Micelle-vesicle transitions in catanionic mixtures of SDS/DTAB induced by salt, temperature, and selective solvents: a dissipative particle dynamics simulation study

The effect of changing the molecular structure of the surfactant on the dissolution of lamellar phases

Dissolution processes of surfactants, especially when in the lamellar phase, into water are important for product formulation. Understanding this process at a molecular level will help to enhance product design and control surfactant processes. The main goal of this study is to examine the effect of different lengths of surfactants and the hydrophobic to hydrophilic ratio on the dissolution process of surfactants. To achieve this goal dissipative particle dynamic (DPD) simulations were used. Lamellar equilibrium simulations were carried out for different surfactant chain lengths at 80 vol% with water. The surfactant chains were each run in a simulation box of dimensions 20 × 20 × 20 until equilibrium was reached. The lamellar phase formed for all different surfactant chain lengths and, after the initial equilibrium the surfactant systems were then simulated with a water box for dissolution. The dissolution process was tracked by visual analysis, local concentration analysis, micelle size, and a zonal model to calculate the diffusion parameter. Results show that as the surfactant chain length increased by adding more of the hydrophobic beads, the dissolution process slowed down. Increasing the hydrophilic part of the surfactant speeds up the dissolution process, but the effect of adding more of the hydrophobic part is greater than the effect of adding more of the hydrophilic part on the dissolution process.

Phase Diagram Study of Catanionic Surfactants Using Dissipative Particle Dynamics

Dissipative particle dynamics (DPD) simulations has been performed to study the phase transition of a mixture of cationic and anionic surfactants in an aqueous solution as a function of the total concentration in water and the relative ratio of surfactants. The impact of the relative difference between the tail lengths of the cationic and anionic surfactants on the phase diagram has been simulated by tuning the number of DPD beads in the simulation model. This research also discusses the impact of the frequently used values of the parameters associated with the harmonic bonds among the bonded DPD beads on the obtained self-assemblies. We find remarkable differences in the resultant self-assemblies based on different choices of harmonic bond parameters. The performed simulations show an enhanced spectrum of self-assemblies with augmented tail lengths and disparate harmonic bond parameters. The obtained self-assemblies are quite unique and can potentially be used in the future for various applications. We also compare the simulation results of the vesicle structures obtained by modeling the electrostatic interaction in the simulation among the charged beads by explicitly introducing charges with a long-range interaction with those obtained by tuning the implicit electrostatic interaction without the long-range interaction. The effects of the chain length of the model and the harmonic bond parameters on the internal density of DPD beads and stress profiles within the vesicles are examined closely. These results are a significant contribution to understanding the stability of the phases and tailoring of the desired vesicles.

Vesicle Geometries Enabled by Semiflexible Polymer

Understanding and controlling vesicle shapes is fundamental challenge in biophysics and materials design. In this paper, we employ the Monte Carlo method to investigate the shape of soft vesicle induced by semiflexible polymer outside in two dimensions. The effect of bending stiffness κ of polymer and the strength εVP of attractive interaction between vesicle and polymer on the shape of vesicle is discussed in detail in the present paper. It is found that the shape of vesicle is influenced by κ and εVP. Typical shape of vesicles is observed, such as circular, cigar-like, double vesicle, and racquet-like. To engineer vesicle shape transformations is helpful for exploiting the richness of vesicle geometries for desired applications.

Dissipative particle dynamics simulations in colloid and Interface science: a review

Dissipative particle dynamics (DPD) is one of the most efficient mesoscale coarse-grained methodologies for modeling soft matter systems. Here, we comprehensively review the progress in theoretical formulations, parametrization strategies, and applications of DPD over the last two decades. DPD bridges the gap between the microscopic atomistic and macroscopic continuum length and time scales. Numerous efforts have been performed to improve the computational efficiency and to develop advanced versions and modifications of the original DPD framework. The progress in the parametrization techniques that can reproduce the engineering properties of experimental systems attracted a lot of interest from the industrial community longing to use DPD to characterize, help design and optimize the practical products. While there are still areas for improvements, DPD has been efficiently applied to numerous colloidal and interfacial phenomena involving phase separations, self-assembly, and transport in polymeric, surfactant, nanoparticle, and biomolecules systems.

Vesicle formation of single-chain amphiphilic 4-dodecylbenzene sulfonic acid in water and micelle-to-vesicle transition induced by wet-dry cycles

Simple single-chain amphiphiles (SCAs) can form vesicular structures in their single-component aqueous solutions, which has attracted great attention, but the understanding of their aggregation behavior is still limited. In this work, the aggregation behavior of 4-dodecylbenzene sulfonic acid (DBSA), a typical simple SCA, in water was investigated. The structure and properties of the aggregates formed were determined. In particular, the effect of wet-dry cycles on the structures of aggregates was examined. The mechanisms of aggregate formation and structural transition were discussed. It was found that the increase of DBSA concentration can drive the occurrence of a micelle-to-vesicle transition, showing a critical micelle concentration and critical vesicle concentration of ∼0.53 and 2.14 mM, respectively. The vesicles formed coexist with micelles in solution, with a unilamellar structure and ∼80 nm size, and exhibit size-selective permeability. In addition, the vesicles show remarkable stability upon long-term storage, exposure to high temperature, and freeze-thaw cycles. The H-bonding interaction between DBSA species and the interdigitated structure of alkyl chains in bilayers play a key role in the formation and stability of DBSA vesicles. Interestingly, it was found that the wet-dry cycle can induce a micelle-to-vesicle transition and an obvious increase in the size of the original vesicles, accompanied by the formation of some multilamellar vesicles. This work provides a better understanding of the aggregation behavior of simple SCAs in their single-component aqueous solutions.

Phase Diagram Study of Sodium Dodecyl Sulfate Using Dissipative Particle Dynamics

Dissipative particle dynamics (DPD) simulations are performed to study the phase transition of sodium dodecyl sulfate (SDS) in aqueous solution, which is an anionic surfactant commonly known as sodium dodecyl sulfate. In this work, the aim is to find a coarse-grained minimal model suitable to produce the full phase diagram of SDS. We examine the coarse-grained models of SDS, which have been used in earlier computational studies to produce the phases as well as for finding the critical micelle concentration (CMC) of SDS. We contrast the results based on these models with the experimental observations to assess their accuracy. Our research also takes into account the importance of sodium ions, which come from the partial dissociation of SDS, when dissolved in water. The effect of sodium ion has not been considered explicitly in the computational work done so far using dissipative particle dynamics. In light of the above explorations, we propose new models for SDS and demonstrate that they successfully produce a compendious SDS phase diagram, which can precisely overlay the experimental results.

Multiscale prediction of functional self-assembled materials using machine learning: high-performance surfactant molecules

Various physical properties of functional materials can be induced by controlling their chemical molecular structures. Therefore, molecular design is crucial in the fields of engineering and materials science. With its remarkable development in various fields, machine learning combined with molecular simulation has recently been found to be effective at predicting the electronic structure of materials (Nat. Commun., 2017, 8, 872 and Nat. Commun., 2017, 8, 13890). However, previous studies have used similar microscale information as input and output data for machine learning, i.e., molecular structures and electronic structures. In this study, we determined whether multiscale data can be predicted using machine learning via a self-assembly functional material system. In particular, we investigated whether machine learning can be used to predict dispersion and viscosity, as the representative physical properties of a self-assembled surfactant solution, from the chemical molecular structures of a surfactant. The results showed that relatively accurate information on these physical properties can be predicted from the molecular structure, suggesting that machine learning can be used to predict multiscale systems, such as surfactant molecules, self-assembled micelle structures, and physical properties of solutions. The results of this study will aid in further development of the application of machine learning to materials science and molecular design.

The effect of temperature, composition and alcohols on the microstructures of catanionic mixtures of sodium dodecylsulfate and cetyltrimethylammonium bromide in water

The influence of mixing protocol, composition, temperature, ageing and added alcohols on the characteristics of the microstructures of sodium dodecylsulfate (SDS) + cetyltrimethylammonium bromide (CTAB) mixtures has been investigated in this paper. In this catanionic mixture (1 weight% total surfactant content) temperature induced microstructural transition occurs, which is (i) a micelle-to-vesicle transition (MVT) if α_SDS (mole fraction of SDS) = 0.7, 0.8 or 0.9 and (ii) a vesicle-to-micelle transition (VMT) if α_SDS = 0.1, 0.2 or 0.3. In the mixture of α_SDS = 0.7, specific conductivity and dynamic light scattering measurements also support the occurrence of MVT. Transition electron microscopy and small angle neutron scattering measurements were also made to assess the characteristics of the microstructures. Alcohols added to the mixture of α_SDS = 0.7 reduced the size of the vesicle, while only monohydric alcohols suppressed the temperature induced transition indicating that the number and location of -OH groups of the alcohols have a dramatic modulating influence on the structural transition occurring in catanionic mixtures. The influence of the alcohols is explained in terms of changes produced in the dielectric constant and hydrophobicity of the medium.

Structure and dynamics of cetyltrimethylammonium chloride-sodium dodecylsulfate (CTAC-SDS) catanionic vesicles: High-value nano-vehicles from low-cost surfactants

HYPOTHESIS

Catanionic vesicles based on large-scale produced surfactants represent a promising platform for the design of innovative, effective and relatively inexpensive nano-vehicles for a variety of actives. Structural, dynamic and functional behavior of these aggregates is finely tuned by the molecular features of their components and can be opportunely tailored for their applications as drug carriers.

EXPERIMENTS

Here we investigate the aggregates formed by CTAC and SDS, two of the most diffused surfactants, by means of Dynamic Light Scattering, Small Angle Neutron Scattering and Electron Paramagnetic Resonance spectroscopy (EPR). The exploitation of these aggregates as nano-vehicles is explored using the poorly water-soluble antioxidant trans-resveratrol (t-RESV), testing t-RESV solubility and antioxidant activity by means of UV, fluorescence spectroscopy and EPR.

FINDINGS

The presence of a large stability region of catanionic vesicles on the CTAC-rich side of the phase diagram is highlighted and interpreted in terms of the mismatch between the lengths of the surfactant tails and of first reported effects of the chloride counterions. CTAC-SDS vesicles massively solubilize t-RESV, which in catanionic vesicles exerts a potent antioxidant and radical-scavenging activity. This behavior arises from the positioning of the active at the surface of the vesicular aggregates thus being sufficiently exposed to the external medium.

Temperature-induced reversible micelle–vesicle transition in aqueous solution of a pseudogemini surfactant without any additive

Temperature-induced reversible micelle–vesicle transition is achieved in aqueous solution of a single pseudogemini surfactant without any additive.

Formation of simple single-tailed vesicles mediated by lipophilic solid surfaces

Adsorption and aggregation of surfactants at solid-liquid interfaces were fairly well understood, but there was limited knowledge regarding the effect of the presence of a solid surface on aggregate structures in bulk solution. Except for the fatty acid system, most simple single-tailed surfactants (STSs) are well known to form micelles but not vesicles in aqueous solution. Herein, we report a novel phenomenon: with the mediation of lipophilic solid surfaces (LSSs), the zwitterionic STS lauryl sulfobetaine (LSB) formed vesicles from its micellar solution without any additives, producing a mixed solution of vesicles and micelles. More interestingly, the STS vesicles coexisted stably with micelles in the solution and were thermally insensitive even after the removal of LSSs. The quantity of LSB vesicles decreases with the addition of ethanol. The pH effects (4.0-9.0) did not have an obvious influence on the formation and stability of the LSB vesicles. Similar results were obtained from the other STSs, suggesting that the LSS-mediated micelle-to-vesicle transition may be a general phenomenon. We proposed a possible mechanism that adsorption, the matrix effect, and interdigitated bilayer structures were probably crucial for the formation and stability of STS vesicles. We expect this work to provide important insights into the effect of the solid/liquid interface on the self-assembly chemistry of surfactants in bulk solution.

Vesicle formation of catanionic mixtures of CTAC/SDS induced by ratio: a coarse-grained molecular dynamic simulation study

A vesicle was formed by disk-like bilayer curling.