Showing papers on "Aggregation number published in 2023"

Hybrid Molecules Consisting of Lysine Dendrons with Several Hydrophobic Tails: A SCF Study of Self-Assembling

Abstract: In this article, we used the numerical self-consistent field method of Scheutjens–Fleer to study the micellization of hybrid molecules consisting of one polylysine dendron with charged end groups and several linear hydrophobic tails attached to its root. The main attention was paid to spherical micelles and the determination of the range of parameters at which they can appear. A relationship has been established between the size and internal structure of the resulting spherical micelles and the length and number of hydrophobic tails, as well as the number of dendron generations. It is shown that the splitting of the same number of hydrophobic monomers from one long tail into several short tails leads to a decrease in the aggregation number and, accordingly, the number of terminal charges in micelles. At the same time, it was shown that the surface area per dendron does not depend on the number of hydrophobic monomers or tails in the hybrid molecule. The relationship between the structure of hybrid molecules and the electrostatic properties of the resulting micelles has also been studied. It is found that the charge distribution in the corona depends on the number of dendron generations G in the hybrid molecule. For a small number of generations (up to G=3), a standard double electric layer is observed. For a larger number of generations (G=4), the charges of dendrons in the corona are divided into two populations: in the first population, the charges are in the spherical layer near the boundary between the micelle core and shell, and in the second population, the charges are near the periphery of the spherical shell. As a result, a part of the counterions is localized in the wide region between them. These results are of potential interest for the use of spherical dendromicelles as nanocontainers for drug delivery.

Dualistic Role of Alcohol in Micelle Formation and Structure from iSAFT Based Density Functional Theory and COSMOplex

Abstract: Alcohols are effective and commonly used additives in surfactant self-assembling systems. Their effect on the behavior of these systems is complex and depends on a balance of the interacting forces. It is known that alcohols can act as cosolvents by altering solvent properties, and can also act as cosurfactants by inserting themselves into the palisade layer of the micelle and coaggregate with surfactants. The most recent models require user input to select the fraction of alcohols partitioning to each role. Two molecular theories iSAFT and COSMOplex are able to capture both roles and successfully predict the effect on micellar structure and critical micelle concentration of different alcohols ranging from short-chain to long-chain in qualitative agreement with experimental data. In this paper, the effect of adding linear 1-alcohols to aqueous solutions of nonionic poly(ethylene oxide) alkyl ether (CxEy) surfactant is investigated. To achieve this, the interfacial Statistical Associating Fluid Theory (iSAFT), a classical density functional theory, is applied to these systems and compared to the newly developed COSMOplex model and to experimental data. iSAFT and COSMOplex can provide detailed density profiles of each component in the system to illustrate the locus of an alcohol within a micellar structure. We observe from DFT that all alcohols studied are more present in the palisade layer than the micelle core, while COSMOplex shows greater accumulation of alcohol in the center of the micelle. Partition coefficients, aggregation numbers, micelle size, and total number of surfactant and alcohol molecules are also predicted and discussed. Furthermore, results from different surfactant architectures are compared and analyzed.

Self-assembly and solution behavior of cationic surfactants in water- trifluoroethanol environment: An experimental and theoretical approach

Abstract: Abstract Self-assembly and micellization performance of cationic conventional surfactant: tetradecyltrimethylammonium bromide (TTAB) and gemini surfactant (GS): N , N′ -ditetradecyl- N , N , N′ , N ′-tetramethyl- N , N′- ethanediyl-diammonium dibromide (14- 2 -14) were investigated in water and water-trifluoroethanol (TFE) solvent mixture using tensiometry and small-angle neutron scattering (SANS) techniques. The micellization parameter critical micelle concentration (CMC) for our examined systems was evaluated at the air-water interface at 303.15 K using tensiometry. It was found that CMC of the cationic surfactants decrease with increasing the concentration of TFE thereby indicating micellization process getting influenced by the addition of surface-active TFE. The shape and aggregation number ( N agg ) of the surfactant micelles were evaluated as a function of TFE using SANS which revealed the decrease in micelle size of cationic surfactants. In addition, the computational simulation study is undertaken to offer an insight into the interactions involved between the examined cationic surfactants and TFE system.

Role of Distance from Equilibrium in the Fragmentation Kinetics of Block Copolymer Micelles

Abstract: The fragmentation kinetics of 1,2-polybutadiene-b-poly(ethylene oxide) (Mn = 17.2 kDa and fPEO = 0.38) block copolymer micelles have been examined with an emphasis on elucidating the role of driving force for micellar fragmentation, represented by the aggregation number ratio Q/Qeq. Large micelles with size Q > Qeq were formed in an ionic liquid [C2mim][TFSI] by the direct dissolution method. A broad range of Q/Qeq was then obtained by altering the solvent quality after micelle formation by addition of a second solvent, selected from a series of imidazolium-based ionic liquids [Cxmim][TFSI] with x = 2, 4, 6, 8, 10, and 12. In order to quantify the change in solvent quality by dilution, the interfacial tension γ between the different ionic liquids and 1,2-polybutadiene homopolymer was determined using the pendant drop method. Micelles in a solution diluted with a second ionic liquid with x ≥ 2 were equilibrated by high-temperature annealing at 170 °C, during which in situ dynamic light-scattering measurements were made to follow the decay of average micelle size with time. Micelles were further characterized using small-angle X-ray scattering and cryogenic transmission electron microscopy to obtain micelle core size distributions. Qeq and γ were found to exhibit a power-law correlation, Qeq ∼ γ6/5, in accordance with the scaling prediction for star-like micelles. The reduction in γ on dilution with a lower γ solvent (x > 2) results in a smaller equilibrium micelle size, enabling access to a higher Q/Qeq, in the range from 1.1 to 5. The rate of fragmentation was found to increase significantly with an increase in Q/Qeq, thus the greater thermodynamic driving force leads to a systematic acceleration of fragmentation kinetics. The detailed mechanism by which micelles with Q ≫ Qeq achieve Qeq remains to be elucidated; the data suggest that it is not a sequential process but concerted.

Micelle Formation in Aqueous Solutions of the Cholesterol-Based Detergent Chobimalt Studied by Small-Angle Scattering

Abstract: The structure and interaction parameters of the water-soluble cholesterol-based surfactant, Chobimalt, are investigated by small-angle neutron and X-ray scattering techniques. The obtained data are analyzed by a model-independent approach applying the inverse Fourier transformation procedure as well as considering a model fitting procedure, using a core-shell form factor and hard-sphere structure factor. The analysis reveals the formation of the polydisperse spherical or moderately elongated ellipsoidal shapes of the Chobimalt micelles with the hard sphere interaction in the studied concentration range 0.17–6.88 mM. The aggregation numbers are estimated from the micelle geometry observed by small-angle scattering and are found to be in the range of 200–300. The low pH of the solution does not have a noticeable effect on the structure of the Chobimalt micelles. The critical micelle concentrations of the synthetic surfactant Chobimalt in water and in H2O-HCl solutions were obtained according to fluorescence measurements as ~3 μM and ~2.5 μM, respectively. In-depth knowledge of the basic structural properties of the detergent micelles is necessary for further applications in bioscience and biotechnology.

Is Structural Precision of Platonic Micelles Controlled Solely by Close-Packed Head Groups or Also by Hydrophobic Tail Packing? A DFT Exploration.

Abstract: Platonic micelles have been defined as structurally precise amphiphilic aggregates with discrete aggregation numbers corresponding to the close packing of spherical caps (representing head groups) on a sphere (representing hydrophobic core), analogous to the Tammes problem in geometry. Here, we use DFT to explore how an actual molecule behaves compared to the idealized picture based on the Tammes problem by also considering the packing of the tails. We modeled micelles of aggregation numbers 4 to 8 generated from the calix[4]arene amphiphile, PACaL3, with the tails forming a close-packed configuration while the headgroups are arranged as in Platonic solids. The DFT calculations reveal that tail packing overwhelmingly influences the equilibrium aggregation number. While the DFT prediction of a PACaL3 micelle of aggregation number 6 agrees with the scattering experiments of the Sakurai group, DFT calculations also suggest small concentrations of micelles of aggregation number 7. More interestingly, DFT calculations reveal that PACaL3 micelle formation occurs even though less than 20% of the hydrophobic tail surface is removed from contact with water, in contrast to the roughly 80% removal observed for classical surfactant micelles. While the close-packed head groups model predicts higher coverage of the hydrophobic surface for aggregation numbers 4 and 6 compared to 5 and 7, the DFT calculations also accounting for tail packing show that the surface coverage for aggregation numbers 5 and 7 is practically no different than that for aggregation number 4. Finally, although both the close-packed head groups model and the DFT calculations agree that the exposed hydrophobic surface area controls the equilibrium micelle aggregation number, the DFT calculations demonstrate how this exposed hydrophobic area is overwhelmingly determined by the tail group packing and not just by the close packing of head groups.

Molecular Dynamics Study on the Aggregation Behavior of Triton X Micelles with Different PEO Chain Lengths in Aqueous Solution

Abstract: The aggregation structure of Triton X (TX) amphiphilic molecules in aqueous solution plays an important role in determining the various properties and applications of surfactant solutions. In this paper, the properties of micelles formed by TX-5, TX-114, and TX-100 molecules with different poly(ethylene oxide) (PEO) chain lengths in TX series of nonionic surfactants were studied via molecular dynamics (MD) simulation. The structural characteristics of three micelles were analyzed at the molecular level, including the shape and size of micelles, the solvent accessible surface area, the radial distribution function, the micelle configuration, and the hydration numbers. With the increase of PEO chain length, the micelle size and solvent accessible surface area also increase. The distribution probability of the polar head oxygen atoms on the surface of the TX-100 micelle is higher than that in the TX-5 or TX-114 micelle. In particular, the tail quaternary carbon atoms in the hydrophobic region are mainly located at the micelle exterior. For TX-5, TX-114, and TX-100 micelles, the interactions between micelles and water molecules are also quite different. These structures and comparisons at the molecular level contribute to the further understanding of the aggregation and applications of TX series surfactants.