Effect of temperature on the surface phase behavior and micelle formation of a mixed system of nonionic/anionic surfactants

The adsorption and micellar behavior of diethylene glycol mono-n-tetradecyl ether (C14E2), sodium 3,6,9,12-tetraoxaoctacosanoate (TOOCNa), and their mixture at a 1:1 molar ratio have been studied by film balance, Brewster angle microscopy (BAM), and surface tensiometry at different temperatures. The monolayers of pure C14E2 and its mixture with TOOCNa show a first-order phase transition with a conspicuous cusp point in their respective adsorption isotherms. This is further confirmed by the observation of bright two-dimensional condensed phase domains visualized by BAM just after the appearance of the phase transition. It is interesting to note here that for C14E2, condensed domains are observed up to 19 degrees C, while in the mixed system, they are observed up to 22 degrees C. To understand why in the mixed system the domains are observed at higher temperatures than for pure C14E2, we have measured the temperature dependency of the equilibrium surface tension at > or = cmc (gammacmc) values of both the pure and the mixed systems. The gammacmc values of pure C14E2 remain almost constant, while those of pure TOOCNa and its mixture with C14E2 decrease appreciably with increasing temperature. It is concluded that higher degree of dehydration of the ethylene oxide (EO) chain reduces the head-group size of TOOCNa, which outweighs the combined effect of the repulsive interactions between the head-groups and the thermal motion of the adsorbed molecules. Furthermore, C14E2 being inserted into the TOOCNa monolayer reduces the electrostatic repulsions between the charged heads, and consequently, the adsorbed monolayers attain closer molecular packing. As a result, the gammacmc values of both pure TOOCNa and its mixture with C14E2 decrease with increasing temperature. This facilitates the formation of condensed domains in the mixed system at higher temperatures, whereas none of the individual members can show any indicative feature of phase transition under the same experimental conditions.     

Molecular-dynamics simulation of the surface layer of a nonionic micelle

A spherical micelle structure has been studied for cationic (n-dodecyltrimethylammonium chloride) and nonionic (hexaethylene glycol mono-n-hexyl ether) surfactants in pure water and a sodium chloride solution. The molecular-dynamics has been used to simulate the self-assembly of aggregates from an initially homogeneous mixture of water and surfactant molecules and to gain insight into the structure of micelles and their surface layers. The radial distribution functions obtained for charged components have been employed to calculate the local electric potentials of the micelles and the contributions from the charges of water atoms, ions, and a surfactant to it. It has been shown that, similarly to previously studied ionic micelles, in nonionic surfactant micelles, the contributions from water molecules and polar groups (and ions in the case of the salt solution) to the electric potential are mutually compensated in the region of the electrical double layer. Therefore, the resultant electric potential of the surface layer rapidly tends to zero.     

Anionic/nonionic mixed surfactants templates preparation of hollow polymer spheres via emulsion polymerization

We herein report a facile, convenient, and economical method to prepare hollow polymer spheres (HSs). By virtue of the phase transformation of nonionic surfactant at its cloud point, hollow spheres of polystyrene were prepared from vesicle templates formed by potassium oleate (KO) and alkyl‐phenol polyoxyethylene (n ) ether (n = 10, OP) at 70–80 °C. The morphologies of the HSs were characterized by field‐emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The diameter of the HSs varies from 200 to 800 nm, and the shell thickness is uniformly c.a. 30–50 nm. The weight ratio of monomer/surfactant was as high as 7/1. The microstructure of the HSs was very stable and remained unchanged after drying or resuspension in water. The mechanism of the formation of HSs was explained on the theory of vesicles. Furthermore, the factors affecting the formation of the hollow structure were discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2533–2541, 2006     

Self-organization of gold-containing hydrogen-bonded rosette assemblies on graphite surface

The self-organization of supramolecular structures, in particular gold-containing hydrogen-bonded rosettes, on highly oriented pyrolytic graphite (HOPG) surfaces was investigated by tapping-mode atomic force microscopy (TM-AFM) and scanning tunneling microscopy (STM). TM-AFM and high-resolution STM results show that these hydrogen-bonded assemblies self-organize to form highly ordered domains on HOPG surfaces. We find that a subtle change in one of the building blocks induces two different orientations of the assembly with respect to the surface. These results provide information on the control over the construction of supramolecular nanoarchitectures in 2D with the potential for the manufacturing of functional materials based on structural manipulation of molecular components.     

Transition metal-chelating surfactant micelle templates for facile synthesis of mesoporous silica nanoparticles

Highly ordered mesoporous silica nanoparticles with tunable morphology and pore-size are prepared by the use of a transition metal-chelating surfactant micelle complex using Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ ions. These metal ions formed a metal-P123 micelle complex in an aqueous solution, while the metal ions are chelated to the hydrophilic domain such as the poly(ethylene oxide) group of a P123 surfactant. The different complexation abilities of the utilized transition metal ions play an important role in determining the formation of nano-sized ordered MSNs due to the different stabilization constant of the metal-P123 complex. Consequently, from a particle length of 1700 nm in the original mesoporous silica materials, the particle length of ordered MSNs through the metal-chelating P123 micelle templates can be reduced to a range of 180–800 nm. Furthermore, the variation of pore size shows a slight change from 8.8 to 6.6 nm. In particular, the Cu²⁺-chelated MSNs show only decreased particle size to 180 nm. The stability constants for the metal-P123 complex are calculated on the basis of molar conductance measurements in order to elucidate the formation mechanism of MSNs by the metal-chelating P123 complex templates. In addition, solid-state ²⁹Si, ¹³C-NMR and ICP-OES measurements are used for quantitative characterization reveal that the utilized metal ions affect only the formation of a metal-P123 complex in a micelle as a template.     

Electrochemical sensor for ultrasensitive determination of ceftazidime using hollow platinum nanoparticles/reduced graphene oxide/pencil graphite electrode

In this paper, an electrochemical sensor was prepared based on the modification of pencil graphite electrode (PGE) by hollow platinum nanoparticles/reduced graphene oxide (HPtNPs/rGO/PGE) for determination of ceftazidime (CFZ). Initially, rGO was electrodeposited on the electrode surface, and then, hollow platinum nanoparticles were placed on the electrode surface via galvanic displacement reaction of Pt(IV) ions with cobalt nanoparticles (CoNPs) that had electrodeposited on the electrode surface. Several significant parameters controlling the performance of the HPtNPs/rGO/PGE were examined and optimized using central composite design as one optimization methodology. The surface morphology and elemental characterization of the bare PGE, rGO/PGE, CoNPs/rGO/PGE, and HPtNPs/rGO/PGE-modified electrodes was analyzed by field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and electrochemical impedance spectroscopy. The electrochemical activity of CFZ on resulting modified electrode was investigated by cyclic voltammetry (CV) and adsorptive differential pulse voltammetry (AdDPV). Adsorptive differential pulse voltammetry indicates that peak current increases linearly with respect to increment in CFZ concentration. CFZ was determined in the linear dynamic range of 5.0 × 10^?13 to 1.0 × 10^?9 M, and the detection limit was determined as 2.2 × 10^?13 M using AdDPV under optimized conditions. The results showed that modified electrode has high selectivity and very high sensitivity. The method was used to determine of CFZ in drug injection and plasma samples.     

Electrolyte effect on mixed micelle and interfacial properties of binary mixtures of cationic and nonionic surfactants

In the present work, the adsorption behavior at the liquid-air interface and micellization characteristics of mixtures of cetyltrimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl) polyoxyethylene (TritonX-100) in aqueous media containing different concentrations of NaBr were investigated by surface tension and potentiometry measurements. From plots of surface tension (gamma) as a function of solution composition and total surfactant concentration, we determined the critical micelle concentration (CMC), minimum surface tension at the CMC (gamma(CMC)), surface excess (Gamma(max)), and mean molecular surface area (A(min)). On the basis of regular solution theory, the compositions of the adsorbed film (Z) and micelles (X(M)) were estimated, and then the interaction parameters in the micelles (beta(M)) and in the adsorbed film phase (beta(sigma)) were calculated. For all mole fraction ratios, the results showed synergistically enhanced ability to form mixed micelles as well as surface tension reduction. Furthermore beta was calculated by considering nonrandom mixing and head group size effects. It was observed that, for both the planar air/aqueous interface and micellar systems, the nonideality decreased as the amount of electrolyte in the aqueous medium was increased. This was attributed to a decrease of the surface charge density caused by increasing the concentration of bromide ions.     

Self-assembly of a nonionic surfactant at the graphite/ionic liquid interface

In this study, we demonstrate by AFM imaging that nonionic surfactants self-assemble into hemicylindrical aggregates at the interface between graphite and the room temperature ionic liquid ethylammonium nitrate. Like aqueous systems, surfactant first adsorbs in a tail-to-tail monolayer arrangement along one of the three symmetry axes of graphite, templating subsequent self-assembly into adsorbed hemicylinders. Longer surfactant tails and higher concentrations are required to produce hemicylindrical aggregates in the ionic liquid than in aqueous solutions.     

Binding on strong polyelectrolytes of mixed ionic and nonionic surfactants below their critical micelle concentration observed by fluorescence

The binding of mixed surfactants of cationic cetyltrimethylammonium bromide (CTAB) and nonionic octaethylene glycol monododecyl ether (C ₁₂E ₈) on anionic polyelectrolyte poly[2-acrylamido-2-methylpropanesulfonic acid (PAMPS)] and fluorophore-labeled copolymers containing about 40 mol% of AMPS was investigated at different mole fractions, Y , of CTAB in the surfactant mixture. The excimer emission of the cationic probe 1-pyrenemethylamine hydrochloride (PyMeA·HCl), nonradiative energy transfer (NRET) between pyrene and naphthalene labels and I ₁/ I ₃ of the pyrene label were determined by varying the total surfactant concentration, c _Surf. The I _E/ I _M value of PyMeA·HCl firstly increases and then decreases to 0 with c _Surf, showing a maximum on every curve. The critical aggregation concentration of the mixed surfactants determined from the I _E/ I _M maximum decreased from 5×10 ^-5 to 1×10 ^-5 mol/l as Y increased from 0.1 to 0.50, and then leveled off as Y increased up to unity. And at least 5×10 ^-6 mol/l CTAB was required for the mixed surfactants to bind on the PAMPS cooperatively. Equimolar binding of CTAB on AMPS was formed at I _E/ I _M=0 when Y =0.25, while at Y =0.1 some CTAB molecules in the mixed micelle were directed to the water phase without binding with AMPS. Both the intramolecular and the intermolecular NRET increased and then decreased with c _Surf, having a maximum on each curve corresponding to the equimolar binding of CTAB and AMPS so long as Y >0, indicating the coiling of the chain and interchain aggregation upon bound surfactants. The I _Py/ I _Np value at the maximum decreased with decreasing Y because more nonionic surfactant C ₁₂E ₈ participated into the polyelectrolyte-mixed surfactant complexes together with bound CTAB.     

Alternate assemblies of platinum nanoparticles and metalloporphyrins as tunable electrocatalysts for dioxygen reduction

Through electrostatic layer-by-layer (LBL) assembly, negatively charged citrate-stabilized platinum nanoparticles (PtNPs) and positively charged [tetrakis(N-methylpyridyl)porphyrinato] cobalt were alternately deposited on a 4-aminobenzoic acid-modified glassy carbon electrode and also on indium tin oxide substrates, directly forming the three-dimensional nanostructured materials. Thus-prepared multilayer films were characterized by UV--visible spectroscopy, surface plasmon resonance (SPR) spectroscopy, atomic force microscopy (AFM), and cyclic voltammetry. Regular growth of the multilayer films is monitored by UV--visible spectroscopy and SPR spectroscopy. AFM provides the morphology of the multilayer films. The PtNPs containing multilayer films exhibit high electrocatalytic activity for the reduction of dioxygen with high stability. Rotating disk electrode voltammetry and rotating ring-disk electrode voltammetry demonstrate that the PtNP-containing multilayer films can catalyze an almost four-electron reduction of O(2) to H(2)O in an air-saturated 0.5 M H(2)SO(4) solution. Furthermore, the electrocatalytic activity of the films could be further tailored by simply choosing different cycles in the LBL process or more specifically the amount of the assembly components in the films. The high electrocatalytic activity and good stability for dioxygen reduction make the PtNP-containing multilayer films potential candidates for the efficient cathode material in fuel cells.     

Multifunctional nanoparticles/silica microsphere assemblies using polyglycidyl methacrylate shells as supports

A general method to prepare functional (or multifunctional) nanoparticles/silica microsphere assemblies is reported in this article. A thin shell of polyglycidyl methacrylate is grafted on the surface of silica through surface-initiated atom transfer radical polymerization technique. And then, various types of nanoparticles, including water-soluble CdTe quantum dots, Au nanoparticles and oil-soluble Fe₃O₄ nanoparticles are assembled on silica microspheres, respectively, or simultaneously. The properties of the assembled nanoparticles are well retained in the nanocomposite assemblies, and the controllable integration of magnetic and fluorescent properties can be achieved through varying the proportion of different nanoparticles assembled on nanoparticle/silica microsphere.     

Preparation of platinum nanoparticles using star-block copolymer with a carboxylic core

Well-defined star polymers containing a functionalized core supply a molecular nanocavity and may be used to control formation of inorganic nanoparticles. Herein, platinum (Pt) nanoparticles of 2-4 nm were prepared by using (poly(acrylic acid)-b-polystyrene)6 (PAA-b-PS)6 amphiphilic star block copolymer as a novel single molecular stabilizer. This PAA core functionalized star polymer was obtained by hydrolysis of (poly(tert-butyl acrylate)-b-polystyrene)6 (PtBA-b-PS)6, which was synthesized by sequential atom transfer radical polymerization (ATRP) of tert-butyl acrylate and styrene with an initiator bearing six 2-bromoisobutyloxyl groups. Pt(IV) ions were loaded by ion exchange to the core of the star polymer and Pt nanoparticle stabilized by single star polymer was produced by a reduction with NaBH4.     

Electrochemical preparation of platinum nanothorn assemblies with high surface enhanced Raman scattering activity

Platinum nanothorn assemblies with sharp tips and edges were prepared, which exhibit high surface enhanced Raman scattering (SERS) activity and yield an enhancement factor as high as 2000 for adsorbed pyridine.     

Thermodynamic studies of mixed ionic/nonionic surfactant systems

Mixtures of alkyltrimethylammonium bromide (CnTAB, n=12, 14, 16, 18) and Triton X-100 were studied at a range of mole fractions of ionic surfactant per nonionic surfactant. For each mixture, the cmc obtained from surface tension measurements differed from that obtained using potentiometry. The behavior of these mixed-surfactant systems showed three different regions with increasing total surfactant concentration. From the surface tension and potentiometry data, we obtained the free monomer concentration of ionic surfactant (mi), the micellar mole fraction of surfactant (xi), and the degree of dissociation (alpha) of ionic surfactant. We also obtained the free monomer concentration of Triton X-100 (m2) using PFG-NMR technique. A new equation was introduced to evaluate the activity coefficient in the micellar phase. The excess free energy (GE) and the synergetic parameters of mixtures were determined at various mole fractions of CnTAB/Triton X-100. Finally, the complexity of the synergism parameters was investigated.     

Polyoxometallates as inorganic templates for electrocatalytic network films of ultra-thin conducting polymers and platinum nanoparticles

We develop a concept of fabrication of the multilayer network films on electrodes by exploring the ability of a Keggin-type polyoxometallate, phosphododecamolybdate (PMo(12)O(40)(3-)), to form stable anionic monolayers (templates) on carbon and metals including platinum. By repeated alternate treatments in the solution of PMo(12)O(40)(3-) (or in the colloidal suspension of polyoxometallate-protected Pt-nanoparticles) and in the solution of monomer (e.g. anilinium) cations, the amount of the material can be increased systematically (layer-by-layer) to form stable three-dimensional assemblies on electrode (e.g. glassy carbon) surfaces. In the resulting hybrid (organic-inorganic) films, the layers of negatively charged polyoxometallate, or polyoxometallate-protected (stabilized) Pt-nanoparticles, are linked or electrostatically attracted by ultra-thin layers of such positively charged conducting polymers as polyaniline (PANI), polypyrrole (PPy) or poly(3,4-ethylenedioxythiophene), PEDOT. Consequently, the attractive physicochemical properties of polymers and reactivity of polyoxometallate or noble metal particles are combined. The films are functionalized and show electrocatalytic properties towards reduction of nitrite, bromate, hydrogen peroxide or oxygen. They are of importance to the chemical and biochemical sensing as well as to the biochemical and medical applications.     

Preparation of nanoparticles by “dissolution” of submicron-sized ionized styrene-methacrylic acid copolymer particles in nonionic emulsifier solution

 Styrene-methacrylic acid copolymer (P(S-MAA)) nanoparticles having high Tg were produced by a dissolution of submicron-sized P(S-MAA) particles as follows. Submicron-sized P(S-MAA) particles having various MAA contents were produced by emulsion copolymerization. Secondly, they were treated in a polyoxyethylene nonylphenyl-ether nonionic emulsifier aqueous solution at pH 13.0 and above 90 °C. The nanoparticles having about 30 nm in diameter were only produced from the particles having MAA contents around 7 mol%, and above the contents, they were not produced. It seems to be based on that emulsifier molecules are not adsorbed onto the polymer molecules enough to dissolve them. The effect of MAA content on such a dissolution behavior was examined using seven kinds of different nonionic emulsifiers having hydrophile-lipophile-balance values between 12.2 and 18.2 at various temperatures and initial pH. Received: 12 June 1996 Accepted: 27 August 1996     

Size-dependent surface reactions of Ag nanoparticles supported on highly oriented pyrolytic graphite

Various sizes of Ag particles were grown on highly oriented pyrolytic graphite (HOPG) surfaces, which had previously been modified with nanopits to act as anchoring sites. Surface reactions of O2, CHCl3, and CCl4 on the Ag particles and bulk Ag(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), and it has been shown that size dependence of O2 and CHCl3 reactions on Ag differs from that of CCl4. Weak reactions of O2 and CHCl3 were observed on the bulk Ag(111) surfaces, while strong reactions occur on Ag particles with medium Ag coverage, suggesting that the reactions are controlled by the number of surface defect sites. On the contrary, the dissociation of CCl4 is mainly determined by the exposed Ag facet area, mainly Ag(111) facet, and strong dissociation reaction happens on the bulk Ag(111) surface. The results suggest that the size effects, which are often discussed in heterogeneous catalysis, are strongly dependent on the reaction mechanism.     

In-situ synthesis of nanoparticles via supersolubilizing micelle self-assembly

In-situ synthesis of nano-particles using the self-assembly of molten salt and super soluble micellae was proposed based on a phenomenon of super solubilization of molten salt in reverse micellae and its self-assembly when the concentration reached up to 95% (w/w). The mechanism of the self-assembly indicates that the self-assembly of molten salt occurs in a reverse micelle where a homogenous phase is established between 5% (w/w) of a surfactant with a VB value of less than 1 and a hydrocarbon species. This synthesis has some unique features, such as being free of water, highly effective deposition and narrow distribution of particle size.     

Fabrication of ordered catalytically active nanoparticles derived from block copolymer micelle templates for controllable synthesis of single-walled carbon nanotubes

We report the use of the block copolymer micelle approach to produce various transition metal nanoparticles such as iron, cobalt, and nickel with precisely controlled size and spacing. These uniformly sized catalyst nanoparticles derived from the block copolymer micelle approach have enabled the synthesis of carbon nanotubes (CNTs) with narrow size distribution. Because of the excellent film forming ability of the polymeric material, metal-bearing surface micelles produced from the solution micelles can be distributed uniformly on a surface, resulting in evenly dispersed catalyst nanoparticles. As a result, high quality and uniformly distributed CNTs have been synthesized. Spatially selective growth of CNTs from a lithographically patterned metal-bearing micelle film has been achieved. The polymer template approach can potentially be extended to synthesize single-metallic and bimetallic catalytically active nanoparticles with uniform size and spacing and is fully compatible with conventional lithographic process. Additionally, catalyst nanoparticles produced from this method do not coalesce at high growth temperature. All these attributes make this approach a promising fabrication pathway for controllable synthesis of CNTs.