Local mobility of micelles of new cationic surfactants and their interactions with hydrophobically modified polyacrylamides

The local dynamics and organization of micelles of new long-chain cationic surfactants with saturated hydrocarbon fragments (from C₁₆ to C₂₂) are investigated via the EPR spin-probe technique. The local mobility of spin probes in the hydrocarbon core of a micelle changes insignificantly, while the order parameter noticeably increases with lengthening of the hydrocarbon fragment of the surfactant molecule. The specific features of the interaction of the surfactants with network junctions of the gels formed by two types of hydrophobically modified polyacrylamides??either containing charged groups (sodium acrylate) in the backbone or lacking these groups??are studied. In both cases, the local mobility of network junctions of the gel increases after the introduction of the surfactant (C₁₈). Moreover, for surfactant with a long alkyl group (C₂), the microscopic viscosity of the gel based on the uncharged polymer decreases, although the local mobility of the network junctions increases. Possible causes of the observed specific features are discussed.     

Effect of salts on local mobility and rheological properties of micelles of new long-chain surfactant

The effects of organic (sodium salicylate) and inorganic (KCl) salts on the rheological properties of micellar solutions and the local characteristics (local mobility and ordering) of micelle cores is studied for a cationic surfactant containing a long (C₁₈) unsaturated alkyl radical. The polar head of the surfactant contains two hydroxyl groups. The local characteristics are determined employing spin probe ESR spectroscopy. It is shown that the incorporation of a salt into a micellar solution reduces the local mobility of radicals of surfactant molecules in micelle cores and increases their local Lordering and the viscosity of the solution. Sodium salicylate has a stronger influence on the solution viscosity and the local characteristics of micelle cores than KCl does. Variations in the local characteristics of micelle cores under the action of the salts are in close correlation with variations in the rheological properties of the micellar solutions.     

Molecular mobility in micellar complexes of a nonionic surfactant and the poly(acrylic acid)-based hydrogel

The interaction between the polyelectrolyte gel of crosslinked poly(acrylic acid) (PAA) and nonionic surfactant Brij 58 based on poly(ethylene glycol) (C₁₆H₃₃(CH₂CH₂O)₂₀OH) is studied. It is established that poly(acrylic acid)-surfactant complexes are formed. Nondissociated carboxyl groups of poly(acrylic acid) and oxygen atoms of the surfactant are involved in the complexation. Surfactant micelles are a kind of bridge that connects polymer chains. The presence of the surfactant decreases the equilibrium swelling of the hydrogel. The spin probe method is employed to determine the local mobility of the hydrocarbon core of a micelle in the complex. It was shown that the local mobility is independent of the hydrogel crosslink density and is much lower in acidic than in alkaline media. In acidic media, much more surfactant molecules of micelles are involved in the complexation than in alkaline media. However, even in alkaline media, surfactant micelles cannot leave the hydrogel, while the spin probes located in micelles are at equilibrium with the spin probes present in the external aqueous medium. The prospects for applying the considered systems as carriers for controlled release drugs are discussed.     

Explaining Fullerene Dispersion by using Micellar Solutions

An effective computational strategy to describe the dispersion of C₆₀ by surfactants is presented. The influence of parameters such as surfactant concentration and molecular length on the final morphology of the system is explored to explain the experimental results and to understand the incorporation of C₆₀ inside micelles. Both neutral and charged amphiphilic molecules are simulated. The long‐discussed problem of the location of fullerenes in micelles is addressed and C₆₀ is found in the hydrocarbon‐chain region of the micelles. If the available hydrophobic space increases, C₆₀ is localized in the inner part of the micellar core. Short, charged amphiphilic stabilizers are more efficient at dispersing fullerenes monomolecularly. Two different phases of C₆₀ are observed as the C₆₀/surfactant ratio varies. In the first, aggregates of C₆₀ are entrapped inside the micelles, whereas, in the second, colloidal nanoC₆₀ is formed with surfactants adsorbed on the surface.     

Spin probes in micellar and polymer self-associating systems

The results of studies of micellar and self-associating polymer systems by spin probe ESR spectroscopy are summarized. The local dynamics and structures of low-molecular-weight micelles built of cationic surfactants bearing long alkyl chains (from C₁₆ to C₂₂), gels of hydrophobically modified polymers, polymer micelles, micellar complexes of nonionic surfactant (Brij58) with hydrogels based on polyacrylic acid, and associates formed in aqueous solutions of poly(diphenylenesulfophthalide) are discussed. Interest in these systems is caused by prospects of their practical use as carriers in drug delivery, in biotechnology, for the enhancement of oil production, and in other purposes.     

Liquid—liquid distribution in reversed micellar systems

The equilibrium distribution of a hydrophilic solute (M^z+ between an aqueous phase and a reversed micellar organic phase (consisting of a surfactant HA with aggregation number x, and dissolved in a hydrocarbon diluent) is analyzed quantitatively by treating the reversed micelles as a pseudophase. It is shown that when the M—A complex is strongly solubilized by the micellar pseudophase, the distribution coefficient (D) has a first-order dependence on the concentration of micellized surfactant (C_s). On the other hand, when the M—A complex is not solubilized by the reversed micelles, a plot of log D versus log C_s has a slope of (z/x); in this case the monomeric species HA is the active extractant and any effect that decreases surfactant aggregation (e.g. low aggregation number, small aggregation equilibrium constant) leads to an increase in the distribution coefficient.     

Effect of Amino‐Acid‐Based Polar Oils on the Krafft Temperature and Solubilization in Ionic and Nonionic Surfactant Solutions

Abstract

The Krafft temperature and solubilization power of ionic and nonionic surfactants in aqueous solutions are strongly affected by added polar oils such as amino‐acid‐based oils (e.g., N‐acylamino acid esters, AAE), because they tend to be solubilized in the surfactant palisade layer. The Krafft temperatures of 5 wt.% sodium dodecyl sulfate (SDS)‐water and octaoxyethylene octadecyl ether (C₁₈EO₈)‐water systems largely decreases upon addition of AAE and 1‐hexanol, whereas it decreases very slightly in isopropyl myristate (IPM) and n‐dodecane. The lowering of the Krafft temperature can be explained by the same mechanism as the melting‐temperature reduction of mixing two ordinary substances. Namely, the polar oils are solubilized in the surfactant palisade layer of micelles and reduce the melting temperature of hydrated solid‐surfactant (Krafft temperature). On the other hand, non‐polar oil such as dodecane is solubilized deep inside micelles and makes an oil pool. The solubilization of non‐polar oil is enhanced by mixing surfactant with AAE due to an increase in micellar size.     

Effect of copolymer structure on micellar behavior of acrylamide-octylphenylpoly (oxyethylene) acrylate surfactant

Acrylamide-octylphenylpoly (oxyethylene) acrylate copolymer (AM-C₈PhEO_nAc) surfactant is the copolymer of acrylamide (AM) and octylphenylpoly (oxyethylene) acrylate macromonomer (C₈PhEO_nAc). The effect of the copolymer structure on the micellar behavior in aqueous solution was studied using dynamic light scattering. It has been found that the length of ethylene oxide (EO) in the branch and the content of C₈PhEO_nAc in the copolymer surfactant have great effects on the size and distribution of the micelles. For AM-C₈PhEO₇Ac copolymer, at the concentration of 5 × 10⁻⁴ g/ml, the micellar size increases with the increase of C₈PhEO₇Ac content. However, for AM-C₈PhEO₁₀Ac copolymer, the result is the opposite; the micellar size decreases with the increase of C₈PhEO₁₀Ac content. Larger C₈PhEO_nAc content leads to narrower micellar distribution. For copolymer surfactants with equal C₈PhEO_nAc content, when the concentration of copolymer solution is the same, the copolymer with longer EO length forms smaller micelles. Received: 2 February 2000 Accepted: 6 October 2000     

Spectroscopic Investigations on the Interaction of Crystal Violet with Nonionic Micelles of Brij and Igepal Surfactants in Aqueous Media

The behavior of the triphenylmethane dye crystal violet in aqueous solutions containing polyoxyethylene nonionic surfactants was investigated using absorption and fluorescence spectroscopic techniques. The interactions of the dye were examined in micellar media in order to prevent dye aggregation and to ensure maximum dye and surfactant interaction. The relative fluorescence enhancements and the binding constants of the dye to the surfactant micelles were determined. The micropolarities of the micellar environment sensed by the pyrene probe were estimated from the I ₁/I ₃ intensity ratios of the fluorescence spectra of pyrene. The fluorescence quenching of pyrene by hexadecylpyridinium chloride was investigated in aqueous surfactant mixtures at a fixed concentration of surfactant in order to determine the aggregation numbers. Attempts were made to correlate the binding constants obtained in this investigation to various micellar parameters.     

Fluorescence decay parameters for pyrene (S1) in micellar and homogeneous liquid dispersions

Relative quantum yields and time constants for the fluorescence from pyrene (S₁) stimulated by UV light have been measured for dispersions of the aromatic in several liquid solvents and aqueous surfactant micelles. Values of the relative radiative decay parameter, k_F, were extracted and its medium dependence tabulated. This parameter was found to vary with medium polarity, being higher in more polar media. This effect, characterized for homogeneous liquid phases, was used to demonstrate that pyrene in surfactant micelles is strongly affected by the polar influences of water molecules which deeply penetrate the micelle in the region of the probe.     

Solubilization of styrene in the catanionic system dodecyltrimethylammonium hydroxide–n′-dodecanephosphonic acid

 The solubilization of styrene in micelles of the catanionic surfactant dodecyltrimethylammonium hydroxide (DTAOH)–n-dodecane-phosphonic acid (DPA) was studied by UV–Vis. spectrometry, as a function of the DTAOH:DPA proportion in the surfactant mixture. The styrene molecules are adsorbed at the surface of the micelles, with the vinyl group closer to the hydrocarbon core than the aromatic ring, which is oriented to the water. In micelles with an excess of DTAOH, the dielectric constant of the water surrounding the micelles was strongly affected by the non-neutralized –N(CH₃)⁺ ₃ groups at the Stem layer. In micelles with an excess of DPA, the –PO₃H₂ groups which are not neutralized by –N(CH₃)⁺ ₃, remain almost unionized and hydrogen-bonded. The effect of the micellar surface on the surrounding water dielectric constant dropped sharply. The dielectric constant in the hydrogen-bonded polar layer is ∼65, rising to the value of pure water very close to the micellar surface. Received: 2 September 1997 Accepted: 20 October 1997     

Catalytic action of CTAB/KBr/C9OH micellar media on the basic hydrolysis of crystal violet

The kinetics of basic hydrolysis of crystal violet (CV) in CTAB/KBr/C₉OH micellar media was investigated under pseudo-first-order conditions. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of CV at 590?nm. It was observed that the pseudo-first-order rate constant increases with increase in C₀. The enhancement of reaction rate with C₀ is explained on the basis of dependence of reaction rate on micellar morphology. Further, the viscosity and DLS analysis supports nonanol-induced morphological transitions. Fluorescence spectroscopy has been used to understand dye–micelles interactions. The enhancement of fluorescence intensity of CV with C₀ suggests an increase in dye–micelles interaction with C_0. The concentration of surfactant and salt had a marked effect on reaction rate. The inhibition of reaction rate at high concentration of surfactant and salt is due to the ionic competition of OH^? and Br^? ions for the reaction center. The influence of [OH^?] on CV hydrolysis was also investigated. The results show that the pseudo-first-order rate constant, k’, increases linearly with hydroxide ion concentration, indicating first-order dependence on [OH^?].     

A critical assessment of capillary condensation and evaporation equations: a computer simulation study

Nonaqueous reverse micelles of brij surfactants are prepared in benzene and ethylammonium nitrate (EAN). The effect of polar head group bulk on reverse micellar size was studied with brij-52, brij-56 and brij-58 whereas the effect of polarity of hydrocarbon chain was investigated taking brij-52 and brij-93 with varying W(s) (W(s)=[EAN]/[surfactant]). Dynamic light scattering (DLS) has been employed to reveal the size and shape of the reverse micelles. Micropolarities of these reverse micelles were investigated by visible spectroscopy using methylene blue (MB) and methyl orange (MO) as molecular optical probes. It has been revealed from the experimental results that with increase in polar head group size reverse micellar size increases. Moreover, it is also observed that with increasing polarity of the hydrocarbon chain the average size of the reverse micelles decreases. It can be concluded that polar head group size and polarity of hydrocarbon chain play important roles in determining reverse micellar size of the brij surfactants apart from the W(s) ratio, nature of the solvent medium, and concentration of the surfactants.     

Self-Assembled Structures of Diglycerol Monolaurate- and Monomyristate in Olive Oil

We have investigated the phase behavior and self-assembled structures of diglycerol monolaurate-and monomyristate (abbreviated as C₁₂G₂ and C₁₄G₂, respectivley) in olive oil over a wide range of temperatures and compositions. At lower temperatures, both the surfactants appear in solid state (α-solid), which does not swell with olive oil. The α-solid transforms into lamellar liquid crystal (L_α) phase upon heating and the solid melting temperature is practically constant at all surfactant/oil compositions, but the C₁₂G₂ melts earlier than the C₁₄G₂. There appear the dispersions of L_α phase and α-solid in the dilute regions of the C₁₂G₂/olive oil and the C₁₄G₂/olive oil systems, respectively, at 25°C. The L_α phase can solubilize some amount of olive oil, but as the oil concentration increases the excess oil separates out from the L_α phase, and there appears L_α dispersion in the dilute surfactant concentration region. The L_α phase eventually transforms into isotropic solutions (reverse micelles) with further heating. The structures (shape and size) of the reverse micelles have been characterized by small-angle x-ray scattering technique. It has found that the C₁₂G₂ and C₁₄G₂ surfactants form reverse rod-like micelles in olive oil above the L_α melting temperature and the micellar size increases with surfactant concentration, but decreases with temperatures.     

Micellar Formation Study of Crown Ether Surfactants

Fluorescence probe and nuclear magnetic resonance (NMR) methods were employed to investigate the micellation of prepared crown ether surfactants, e.g. decyl 15‐crown‐5 and decyl 18‐crown‐6. Pyrene was employed as the fluorescence probe to evaluate the critical micellar concentration (CMC) of these surfactants in aqueous solutions while spin lattice relaxation times (T₁) and chemical shifts of H‐1 NMR were applied in non‐aqueous solutions. Decyl 15‐crown‐5 with lower CMC forms micelles much easier than decyl 18‐crown‐6 with higher CMC in aqueous solutions, whereas decyl 18‐crown‐6 forms micelles easier than decyl 15‐crown‐5 in nonaqueous solutions. Comparison of the CMC of crown ether surfactants and other polyoxyethylene surfactants such as decylhexaethylene glycol was made. Effects of salts and solvents on the micellar formation were also investigated. In general, additions of both alkali metal salts and polar organic solvents into the aqueous surfactant solutions increased in the CMC of these surfactants. The formation of micelles in organic solvents such as methanol and acetonitrile was successfully observed by the NMR method while it was difficult to study these surfactants in organic solutions by the pyrene fluorescence probe method. The NMR study revealed that the formation of micelles resulted in the decrease in all H‐1 spin lattice relaxation times (T₁) of hydrophobic groups, e.g. CH₃ and CH₂, and hydrophilic group OCH₂ of these surfactants. However, upon the micellar formation, the H‐1 chemical shifts (δ) of these surfactant hydrophobic groups were found to shift to downfield (increased δ) while the chemical shift of the hydrophilic group OCH₂ moved to up‐field. Comparison of the spin lattice relaxation time and H‐1 chemical shift methods was also made and discussed.     

Molecular structure and dynamics of poly(methacrylic acid) and poly(acrylic acid) complexes with dodecyl-substituted poly(ethylene glycol)

The molecular dynamics and the structure of molecular complexes formed by micelles of dodecyl-substituted poly(ethylene glycol) with poly(methacrylic acid) and poly(acrylic acid) in aqueous solutions were studied by viscosimetry, pH measurement, and electron spin resonance spin-probe techniques. At low surfactant concentrations, the conformation of the complex is a compact globule. The local mobility of surfactant molecules in such a complex is much slower than that in the free micelle. At high surfactant concentration, the nonionic micelles and polyacids form hydrophilic associates. The associates have the conformation of extended coils. In an associate, a major part of the micellar poly(ethylene glycol) groups is free. The local mobility of the micellar phase depends on the number of micelles involved in an associate. The mobility of surfactant molecules is slower in the complexes of poly(methacrylic acid) than in the complexes of poly(acrylic acid).     

Activity coefficients of alkyl sulfate and alkylsulfonate ions in aqueous and water-salt premicellar solutions

Potentiometry has been employed to determine the activity coefficients of n-decyl sulfate and n-alkyl(C₅–C₁₀)sulfonate ions in aqueous solutions of their sodium salts containing NaCl or Na₂SO₄ as a background electrolyte. The selectivity factors have been estimated for ion-selective electrodes exhibiting responses to surfactant ions in the presence of inorganic salts. The pattern of the dependences of the activity coefficients of the surfactant anions on the length of their hydrocarbon radicals and the nature of functional groups has been revealed. The results obtained have been compared with the data on long-chain carboxylates.     

CORRELATION BETWEEN INTERFACIAL TENSIONS AND MICELLAR STRUCTURES

It is shown that results of surface and interfacial tension measurements can be used to predict the type of micelles and of liquid crystalline phases which are formed in binary and ternary surfactant solutions. In particular it is possible to predict the position of l.c. cubic phases in ternary systems consisting of surfactant, hydrocarbon and water. Data to demonstrate the conclusions were obtained on the surfactants Alkyltrimethylammoniumbromides, Alkyldimethylaminoxides and Alkyldimethylphosphinoxides. It was found that the interfacial tension of a dilute micellar solution against a reference hydrocarbon is a most sensitive and indicative parameter for the prediction of the different structures. Large changes of the interfacial tension were observed for the three systems having the same hydrocarbon chainlength. The value of the interfacial tension directly reflects also the amount of hydrocarbon which can be solubilized in the micellar solution. Interfacial tensions larger than 1mN/m are indicative of globular micelles while interfacial tensions between 0.1 and 1 mN/m indicate the formation of rods. Values below 0.1 mN/m indicate disclike micelles or lamellar phases.

The interfacial tension depends somewhat on the kind of hydrocarbon which is used for the measurements. It is observed that for several surfactant solutions the interfacial tension passes through a shallow minimum when the chainlength of the hydrocarbon is increased from six to sixteen.     

三种不同分子结构阴离子表面活性剂胶束微结构的NMR研究

用核磁共振测定自旋-晶格、自旋-自旋弛豫时间(t₁,t₂)、自扩散系数(D),用2DNOESY技术对正十四烷基硫酸钠、β-戊基壬烷基硫酸钠和β-戊基壬烷基聚氧乙烯醚(4)硫酸钠三类阴离子表面活性剂水溶液进行了观测,烷烃链各基团的t₂/t₁值给出了这三类分子形成各自胶束的水合层位点信息以及烷烃链在胶束内核中堆积程度的比较,自扩散系数结果表明,β-戊基壬烷基硫酸钠比正十四烷基硫酸钠形成的胶束的水合动力学半径小,但β-戊基壬烷基聚氧乙烯醚(4)硫酸钠形成的胶束水合动力学半径明显大于其它两类表面活性剂胶束,2DNOESY谱图提供了β-戊基壬烷基聚氧乙烯醚(4)硫酸钠分子中聚氧乙烯基键在胶束外层卷曲排列的信息.     

Zwitterionic surfactants in ion binding and catalysis

Zwitterionic surfactants have unique properties for applications in separation methods and catalysis. Their properties and efficiencies depend on two main factors: surfactant structure and preferential interactions of zwitterionic surfactant interfaces with anions. Structural changes are related to hydrocarbon chain length, distance between charges, and type and order of functional groups in the polar head. Interactions of anions with zwitterionic micelles follow the Hofmeister series and change the surface charge. The interactions between surfactants and molecules/ions allow the rational control of separation by chromatography and micellar capillary electrophoresis; cloud point extraction; and stabilization and catalytic activity of biomolecules and nanoparticles.