Average Aggregation Number Research Articles

Evaporation-induced self-assembly of Janus pyramid molecules from fractal network to core-shell nanoclusters evidenced by small-angle X-ray scattering

Self-assembly of nanoclusters (NCs) is an effective synthetic method for preparing functionalized nanomaterials. However, the assembly process and mechanisms in solutions still remain ambiguous owing to the limited strategies to monitor intermediate assembled states. Herein, the self-assembly process of amphiphilic molecule 4POSS-DL-POM (consisting of four polyhedral oligomeric silsesquioxanes, a dendritic linker, and one polyoxometalate) by evaporation of acetone in a mixed acetone/n-decane solution is monitored by time-resolved synchrotron small-angle X-ray scattering (SAXS). Scattering data assessments, including Kratky analysis, pair distance distribution function, and model fitting, track the self-assembly process of 4POSS-DL-POM from a fractal network to compact NCs, then to core–shell NCs, and finally to superlattice structure. The calculated average aggregation number of a core–shell NC is 11 according to the parameters obtained from core-shell model fitting, in agreement with electron microscopy. The fundamental understanding of the self-assembly dynamics from heterocluster into NCs provides principles to control building block shape and guide target aggregation, which can further promote the design and construction of highly ordered cluster-assembled functional nanomaterials.

Controllable Supramolecular Aggregation of Hydroazaheptacene Tetraimides and Derivatives in Nonpolar and Polar Solvents.

Rational control of the supramolecular aggregation of π-conjugated molecules plays an important role in determining their optoelectronic properties and applications. Here, we report a systematic study of the factors, including solvent polarity, concentration, and surfactants, that affect the aggregation behavior of a brominated hydroazaheptacene tetraimide (HATI) and its thiophene-substituted derivative, Th-HATI, as near-infrared fluorophores, in both nonpolar and polar solvents. The thermal stability of the aggregates is also studied by monitoring their optical absorption against temperature change. Our results indicate that the aggregation of HATI is highly sensitive to the solvent polarity. Moreover, the average aggregation number of HATI inside the colloidal nanoparticles formed in aqueous media can be controlled by surfactants. The substitution of the bromo groups in HATI by thiophene units induces a slight blue shift of the optical absorption, enhanced crystallinity, distinct aggregation behavior in both nonpolar and polar solvents, and improved thermal stability. The multifacet understanding of the supramolecular aggregation of these systems may offer insight for other π-conjugated molecular chromophores with various optoelectronic properties and applications.

Micellization of dodecyldimethyl-N-2-phenoxyethylammonium bromide (domiphen) in aqueous solution. Comparison with other alkyl ammonium surfactants

Dodecyldimethyl-N-2-phenoxyethylammonium bromide (domiphen) is a quaternary ammonium salt which has been used in oral hygiene long time ago. The published studies on its characterization as surfactant evidence several contradictory results. In this paper, measurements of surface tension, conductimetry, fluorescence and mainly isotermal titration calorimetry (ITC) have been performed at different temperatures. The enthalpy of demicellization, ΔHdemo, varies linearly with temperature in the interval 15–45 °C, from which a value of 704 ± 39 J mol−1K−1 in water has been determined for the change of heat capacity, ΔCP,demo. The obtained results are compared with published values for alkyltrimethylammonium bromides with different alkyl chain lengths (CnTAB). For instance, its critical micelle concentration, cmc, is almost ten times lower than that of C12TAB. The substitution of one methyl group by the phenoxyethyl has a strong influence on the behavior of domiphen making it equivalent to a CnTAB surfactant with n = 15–19 methylene groups, the number depending on the studied property. From ITC an average aggregation number of 45 ± 3 is obtained in good agreement with the one from fluorescence quenching of pyrene equal to 44.5–47.6. From the analysis of specific conductivity, the fraction of bound counterions to micelles was obtained, the values linearly decreasing with temperature. Below cmc domiphen behaves as a strong 1:1 electrolyte without association of monomers. Other themodynamic parameters have also been obtained. The fjord and reef hydration models are used to propose a structure for domiphen micelles.

Estimation of the Compositional Proportions of Different Micelle Species in Water: Application of One-dimensional Aggregate Theory to Small Angle Neutron Scattering Analyzed Data

Abstract The one-dimensional aggregation theory was applied to the small angle neutron scattering (SANS) analyzed data of surfactant solutions. Plots of the hydrophobic bonding energy E to form an aggregate against the number-average aggregation number n were found to be powerful to estimate the compositional ratios of different aggregate species co-existing in a gross sample solution.

Combination of molecular dynamics simulation and COSMO to understand asphaltenes aggregation

ABSTRACT Asphaltenes interaction produces problems in oil transport and recovery, and understanding the association process is vital to improving economic and industrial production. Here, we propose the COSMO model in combination with molecular dynamics (MD) simulations to complement the results obtained by Newtonian mechanics with quantum results to get reliable association data. Four asphaltene molecules were studied with toluene and heptane solvents. COSMO showed molecular interaction tendencies, which were later verified with MD. The average aggregation number was calculated through MD and turned out to be entirely dependent on the aromaticity due to π-π stacking. The aromatic rings are responsible for aggregation, while the sulfur heteroatom plays a less fundamental role. Viscosity is also related to the aggregation process; a higher tendency in aggregation increases the dynamic viscosity. Diffusion coefficients were correlated with the aromaticity, dipole moment, and molecular weight. Finally, a mathematical comparison between COSMO and MD results is accomplished.

General Adsorption Model to Describe Sigmoidal Surface Tension Isotherms of Binary Liquid Mixtures

Surface tension (σ) isotherms of liquid mixtures can be divided into Langmuir-type (L-type, including LI- and LII-type) and sigmoid-type (S-type, including SI- and SII-type). Many models have been developed to describe the σ-isotherms. However, the existing models can well describe the L-type isotherms, but not the S-type ones. In the current work, a thermodynamic model, called the general adsorption model, was developed based on the assumption of surface aggregation occurring in the surface layers, to relate the surface composition with the bulk one. By coupling the general adsorption model with the modified Eberhart model, a two-parameter equation was developed to relate the σ with the bulk composition. Its rationality was examined using the σ data of 10 binary mixtures. The results indicate that the new model can accurately describe the S- and L-type isotherms of binary liquid mixtures, showing a good universality. One advantage of the model is that its two parameters, i.e., the adsorption equilibrium constant (K) and the average aggregation number (n), can be estimated by linear fitting experimental σ data, thereby obtaining unique values. This model suggests that the S- and LII-type isotherms arise from the surface aggregation (n ≠ 1). In addition, the standard molar Gibbs free energy of surface adsorption (ΔG̃ad0) and the apparent surface layer thickness (τ) were analyzed for 10 binary mixtures. The ΔG̃ad0 data suggest that the order of adsorption tendency is LI-type ≫ SI-type ≈ SII-type > LII-type, and the strong adsorption usually corresponds to large τ. This work provides a feasible model for describing the S-type isotherms and a better understanding of the surface properties of liquid mixtures.

Self-Assembly of Amphiphilic Cubes in Suspension.

We study the self-assembly of amphiphilic cubic colloids using molecular dynamics as well as rejection-free kinetic Monte Carlo simulations. We vary both the number and location of the solvophobic faces (patches) on the cubes at several colloid volume fractions and determine the resulting size and shape distributions of the self-assembled aggregates. When the binding energy is comparable to the thermal energy of the system, aggregates typically consist of only few spontaneously associating/dissociating colloids. Increasing the binding energy (or lowering the temperature) leads to the emergence of highly stable aggregates, e.g., small dimers in pure suspensions of one-patch cubes or large (system-spanning) aggregates in suspensions of multipatch colloids. In mixtures of one- and multipatch cubes, the average aggregation number increases with increasing number of solvophobic faces on the multipatch cubes as well with increasing fraction of multipatch cubes. The resulting aggregate shapes range from elongated rods over fractal objects to compact spheres, depending on the number and arrangement of solvophobic patches on the cubic colloids. Our findings establish the complex self-assembly pathways for a class of building blocks that combine both interaction and shape anisotropy, with the potential of forming hierarchically ordered superstructures.

Mesoscale Modeling of Agglomeration of Molecular Bottlebrushes: Focus on Conformations and Clustering Criteria.

Using dissipative particle dynamics, we characterize dynamics of aggregation of molecular bottlebrushes in solvents of various qualities by tracking the number of clusters, the size of the largest cluster, and an average aggregation number. We focus on a low volume fraction of bottlebrushes in a range of solvents and probe three different cutoff criteria to identify bottlebrushes belonging to the same cluster. We demonstrate that the cutoff criteria which depend on both the coordination number and the length of the side chain allows one to correlate the agglomeration status with the structural characteristics of bottlebrushes in solvents of various qualities. We characterize conformational changes of the bottlebrush within the agglomerates with respect to those of an isolated bottlebrush in the same solvents. The characterization of bottlebrush conformations within the agglomerates is an important step in understanding the relationship between the bottlebrush architecture and material properties. An analysis of three distinct cutoff criteria to identify bottlebrushes belonging to the same cluster introduces a framework to identify both short-lived transient and long-lived agglomerates; the same approach could be further extended to characterize agglomerates of various macromolecules with complex architectures beyond the specific bottlebrush architecture considered herein.

Flash Colloidal Gold Nanoparticle Assembly in a Milli Flow System: Implications for Thermoplasmonic and for the Amplification of Optical Signals

The assembly and stabilization of a finite number of nanocrystals in contact in water could maximize the optical absorption per unit of material. Some local plasmonic properties exploited in applications, such as photothermia and optical signal amplification, would also be maximized which is important in the perspective of mass producing nanostructures at a lower cost. The main lock is that bringing charged particles in close contact requires the charges to be screened/suppressed, which leads to the rapid formation of micrometric aggregates. In this article, we show that aggregates containing less than 60 particles in contact can be obtained with a milli-flow system composed of turbulent mixers and flow reactors. This process allows to stop a fast non-equilibrium colloidal aggregation process at millisecond times after the initiation of the aggregation process which allows to control the aggregation number. As a case study, we considered the rapid mixing of citrate coated gold nanoparticles (NP) and AlCl3 in water to initiate a fast aggregation controlled by diffusion. Injecting a solution of polycation using a second mixer allowed us to arrest the aggregation process after a reaction time by formation of overcharged cationic aggregates. We obtained within seconds stable dispersions of a few milliliters composed of particle aggregates. Our main result is to show that it is possible to master the average aggregation number between 2 and 60 NP per aggregate by varying the a reaction time between 10 ms and 1 s.

Binary micellar schemes of cationic gemini and conventional surfactants: chain length effect

Binary micellar schemes have been appropriated to ascertain the effect of chain length on micellization and interfacial properties. Mixed micellar systems of cationic gemini surfactant, bis-N,N-tetradecyldimethyl-m-phenylenediammonium dibromide (14-m-ɸ-14), and conventional cationic surfactants, viz., dodecyltrimethylammonium bromide (C12TAB), tetradecyltrimethylammonium bromide (C14TAB) and hexadecyltrimethylammonium bromide (C16TAB), in aqueous media were investigated by conductometric, tensiometric, fluorimetric and dynamic light scattering (DLS) techniques at 25 °C. The behavior of the binary mixtures under regular solution theory have been explained in light of Clint’s and Rubingh’s models. A non-ideal behavior was indicated. This was further confirmed by interaction parameter (β) values and activity coefficients. The synergism, an attractive interaction, between the surfactant molecules was ascertained. The average aggregation number () was evaluated by steady-state fluorescence quenching method. The increased with increase in mole fraction of CnTAB (n = 12, 14, 16). The size of the aggregates varied with length of conventional surfactants as exhibited by DLS. The zeta potential values of the mixed micelles increase with increase in mole fractions of CnTAB.

Adsorption of Cetylpyridinium Chloride at Silica Nanoparticle/Water Interfaces (II): Dependence of Surface Aggregation on Particle Size

Herein, we report a thermodynamic model that relates the adsorption (aggregation) parameters of surfactants at solid/liquid interfaces to particle radius (r). The adsorption (aggregation) parameters include adsorption amounts, equilibrium constants (or the standard Gibbs free energy changes), the critical surface micelle concentration (csmc), and the average aggregation number of surface micelles (n). The model predicts the size dependence of the surface aggregation of surfactants, which is determined by the changes in the interfacial tension and the molar volume of surface components caused by adsorption. In addition, the adsorption of cetylpyridinium chloride (CPyCl), a cationic surfactant, on silica nanoparticles with different r values (ca. 6-61 nm) was determined at 298 K and pH 4, showing an obvious size dependence, consistent with the prediction of the model. With an increase in r, the adsorption isotherm changes from the double-plateau type to the Langmuir type, accompanied by obvious changes in the adsorption parameters. The size-dependent adsorption data can be well described using the model equations, indicating that the model presented here is acceptable. In addition, the model can extract information on the interfacial tensions from adsorption data. We think that the model deepens the understanding of the aggregation phenomena of surfactants at solid/liquid interfaces.

Nonlinear dynamics in micellar surfactant solutions. II. Diffusion

We discuss diffusion in micellar surfactant solutions in a form appropriate for analyzing experiments that involve large deviations from equilibrium. A general nonlinear dynamical model for inhomogeneous systems is developed that describes the effects of diffusion and micelle kinetics as a set of coupled partial differential equationsfor unimer concentration, micelle number concentration, average micelle aggregation number, and, optionally, the variance of the micelle aggregation number. More specialized models are developed to describe slow dynamics in situations in which the system stays in a state of partial local equilibrium or full local equilibrium. As an illustrative example of a nonlinear transport phenomenon, we discuss a simple model of diffusion from an initially homogeneous micellar solution to a rapidly created absorbing interface with fast unimer adsorption.

Nonlinear dynamics in micellar surfactant solutions. I. Kinetics.

This is the first of a pair of articles that present the theory of kinetic and transport phenomena in micelle-forming surfactant solutions in a form that facilitates discussion of large deviations from equilibrium. Our goal is to construct approximate but robust reduced models for both homogeneous and inhomogeneous systems as differential equationsfor unimer concentration c_{1}, micelle number concentration c_{m}, average micelle aggregation number q and (optionally) aggregation number variance σ_{m}^{2}. This first article discusses kinetics in homogeneous solutions. We focus particularly on developing models that can describe both weakly perturbed states and states in which c_{1} is suppressed significantly below the critical micelle concentration, which leads to rapid shrinkage and dissociation of any remaining micelles. This focus is motivated by the strong local suppression of c_{1} that is predicted to occur near interfaces during some adsorption processes that are considered in the second article. Toward this end, we develop a general nonlinear theory of fast stepwise processes for systems that may be subjected to large changes in q and c_{1}. This is combined with the existing nonlinear theory of slow association and dissociation processes to construct a general model for systems governed by stepwise reaction kinetics. We also consider situations in which the slow process of micelle creation and destruction instead occurs primarily by micelle fission and fusion, and analyze the dependencies of micelle lifetime and the slow relaxation time upon surfactant concentration in systems controlled by either association-dissociation or fission-fusion mechanisms.

Halogen-free COOH functionalized surface-active ionic liquids: surface activity, aggregation behavior, and antimicrobial activity

In the field of interphase and colloidal science, the interest in the research of ionic liquid (IL) has been growing specifically when surface active ionic liquids (SAILs) emerged in this field. Herein, we report the synthesis of Halogen-free COOH-functionalized Imidazolium and Pyridinium-based SAILs. The surface activity and aggregation behavior of synthesized SAILs were investigated using the electric conductivity, surface tension, fluorescence quenching, density, and speed of sound. In addition, the thermodynamic parameters and the critical micellar concentration dependency on temperature were also investigated using charged pseudo-phase model. The micellization process was found spontaneous and entropy-driven in the temperature range of 25°–45 °C of synthesized SAILs. Furthermore, the average aggregation number was determined by the speed of sound, fluorescence quenching, and density. In addition, the antimicrobial activity of the synthesized SAILs was studied, which shows a broad spectrum of antimicrobial activity against Gram-positive and Gram-negative bacteria. The introduction of an acid group enhanced the antimicrobial activity significantly as compared to non-functionalized ILs.

Investigation of the aggregation behaviour of the anionic surfactant sodium dodecyl sulfate in ionic liquids 1-allyl-3-methylimidazolium chloride and 1-Ethyl-3-methylimidazolium diethyl phosphate

We investigated the aggregation behaviour of the anionic surfactant sodium dodecyl sulfate (SDS) in 1-allyl-3-methylimidazolium chloride (AMIMCl) ionic liquid at three temperatures 323 K, 333 K and 343 K using 1H Nuclear Magnetic Resonance (NMR) chemical shift and Pulsed-Field Gradient (PFG) spin-echo NMR self-diffusion measurements. Chemical shifts of SDS protons and diffusion coefficients of SDS as a function of the surfactant concentration have breakpoints, which indicate an onset of the surfactant micellisation. We estimated the critical micelle concentration (cmc) with high accuracy from SDS methyl protons' chemical shifts and the micelles' average aggregation number from diffusion coefficients. The formation of micelles does not influence the diffusion and chemical shifts of AMIMCl.We determined water concentration in the samples from the NMR spectra. We did not find evidence that water affects the aggregation process since water content practically does not influence the SDS protons' chemical shifts and diffusion coefficients. However, water appears to influence the diffusion and chemical shifts of AMIMCl.We also investigated the behaviour of SDS in 1-Ethyl-3-methylimidazolium diethyl phosphate (EMIMDEP) hydrophilic ionic liquid. The value of the Gordon parameter of EMIMDEP indicates that it does not possess the ability to support the self-assembly of amphiphiles. Within this work, we treat the system SDS/EMIMDEP as a reference one.SDS forms a lamellar phase in AMIMCl at high concentration, as revealed by Small Angle X-ray Scattering (SAXS) and Polarised Optical Microscopy (POM) investigations. At about 358 K, a transition from the lamellar phase to the isotropic liquid phase occurs. No other liquid-crystalline phase in this system was determined.

Effects of inhibitor concentration and thermodynamic conditions on n-octylphenol-asphaltene molecular behaviours

The asphaltene stability in crude oil can be disturbed due to changes to thermodynamic conditions during production, raising flow assurance concerns. The addition of chemical inhibitors, such as surfactants, to a crude oil can improve the asphaltene stability. The impact of chemical inhibitors on the asphaltene aggregation intensity and aggregate characteristics, which is needed for design of an efficient inhibitor for a target crude oil, is poorly understood. In this paper, a molecular dynamics simulation tool is employed to simulate the molecular behaviour of the asphaltene and n-octylphenol (OP) at various concentrations of OP (0–15 wt%) and thermodynamic conditions (T = 300–360 K and P = 1–60 bar). To meet the objectives, the average aggregation number of different asphaltenes and asphaltene mixtures is measured besides the aggregate characteristics, including aggregate gyration radius, density, and shape. The results show that the archipelago asphaltenes (A1) do not self-aggregate severely, and the gyration radius distribution of the aggregates is similar after the OP addition. Nevertheless, the average aggregation number and the aggregate density analysis show that the aggregation intensity reduces slightly when the OP concentration is increased. Because the continental asphaltene (A2) can form hydrogen bonds, the addition of OP increases the aggregate dispersity significantly, which is identified with the highest peaks at 28 and 35 Å after the addition of 7 and 15 wt% OP, respectively. It is found that OP increases the variations in the aggregate shape and type for A2 aggregates. It is concluded that a high concentration of OP (above 7 wt%) is needed to avoid the asphaltene aggregation. The pressure increase does not change the aggregate shape both with or without the OP. However, adding OP increases the aggregate gyration radius and decreases the aggregate density at 30 bar. Increasing temperature results in a severe aggregation in the system without the inhibitor as the aggregate gyration radius increases, and aggregates become more compact and spherical. However, by adding OP, both the average aggregation number and aggregate gyration radius decrease (especially at 360 K). According to the sensitivity analysis results, integrating GPUs with CPUs can speed up the simulation approximately three times, compared to the system processing with only CPUs. Outcomes from this research work can help for a better understanding of the molecular behaviour of asphaltene and inhibitors at various thermodynamic situations, leading to the improvement of inhibitor design. This paper highlights the competence of molecular dynamics for exploring the optimal inhibitor concentration and the thermodynamic conditions in which the inhibitor has the highest impact on asphaltene aggregation.

Study of the premicellar state in aqueous solutions of sodium dodecyl sulfate by nuclear magnetic resonance diffusion.

Self-diffusion coefficients of sodium dodecyl sulfate (SDS) were measured in aqueous solutions in the premicellar range of the SDS concentrations 7-34.7 mM and temperatures 30-90°C. Average effective hydrodynamic radii and aggregation numbers of SDS in the premicellar region were determined. At C < CMC at all temperatures, the SDS solution is the solution of monomers. At C > CMC, the increase of temperature leads to decrease in the effective hydrodynamic radii and the average aggregation numbers. At C > > CMC, it is impossible to reach the monomeric state by increasing the temperature.

Cation isomerism effect on micellization of pyridinium based surface-active ionic liquids

In this study, four pyridinium-based surface-active ionic liquids (SAILs) with small changes in the cation structure were synthesized, namely N-dodecylpyridinium bromide, N-dodecyl-2-methylpyridinium bromide, N-dodecyl-3-methylpyridinium bromide, and N-dodecyl-4-methylpyridinium bromide. Their micellization behaviour in aqueous solutions was investigated by isothermal titration calorimetry (ITC) in the temperature range between 278.15 K and 318.15 K. The experimental ITC data were analysed by applying an improved mass-action model, which provides the values for the critical micelle concentration (cmc), the degree of counterion binding (β), the average aggregation number (n), and the standard heat capacity (ΔMcpθ), enthalpy (ΔMHθ), entropy (ΔMSθ) and Gibbs free energy (ΔMGθ). Only minor differences were observed for the values of ΔMHθ, ΔMSθ and ΔMGθ, while some significant differences were found for the values of the average aggregation number (n) and ΔMcpθ.From MD simulations it was found that the presence of the methyl group on ortho-, meta- or para- position significantly affects the water organization around the pyridinium ring. Moreover, the hydration of the para-isomer is most pronounced and the hydration shell on this cation may also prevent the monomers from approaching each other. On the other hand, it turned out that the ortho-substituted pyridinium based SAIL has the most negative value of ΔMcpθ, which can be ascribed to the partial dehydration of the methyl group on the ortho- position during micelle formation. In contrast, for other systems, the alkyl chain was found to be mainly dehydrated while the pyridinium ring remains completely in contact with water even upon micellization.

Spectroscopic and docking molecular study of new anticancer Pt complex binding with human serum albumin

After synthesizing and identifying the nature of the new complex based on platinum metal, [Pt(NH3)2(butylgly)]NO3, the interaction of this complex with human serum albumin (HSA) was performed by spectroscopy and molecular docking methods at two temperatures of 27 and 37 °C and under physiological conditions of the body. The toxicity test of this complex was performed on the MCF-7 cell line (IC50 = 300 µM). Enthalpy, entropy, Gibbs free energy, binding constant, number of complex binding sites on the HSA, Scatchard diagrams, Hill coefficient, and Hill constant were calculated and then plotted via UV/Vis. According to the Gibbs free energy obtained at two temperatures of 27 and 37 °C (−20.6, −21.2 kJ mol−1), the interaction was done spontaneously. Moreover, the melting temperature of human serum albumin with this complex; and the kinetics of this interaction (the second-order) were calculated. Using fluorescence at three temperatures of 25, 27, and 37 °C, the binding constant (2.9 × 104, 1.0 × 104, and 5.7 × 103 M−1), the quenching constant, average aggregation number of HSA, and the number of binding sites of the complex on the protein were obtained. As well, the static quenching mechanism was also observed. Molecular docking results showed that the site of binding of this complex to the HSA, is the site II subdomain IIIA, and the hydrogen and hydrophobic bonds are superior.

Equilibrium model for estimating micellar aggregation number from surface equation

The Aggregation Surface Equation (ASE) is attained by coupling the Gibbs adsorption equation and surface aggregation isotherm. The resulting equation can be used to study fluid-fluid interfaces in connection with the micelle formation process. This new equation contains two boundary conditions in the vicinity of the saturation region: (i) the critical micelle concentration (CMC), which corresponds to the onset of micelle formation; and (ii) the critical spherical micelle concentration (CSC) (i.e. the completion of spherical micellar aggregates). These boundary conditions, which appear within a narrow surfactant concentration range, provide the basis to calculate the average aggregation number under the assumption of constant average aggregation number and spherical micelles geometry. The resulting Surface Equation (SE) contains the maximum surface concentration of the surface monolayer, the critical micelle concentration and the micelle average aggregation number. The mass action law allows extending the ASE as a function of the free monomer, as opposed to the total or nominal surfactant concentration, which was recently performed in the STAND model (Langmuir, 32, (16), (2016), 3917-3925). This paper presents a simple and direct method to calculate the micellar average aggregation number from surface tension data and the proposed surface equation.