Average Aggregation Number Research Articles

Breakdown of the Stokes-Einstein Equation for Solutions of Water in Oil Reverse Micelles.

An experimental study is presented for the reverse micellar system of 15% by mass polydisperse hexaethylene glycol monodecylether (C10E6) in cyclohexane with varying amounts of added water up to 4% by mass. Measurements of viscosity and self-diffusion coefficients were taken as a function of temperature between 10 and 45 °C at varying sample water loads but fixed C10E6/cyclohexane composition. The results were used to inspect the validity of the Stokes-Einstein equation for this system. Unreasonably small reverse average micelle radii and aggregation numbers were obtained with the Stokes-Einstein equation, but reasonable values for these quantities were obtained using the ratio of surfactant-to-cyclohexane self-diffusion coefficients. While bulk viscosity increased with increasing water load, a concurrent expected decrease of self-diffusion coefficient was only observed for the surfactant and water but not for cyclohexane, which showed independence of water load. Moreover, a spread of self-diffusion coefficients was observed for the protons associated with the ethylene oxide repeat unit in samples with polydisperse C10E6 but not in a sample with monodisperse C10E6. These findings were interpreted by the presence of reverse micelle to reverse micelle hopping motions that with higher water load become increasingly selective toward C10E6 molecules with short ethylene oxide repeat units, while those with long ethylene oxide repeat units remain trapped within the reverse micelle because of the increased hydrogen bonding interactions with the water inside the growing core of the reverse micelle. Despite the observed breakdown of the Stokes-Einstein equation, the temperature dependence of the viscosities and self-diffusion coefficients was found to follow Arrhenius behavior over the investigated range of temperatures.

Clustering of asymmetric dumbbell-shaped silica/polystyrene nanoparticles by solvent-induced self-assembly

We report a quite simple strategy to assemble silica/polystyrene dumbbell-shaped nanoparticles into clusters with an aggregation number from two to more than 30. The polystyrene lobe serves as a patch that is made sticky and ready to merge with similar ones when the dumbbells are dispersed in an ethanol/DMF mixture. Thanks to transmission electron microscopy experiments, we describe qualitatively and quantitatively the influence of several experimental parameters such as the solvent quality, i.e. DMF fraction, and the patch-to-particle size ratio. We show that the DMF fraction range (30–50 vol%) for the sticky regime can be extended if the incubation process is completed by a centrifugation step. We also demonstrate an unexpected evolution in the average aggregation number with the patch-to-particle size ratio that may be explained by the molar mass distribution within the polystyrene core of the clusters. Lastly, we show that this assembly route may be extended to gold-coated clusters.

Effect of Molecular Crowding on Conformation and Interactions of Single-Chain Nanoparticles

Single-chain nanoparticles (SCNPs) are soft disordered nano-objects synthesized by intra-molecular bonding of linear polymer chains with a finite fraction of reactive groups allowing internal crosslinking of the molecule. They may be investigated as a mimick of, e.g., intrinsically disordered proteins, in particular with respect to the effect of the crowded cellular environment on macromolecular properties. Here the conformation of synthetic SCNPs in presence of linear polystyrene crowding molecules has been studied by small-angle neutron scattering under contrast-matching of the crowders[1]. A model describing the scattering of aggregating polydisperse SCNPs (see Figure 1) has been developed, resulting in the determination of the potentially squeezed size of the individual SCNPs within aggregates, their local chain statistics, and the average aggregation number, as a function of crowding. Two different crowders – of low and high molecular weight, respectively – are shown to have a different effect: while long chains tend to impede their aggregation above their overlap concentration, short ones are found to mediate depletion interactions leading to aggregation. Self-imposed crowding within the aggregates has a similar impact on chain conformation independently of the crowding of the surrounding medium. Our results are compared to recent simulations and is hoped to contribute to the microscopic understanding of the phase behavior of soft intrinsically disordered nano-objects, and in particular the effect of crowding on biomacromolecules. Figure 1: Schematic representation of the aggregation process of two SCNPs of monomer numbers i1 and i2 described by the addition of their individual SCNP masses, and obeying the same local chain statistics.

Electronic and fluorescent studies on the interaction of DNA and BSA with a new ternary praseodymium complex containing 2,9-dimethyl 1,10-phenanthroline and antibacterial activities testing

In this study, fluorescence emission spectra, UV–vis absorption spectra, ethidium bromide (EB)-competition experiment, and iodide quenching experiment were used for the interaction study of the Fish salmon DNA (FS-DNA) with [Pr(dmp)2Cl3(OH2)] where dmp is 2,9-dimethyl 1,10-phenanthroline. The binding constant and the number of binding sites of the complex with FS-DNA were 6.09 ± 0.04 M−1 and 1.18, respectively. The free energy, enthalpy, and entropy changes (ΔG°, ΔH°, and ΔS°) in the binding process of the Pr(III) complex with FS-DNA were –8.02 kcal mol−1, +39.44 kcal mol−1, and +159.56 cal mol−1 K−1, respectively. Based on these results, the interaction process between FS-DNA with [Pr(dmp)2Cl3(OH2)] was spontaneous and the main binding interaction force was groove binding mode. Also, Fluorescence and electronic absorption spectroscopy were used in order to evaluate the binding characteristics, stoichiometry, and interaction mode of praseodymium(III) (Pr(III)) complex with bovine serum albumin (BSA). Title complex showed good binding propensity to BSA presenting moderately high Kb values. The fluorescence quenching of BSA by Pr(III) complex has been observed to be the static process. The positive ΔH° and ΔS° values showed that the hydrophobic interaction is the main force in the binding of Pr(III) complex and BSA. Eventually, the average aggregation number, <J>, of BSA potentially induced by title complex confirmed the 1:1 stoichiometry for title complex-BSA adducts. In vitro, antimicrobial activity of title complex was indicated that the complex is more active against both Escherichia coli and Enterococcus faecalis bacterial strains than Staphylococcus aureus, and Pseudomonas aeruginosa.Communicated by Ramaswamy H. Sarma

Aggregation in Biocompatible Linear Block Copolymers: Computer Simulation Study

The self-organization of molecules of linear diblock copolymer poly(ethylene oxide)–polylactide and triblock copolymer polylactide–poly(ethylene oxide)–polylactide in aqueous solution is studied by the dissipative particle dynamics method, and quantitative comparison with the experimental data is performed. It is shown that the diblock copolymers are aggregated to spherical micelles, their average aggregation number increases with increasing both polymer concentration and hydrophobic block length. For the given type of molecules, the simulation results agree well with the experiment. For the case of triblock copolymers it is predicted that stable supramolecular aggregates of various morphologies exist in solution even at low concentrations.

Aggregation properties of sucrose fatty acid esters and some other sugar-based surfactants at different temperatures

Measurements of density, viscosity and light scattering of aqueous solutions of food additive antimicrobial surfactants: sucrose capric acid ester (SMD) and sucrose lauric acid ester (SML) and some other sugar-based ones (n‑octyl‑β‑d‑glucopyranoside (OGP), n‑dodecyl‑β‑d‑glucopyranoside (DDGP), n‑dodecyl‑β‑d‑maltoside (DM)) as well as the fluorescence intensity of 8‑Anilino‑1‑naphthalene-sulfonic acid (ANS) and pyrene (Py) in aqueous solutions of the studied surfactants were carried out at different temperatures. On the basis of the obtained results the critical micelle concentration, average aggregation number, apparent and partial molar volume as well as the standard Gibbs free energy, standard enthalpy and entropy of micellization were determined. From the obtained thermodynamic parameters of micellization of studied sugar surfactants under different conditions, the temperature and type of surfactant impact on their aggregation properties were deduced. From the changes of the standard Gibbs free energy of micellization it results that the tendency of studied surfactants molecules to form micelles comes down as the temperature and surfactant tail length decrease. The presence and strength of the hydrogen bonds between the water molecules and sugar moiety in a surfactant molecule plays a decisive role in the micellization process of the studied sugar-based surfactants.

Liquid-Crystalline Ordering of Filaments Formed by Bidisperse Amphiphilic Macromolecules

Self-organization in mixtures of amphiphilic macromolecules with helical backbone having two different degrees of polymerization was studied by means of molecular dynamics simulations. It was found that in semi-dilute solutions bidispersity leads to increase of average aggregation number compared with monodisperse solutions and promotes formation of long wire-like aggregates, containing both short and long macromolecules. In concentrated mixtures, the structure of the aggregates is similar, they also contain both types of macromolecules and are nematically ordered. The values of orientational order parameter of both types of macromolecules in concentrated solutions are identical.

Application of cross-linked stearic acid nanoparticles with dialysis membranes for methylene blue recovery

Abstract The interfacial properties of conjugated linoleic acid (CLA), and the ability to polymerize it, make it a potential precursor to nano-scale materials for water and wastewater treatment. Hence, the properties of CLA and of cross-linked linoleic acid micelles, which are basically stearic acid (SA) nanoparticles, were investigated in this study. The acid dissociation constant (pKa) of CLA was measured to be 4.93. The surface tension (γ) and the critical micelle concentration (CMC) of CLA (potassium salt) at different temperatures and ionic strengths were measured. Higher temperatures and ionic strengths both led to lower CMC and surface tension values. The average diameter and aggregation number of 4 mM thermally cross-linked micelles (4.55 nm and 135, respectively) were both larger than that of self-assembled 4 mM CLA micelles at 25 °C (2.83 nm and 44, respectively) in 0.1 M KOH. The pure cross-linked SA nanoparticles were obtained by repeated dialysis, and the stability of the cross-linked nanoparticles was confirmed by further dialysis and from equilibration studies in different solvents. The potential application of SA nanoparticles with dialysis membranes was tested by removing a cationic organic material, methylene blue, from water.

Combining Freezing Point Depression and Self-Diffusion Data for Characterizing Aggregation.

The colligative property freezing point depression is evaluated as a means for estimating the extent of aggregation for solutions of poly(ethylene oxide) alcohol (C10E6) nonionic surfactant in cyclohexane. Combined with additional measurements of self-diffusion coefficients, it is shown that both unaggregated C10E6 as well as reverse micelles are significantly present for the entire range of measured C10E6 concentration (0.048-2.35 mol kg-1). A change in speciation near 0.2 mol kg-1 is indicated by the results from both freezing point depression and self-diffusion coefficient measurements. It is shown that average reverse micelle radii and aggregation numbers obtained from the ratio of solvent and C10E6 self-diffusion coefficients are consistent with prior reported results. However, unreasonably small radii for the reverse micelles as well as for the cyclohexane were obtained from analysis of the results by the Stokes-Einstein equation using additional measured solution viscosities. The concentration of reverse micelles and unaggregated C10E6 was calculated from the freezing point depression results using the aggregation numbers obtained from ratio of self-diffusion coefficients. These concentrations indicate that the reverse micelles become smaller in average size and increase in number with increasing temperature without an increase in unaggregated C10E6.

Extraction Selectivity of a Quaternary Alkylammonium Salt for Trivalent Actinides over Trivalent Lanthanides: Does Extractant Aggregation Play a Role?

ABSTRACTThe extraction behavior of a quaternary alkylammonium salt extractant was investigated for its selectivity for trivalent actinides over trivalent lanthanides in nitrate and thiocyanate media. The selectivity was evaluated by solvent extraction experiments through radiochemical analysis of 241Am and 152/154Eu. Solvent extraction distribution and slope-analysis experiments were performed with americium(III) and europium(III) with respect to the ligand (nitrate and thiocyanate), extractant, and metal (europium only) concentrations. Further evaluation of the equilibrium expression that governs the extraction process indicated the appropriate use of the saturation method for estimation of the aggregation state of quaternary ammonium extractants in the organic phase. From the saturation method, we observed an average aggregation number of 5.4 ± 0.8 and 8.5 ± 0.9 monomers/aggregate for nitrate and thiocyanate, respectively. Through a side-by-side comparison of the nitrate and thiocyanate forms, we discuss the potential role of the aggregation in the increased selectivity for trivalent actinides over trivalent lanthanides in thiocyanate media.

Amphiphilic copolymers and surface active ionic liquid systems in aqueous media – Surface active and aggregation characteristics

The surface active and micellar behavior of a di- and tri-block copolymer of EB and EBE, where E is oxyethylene and B is oxybutylene moieties and surface active ionic liquids (SAILs) namely 4-dodecyl-4-methylmorpholinium chloride, [C12mmor][Cl] and 1-dodecyl-1-methylpyrrolidinium chloride, [C12mpyrr][Cl]) is studied in aqueous media by surface tension, steady-state florescence and particle size measurements. The critical micelle concentration (CMC), surface active parameters (adsorption efficiency, surface excess, surface tension reduction, minimum area per molecule at air/water interface, etc.), average translation diffusion coefficient, size parameters and aggregation numbers are calculated. The effect of both SAILS and also copolymers as additives on the surface active and micellar behavior of one another is studied. The shape and size parameters of the micelles of the pure copolymer or SAILs are determined from translational diffusion coefficients at infinite dilution. The mixed micelles of copolymer–SAIL systems are treated in terms of spheres and it is observed that their hydrodynamic radii decreased with the increase in the proportion of SAIL. The aggregation number of mixed micelles as determined from steady state florescence quenching method initially increased, reached a peak value and decreased drastically at high SAIL mole fractions. It is rationalized that the SAILs in general facilitate the dissolution of copolymer micelles but the mixed micelles predominantly consist of SAIL molecules or micelles over most of the mole fraction range. Regular solution theory formulations are tested on copolymer–SAIL mixture solutions and micellar mole fraction, interaction parameter (β) and activity coefficients (f1, f2) are calculated. The values of β ranged from −3.3 to −5.9 suggesting synergistic interactions between the two amphiphiles.

Molecular modeling of ionic aggregates at several concentrations of SDS in aqueous solution

The results of molecular modeling of ionic surfactant self-aggregation in sodium dodecyl sulphate (SDS) aqueous solutions, both salt-free and with addition of NaCl, are reported. The modeling has been based on all-atom molecular dynamics simulations at several SDS concentrations. Formation of one or three ionic aggregates of different size has been observed in the molecular dynamics runs. The computed trajectories of molecules and ions have been used to study the effects of aggregation on local densities of water molecules and counterions, and on the diffusivities of the aggregates themselves. With finding the mean force potential, the degree of counterion binding for aggregates with different aggregation numbers has been estimated. The results have been compared with the classical square-gradient density functional computations for a single ionic micelle in polar solvent. In addition, the influence of size of the simulation box on the transport properties of SDS aggregates has been investigated. In particular, we have found how the diffisivity of an ionic aggregate with a given aggregation number depends on the total surfactant concentration. The diffusivities of aggregates with aggregation numbers 16, 32, 48, and 64 at fixed total surfactant concentration have been computed. With the help of the Stokes-Einstein relation, the viscosity of the micellar solution at different total surfactant concentrations has been calculated. Time dependencies of the number of aggregates in the simulation box and of the average aggregation number during the self-assembly of surface active ions have been studied.

Simulation of Asphaltene Aggregation through Molecular Dynamics: Insights and Limitations

We report classical atomistic molecular dynamics simulations of four structurally diverse model asphaltenes, a model resin, and their respective mixtures in toluene or heptane under ambient conditions. Relatively large systems (∼50 000 atoms) and long time scales (>80 ns) are explored. Wherever possible, comparisons are made to available experimental observations asserting the validity of the models. When the asphaltenes are dissolved in toluene, a continuous distribution of cluster sizes is observed with average aggregation number ranging between 3.6 and 5.6, monomers and dimers being the predominant species. As expected for mixtures in heptane, the asphaltene molecules tend to aggregate to form a segregated phase. There is no evidence of the distinct formation of nanoaggregates, and the distribution of clusters is found to be continuous in character. Analysis of the shape of the clusters of asphaltenes suggests that they are generally spherical in character, with the archipelago models favoring longer p...

Structure-based modelling of hemocyanin allergenicity in squid and its response to high hydrostatic pressure

The secondary, tertiary, and quaternary structures of squid hemocyanin (Hc) were characterised, and the relationship between Hc structure and allergenicity responses to high hydrostatic pressure (HHP) was modelled. The Hc allergenicity varied with its protein structure. Electrophoresis analysis revealed that HHP treatment significantly decreased the band intensity of Hc when increasing pressure from 200 and 400 MPa to 600 MPa. The protein structure analysis of squid Hc showed that while HHP treatment decreased the α-helix content, free sulfhydryl content, and Rg, it increased the random coil content, surface hydrophobicity index (Ho), Guinier aggregation number (〈Nagg〉G) and average aggregation number (〈Nagg〉Q). The α-helix and random coil contents of the 600 MPa treated samples were 23.67% and 37.54%, respectively, compared to 32.37% and 32.02% in the control, respectively. HHP treatment decreased the IgE and IgG-binding capacities, indicating a significant decrease in the allergenicity (P< 0.05) of squid Hc. This study provided meaningful information of applying HHP to reduce allergenicity, and explained the responses of Hc protein structure to HHP for lowering the allergenicity of squid.

Determination of the Water/AOT/Isooctane Reverse Micelles Size Parameters from Their Refractive Index Data

The present paper aims to study the proprieties of water confined in water/AOT/isooctane reverse micelles from their refractive indices at 298.5 K. The refractive indices of the microemulsions were investigated at increasing water concentration and at different micellar volume fractions, Φd. The refractive index of micellar water was deduced for a large water to AOT molar ratio, W0. The refractive index of interfacial water was also calculated. Then, the molar faction of interfacial water, α, was deduced for different W0 values and compared with those measured by NMR as reported in the literature. The results show qualitative agreement. Finally, the average aggregation number, nagg, the area per surfactant headgroup, σ, as well as the interfacial thickness, di, of the reverse micelles were determined for the W0 values studied. The values of these structural parameters also show good qualitative agreement with NMR and small angle X-ray scattering data reported in the literature. Thus, the refractive index method can be a cheap and fast alternative for these two methods.

Molecular dynamics study of micellization thermodynamics in AOT/hexane system

The thermodynamics of reverse micelle formation from an ionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT, AOT), in hexane is studied by molecular dynamics simulation. A change in the Gibbs free energy upon the addition of one AOT molecule to a reverse micelle is calculated as depending on aggregation number N by the thermodynamic integration method. This dependence has a minimum at N ≈ 20 and maximum at N ≈ 35 and predetermines the monotonically decreasing character of the standard chemical potential of AOT in a micelle with the increase of the aggregation number. The simulation results predict the formation of reverse AOT micelles with an average aggregation number of ≈30, which is in good agreement with experimental data.

Effect of Ionic Liquids on Surface and Aggregation Properties of Non‐ionic Surfactant Triton™ X‐100 in Aqueous Media

AbstractThe effect of the hydrophilic ionic liquids (ILs) 1‐butyl‐2,3‐dimethylimidazolium bromide [bdmim][Br] and 1‐hexyl‐2,3‐dimethylimidazolium bromide [hdmim][Br] on the aggregation and surface active behaviour of the non‐ionic surfactant Triton™ X‐100 (TX‐100) was studied in aqueous media. Several aggregation properties of TX‐100 + IL/water systems, such as critical micelle concentration (CMC), surface active parameters, aggregation number (Nagg) and aggregate size, were determined by surface tension, fluorescence and dynamic light scattering (DLS) techniques. It was found that the average micellar size and aggregation number decrease, whereas the CMC increases with increasing concentration of ILs. Interestingly, the CMC value of TX‐100 is reduced slightly below 0.5 wt% of both the ILs in the medium. At higher wt% of IL in the system the CMC increases. It was demonstrated that ILs [bdmim][Br] and [hdmim][Br] can be judiciously used at different wt% for modifying the physico‐chemical properties of TX‐100.

Elucidation of adsorption mechanisms of solvent molecules with distinct functional groups on amylose tris(3,5-dimethylphenylcarbamate)-based sorbent

Although polysaccharide derivative-based sorbents have been widely used for chiral separation, for a long time it remained unclear how these CSPs interact with the molecules associated with different functional groups. In this study, six molecules were chosen for retention behavior studies: acetone (AC), tetrahydrofuran (THF), methanol (MET), isopropanol (IPA), tert-butanol (TBA), and benzene (BZN). An immobilized amylose carbamate stationary phase, amylose tris(3,5-dimethylphenylcarbamate)-based sorbent, or Chiralpak IA, was used. Van’t Hoff plots of ln k versus 1/T showed that alcohol molecules may simultaneously form two H-bonds with the IA sorbent. The results of density functional theory simulations and IR spectra support this inference showing that alcohol may bind with amide groups in three possible configurations. Frontal tests of AC and IPA were performed to estimate adsorbed solute concentration. Langmuir isotherm for IPA adsorption and mass action model for IPA self-aggregation were used for analyzing the IPA frontal results. Average IPA aggregation numbers range from 1.4 to 2.3. More than fifty percent of IPA molecules were found to be in aggregate form. From the frontal test results, thermodynamic properties of the adsorptions were determined. Retention behaviors of the five solutes as a function of IPA concentration were investigated. The absolute values B of the slopes from plots of the logarithms of the retention factor versus the logarithms of the IPA concentration increase in the order THF<AC<TBA<MET. U-shaped retention curves were observed for BZN, indicating that π-interactions of BZN aromatic group with the IA sorbent may involve both electrostatic and hydrophobic interactions. For THF, the slightly concave upward retention curves also suggested potential hydrophobic interactions between THF and IA. A monovalent retention model was used to estimate equilibrium constants (KSL-A) of solute-IPA complexation. The retention factors of the solutes with larger KSL-A values increase more rapidly with a decrease in the IPA concentration. Hence, a combination of the thermodynamic analysis and retention model has been shown to be reliable approach for elucidating the adsorption mechanisms of monovalent molecule containing different functional groups.

STAND: Surface Tension for Aggregation Number Determination.

Taking advantage of the extremely high dependence of surface tension on the concentration of amphiphilic molecules in aqueous solution, a new model based on the double equilibrium between free and aggregated molecules in the liquid phase and between free molecules in the liquid phase and those adsorbed at the air/liquid interface is presented and validated using literature data and fluorescence measurements. A key point of the model is the use of both the Langmuir isotherm and the Gibbs adsorption equation in terms of free molecules instead of the nominal concentration of the solute. The application of the model should be limited to non ionic compounds since it does not consider the presence of counterions. It requires several coupled nonlinear fittings for which we developed a software that is publicly available in our server as a web application. Using this tool, it is straightforward to get the average aggregation number of an amphiphile, the micellization free energy, the adsorption constant, the maximum surface excess (and so the minimum area per molecule), the distribution of solute in the liquid phase between free and aggregate species, and the surface coverage in only a couple of seconds, just by uploading a text file with surface tension vs concentration data and the corresponding uncertainties.

Effect of ethylene glycol and propylene glycol on the aggregation of alkanediyl-α,ω-bis(tetradecyldimethylammonium bromide) (14–s–14; s = 4–6) gemini surfactants

The effect of addition of two co-solvents (ethylene glycol (EG) and propylene glycol (PG)) on the aggregation properties of three cationic gemini surfactants (viz. butanediyl-1,4-bis(tetradecyldimethylammonium bromide) (14–4–14), pentanediyl-1,5-bis(tetradecyldimethylammonium bromide) (14–5–14) and hexanediyl-1,6-bis(tetradecyldimethylammonium bromide) (14–6–14)) has been studied by conductance, steady state fluorescence and dynamic light scattering measurements at 298K. The weight percentage of these co-solvents varied in between 0 and 50% at an interval of 5% up to 20% and then 10% up to 50%. Micellization is delayed by the addition of co-solvents; consequently, a progressive increase was observed in critical micelle concentration (cmc) and Gibbs free energy of micellization (ΔGm0), a rough increase was observed in the values of degree of counter ion dissociation (α) and a decrease was obtained in values of average aggregation number (Nagg) and Stern–Volmer constant (KSV). At low weight percentage (up to 15%) of co-solvents, 14–s–14 (s=4–6) geminis are found to be almost equally prone to micellization both in EG–water (EG–WR) and in PG–water (PG–WR) mixed media whereas at high weight percentages they are more prone to micellization in EG–WR than in PG–WR mixed media. The measurements of hydrodynamic radius (dH) performed at a concentration of 10mmolkg−1 shows that the hydrodynamic diameter (dH) is less than 70nm for most of the system except at some of the composition where slightly higher values were obtained.