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Fullerene-based amphiphiles are new types of monomers that form self-assemblies with profound applications. The conical fullerene amphiphiles (CFAs) have attracted attention for their uniquely self-assembled structures and have opened up a new field for amphiphile research. The CFAs and CFAs with different substances embedded in cavities are designed and their self-assembly behaviors are investigated using molecular dynamics (MD) simulations. The surface and internal structures of the micelles are analyzed from various perspectives, including micelle size, shape, and solvent-accessible surface area (SASA). The systems studied are all oblate micelles. In comparison, embedding Cl− or embedding Na+ in the cavities results in larger micelles and a larger deviation from the spherical shape. Two typical configurations of fullerene surfactant micelles, quadrilateral plane and tetrahedral structure, are presented. The dipole moments of the fullerene molecules are also calculated, and the results show that the embedded negatively charged Cl− leads to a decrease in the polarity of the pure fullerene molecules, while the embedded positively charged Na+ leads to an increase. [ABSTRACT FROM AUTHOR]

Betaine is a kind of zwitterionic surfactant with both positive and negative charge groups on the polar head, showing good surface activity and aggregation behaviors. The interfacial adsorption, structures and properties of n-dodecyl betaine (NDB) at different surface coverages at the air–water interface are studied through molecular dynamics (MD) simulations. Interactions between the polar heads and water molecules, the distribution of water molecules around polar heads, the tilt angle of the NDB molecule, polar head and tail chain with respect to the surface normal, the conformations and lengths of the tail chain, and the interfacial thickness of the NDB monolayer are analyzed. The change of surface coverage hardly affects the locations and spatial distributions of the water molecules around the polar heads. As more NDB molecules are adsorbed at the air–water interface, the number of hydrogen bonds between polar heads and water molecules slightly decreases, while the lifetimes of hydrogen bonds become larger. With the increase in surface coverage, less gauche defects along the alkyl chain and longer NDB chain are obtained. The thickness of the NDB monolayer also increases. At large surface coverages, tilted angles of the polar head, tail chain and whole NDB molecule show little change with the increase in surface area. Surface coverages can change the tendency of polar heads and the tail chain for the surface normal. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract Molecular dynamics (MD) and metadynamics simulations were used to give an understanding on the cloud point phenomenon of ionic surfactants. Meanwhile, the differences between tetrabutylammonium interactions with dodecyl sulfate (DS−) and alkyl carboxylate (C12) micelles are discussed. Micelle ionization degree (α), binding patterns between the counterions and micelle, potential of mean force (PMF), and intermicellar interactions were characterized to present a reasonable explanation on the different behaviors of tetrabutylammonium alkyl carboxylate (TBAC) and tetrabutylammonium dodecyl sulfate (TBADS). Free energy profiles indicate that three bridging models exist between TBADS micelles. This will be the fundamental intermicellar model to interpret the cloud point phenomenon of TBADS ionic surfactants and other analogous systems. Highlights • Metadynamics simulation is used to study the intermicellar interactions. • Different intermicellar behaviors of TBAC and TBADS is explained. • Explicit bridging models between TBADS micelles is proposed. [ABSTRACT FROM AUTHOR]

Zwitterionic betaines are widely used as foam boosters due to these can enhance the stability of foam films. In this paper, mechanistic insights of betaine to improve the stability of alkyl-polyoxyethylene carboxylate (AEC) foam are provided by molecular simulation. In the simulation, we observe the electropositive nitrogen atoms in betaine interact with the electronegative sulfur atoms, an electrostatic structure is formed at the air/water interface. Interaction energies of the mixed surfactants are calculated by the quantum chemistry methods. The calculations show betaine-AEC and betaine–betaine possess attractive interaction, and that AEC–AEC has repulsion to each other. In the other words, the repulsion between the headgroups of anionic surfactants is relaxed by betaine. Additionally, the influence of concentration of betaine on the stability of foam films is also simulated. The RDF and coordination numbers show that the electrostatic structures become denser with the increasing concentration of betaine. Therefore, entry barrier is enhanced accordingly. The SMD simulation also demonstrates the same variation tendency of entry barrier. The simulation details provide vital supplements to experiments. [ABSTRACT FROM AUTHOR]

Different Tetraalkylammonium (TAA + ) counterions including tetramethylammonium (TMA + ), tetraethylammonium (TEA + ), tetrapropylammonium (TPA + ) and tetrabutylammonium (TBA + ) with dodecyl sulfate (DS − ) at the air/water interface were studied with atom molecular dynamics (MD) simulations. Structural properties such as the distribution of polar head at the interface, probability of gauche defects, order parameters, length and angle distributions were investigated. The counterions and DS − form mixed adsorption layer. Our results show that weak C H O hydrogen bonds are formed between the polar head and alkane chains of counterions. The weak hydrogen bonds play an important role in the formation of mixed adsorption layer. Mainly four interaction patterns exit between TAA + and DS − ions, in which binding pattern II, representing that two terminal methyl groups of TAA + penetrate into the monolayer, is the most favorable one. [ABSTRACT FROM AUTHOR]

The state of surfactant in different environments and the conformational change of surfactant self in different mass transport processes are investigated and analysed. Interfacial adsorption and bulk micellization of surfactant are common in natural world, and have prominent applications both in production and life. Surfactant molecules circularly flux among solution bulk, interface, and micelle in surfactant solution. Molecular dynamics (MD) simulations are carried out to compare the state of surfactant among solution monomer, interfacial monolayer, and micelle. Surface coverages from extremely dilute adsorption to saturated adsorption are all considered. Using the quasi harmonic approximation method, configurational entropies of N-dodecyl betaine (NDB) in different environments are obtained. The NDB monomers possess the largest freedom owing to spatial and non-aggregated superiorities. Entropy decomposition, including translational, rotational and vibrational entropies, is calculated to give further comparison and analysis. A new Born-Haber cycle is proposed to depict the state of NDB in different environments and to present entropy change of NDB itself in different mass transport processes. In particular, entropy changes of NDB in physical adsorption process below and above critical micelle concentration are obtained. The NDB molecule exhibits entropy decrease in interfacial adsorption, bulk micellization, and the transport of surfactant from micelle to surface layer. [ABSTRACT FROM AUTHOR]

We investigated the dissolution state of cellulose in NaOH/urea aqueous solution using molecular dynamics simulations. All the components, including cellulose, NaOH, urea and H 2 O were combined into one simulation. A clear and detailed description for the formation of cellulose inclusion complexes has been stated. The simulation results showed that the cellulose inclusion complexes exhibited anisotropic properties. In this system, the sodium ions and hydroxide ions located at the regions of hydroxyl and hydroxymethyl groups in cellulose molecules. However, the urea molecules occupied the faces of the hydrophobic pyranose rings. The hydrogen-bonding configuration and lifetime of hydrogen bonds in cellulose inclusion complexes were also systematically characterized. The spatial structure of inclusion complexes, the hydrogen-bonding interaction between cellulose and solvent molecules, the diffusive property of solvent molecules and the distribution of water–water angles in cellulose inclusion complexes were discussed in details. The NaOH, urea and H 2 O molecules surrounding the cellulose chain within radial distance 0.35 nm have smaller diffusion coefficients than those in bulk state. The water–water angles provide a quantitative estimate of hydrophobic effect of cellulose. [ABSTRACT FROM AUTHOR]

As the major stabilizer for crude oil emulsion in petroleum industry, asphaltene has been concerned more widespread with the increasing demand of energy in the world. The in-depth understanding on its properties at crude oil-water interface has some beneficial effect on dealing with the emulsion in oil industry. To investigate these interfacial properties at molecular level, molecular dynamics (MD) simulation was utilized. The results show that asphaltene molecules tend to different orientation between them at this interface so as to decrease the repulsive force of aggregation process. Moreover, the contributors to asphaltene stability at crude oil-water interface also were discussed. It mainly includes hydrogen bond networks around asphaltene polar group. The hydrogen bond networks behave like an anchor to grab hold of asphaltene at the interface. The aggregation properties of asphaltene at oil-water interface were obtained by using the umbrella sampling method. The results indicate that aggregation state is most stable for asphaltene molecules at oil-water interface and the system free energy will increase after the aggregate be broken. [ABSTRACT FROM AUTHOR]

All R s values become larger with adsorbed counterions, however, organic counterions adsorbed onto micelles can decrease the SASA, which is contrary to the inorganic Cl− ions. [Display omitted] • Absorbed PO− and BZ− ions lead to the decrease in SASA. • The Cl− ions are adsorbed on micelle surface while PO− and BZ− ions penetrate deeper into the micellar interior. • Water molecules penetrate into the deepest position in [C 14 mim][Cl] micelle. Self-assembly of ionic liquids (ILs) in solution has attracted wide attention due to their potential applications in various fields. The nature of counterions, especially aromatic counterions, will significantly affect the aggregation and morphological properties of ILs surfactants. In this paper, molecular dynamics (MD) simulations are applied to study the influence of three organic and inorganic counterions (phenolate (PO−), benzoate (BZ−), and Cl−) on the micellar structure of imidazolium (C 14 mim+) based ILs surfactant in aqueous solutions. Three micelles all possess prolate shape. The PO−, BZ−, and Cl− ions hardly affect the orientation and length of hydrophobic chains in micellar interior. Hydration numbers and average radial density distributions analyses show that interactions between micelles and water molecules are different in studied micelles. Surrounding water molecules penetrate into the deepest region in [C 14 mim][Cl] micelle. However, adsorbed Cl− ions are mainly located on micellar surface, not penetrating into micellar interior. Due to the intrinsic hydrophobicity of organic ions, both PO− and BZ− ions enter into deeper positions in micelles. Unlike Cl− ions, there exist π-interactions between adsorbed PO− and BZ− ions themselves. One interesting finding is that adsorbed PO− and BZ− ions can make micelle radius (R g) increase but make solvent accessible surface area (SASA) decrease. Conversely, adsorbed Cl− ions make both R g and SASA increase. Three MD models for different C 14 mim+ based micelles are proposed. [ABSTRACT FROM AUTHOR]

Micelle plays an important role both in scientific research and practical applications. However, the problem about the state of micelle interior has not been completely solved. Based on the molecular dynamics (MD) simulation, absolute configurational entropies of hydrophobic chain are calculated to give a correct description of micelle hydrophobic interior. N-dodecyl betaine (NDB) and n-tridecane (C13), which have the same number of methylene group, are investigated in the present study. Results including total entropy, translational entropy, rotational entropy as well as vibrational entropy are calculated and compared between alkyl chains of NDB micelle and liquid C13. Local entropies along NDB alkyl chain and those between micelle and C13 are also considered. Through these comparisons, we explicitly propose that innermost groups of hydrophobic chains, where carbon number is no more than two, are identical to pure alkane whilst the outer core in the micelle shows the liquidlike state at the molecular level. The detailed information of micelle interior is the foundation for understanding micelle applications, such as solubilization, drug delivery and catalysis. What is the packing state of micelle interior? Unlabelled Image • A molecular dynamics model of the packing state of micelle interior is proposed. • Inhomogeneous micelle core is defined from the local entropy along alkyl chain. • The configuration entropy of selected alkyl chain is calculated and decomposed. [ABSTRACT FROM AUTHOR]

Different aggregation configurations of SC4AP micelles. [Display omitted] • Two adsorption modes of Na+ ions on SC4AP micelle are proposed. • The cavity of each SC4AP includes one adsorbed Na+ ion and one water molecule. • Representative configurations of tetramer and hexamer SC4AP micelle are presented. Calixarene-based amphiphiles are new types of monomers to form self-assemblies with profound applications. Micelles formed by these amphiphiles are classified as Platonic micelles. The effects of different NaCl concentrations of 10, 15 and 100 mM on sulfonated amphiphilic calix [4] arene (SC4AP) micelles are studied by all-atom molecular dynamics (MD) simulation. Structural characteristics including micelle shape and size, solvent accessible surface area, radial distribution function, hydration number, the adsorption of sodium (Na+) ion on micelles and micellar configuration, are analyzed at the molecular level. The MD results show that all micelles are not perfectly spherical. With the increase of NaCl concentration, more Na+ ions are absorbed on micelle surface. However, the increasing extent of adsorbed Na+ ions decreases at large NaCl concentration. Two adsorption modes of Na+ ions on micelles are found. One attracting point is that each cavity of SC4AP molecule include one adsorbed Na+ ion with one hydrated water molecule. Representative configurations of tetramer and hexamer SC4AP micelles under different NaCl concentrations are proposed. At 10 mM NaCl concentration, tetramer SC4AP micelles switch between tetrahedron and planar quadrilateral structures during simulations. [ABSTRACT FROM AUTHOR]

Mixture of different surfactant molecules possess better performances than single surfactant, which has importantly scientific and practical significance. In this paper, the effects of different mole ratios on the packing states of sodium dodecyl sulfate (SDS) and n-dodecyl betaine (NDB) in mixed micelles are studied through molecular dynamics simulations. The packing state is characterized by calculating and comparing the configuration entropy and entropy decomposition of SDS and NDB. Results show that mixing with NDB molecules can cause the increase of SDS packing order in mixed micelles, while mixing with large proportions of SDS molecules can cause the decrease of NDB packing order in mixed micelles. Moreover, structural properties that are closely related to the packing state are analyzed. The surface hydrophobicity and hydrophilicity of SDS molecules in micelle can be reversed through changing its mole fraction. Both SDS and NDB terminal carbon atoms in mixed micelles possess opposite distribution tendency to those in pure SDS or NDB micelle. As mixed ratio changes, dominant packing patterns of SDS and NDB in mixed micelles are different. Two represent models of mixed micelle with the change of mole ratios are presented. What is the effect of surfactant mixture on the packing state of surfactant molecules in micelles and micellar structure? [Display omitted] • Packing order of SDS is more affected for SDS+NDB mixture in similar proportion. • The surface hydrophobicity and hydrophilicity of SDS in micelle can be reversed. • Two represent models of mixed micelles are obtained. [ABSTRACT FROM AUTHOR]

Ionic liquids (ILs) have good designability and promising prospect of application. By changing different cations or anions, ILs possessing better and specific properties can be synthesized. In this paper, molecular dynamics (MD) simulation is applied to study ILs with the anion of 2,5,8,11-tetraoxatridecan-13-oate (TOTO), which are named as TOTO-based ILs. Attention is mainly paid to the effect of five different types of cations, including organic choline (Ch), tetraethylammonium (TEA), tetrapropylammonium(TPA), and tetrabutylammonium (TBA) and inorganic sodium (Na) ions, on the structure and properties of TOTO-based ILs. Results about conformation and length distribution of TOTO chain, distance between cations and TOTO anions, distribution of coordination number for cation-anion, and dynamic coordination are analysed, showing that changing cations makes no considerable influence on gauche probability along TOTO chain. All organic cation-TOTO ILs have similar TOTO chain length distribution, and similar radical function distributions between cations and anions. No matter inorganic cations and organic cations, these cations prefer to interact with carboxyl oxygen atoms. For organic cation-TOTO systems, interionic coordination interactions are weakened with the increase of cation size. Due to the hydroxyl group in one Ch alkyl chain, Ch-TOTO system exhibits some particular properties. The introduce of functional group can tune ILs properties and promote the design of task-specific ILs. [ABSTRACT FROM AUTHOR]