Microscopic Origin of Surfactant Irritation: An Experimental and Computational Study

Surfactants, which are widely used in skin care products and cleansers, can cause skin irritation. The skin irritation potential of surfactants is fundamentally determined by their molecular structure and is directly related to their microscopic aggregation structure and specific interactions with the skin. The microscopic origin of the irritation of the surfactants remains unknown. In this work, irritation properties of four surfactant solutions were measured, and their microscopic aggregation behavior was systematically analyzed. The results indicate that the surfactants self-assembled in aqueous solution to form aggregates with different morphologies, where the head groups of surfactants were closer to each other. Furthermore, surfactants that can form larger and more stable aggregate structures in aqueous solutions will exhibit less irritation. These findings hold significant implications for the design and expanded applications of mild surfactants.

Review of the Interfacial Structure and Properties of Surfactants in Petroleum Production and Geological Storage Systems from a Molecular Scale Perspective

Surfactants play a crucial role in tertiary oil recovery by reducing the interfacial tension between immiscible phases, altering surface wettability, and improving foam film stability. Oil reservoirs have high temperatures and high pressures, making it difficult and hazardous to conduct lab experiments. In this context, molecular dynamics (MD) simulation is a valuable tool for complementing experiments. It can effectively study the microscopic behaviors (such as diffusion, adsorption, and aggregation) of the surfactant molecules in the pore fluids and predict the thermodynamics and kinetics of these systems with a high degree of accuracy. MD simulation also overcomes the limitations of traditional experiments, which often lack the necessary temporal-spatial resolution. Comparing simulated results with experimental data can provide a comprehensive explanation from a microscopic standpoint. This article reviews the state-of-the-art MD simulations of surfactant adsorption and resulting interfacial properties at gas/oil-water interfaces. Initially, the article discusses interfacial properties and methods for evaluating surfactant-formed monolayers, considering variations in interfacial concentration, molecular structure of the surfactants, and synergistic effect of surfactant mixtures. Then, it covers methods for characterizing microstructure at various interfaces and the evolution process of the monolayers' packing state as a function of interfacial concentration and the surfactants' molecular structure. Next, it examines the interactions between surfactants and the aqueous phase, focusing on headgroup solvation and counterion condensation. Finally, it analyzes the influence of hydrophobic phase molecular composition on interactions between surfactants and the hydrophobic phase. This review deepened our understanding of the micro-level mechanisms of oil displacement by surfactants and is beneficial for screening and designing surfactants for oil field applications.

Synthesis and Aggregation Behavior of Hexameric Quaternary Ammonium Salt Surfactant Tz-6C12QC

A hexameric quaternary ammonium salt surfactant Tz-6C12QC featuring a rigid triazine spacer and six ammonium groups was synthesized. The molecular structure and aggregation behavior of Tz-6C12QC were characterized by nuclear magnetic resonance spectroscopy, surface tension, conductivity, dynamic light scattering, and transmission electron microscopy, etc. Dissipative particle dynamics (DPD) simulation was employed to investigate the self-assembly behavior of Tz-6C12QC at different concentrations. The rheological behavior of the polyacrylamide/Tz-6C12QC system was characterized by shear rheology. The results indicated that Tz-6C12QC exhibited superior surface activity and lower surface tension compared to conventional surfactants. Rheology analysis revealed that Tz-6C12QC had a significant viscosity reduction effect on polyacrylamide. DLS and TEM indicated that, as the concentration of Tz-6C12QC increased, monomer associations, spherical aggregations, vesicles, tubular micelles, and bilayer vesicles were sequentially formed in the solution. This study presents a synthetic approach for polysurfactants with a rigid spacer and sheds light on the self-assembly process of micelles.

Rationalizing the Design of Pluronics-Surfactant Mixed Micelles through Molecular Simulations and Experiments

Aqueous systems comprising polymers and surfactants are technologically important complex fluids with tunable features dependent on the chemical nature of each constituent, overall composition in mixed systems, and solution conditions. The phase behavior and self-assembly of amphiphilic polymers can be changed drastically in the presence of conventional ionic surfactants and need to be clearly understood. Here, the self-aggregation dynamics of a triblock copolymer (Pluronics L81, EO₃PO₄₃EO₃) in the presence of three cationic surfactants (with a 12C long alkyl chain but with different structural features), viz., dodecyltrimethylammonium bromide (DTAB), didodecyldimethylammonium bromide (DDAB), and ethanediyl-1,2-bis(dimethyldodecylammonium bromide) (12-2-12), were investigated in an aqueous solution environment. The nanoscale micellar size expressed as hydrodynamic diameter (D_h) of copolymer-surfactant mixed aggregates was evaluated using dynamic light scattering, while the presence of a varied micellar geometry of L81-cationic surfactant mixed micelles were probed using small-angle neutron scattering. The obtained findings were further validated from molecular dynamics (MD) simulations, employing a simple and transferable coarse-grained molecular model based on the MARTINI force field. L81 remained molecularly dissolved up to ∼20 °C but phase separated, forming turbid/translucent dispersion, close to its cloud point (CP) and existed as unstable vesicles. However, it exhibited interesting solution behavior expressed in terms of the blue point (BP) and the double CP in the presence of different surfactants, leading to mixed micellar systems with a triggered morphology transition from unstable vesicles to polymer-rich micelles and cationic surfactant-rich micelles. Such an amendment in the morphology of copolymer nanoaggregates in the presence of cationic surfactants has been well observed from scattering data. This is further rationalized employing the MD approach, which validated the effective interactions between Pluronics-cationic surfactant mixed micelles. Thus, our experimental results integrated with MD yield a deep insight into the nanoscale interactions controlling the micellar aggregation (Pluronics-rich micelles and surfactant-rich micelles) in the investigated mixed system.

Coarse-Grained Model Incorporating Short- and Long-Range Effective Potentials for the Fast Simulation of Micelle Formation in Solutions of Ionic Surfactants

A coarse-grained model comprising short- and long-range effective potentials, parametrized with the iterative Boltzmann inversion (IBI) method, is presented for capturing micelle formation in aqueous solutions of ionic surfactants using as a model system sodium dodecyl sulfate (SDS). In the coarse-grained (CG) model, each SDS molecule is represented as a sequence of four beads while each water molecule is modeled as a single bead. The proposed CG scheme involves ten potential energy functions: four of them describe bonded interactions and control the distribution functions of intramolecular degrees of freedom (bond lengths, valence angles, and dihedrals) along an SDS molecule while the other six account for intermolecular interactions between pairs of SDS and water beads and control the radial distribution functions. The nonbonded effective potentials between coarse-grained SDS molecules extend up to about 12 nm and capture structural and morphological features of the micellar solution both at short and long distances. The long-range component of these potentials, in particular, captures correlations between surfactant molecules belonging to different micelles and is essential to describe ordering associated with micelle formation. A new strategy is introduced for determining the effective potentials through IBI by using information (target distribution functions) extracted from independent atomistic simulations of a micellar reference system (a salt-free SDS solution at total surfactant concentration c_T equal to 103 mM, temperature T equal to 300 K, and pressure P equal to 1 atm) obtained through a multiscale approach described in an earlier study. It employs several optimization steps for bonded and nonbonded interactions and a gradual parametrization of the short- and long-range components of the latter, followed by reparametrization of the bonded ones. The proposed CG model can reproduce remarkably accurately the microstructure and morphology of the reference system within only a few hours of computational time. It is therefore very promising for future studies of structural and morphological behavior of various liquid surfactant formulations.

Design and Self-Assembly of Sugar-Based Amphiphiles: Spherical to Cylindrical Micelles

Sugar-based amphiphiles are a relevant natural alternative to synthetic ones due to their biodegradable properties. An understanding of their structure-assembly relationship is needed to allow the concrete synthesis of suitable derivatives. Here, four different mannose-derivative surfactants are characterized by pendant drop, dynamic light scattering, small-angle X-ray scattering, cryotransmission electron microscopy, and molecular dynamics techniques in aqueous media. Measurements denote how the polysaccharide average degree of polymerization (DP¯) and the addition of a hydroxyl group to the hydrophobic tail, and thus the presence of a second hydrophilic moiety, affect their self-assembly. A variation in the DP¯ of the amphiphile has no effect in the critical micelle concentration in contrast to a change in the hydrophobic molecular region. Moreover, high-DP¯ amphiphiles self-assemble into spherical micelles irrespective of the hydroxyl group presence. Low-DP¯ amphiphiles with only one hydrophilic moiety form cylindrical micelles, while the addition of a hydroxyl group to the tail leads to a spherical shape.

Scission energy and topology of micelles controlled by the molecular structure of additives

We employ coarse-grained (CG) molecular dynamics simulations (MD) to investigate the effects of the molecular structure of additives on the scission energy and morphology of charged micelles. Considering sodium dodecyl sulfate (SDS) as a representative charged surfactant and taking trimethylphenylammonium chloride (TMPAC) and octyltrimethylammonium bromide (OTAB) as oppositely charged additives, we show that the scission energy and topology of micelles vary significantly depending on the molecular structure of the hydrophobic part of the additives. The cyclic aromatic tail of the TMPAC disrupts the core structure of the SDS micelle and hence decreases the micelle scission energy, whereas the linear alkyl tail of the OTAB packs very well with the micelle core and increases the scission energy. Although both the additives have similar head structures, they lead to very different micelle morphologies because of the difference in the shape of their tail structures; ring-like or toroidal shaped micelles are formed in SDS/TMPAC solution whereas bicelle-like structures are formed in SDS/OTAB solution when the additive to surfactant ratio is higher than a certain value.

Self-assembly and viscosity changes of binary surfactant solutions: A molecular dynamics study

HYPOTHESIS

Structure and self-assembly of surfactants in the solution shows a fundamental influence on its viscosity. Through molecular simulations using Martini force field, synergistic effects in aggregation as well as the viscosity changes of a binary ionic surfactant system can be modelled. Simulations: Coarse-grained molecular dynamics simulations are performed to model the SDS/CAPB binary surfactant solution, and both equilibrium and non-equilibrium methods are used to calculate the viscosity of the equilibrated micellar systems.

FINDINGS

Our simulation results indicate that the new version of the Martini force field can provide more reasonable self-assembly of surfactant, both single and binary system. Synergistic effects in micelle formation for SDS/CAPB have been successfully reproduced, that is, the formation of cylindrical micelles or even wormlike micelles at a lower concentration when compared with the pure system. Meanwhile, both equilibrium and non-equilibrium methods provide quantitatively comparable viscosity for each system. For pure micellar system, the viscosity linearly increases with the total concentration. Nevertheless, our simulation fails to capture the viscosity enhancement of the solution in corresponding with the formation of rodlike or wormlike micelles, and a full parameter optimization of force field is still necessary.

Effect of the Janus Amphiphilic Wall on the Viscosity Behavior of Aqueous Surfactant Solutions

The effects of the chemical nature of an interface are one of the key parameters which can affect self-assembly and rheological behavior. To date, several studies have reported self-assembled structures and rheological behaviors in the development of various functional materials. In this study, we investigated the self-assembly and viscosity behavior of aqueous surfactant solutions confined in three types of Janus amphiphilic nanotubes (JANTs), which have two, four, and eight sequential domains, respectively, using molecular simulation. We found that the viscosity behavior depends on the surfactant concentration and the chemical nature of the wall surface. For instance, although the concentration levels of the surfactants are the same (c = 10%), completely different viscosity behaviors were observed in the two sequential domains (Newtonian-like) and the four and eight sequential domains (strong shear-thinning) of the JANTs. Our simulations demonstrated how the rheological properties of aqueous surfactant solutions, including viscosity and velocity profiles, can be controlled by the chemical nature of the JANT wall surface, effect of confinement, and their self-assembly structures. Considering the foregoing, we hope that our study offers new knowledge on nanofluid systems.

Micellar structure and transformations in sodium alkylbenzenesulfonate (NaLAS) aqueous solutions: effects of concentration, temperature, and salt

We investigate the shape, dimensions, and transformation pathways of micelles of linear sodium alkylbenzenesulfonate (NaLAS), a common anionic surfactant, in aqueous solution. Employing Small Angle Neutron Scattering (SANS) and surface tensiometry, we quantify the effects of surfactant concentration (0.6-15 wt%), temperature (5-40 °C) and added salt (≤0.35 M Na2SO4). Spherical micelles form at low NaLAS (≤2.6 wt%) concentration in water, and become elongated with increasing concentration and decreasing temperature. Addition of salt reduces the critical micelle concentration (CMC) and thus promotes the formation of micelles. At fixed NaLAS concentration, salt addition causes spherical micelles to grow into cylindrical micelles, and then multilamellar vesicles (MLVs), which we examine by SANS and cryo-TEM. Above a threshold salt concentration, the MLVs reach diameters of 100 s of nm to few μm, eventually causing precipitation. While the salt concentrations associated with the micelle-to-cylinder transformation increase only slightly with temperature, those required for the cylinder-to-MLV transformation exhibit a pronounced, linear temperature dependence, which we examine in detail. Our study establishes a solution structure map for this model anionic surfactant in water, quantifying the combined roles of concentration, temperature and salt, at practically relevant conditions.

Molecular Dynamics Simulations and Density Functional Theory on Unraveling Photoresponsive Behavior of Wormlike Micelles Constructed by 12-2-12·2Br- and trans-ortho-Methoxy Cinnamate

Photoresponsive systems with controllable self-assembly morphologies and adjustable rheological properties have attracted widespread interest by researchers in the past few years. Among them, the photoresponsive systems consisting of ortho-methoxycinnamic (OMCA) and Gemini surfactants are endowed with rich self-assemblies with different states and in different scales including spherical micelles, wormlike micelles, vesicles, aqueous two-phase system (ATPS), etc. All these self-assemblies display excellent photoresponsive behavior. However, the mechanism of these photoresponsive behaviors has not been unraveled systematically so far. In this study, molecular dynamics (MD) simulations, density functional theory (DFT) calculations, transmission electron microscopy, and rheology are employed to investigate the photoresponsive behaviors of wormlike micelles caused by photoisomerization of trans-OMCA in 12-2-12·2Br^-/trans-OMCA solutions and to unravel the underlying mechanisms of these photoresponsive behaviors. The experimental results show that 12-2-12·2Br^-/trans-OMCA micelles display photoresponsiveness after UV-light irradiation, with the transformation of micellar morphologies from wormlike micelle to spherical micelles. In MD simulations, certain micelle morphologies in experiments and the specific packing between 12-2-12·2Br^-/OMCA were successfully captured. The larger three-dimensional structure and steric hindrance of cis-OMCA disturb the interior structure of micelles. The stronger hydrophilicity of cis-OMCA induces the escape of cis-OMCA from the interval of micelles to the solution. The energy results prove that trans-OMCA associates more strongly with 12-2-12·2Br^- than cis-OMCA. These causes lead to the fission and repacking of wormlike micelles.

Replica exchange dissipative particle dynamics method on threadlike micellar aqueous solutions

The self-assembly of surfactant molecules can spontaneously result in a variety of micelle morphologies, such as spherical micelles, threadlike micelles, and vesicles, and it is therefore crucial to predict and control the self-assembly to achieve a helpful process in the fields of materials chemistry and engineering. A dissipative particle dynamics (DPD) method used in a coarse-grained molecular simulation is applied to simulate various self-assembling soft matter systems because it can handle greater length and time scales than a typical molecular dynamics simulation (MD). It should be noted that the thorough sampling of a system is not assured at low temperatures because of large complex systems with coarse-grained representations. In this article, we demonstrate that the replica exchange method (REM) is very effective for even a DPD in which the energy barrier is comparatively lower than that of a MD. A replica exchange on DPD (REDPD) simulation for threadlike micellar aqueous solutions was conducted, and the values of the potential energy and the mean aggregation number were compared. As a result, the correct values and a self-assembled structure within a low-temperature range can only be obtained through the REDPD.

Stretch and Breakage of Wormlike Micelles under Uniaxial Strain: A Simulation Study and Comparison with Experimental Results

We use coarse-grained (CG) molecular dynamics simulations to determine the effect of uniaxial strain on the stress, scission stress, and scission energy of solutions of wormlike micelles of cetyltrimethylammonium chloride/sodium salicylate (NaSal). We find that the breaking stress, stretch modulus, and scission energy of the charged micelles are nonmonotonic functions of oppositely charged hydrotrope (NaSal) concentration. While the stretch modulus shows a peak at a value of surfactant-to-hydrotrope concentration ratio ( R) close to unity as expected due to neutralization of head-group charge at R = 1, the breaking stress and scission energy produce a peak at R < 1.0 because of thinning of the micelle diameter with increased R. The breaking stress from the simulations depends on the rate of deformation and roughly agrees with the experimental values of Rothstein ( J. Rheol. 2003 , 47 , 1227 ) after extrapolation to the much lower experimental rates. The method and results can be used to predict the effects of flow and mechanical stress on rates of micellar breakage, which is important in the rheology of wormlike micellar solutions.

Asymmetric cationic gemini surfactants: an improved synthetic procedure and the micellar and surface properties of a homologous series in the presence of simple salts

The micellar and morphological properties of symmetric, cationic gemini surfactants have been well studied in the literature as a function of nature and type of the spacer group and the length and type of hydrophobic chain. In this paper, we have examined the effects of tail asymmetry on the properties of a series of cationic surfactants, the N-alkyl-1-N′-alkyl-2-N,N,N′,N′-tetramethyldiammonium dibromide. A novel synthetic method is used to prepare a series of these surfactants and the consequences of asymmetry on micellar properties are presented. This new method has been shown to be more efficient, with higher yields of the asymmetric surfactants than the yields of the accepted literature method. The critical micelle concentration values and the micelle sizes of the asymmetric gemini surfactants, 12-4-12, 12-4-10, 12-4-8, and 12-4-6 gemini surfactants, were obtained from conductivity and dynamic light scattering. With increasing chain asymmetry, the size of the micelle increased due to the formation of loose micelles. The addition of NaCl and Na2SO4 to the surfactant solutions increased the aggregate size, and this effect was more pronounced with increasing salt concentrations. These results are interpreted in terms of the effect these ions have on the “compactness” of the micelle structure.

Janus or homogeneous nanoparticle mediated self-assembly of polymer electrolyte fuel cell membranes

The functionality of polymer electrolyte fuel cell membranes depends on the self-assembled structure of the graft polymer. To control self-assembly, nanoparticles (NPs) are often used as catalysts. Hence, we investigate the effect of hydrophilic (HI), hydrophobic (HO), and Janus nanoparticles (JNPs) for the self-assembly of graft polymers using dissipative particle dynamics (DPD) simulations. We found that the differences that appeared among the self-assembled structures of water depended on the concentration of PEFC. We also calculated the diffusion constant of water (D(H2O)) from the slopes of the time-averaged mean square displacement (MSD) curves. HI NPs had the largest effect in suppressing the diffusion of water because the HI NPs incorporated into the water particles. It was also seen that D(H2O) with various NPs gradually decreased as the number of NPs increased for three PEFC concentrations (70%, 80%, and 90%). Thus, a close correlation between the position and chemical composition of NPs in polymer electrolyte fuel cell (PEFC) membrane systems has been found. Moreover, the mean square radius of gyration 〈R g〉 and the mean square end-to-end distance 〈R〉 was calculated to analyse the self-assembled structures of PEFC. The 〈R g〉 and 〈R〉 increased as the concentration of PEFC was increased, with and without various NPs.

Clear distinction between CAC and CMC revealed by high-resolution NMR diffusometry for a series of bis-imidazolium gemini surfactants in aqueous solutions

The aggregation behavior in the transition region was studied for a series of dicationic surfactants 3,3′-[α,ω-(dioxaalkane)]bis(1-dodecylimidazolium)dichlorides with varied spacer length from two to twelve carbon atoms. We employed Nuclear Magnetic Resonance diffusometry and Bayesian DOSY analysis to obtain the aggregate size distribution in the transition region. The critical concentrations CC were independently obtained from surface tension, electric conductivity, UV-Vis and NMR methods. The micelle aggregation numbers were estimated from the self-diffusion coefficients and were independently confirmed using steady-state fluorescence quenching. The morphology of the aggregates was characterized by small-angle scattering of synchrotron radiation and molecular dynamics simulations. The obtained CC values are identified as critical aggregation concentrations CAC. A broad transition region was observed, and stable micelles were obtained at much higher concentrations than CAC. The accurate CMC values could not be identified for the systems in the study. We indicated that the distribution of aggregate size becomes small and the system becomes homogeneous at much larger concentrations than CAC (typically 15–20 mM). The existence of a slow exchange between two environments, an aggregate and aqueous environment, was confirmed by ¹H NMR and 2D HSQC NMR spectroscopy.

The aggregation behavior in the transition region was studied for a series of dicationic surfactants 3,3′-[α,ω-(dioxaalkane)]bis(1-dodecylimidazolium)dichlorides with varied spacer length from two to twelve carbon atoms.