Molecular morphology of modified partially hydrolyzed polyacrylamide (MHPAM) on mica substrates and Langmuir-Blodgett films of MHPAM/CTAB complexes as observed by AFM

Direct observation of molecular morphology of modified partially hydrolyzed polyacrylamide (MHPAM) has been studied in this paper by atomic force microscopy (AFM). The MHPAM molecule chains are stretched forming worm-like coils due to the electrostatic repulsion between randomly distributed positive charges along the chain at pH 3. The further data analysis demonstrates single MHPAM molecules are being imaged. In addition, morphology of Langmuir-Blodgett films of MHPAM/CTAB complexes on mica surfaces also investigated as a function of the subphase pH. At pH 6, MHPAM/CTAB molecules covered on mica surfaces adopted the worm-like nanostructures, and at pH 7, conformational transition of polymer complexes appeared, showing the morphology of bump-like to some extent owing to aggregation of intrachains and interchains. Eventually, more compact globule morphology was formed at pH 12.     

Structural evolution in cationic micelles upon incorporation of a polar organic dopant

Micelles of cetyltrimethylammonium bromide (CTAB), when doped with increasing levels of 4-ethylphenol, show microstructural transitions from spherical micelles to elongated wormlike micelles, disks, and subsequently to globular and then to tubular vesicles. Wormlike micelles are observed at a dopant-to-CTAB molar ratio of 1:3. At higher dopant ratios (1:1), globular vesicles are observed which transition to tubular vesicles when the dopant becomes the predominant species at a ratio of 3:1. These transitions are reflected in small-angle neutron scattering analysis and, interestingly, can be directly observed through cryo-transmission electron microscopy. The para-substituted phenol is interfacially active and modulates interfacial curvature of the micelles. The observations of microstructure modifications have relevance to the synthesis of mesoporous materials using CTAB as the template.     

Langmuir Aggregation of Stain on Ionic Surfactant and its Application: Interaction of Bromo‐Pyrogallol Red with Cetyl Trimethylammonium Bromide (CTAB) and Determination of Molybdenum Traces

The microphase adsorption‐spectral correction (MPASC) technique was de scribed and applied to study the interaction of bromo‐pyrogallol red (BPR) with cetyl trimethylammonium bromide (CTAB). The synergism mechanism of micelles was analyzed and discussed. The large electrostatic micelle adsorbs the chromophore in only the monolayer, and the adsorption obeys the Langmuir adsorption. The formation of the (BPR₂·CTAB)₇₈ aggregate accelerates the Mo‐BPR complex reaction. Experimental results showed the adsorption constant of the adsorption aggregate 6.20 × 10⁵ and its molar absorptivity ?_r^610nm = 1.40 × 10⁶ lmol^?1cm^?1. For analysis of samples, the recovery of Mo was between 97.4 and 107% and the RSD was less than 4.39%.     

Freezing of fluids confined between mica surfaces

Using grand ensemble simulations, we show that octamethyl-cyclo-tetra-siloxane (OMCTS) confined between two mica surfaces can form a variety of frozen phases which undergo solid-solid transitions as a function of the separation between the surfaces. For atomically smooth mica surfaces, the following sequence of transitions 1[triangle up] --> 1[triangle up]b --> 2B --> 2 square --> 2[triangle up] are observed in the one- and two-layered regimes, where n[triangle up], n[square], and nB denote triangular, square, and buckled phases, respectively, with the prefix n denoting the number of confined layers. The presence of potassium on mica is seen to have a strong influence on the degree of order induced in the fluid. The sequence of solid-solid transitions that occurs with the smooth mica surface is no longer observed. When equilibrated with a state point near the liquid-solid transition, a counterintuitive freezing scenario is observed in the presence of potassium. Potassium disrupts in-plane ordering in the fluid in contact with the mica surface, and freezing is observed only in the inner confined layers. The largest mica separations at which frozen phases were observed ranged from separations that could accommodate six to seven fluid layers. The extent of freezing and the square-to-triangular lattice transition was found to be sensitive to the presence of potassium as well as the thermodynamic conditions of the bulk fluid. The implications of our results on interpretation of surface force experiments as well as the generic phase behavior of confined soft spheres is discussed.     

Experimental studies on the adsorption of two surfactants on solid-aqueous interfaces: adsorption isotherms and kinetics

A quartz crystal microbalance with dissipation (QCM-D) was used to measure the adsorption from aqueous solutions of CTAB (cationic) and C(12)E(6) (nonionic) surfactants on gold and silica surfaces. QCM-D allows for the determination of adsorption isotherms and also the monitoring of the dynamics of adsorption in real time. By considering the atomic-scale roughness of the solid surfaces and the surface area per head group at the air/water interface, our experiments indicate that at bulk concentrations above the critical micelle concentration adsorbed C(12)E(6) forms a monolayer-like structure on both surfaces and CTAB yields a bilayer-like structure. Although our measurements do not allow us to discriminate between the morphology of the aggregates (i.e., between flat monolayers, hemicylinders, or hemispheres in the case of C(12)E(6) and between flat bilayers, cylinders, or spheres in the case of CTAB), these results are particularly significant when compared to recent QCM-D data reported by Macakova et al. (Macakova, L.; Blomberg, E.; Claesson, P. M. Langmuir 2007, 23, 12436). These authors reported that QCM-D overestimates the amount of CTAB adsorbed on silica by as much as 30-40% as a result of entrapped water. Our analysis suggests that the effect of entrapped solvent is not as important as previously assumed and, in fact, QCM-D may not overestimate the amount of CTAB adsorbed when roughness is considered. Results for the kinetics of adsorption suggest that the aggregate structure as well as whether micelles are present may influence the adsorption mechanism. We discuss our results in the perspective of molecular theories for both the equilibrium and kinetics of surfactant adsorption.     

Zeta-potentials of self-assembled surface micelles of ionic surfactants adsorbed at hydrophobic graphite surfaces

The zeta-potentials of the self-assembled surface ionic surfactants (sodium dodecyl sulfate—SDS and hexadecyltrimethyl ammonium bromide—CTAB) on graphite surfaces were determined both from streaming potential and electrophoretic mobility measurements. The adsorption of the surfactants at graphite–liquid interfaces has been studied using atomic force microscopy (AFM) soft-contact imaging which shows the formation of linear, parallel hemicylinders with headgroups oriented towards the solution. The magnitude of the zeta-potential increased with an increase in surfactant concentration, reaching a constant value at a concentration corresponding to the point of surface micelle formation as confirmed from AFM imaging. The streaming potential and electrophoretic mobility measurements showed that the zeta-potentials of SDS and CTAB surface micelles adsorbed at the graphite surface were about −75 and +70 mV, respectively, well in agreement with the values reported for bulk phase micelles in the literature.     

Influence of DNA adsorption and DNA/cationic surfactant coadsorption on the interaction forces between hydrophobic surfaces

The forces between hydrophobic surfaces with physisorbed DNA are markedly and irreversibly altered by exposure to DNA/cetyltrimethylammonium bromide (CTAB) mixtures. In this colloidal probe atomic force microscopy study of the interactions between a hydrophobic polystyrene particle and an octadecyltrimethylethoxysilane-modified mica surface in sodium bromide solutions, we measure distinct changes in colloidal forces depending on the existence and state of an adsorbed layer of DNA or CTAB-DNA complexes. For bare hydrophobic surfaces, a monotonically attractive approach curve and very large adhesion are observed. When DNA is adsorbed at low bulk concentrations, a long-range repulsive force dominates the approach, but on retraction some adhesion persists and DNA bridging is clearly observed. When the DNA solution is replaced with a CTAB-DNA mixture at relative low CTAB concentration, the length scale of the repulsive force decreases, the adhesion due to hydrophobic interactions greatly decreases, and bridging events disappear. Finally, when the surface is rinsed with NaBr solution, the length scale of the repulsive interaction increases modestly, and only a very tiny adhesion remains. These pronounced changes in the force behavior are consistent with CTAB-induced DNA compaction accompanied by increased DNA adsorption, both of which are partially irreversible.     

Structural Transitions of CTAB Micelles in a Protic Ionic Liquid

Micellar solutions of hexadecyltrimethylammonium bromide (CTAB) in a protic ionic liquid, ethylammonium nitrate (EAN), are studied by shear rheology, polarizing optical microscopy (POM), conductivity measurements, and small angle neutron scattering (SANS). Three concentration regimes are examined: A dilute regime (with concentrations [CTAB] < 5 wt %) consisting of noninteracting spherical micelles, a semidilute regime (5 wt % ≤ [CTAB] ≤ 45 wt %) where micelles interact via electrostatic repulsions, and a concentrated regime (45 wt % < [CTAB] ≤ 62 wt %) where a reversible, temperature-dependent isotropic (L(1)) to hexatic (Hex) phase transition is observed. The L(1)-Hex transition, which has been predicted but not previously observed, is characterized by (1) a sharp increase in the shear viscosity, (2) the formation of focal conical birefringence textures (observed by POM), and (3) enhancement of the crystalline order, evidenced by the appearance of Bragg reflections in the SANS profiles. Ionic conductivity is not sensitive to the L(1)-Hex transition, which corroborates the absence of topological transitions.     

Pseudo-phase ion-exchange model for micellar catalysis in the acid hydrolysis of vinyl ethers

The rate of the perchloric acid hydrolysis of aqueous ethyl and butyl vinyl ethers at 25.0°C, in the presence of micellar aggregates [anionic, sodium dodecyl sulfate (SDS); cationic, cetyl trymethyl ammonium bromide (CTAB); and nonionic, polyoxyethylen? 23? dodecanol, (Brij 35)], has been studied. Negligible effects were observed in the cases of cationic and nonionic micelles. Anionic micelles produce an enhancement in the reaction velocity, and the rate constants go through maxima with increasing SDS concentration. These maxima disappear in the presence of excess sodium perchlorate. All these facts are interpreted quantitatively by means of the pseudo-phase ion-exchange model.     

Cascade of coil-globule conformational transitions of single flexible polyelectrolyte molecules in poor solvent

We show that hydrophobic flexible polyelectrolyte molecules of poly(2-vinylpyridine) and poly(methacryloyloxyethyl dimethylbenzylammonium chloride) are trapped and frozen due to adsorption on the mica surface, and the observed AFM single molecule structures reflect the molecular conformation in solution. An increase of the ionic strength of the solution induces the cascade of abrupt conformational transitions due to the intrachain segregation from elongated coil to compact globule conformations through intermediate pearl necklace-globule conformations with different amounts of beads per chain. The length of the necklaces and the number of beads decrease, while the diameter of beads increases with the increase of ionic strength. Coexistence at the same time of extended coils, necklaces with different amounts of beads, and compact globules indicates the cascade of the first-order-type phase transitions.     

Depletion flocculation of latex dispersion in ionic micellar systems

The flocculation behavior of anionic and cationic latex dispersions induced by addition of ionic surfactants with different polarities (SDS and cetyltrimethylammonium bromide (CTAB)) have been evaluated by rheological measurements. It was found that in identical polar surfactant systems with particle surfaces of SDS + anionic lattices and CTAB + cationic lattices, a weak and reversible flocculation has been observed in a limited concentration region of surfactant, which was analyzed as a repletion flocculation induced by the volume-restriction effect of the surfactant micelles. On the other hand, in oppositely charged surfactant systems (SDS + cationic lattices and CTAB + anionic lattices), the particles were flocculated strongly in a low surfactant concentration region, which will be based on the charge neutralization and hydrophobic effects from the adsorbed surfactant molecules. After the particles stabilized by the electrostatic repulsion of adsorbed surfactant layers, the system viscosity shows a weak maximum again in a limited concentration region. This weak maximum was influenced by the shear rate and has a complete reversible character, which means that this weak flocculation will be due to the depletion effect from the free micelles after saturated adsorption.     

Studies on the antioxidant effect of rutin in the microenvironment of cationic micelles

UV-vis spectra, fluorescence emission spectra, cyclic voltammetric and electron spin resonance are employed to study the location of rutin in cetyltrimethylammonium bromide (CTAB) micelles with different microstructures and microenvironments, and to gain information about the antioxidant capacity (hydroxyl radical scavenge activity) of rutin in CTAB micelles. Rutin molecules can be partly solubilized in CTAB spherical micelles through electrostatic attraction and hydrophobic force, which leads to the bathochromic shift of absorption spectra, decrease of the microenvironment polarity of pyrene and the hydroxyl radical scavenge activity. However, CTAB rod-like micelles congregate rutin molecules on their surfaces, which results in the hypochromic shift of absorption spectra compared to the spectrum in CTAB spherical micelles and also heightens the antioxidant capacity of rutin. Correspondence: Rong Guo, School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P.R. China     

Effects of Cetyltrimethylammonium Bromide and Heptane on the Surface Properties and CO2 Adsorption of Zeolite NaX

Zeolite X was synthesized by the gelation method for use as a CO₂ adsorbent. The zeolite was prepared from a sodium silicate solution and Al(OH )₃ as silicon and aluminum source, respectively. The effects of cetyltrimethylammonium bromide (CTAB ) and heptane addition on surface properties, morphology, and CO₂ adsorption capacities were investigated. The CO₂ adsorption capacity of the synthesized zeolite with the addition of CTAB and heptane increased as demonstrated by the higher CO₂ uptake (3.00 mmol/g), which was due to the higher surface area and pore volume. Interaction between CTAB micelles and sodium cations with aluminosilicate anions led to an increase in the nucleation rate and smaller crystalline sizes. The addition of heptane resulted in higher positively charged CTAB micelles and improved the affinity between the micelles and aluminosilicate anions.     

Kinetics of hydrolysis of procaine in aqueous and micellar media

The kinetics of alkaline hydrolysis of procaine under the pseudo–first‐order condition ([OH^?] ? [procaine]) has been carried out. N,N‐Diethylaminoethanol and p‐aminobenzoate anion were obtained as the hydrolysis product. The rate of hydrolysis was found to be linearly dependent upon [NaOH]. The addition of cationic cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DDTAB) and tetradecyltrimethylammonium bromide, and anionic sodium dodecyl sulfate (SDS) micelles inhibited the rate of hydrolysis. The maximum inhibitive effect on the reaction rate was observed for SDS micelles, whereas among the cationic surfactants, CTAB inhibited most. The variation in the rate of hydrolysis of procaine in the micellar media is attributed to the orientation of a reactive molecule to the surfactant and the binding constant of procaine with micelles. The rate of hydrolysis of procaine is negligible in DDTAB micelles. The observed results in the presence of cationic micelles were treated on the basis of the pseudophase ion exchange model. The results obtained in the presence of anionic micelles were treated by the pseudophase model, and the various kinetic parameters were determined. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 1–9, 2013     

Effect of ionic micellar medium on kinetics and mechanism of oxidation of bovine serum albumin: A pulse radiolysis study

Effect of protein–micelle interaction on bovine serum albumin (BSA) oxidation by trichloromethyl peroxyl radical (CCl₃O₂^·) in anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethyl ammonium bromide (CTAB) micellar media has been studied using nanosecond pulse radiolysis technique. Viscosity measurement and light scattering studies have suggested that SDS and CTAB micelles produce BSA–micelle aggregates of different sizes and polydispersity. Oxidation kinetics and transients have been affected both by anionic SDS and cationic CTAB micelles but in a different manner. Tryptophanyl-CCl₃O₂^· adduct radical to tyrosyl radical transformation in BSA has been observed in anionic SDS micelles but not in cationic CTAB micelles. Similar studies have also been done with tryptophan and tyrosine amino acids, which undergo oxidation in BSA. The study suggests that Coulombic and hydrophobic interactions between micelles and protein affect the structure of the protein to shield its functional amino acids, like tryptophan and tyrosine, to neutral oxidizing radical.     

Kinetics and mechanism of interaction of dipeptide (glycyl–glycine) with ninhydrin in aqueous micellar media

The rates of reaction between ninhydrin and dipeptide glycyl–glycine (Gly–Gly) have been determined by studying the reaction spectrophotometrically at 70°C and pH 5.0 in aqueous and in aqueous cationic micelles of cetyltrimethylammonium bromide (CTAB). The reaction follows first‐ and fractional‐order kinetics, respectively, in [Gly–Gly] and [ninhydrin]. The observed rate constant is affected by [CTAB] changes and the maximum rate enhancement is ca. three‐fold. As the k_ψ ? [CTAB] profile shape is characteristic of bimolecular reactions catalyzed by micelles, the catalysis is explained in terms of the pseudo‐phase model of the micelles (proposed by Menger and Portnoy and developed by Bunton and Romsted). The presence of inorganic salts (NaCl, NaBr, Na₂SO₄) does not reveal any regular effect but the data with organic salts (NaBenz, NaSal) show an increase in the rate followed by a decrease. The kinetic data have been used to calculate the micellar binding constants K_S for Gly–Gly and K_N for ninhydrin and the respective values are 317 and 69 mol^?1 dm³. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 643–650, 2006     

Stabilizing bolaform amphiphile interfacial assemblies by introducing mesogenic groups

We describe the synthesis and characterization of the mesogen-bearing bolaform amphiphile 4,4'-dihydroxybiphenylbis(11-pyridinium-N-yl-undecanoic ester) dibromide (BP-10) and its solid/liquid interfacial self-assembly. Cylindrical micelles are directly observed by atomic force microscopy (AFM) at the interface between mica and the aqueous solution above the critical micelle concentration (cmc). In situ and ex situ AFM studies indicate that the cylindrical micelles are stable both at the mica/solution interface and in the dry state. The enhanced stability of the micellar structures enables a detailed investigation of their self-assembly behavior and supramolecular structures at the interface. The adsorption model proposed here is supported by the variation of the interfacial self-assemblies on changing the solution concentration and substrate temperature.     

Anionic polyelectrolyte adsorption on mica mediated by multivalent cations: a solution to DNA imaging by atomic force microscopy under high ionic strengths

Adsorption of DNA molecules on mica, a highly negatively charged surface, mediated by divalent or trivalent cations is considered. By analyzing atomic force microscope (AFM) images of DNA molecules adsorbed on mica, phase diagrams of DNA molecules interacting with a mica surface are established in terms of concentrations of monovalent salt (NaCl) and divalent (MgCl2) or multivalent (spermidine, cobalt hexamine) salts. These diagrams show two transitions between nonadsorption and adsorption. The first one arises when the concentration of multivalent counterions is larger than a limit value, which is not sensitive to the monovalent salt concentration. The second transition is due to the binding competition between monovalent and multivalent counterions. In addition, we develop a model of polyelectrolyte adsorption on like-charged surfaces with multivalent counterions. This model shows that the correlations of the multivalent counterions at the interface between DNA and mica play a critical role. Furthermore, it appears that DNA adsorption takes place when the energy gain in counterion correlations overcomes an energy barrier. This barrier is induced by the entropy loss in confining DNA in a thin adsorbed layer, the entropy loss in the interpenetration of the clouds of mica and DNA counterions, and the electrostatic repulsion between DNA and mica. The analysis of the experimental results provides an estimation of this energy barrier. We then discuss some important issues, including DNA adsorption under physiological conditions.     

AFM study on the wettability of mica and graphite modified with surfactant DTAB

In this work atomic force microscopy (AFM) was applied to study the wettability of mica and graphite modified with surfactant dodecyltrimethylammonium bromide (DTAB) at varying DTAB concentrations (below the cmc) and adsorption time. The coverage states of DTAB on surfaces were analyzed from the AFM images, while the contact angle measurement was made for the wettability of DTAB-modified surfaces. The experimental results have shown that the adsorption aggregates formed as needle-like dots covering on the mica surface with the surfactant concentration of 10^?6–10^?4?mol/L. The coverage of DTAB aggregates increased with the increasing concentration, leading to a strong hydrophobicity on the surfaces. However, the large aggregates which might be caused by bilayer adsorption of surfactant occurred on mica surface at surfactant concentration of 10^?3?mol/L, resulting in the reverse of the wettability as the adsorption time extended. In the case of hydrophobic graphite, DTAB aggregates mainly formed as stripe covering on the surfaces, leading to the reduction of hydrophobicity. This reduction became stronger as more DTAB aggregates covered on graphite surfaces.     

The effect of β-cyclodextrin on the properties of cetyltrimethylammonium bromide micelles

The effect of ß-cyclodextrin (ß-CD) on cetyltrimethylammonium bromide (CTAB) micellar properties was studied by the determination of the diffusion coefficient, D. When the CTAB micelles have a spherical structure, D firstly increased and then remained unchanged, while the micellar aggregation number, N, decreased with the addition of ß-CD. When the CTAB concentration was less than the first critical micellar concentration, CTAB molecules could be included into ß-CD cavities with the molar ratio of CTAB to ß-CD being about 1:1. However, when the CTAB concentration was higher than the first critical micellar concentration, mixed spherical micelles were formed with the molar ratio of CTAB to ß-CD being 9:1.