of MAOTs

Simple procedures and characterization of a series of well-defined precursors are described for preparation of a unique microenvironment in nanoreactors, reverse micelles. The Na(+), K(+), Rb(+), Cs(+), and Mg(2+) surfactants were prepared using liquid-liquid ion exchange using chloride and nitrate salts. The surfactants were characterized using (1)H NMR spectroscopy and a variety of other techniques. (1)H NMR spectroscopy was found to be a sensitive probe for characterization of the size of the nanoreactor as well as its water content. (1)H NMR spectra can be used for detailed characterization of reactions in confined environments when counterion effects are likely to be important. (1)H NMR spectroscopy revealed two separate peaks corresponding to water in Mg(AOT)2 samples; one peak arises from water coordinated to the Mg(2+) ion while the other peak arises from bulk water. The two water signals arise directly from the slow exchange of the water coordinated to Mg(2+) in these microemulsions with water in the water pool, and provide an opportunity to study hydration of Mg(2+). This work thus extends the potential use of MAOT microemulsions for applications such as in green chemistry.

Role of Physical Structures in Bulk Oils on Lipid Oxidation

Lipid oxidation is important to food manufacturers especially when they increase unsaturated lipids in their products to improve nutritional profiles. Unfortunately, the number of antioxidants available to food manufacturers to control oxidative rancidity is limited and the approval of new antioxidants is unlikely due to economic barriers in obtaining government approval for new food additives. Therefore, new antioxidant technologies are needed for food oils. This paper reviews the current knowledge of lipid oxidation in foods with emphasis on how physical properties of food systems impact oxidation chemistry. In particular, the role of association colloids in bulk oils on lipid oxidation chemistry is discussed in an attempt to understand mechanisms of oxidation. Increasing the understanding of how physical properties impact lipid oxidation could lead to the development of novel antioxidant technologies that not only protect the oil against oxidation and increase shelf-life but also allow food manufacturers to include more nutritionally beneficial fatty acids in their products.

Ultrafast dynamics of water in cationic micelles

The effect of confinement on the dynamical properties of liquid water is investigated for water enclosed in cationic reverse micelles. The authors performed mid-infrared ultrafast pump-probe spectroscopy on the OH-stretch vibration of isotopically diluted HDO in D(2)O in cetyltrimethylammonium bromide (CTAB) reverse micelles of various sizes. The authors observe that the surfactant counterions are inhomogeneously distributed throughout the reverse micelle, and that regions of extreme salinity occur near the interfacial Stern layer. The authors find that the water molecules in the core of the micelles show similar orientational dynamics as bulk water, and that water molecules in the counterion-rich interfacial region are much less mobile. An explicit comparison is made with the dynamics of water confined in anionic sodium bis(2-ethythexyl) sulfosuccinate (AOT) reverse micelles. The authors find that interfacial water in cationic CTAB reverse micelles has a higher orientational mobility than water in anionic AOT reverse micelles.

Preparation of organic nanoparticles using microemulsions: their potential use in transdermal delivery

Organic nanoparticles of cholesterol and retinol have been synthesized in various AOT (Aerosol OT; sodium bis(2-ethylhexyl) sulfosuccinate)/heptane/water microemulsions by direct precipitation of the active principle in the aqueous cores. The nanoparticles are observed by transmission electron microscopy (TEM) using the adsorption of a contrasting agent, such as iodine vapor. The size of the nanoparticles can be influenced, in principle, by the concentration of the organic molecules and the diameter of the water cores, which is related to the ratio R=[H2O]/[surfactant]. The particles remain stable for several months. The average diameter of the cholesterol nanoparticles varies between 3.0 and 7.0 nm, while that of retinol varies between 4.0 and 10 nm. The average size of the cholesterol nanoparticles does not change much either as a function of the ratio R or as a function of the concentration of cholesterol. The constant size of the nanoparticles can be explained by the thermodynamic stabilization of a preferential size of the particles. Chloroform is used to carry the active principle into the aqueous cores. Retinol molecules form J-complexes composed of two or three molecules, as detected by UV-visible spectroscopy.

Study of reverse micelles of di-isobutylphenoxyethoxyethyldimethylbenzylammonium methacrylate in benzene by nuclear magnetic resonance spectroscopy

(1)H NMR spectroscopy was used to investigate the aggregation of the surfactant di-isobutyl-phenoxy-ethoxy-ethyl-dimethyl-benzyl ammonium methacrylate (Hyamine-M) in benzene. Adding water makes swollen reverse micelles (microemulsion droplets). The droplets also contain cadmium ions and the sodium salt of the methacrylic acid. The critical micelle concentration of Hyamine-M was determined by NMR to be 3.95 mM under the current conditions. Two-dimensional NMR NOESY spectra were used to study the conformation of the surfactant in the micelle and the spatial localization of water and counterions. We found that the surfactant molecules are folded with both phenyl fragments oriented toward the micelle exterior and the oxyethylene and NCH(3) groups in the micelle core. The water molecules and counterions are distributed around the surfactant polar groups in the micelle interior and penetrate up to both aromatic rings. The investigated system can be further utilized as a microemulsion matrix for the synthesis of cadmium-containing semiconductor nanocrystals, eventually capped with a polymer shell, or of polymer nanoparticles.

Mechanism of formation of inorganic and organic nanoparticles from microemulsions

This chapter essentially deals with the preparation of nanoparticles using microemulsions. The preparation of inorganic nanoparticles--Ni2B, Pt, Au, Pt-Au, AgX--and the synthesis of organic nanoparticles--cholesterol, rhovanil, rhodiarome--are systematically studied as a function of the concentration of the precursor molecules, the size of the inner water cores, and the manner of mixing the various solutions. Two different behaviors are observed in the various systems. The first case shows a dependence of the nanoparticle size on the various physicochemical parameters. Either a monotonous increase of the size or the presence of a minimum is observed as a function of the concentration of the precursor molecules. This case can be easily explained following the LaMer diagram, where the nucleation of the nanoparticles is separated from the particle growth. The second case does not show any dependence of the nanoparticle size on the physicochemical parameters. The size remains constant in all experimental conditions. The constant character of the size can be explained only by thermodynamic stabilization, where particles with a certain size are better stabilized. It should be emphasized that the size distribution is small in all the cases studied. Finally, the aging of the nanoparticles was also checked, especially for the organic nanoparticles. It is concluded that these particles remain stable for months in the microemulsion.

Ion pairs of crystal violet in sodium bis(2-ethylhexyl)sulfosuccinate reverse micelles

The interfacial localization and the ion pair formation of the positively charged dye crystal violet (CV) in sodium bis(2-ethylhexyl)sulfosuccinate reverse micelles (AOT RMs) were studied by several structural and spectroscopic techniques and by quantum chemical calculations. The size and shape of the AOT RMs in the presence of CV were investigated by small-angle X-ray scattering, showing that CV does not significantly change the RM structure. CV localization as a function of the water to surfactant molar ratio (w(0)) was characterized by H(1) and (13)C NMR, indicating the close proximity of CV to the sulfosuccinate group of AOT at small and large w(0) values. These results were confirmed by calculation of magnetic shielding constants using the gauge-independent atomic orbital method with the HF/6-31G(d) basis set. Two different types of ion pairs between AOT and CV, i.e., contact ion pair (CIPs) and solvent-separated ion pair (SSIPs), were characterized by UV-vis spectroscopy and quantum chemical calculations using the semiempirical ZINDO-CI method. In nonpolar isotropic solvents CIPs are formed with an association constant (K(ASSOC)) of 2 x 10(4) mol(-1) L in isooctane and 750 mol(-1) L in chloroform. In AOT RMs at low w(0), CV-AOT CIPs are also formed. By increasing w(0), there is a sharp decrease in the CIP association free energy, and SSIPs are formed. (CV(+))(H(2)O)(AOT(-)) SSIPs are stable in the AOT RM up to the largest w(0) tested (w(0) = 33).

Solvolysis of benzoyl halides in water/NH4DEHP/isooctane microemulsions

A study was carried out on the solvolysis reactions of different benzoyl halides in microemulsions of water/NH4DEHP/isooctane, where NH4DEHP is ammonium bis(2-ethylhexyl) phosphate. Because of the low solubility of benzoyl halides in water, they are distributed between the continuous medium and the interface of the microemulsion, where the reaction takes place. The application of the pseudophase model has allowed us to obtain the distribution constants and the rate constants at the interface for the benzoyl halides. Reaction mechanisms and the changes in these mechanisms in terms of the water content of the microemulsion have been determined on the basis of kinetic data. The influence of the substituent and the leaving group on the reaction rate has been investigated. A comparison of kinetic results with those previously obtained in water/AOT/isooctane microemulsions allows a kinetic evaluation of the change in the microemulsion properties with the surfactant.

Cooperative Hydration of Pyruvic Acid in Ice

About 3.5 +/- 0.3 water molecules are still involved in the exothermic hydration of 2-oxopropanoic acid (PA) into its monohydrate (2,2-dihydroxypropanoic acid, PAH) in ice at 230 K. This is borne out by thermodynamic analysis of the fact that QH(T) = [PAH]/[PA] becomes temperature independent below approximately 250 K (in chemically and thermally equilibrated frozen 0.1 < or = [PA]/M < or = 4.6 solutions in D2O), which requires that the enthalpy of PA hydration (DeltaHH approximately -22 kJ mol(-1)) be balanced by a multiple of the enthalpy of ice melting (DeltaHM = 6.3 kJ mol(-1)). Considering that: (1) thermograms of frozen PA solutions display a single endotherm, at the onset of ice melting, (2) the sum of the integral intensities of the 1deltaPAH and 1deltaPA methyl proton NMR resonances is nearly constant while, (3) line widths increase exponentially with decreasing temperature before diverging below approximately 230 K, we infer that PA in ice remains cooperatively hydrated within interstitial microfluids until they vitrify.

Testing the Core/Shell Model of Nanoconfined Water in Reverse Micelles Using Linear and Nonlinear IR Spectroscopy

A core/shell model has often been used to describe water confined to the interior of reverse micelles. The validity of this model for water encapsulated in AOT/isooctane reverse micelles ranging in diameter from 1.7 to 28 nm (w0 = 2-60) and bulk water is investigated using four experimental observables: the hydroxyl stretch absorption spectra, vibrational population relaxation times, orientational relaxation rates, and spectral diffusion dynamics. The time dependent observables are measured with ultrafast infrared spectrally resolved pump-probe and vibrational echo spectroscopies. Major progressive changes appear in all observables as the system moves from bulk water to the smallest water nanopool, w0 = 2. The dynamics are readily distinguishable for reverse micelle sizes smaller than 7 nm in diameter (w0 = 20) compared to the response of bulk water. The results also demonstrate that the size dependent absorption spectra and population relaxation times can be quantitatively predicted using a core-shell model in which the properties of the core (interior of the nanopool) are taken to be those of bulk water and the properties of the shell (water associated with the headgroups) are taken to be those of w0 = 2. A weighted sum of the core and shell components reproduces the size dependent spectra and the nonexponential population relaxation dynamics. However, the same model does not reproduce the spectral diffusion and the orientational relaxation experiments. It is proposed that, when hydrogen bond structural rearrangement is involved (orientational relaxation and spectral diffusion), dynamical coupling between the shell and the core cause the water nanopool to display more homogeneous dynamics. Therefore, the absorption spectra and vibrational lifetime decays can discern different hydrogen bonding environments whereas orientational and spectral diffusion correlation functions predict that the dynamics are size dependent but not as strongly spatially dependent within a reverse micelle.

Acid titrations of poly(dG-dC).poly(dG-dC) in aqueous solution and in a w/o microemulsion

The model polynucleotide poly(dG-dC).poly(dG-dC) (polyGC) was titrated with a strong acid (HCl) in aqueous unbuffered solutions and in the quaternary w/o microemulsion CTAB/n-pentanol/n-hexane/water. The titrations, performed at several concentrations of NaCl in the range 0.005 to 0.600 M, were followed by recording the modifications of the electronic absorption and of the CD spectra (210< or = lambda < or =350 nm) upon addition of the acid. In solution, the polynucleotide undergoes two acid-induced transitions, neither of which corresponds to denaturation of the duplex to single coil. The first transition leads to the Hoogsteen type synG.C+ duplex, while the second leads to the C+.C duplex. The initial B-form of polyGC was recovered by back-titration with NaOH. The apparent pKa values were obtained for both steps of the titration, at all salt concentrations. A reasonably linear dependence of pKa1 and pKa2 from p[NaCl] was obtained, with both pKa values decreasing with increasing ionic strength. In microemulsion, at salt concentrations < or = 0.300 M, an acid-induced transition was observed, matching the first conformational transition recorded also in solution. However, further addition of acid led to denaturation of the protonated duplex. Renaturation of polyGC was obtained by back-titration with NaOH. At salt concentrations > 0.300 M, polyGC is present as a mixture of B-form and psi- aggregates, that slowly separate from the microemulsion. The acid titration induces at first a conformational transition similar to the one observed at low salt or in solution, then denaturation occurs, which is however preceded by the appearance of a transient conformation, that has been tentatively classified as a left-handed Z double helix.