Critical Micelle Concentrations in Surfactant Mixtures and Blends by Simulation

Waste cooking oils as a sustainable feedstock for bio-based application: A systematic review

Waste cooking oils (WCOs) are common wastes and promising green, eco-friendly and sustainable feedstocks for bio-based applications. While the primary valorisation strategy revolves around the concept of waste-to energy, new research trends have emerged in the last decade. This systematic review provides a comprehensive analysis of the current state of the art in the conversion of WCOs into bio-based molecules. Based on the PRISMA methodology, 64 papers were selected using different databases and sources, such as: PubMed, ScienceDirect, Scopus and MDPI. The data extraction process focused on studies reporting the biological and chemical conversion of WCOs into value-added bioproducts. Many of the selected publications deal with the development of bioactive molecules, including biosurfactants, with application in pharmaceuticals, food, cosmetics, and bioremediation. Bioconversion processes mainly featured engineered Yarrowia lipolytica and Escherichia coli strains, even if additional microorganisms were also employed. In the same way, different chemical processes have been thoroughly studied. A smaller segment of research is directed to the production of feed supplements and soaps. Regulatory constraints limit further development in feed supplements due to potential contaminants, while soap production needs further stability studies. The present systematic review shows promising outcomes in the valorisation of WCOs through the development of value-added molecules and products. Despite the wide range of applications, these findings identify that the scalability and economic sustainability of the selected processes require further investigation. This study seeks to summarize the current state of the art and identify potential gaps to advance the industrialization of WCOs valorisation.

A Coarse-Grained Model Describing the Critical Micelle Concentration of Perfluoroalkyl Surfactants in Ionic Aqueous Phase

In this study, dissipative particle dynamics (DPD) simulations were employed to determine the critical micelle concentration (CMC) of perfluoroalkyl and polyfluoroalkyl substances (PFAS) in ionic aqueous solutions. This approach provides precise CMC data for PFAS surfactants in the presence of various ionic species, thereby addressing a gap in the current literature. Additionally, this study contributes to the development of open-source molecular force fields for charged perfluorinated compounds, which are currently limited. These models incorporate hydration free energy values obtained from density functional theory (DFT) and account for ionic interactions through a well-established linear relationship. Hydrophobic interactions between the surfactant tail and water were fine-tuned to match the CMC of chosen surfactants. Then, the DPD models successfully predicted CMC values for a diverse range of surfactants, including those based on hydrocarbons and PFAS, demonstrating the ability to represent realistic salinities encountered in natural waters. Experimental validation of the methodology was conducted using sodium n-nonyl sulfate (SNS) and sodium n-dodecyl sulfate (SDS) via interfacial tension measurements, confirming the accurate representation of the CMC changes with salinity. This study enhances our understanding of the behavior of PFAS surfactants in ionic aqueous solutions and provides a valuable tool for predicting CMC values in complex environmental systems.

A Meticulous Focus on the Determination of Critical Micelle Concentration Employing Fluorescence Spectroscopy

Micelles are small spherical structures formed by one or a combination of amphiphilic structures known as surfactants. Surfactants are amphiphilic molecules with both hydrophilic and hydrophobic regions and can self-assemble into micelles above the critical micelle concentration, with the hydrophobic tails oriented inwards and hydrophilic head outwards. In this review article, critical micelle concentration determination using the fluorescence spectroscopy technique is described for both single and mixed micellar systems with the preparation of samples, working principle, and also about theoretical aspects. Fluorescence measurements using direct i.e. intrinsic fluorescence of surfactant molecule and indirect i.e. using extrinsic fluorescence probes have been discussed. Fluorescence spectroscopy offers a sensitive and valid approach for the characterization of surfactant behaviors in aqueous solutions. Various fluorescence parameters are measured at specific wavelengths with an increase in surfactant concentration and a plot is generated from which the critical micelle concentration is determined. Furthermore, the calculation of critical packing parameter is also described which gives an idea about the geometrical arrangement of the surfactant molecules in a micellar structure. This value also provides valuable insights into the micelle's shape and structure. In conclusion, the effectiveness of the fluorescence spectroscopy technique in determining the critical micelle concentration and the critical packing parameter is described in detail in this review article.

Modeling Contact Angles with Chemically Specific Dissipative Particle Dynamics

We explore how walls can be introduced into chemically specific dissipative particle dynamics models such that the surface energies can be chosen to obtain a desired contact angle on a given substrate, for example, for an oil/water interface. We certify the methodology for determining the surface energy, which can be positive or negative, such that the Young equation is automatically satisfied. We validate the approach against direct numerical simulation of cylindrical droplets of dodecane in water on the surface and test it against an experimental model of a water droplet in dodecane on a surface-adsorbed monolayer on silica.

Barrier properties of polylactic acid to binary aqueous solution systems

The barrier properties of polylactic acid (PLA) to four typical binary aqueous solutions with sodium chloride (NaCl), sodium lauryl polyoxyethylene ether sulfate (SELS), acetic acid (AcOH), and ethanol (EtOH) as solute respectively were studied by investigating the adhesion work from contact angle and surface tension, measuring the water diffusion coefficient (DATR) from time-resolved infrared spectroscopy (TRIS), and detecting the solute permeation rate (Pcell) from permeation. The results illustrate that, among the four solutions and the single water, the single water shows the biggest adhesion work to the PLA, being 89.2 J/m2, and the interaction between the solution and PLA is closely correlated with the structure of the solution. The DATR of the NaCl solution (0.49 × 10-8 cm2/s of 25 mass%) and the SELS solution (0.12 × 10-8 cm2/s of 25 %) is lower than that of the single water (1.13 × 10-8 cm2/s), while the DATR of the AcOH solution (4.22 × 10-8 cm2/s of 25 mass%) and the EtOH solution (1.84 × 10-8 cm2/s of 25 mass%) is higher than that of the single water. Result of Pcell reveals that the permeation of the solution through PLA is positively correlated with the solution concentration within the mass concentration of 25 %. A comprehensive barrier property of PLA to the four binary aqueous solutions exhibits the following order of resistance: SELS solution > NaCl solution > AcOH solution > EtOH solution.

Enhancing cancer therapy: advanced nanovehicle delivery systems for oridonin

Oridonin (ORI), an ent-kaurane diterpenoid derived from Rabdosia rubescens (Hemsl.) H.Hara, serves as the primary bioactive component of this plant. It demonstrates a broad spectrum of therapeutic activities, including moderate to potent anticancer properties, alongside anti-inflammatory, antibacterial, antifibrotic, immunomodulatory, and neuromodulatory effects, thus influencing diverse biological processes. However, its clinical potential is significantly constrained by poor aqueous solubility and limited bioavailability. In alignment with the approach of developing drug candidates from natural compounds, various strategies, such as structural modification and nanocarrier systems, have been employed to address these challenges. This review provides an overview of ORI-based nano-delivery systems, emphasizing their potential to improve the clinical applicability of oridonin in oncology. Although some progress has been made in advancing ORI nano-delivery research, it remains insufficient for clinical implementation, necessitating further investigation.

Experimental and simulation study of reverse micelles formed by aerosol-OT and water in non-polar solvents

The formation of reverse micelles by aerosol-OT [sodium bis(2-ethylhexyl) sulfosuccinate] in hydrocarbon solvents, and in the presence of water, is studied using a combination of atomistic molecular-dynamics simulations and small-angle neutron scattering (SANS). There have been many previous studies of aerosol-OT and its self-assembly in both water and non-aqueous solvents, but this work is focused on a combined experimental and simulation study of reverse-micelle formation. The effects of hydration (with water-to-surfactant molar ratios in the range 0-60) and solvent (cyclohexane and n-dodecane) are investigated. A force field is adapted that results in spontaneous formation of reverse micelles starting from completely randomized configurations. The computed dimensions of the reverse micelles compare very favourably with those determined in SANS experiments, providing validation of the simulation model. The kinetics of reverse-micelle formation are studied with a 50-ns, 1.7-million-atom system which contains, in the steady state, about 50 reverse micelles. The internal structures of reverse micelles are characterized with mass density profiles, and the effects of solvent, and the structural crossover from highly structured water to 'bulk' water in the core, are detailed. The corresponding changes in the molecular reorientation times of sequestered water are also determined. Overall, the combination of experiment and simulation gives a detailed picture of reverse-micelle self-assembly and structure.

A comprehensive review on sustainable surfactants from CNSL: chemistry, key applications and research perspectives

Surfactants, a group of amphiphilic molecules (i.e. with hydrophobic(water insoluble) as well as hydrophilic(water soluble) properties) can modulate interfacial tension. Currently, the majority of surfactants depend on petrochemical feedstocks (such as oil and gas). However, deployment of these petrochemical surfactants produces high toxicity and also has poor biodegradability which can cause more environmental issues. To address these concerns, the current research is moving toward natural resources to produce sustainable surfactants. Among the available natural resources, Cashew Nut Shell Liquid (CNSL) is the preferred choice for industrial scenarios to meet their goals of sustainability. CNSL is an oil extracted from non-edible cashew nut shells, which doesn't affect the food supply chain. The unique structural properties and diverse range of use cases of CNSL are key to developing eco-friendly surfactants that replace petro-based surfactants. Against this backdrop, this article discusses various state-of-the-art developments in key cardanol-based surfactants such as anionic, cationic, non-ionic, and zwitterionic. In addition to this, the efficiency and characteristics of these surfactants are also analyzed and compared with those of the synthetic surfactants (petro-based). Furthermore, the present paper also focuses on various market aspects and different applications in various industries. Finally, this article describes various future research perspectives including Artificial Intelligence technology which, of late, is having a huge impact on society.

Determining interfacial tension and critical micelle concentrations of surfactants from atomistic molecular simulations

Hypothesis Atomistically-detailed models of surfactants provide quantitative information on the molecular interactions and spatial distributions at fluid interfaces. Hence, it should be possible to extract from this information, macroscopical thermophysical properties such as interfacial tension, critical micelle concentrations and the relationship between these properties and the bulk fluid surfactant concentrations. Simulations and Experiments Molecular-scale interfacial of systems containing n-dodecyl β-glucoside (APG12) are simulated using classical molecular dynamics. The bulk phases and the corresponding interfacial regions are all explicitly detailed using an all-atom force field (PCFF+). During the simulation, the behaviour of the interface is analyzed geometrically to obtain an approximated value of the critical micelle concentration (CMC) in terms of the surfactant area number density and the interfacial tension is assessed through the analysis of the forces amongst molecules. New experimental determinations are reported for the surface tension of APG12 at the water/air and at the water/n-decane interfaces. Findings We showcase the application of a thermodynamic framework that inter-relates interfacial tensions, surface densities, CMCs and bulk surfactant concentrations, which allows the in silico quantitative prediction of interfacial tension isotherms.

Computational predictions of interfacial tension, surface tension, and surfactant adsorption isotherms

All-atom (AA) molecular dynamics (MD) simulations are employed to predict interfacial tensions (IFT) and surface tensions (ST) of both ionic and non-ionic surfactants. The general AMBER force field (GAFF) and variants are examined in terms of their performance in predicting accurate IFT/ST, γ, values for chosen water models, together with the hydration free energy, ΔGhyd, and density, ρ, predictions for organic bulk phases. A strong correlation is observed between the quality of ρ and γ predictions. Based on the results, the GAFF-LIPID force field, which provides improved ρ predictions is selected for simulating surfactant tail groups. Good γ predictions are obtained with GAFF/GAFF-LIPID parameters and the TIP3P water model for IFT simulations at a water-triolein interface, and for GAFF/GAFF-LIPID parameters together with the OPC4 water model for ST simulations at a water-vacuum interface. Using a combined molecular dynamics-molecular thermodynamics theory (MD-MTT) framework, a mole fraction of C12E6 molecule of 1.477 × 10-6 (from the experimental critical micelle concentration, CMC) gives a simulated surface excess concentration, ΓMAX, of 76 C12E6 molecules at a 36 nm2 water-vacuum surface (3.5 × 10-10 mol cm-2), which corresponds to a simulated ST of 35 mN m-1. The results compare favourably with an experimental ΓMAX of C12E6 of 3.7 × 10-10 mol cm-2 (80 surfactants for a 36 nm2 surface) and experimental ST of C12E6 of 32 mN m-1 at the CMC.

Molecularly informed field theory for estimating critical micelle concentrations of intrinsically disordered protein surfactants

The critical micelle concentration (CMC) is a crucial parameter in understanding the self-assembly behavior of surfactants. In this study, we combine simulation and experiment to demonstrate the predictive capability of molecularly informed field theories in estimating the CMC of biologically based protein surfactants. Our simulation approach combines the relative entropy coarse-graining of small-scale atomistic simulations with large-scale field-theoretic simulations, allowing us to efficiently compute the free energy of micelle formation necessary for the CMC calculation while preserving chemistry-specific information about the underlying surfactant building blocks. We apply this methodology to a unique intrinsically disordered protein platform capable of a wide variety of tailored sequences that enable tunable micelle self-assembly. The computational predictions of the CMC closely match experimental measurements, demonstrating the potential of molecularly informed field theories as a valuable tool to investigate self-assembly in bio-based macromolecules systematically.

Predicting surfactant phase behavior with a molecularly informed field theory

HYPOTHESIS

The computational study of surfactants and self-assembly is challenging because 1) models need to reflect chemistry-specific interactions, and 2) self-assembled structures are difficult to equilibrate with conventional molecular dynamics. We propose to overcome these challenges with a multiscale simulation approach where relative entropy minimization transfers chemically-detailed information from all-atom (AA) simulations to coarse-grained (CG) models that can be simulated using field-theoretic methods. Field-theoretic simulations are not limited by intrinsic physical time scales like diffusion and allow for rigorous equilibration via free energy minimization. This approach should enable the study of properties that are difficult to obtain by particle-based simulations.

SIMULATION WORK

We apply this workflow to sodium dodecylsulfate. To ensure chemical fidelity we present an AA force field calibrated against interfacial tension experiments. We generate CG models from AA simulation trajectories and show that particle-based and field-theoretic simulations of the CG model reproduce AA simulations and experimental measurements.

FINDINGS

The workflow captures the complex balance of interactions in a multicomponent system ultimately described by an atomistic model. The resulting CG models can study complex 3D phases like double or alternating gyroids, and reproduce salt effects on properties like aggregation number and shape transitions.

Rhamnolipid Self-Aggregation in Aqueous Media: A Long Journey toward the Definition of Structure–Property Relationships

The need to protect human and environmental health and avoid the widespread use of substances obtained from nonrenewable sources is steering research toward the discovery and development of new molecules characterized by high biocompatibility and biodegradability. Due to their very widespread use, a class of substances for which this need is particularly urgent is that of surfactants. In this respect, an attractive and promising alternative to commonly used synthetic surfactants is represented by so-called biosurfactants, amphiphiles naturally derived from microorganisms. One of the best-known families of biosurfactants is that of rhamnolipids, which are glycolipids with a headgroup formed by one or two rhamnose units. Great scientific and technological effort has been devoted to optimization of their production processes, as well as their physicochemical characterization. However, a conclusive structure–function relationship is far from being defined. In this review, we aim to move a step forward in this direction, by presenting a comprehensive and unified discussion of physicochemical properties of rhamnolipids as a function of solution conditions and rhamnolipid structure. We also discuss still unresolved issues that deserve further investigation in the future, to allow the replacement of conventional surfactants with rhamnolipids.

Modeling Alkyl Aromatic Hydrocarbons with Dissipative Particle Dynamics

Building on previous work studying alkanes, we develop a dissipative particle dynamics (DPD) model to capture the behavior of the alkyl aromatic hydrocarbon family under ambient conditions of 298 K and 1 atmosphere. Such materials are of significant worldwide industrial importance in applications such as solvents, chemical intermediates, surfactants, lubricating oils, hydraulic fluids, and greases. We model both liquids and waxy solids for molecules up to 36 carbons in size and demonstrate that we can correctly capture both the freezing transition and liquid-phase densities in pure substances and mixtures. We also demonstrate the importance of including specialized bead types into the DPD model (rather than solely relying on generic bead types) to capture specific local geometrical constructs such as the benzene ring found in the benzyl chemical group; this can be thought of as representing subtle real-world many-body effects via customized pairwise non-bonded potentials.

Rhamnolipid Biosurfactants for Oil Recovery: Salt Effects on the Structural Properties Investigated by Mesoscale Simulations

Rhamnolipids (RLs) are biosurfactants produced by Pseudomonas. The biodegradability and the variety of their functionality make them suitable for environmental remediation and oil recovery. We use dissipative particle dynamics simulations to investigate the aggregation behaviors of ionic RL congeners with nonane in various operating conditions. Under zero-salinity conditions, all RL congeners studied here form small ellipsoidal clusters with detectable free surfactants. When salt ions are present, the electrostatic repulsion between the ionized heads is overcome, resulting in the formation of larger aggregates of unique structures. RLs with C10-alkyl tails tend to form elongated wormlike micelles, while RLs with C16-alkyl tails tend to form clusters in spherical symmetry, including vesicles. Di-rhamnolipids (dRLs) require stronger solvation than monorhamnolipids (mRLs) to form clusters, and the resulting size of micelles is decreased. The morphology of the mixed dRL/mRL/oil systems is controlled based on the type of the congeners and the oil contents. In addition, the divalent calcium ions are found to be influential to the structure of the micelles through different mechanisms. For 5 wt % salinity, the ionic RLs can form oil-swollen micelles up to a 1:1 surfactant-to-oil ratio, suggesting that ionic RLs are superb to act as cleaning agents for petroleum hydrocarbons in the marine area. These key findings may guide the design for RL-based washing techniques in enhanced oil recovery.

Structural properties of cationic surfactant-fatty alcohol bilayers: insights from dissipative particle dynamics

Bilayers, self-assembled by cationic surfactants and fatty alcohols in water, are the basic units of lamellar gel networks - creamy formulations extensively used in cosmetics and pharmaceutics. Mesoscopic modelling and study of the bilayers formed by single- or double-tail cationic surfactants (CTAC or DHDAC), and fatty alcohols (FAs) in the lamellar fluid and gel phases were employed. Fatty alcohols with alkyl tail equal to or greater than the surfactant alkyl tail, i.e., C₁₆FA or C₁₈FA and C₂₂FA, were considered. A model formulation was explored with the FA concentration greater than that of the surfactant and the structure of the fluid and gel bilayers in tensionless state characterised via the density profiles across the bilayers, orientational order parameters of the surfactant and FA chains, intrinsic analysis of the bilayer interfaces, and bending rigidity. The intrinsic analysis allows identification and quantification of the coexistence of the interdigitated and non-interdigitated phases present within the gel bilayers. The FA chains were found to conform the primary scaffolding of the bilayers while the surfactant chains tessellate bilayer monolayers from their water-hydrophobic interface. Further, the overlap of the FA chains from the apposed monolayers of the fluid bilayers rises with increasing FA length. Finally, the prevalence of the non-interdigitated phase over the interdigitated phase within the gel bilayers becomes enhanced upon the FA length increase with a preference of the surfactant chains to reside in the non-interdigitated phase rather than the interdigitated phase.