Self-aggregation of the SDS surfactant at a solid-liquid interface

Molecular dynamics simulations of sodium dodecyl sulfate (SDS) molecules on a graphite surface are presented. The simulations were conducted at low and high surface coverage to study aggregation at the water/graphite interface. Results showed that at low surface coverage, the SDS molecules form hemicylindrical aggregates, in agreement with AFM experiments, whereas at high surface coverage, the surfactants form full cylinders. The latter aggregates have not been reported in systems of SDS on hydrophobic substrates, such as graphite. The unexpected results are explained in terms of a water layer adsorbed at the solid surface which was the responsible for the formation of these aggregates. Moreover, the SDS tails in the full cylindrical configuration became straighter than those of the hemicylindrical aggregate. Hydrogen bond formation between water and surfactant head groups was also studied, and it was found that they did not depend on the surfactant concentration.     

Adsorption and morphology transition of surfactants on hydrophobic surfaces: a lattice Monte Carlo study

In this work, we first show that there are only five independent interchange parameters in the surfactant-solvent-interface system in Larson's model, and then adsorption and morphology transition of surfactants on hydrophobic surfaces are studied by extensive lattice Monte Carlo simulations. In our simulations, we found that there exist six adsorbed morphologies: (1) premature admicelle, (2) hemisphere, (3) hemisphere-hemicylinder mixture, (4) wormlike hemicylinder, (5) perforated monolayer, and (6) monolayer. The surface morphologies and the amount of adsorption on hydrophobic surfaces are found to be affected obviously by two interchange parameters. One is the attractive interaction between tail groups and surface (chiTS), and the other is the solubility of head groups in bulk (chiHW). Phase diagrams in chiHW versus chiTS planes for surfactants with different hydrophobicities (chiTW) and for surfactants with different molecular structures are determined in this work, from which the transitions of surface morphologies and adsorption behaviors are discussed.     

Molecular interactions of cationic and anionic surfactants in mixed monolayers and aggregates

The properties of anionic-rich and cationic-rich mixtures of CTAB (cetyltrimethylammonium bromide) and SDS (sodium dodecyl sulfate) were investigated with conductometry and surface tension measurements and by determining the surfactant NMR self-diffusion coefficients. The critical aggregate concentration (CAC), surface tension reduction effectiveness(gamma(CAC)), surface excess(Gamma(max)), and mean molecular surface area (A(min)) were determined from plots of the surface tension (gamma) as a function of the total surfactant concentration. The compositions of the adsorbed films (Z) and aggregates (chi) were estimated by using regular solution theory, and then the interaction parameters in the aggregates (beta) and the adsorbed film phases (beta(sigma)) were calculated. The results showed that the synergism between the surfactants enhances the formation of mixed aggregates and reduces the surface tension. Further, the nature and strength of the interaction between the surfactants in the mixtures were obtained by calculating the values of the following parameters: the interaction parameter, beta, the size parameter, rho, and the nonrandom mixing parameter, P*. These results indicate that in ionic surfactant mixtures the optimized packing parameter has the highest value and that the size parameter can be used to account for deviations from the predictions of regular solution theory. It was concluded that, for planar air/aqueous interfaces and aggregation systems, this nonideality increases as the temperature increases. This trend is attributed to the increased dehydration of the surfactant head groups that results from increases in temperature. Further, our conductometry measurements show that the counterion binding number of mixed micelles formed in mixtures with a high CTAB content is different to those with a high SDS content. This difference is due to either their different aggregation sizes or the different interactions between the head groups and the counterions.     

Effect of liquid paraffin on the stability of aqueous foam in the presence and absence of electrolytes

The foam stability of dodecyl diphenyl ether disulfonate solution with liquid paraffin droplets in the presence and absence of electrolytes was evaluated, and the stabilization mechanisms were deduced. The foam film is stabilized when the monovalent and divalent counterion concentration is lower than a critical value. However, the foam stability declined with the addition of trivalent counterions. There are two mechanisms we have speculated. Firstly, the counterions reduce the repulsive interaction between the polar groups of surfactants adsorbed at the air/water and oil/water interfaces in the pseudo-emulsion film. Secondly, comparing with the monovalent counterions, the multivalent counterions are not only able to interconnect head groups of adjacent ionic surfactant molecules which are adsorbed at the air/water or oil/water interface, but also to attract the couples of neighboring surfactant head groups adsorbed at the air/water and oil/water interfaces. The attractive interaction between both the interfaces promotes the emulsified droplets piercing the air/water interface.     

十二烷基苯磺酸钠在SiO2表面聚集的分子动力学模拟

采用分子动力学方法研究了阴离子表面活性剂十二烷基苯磺酸钠(SDBS)在无定形SiO2固体表面的吸附. 设置不同的水层厚度, 观察固液界面和气液界面吸附的差异. 模拟发现表面活性剂分子能够在短时间内吸附到SiO2表面, 受碳链和固体表面之间相互作用的影响形成表面活性剂分子层, 并依据吸附量的大小形成不同的聚集结构; 在水层足够厚的情况下, 由于有较多的表面活性剂分子吸附在固体表面,从而形成带有疏水核心的半胶束结构; 计算得到的成对势表明极性头与钠离子或水分子之间的结合或解离与二者之间的能垒有关, 解离能垒远大于结合能垒, 引起更多Na+聚集在极性头周围而只有少数Na+存在于溶液中; 无论气液还是固液界面, 极性头均伸向水相, 与水分子形成不同类型的氢键. 模拟表明, 分子动力学方法可以作为实验的一种补充, 为实验提供必要的微观结构信息.     

Atomistic structure of a micelle in solution determined by wide Q-range neutron diffraction

The accepted picture of the structure of a micelle in solution arises from the idea that the surfactant molecules self-assemble into a spherical aggregate, driven by the conflicting affinity of their head and tail groups with the solvent. It is also assumed that the micelle's size and shape can be explained by simple arguments involving volumetric packing parameters and electrostatic interactions. By using wide Q-range neutron diffraction measurements of H/D isotopically substituted solutions of decyltrimethylammonimum bromide (C(10)TAB) surfactants, we are able to determine the complete, atomistic structure of a micelle and its surroundings in solution. The properties of the micelle we extract are in agreement with previous experimental studies. We find that ~45 surfactant molecules aggregate to form a spherical micelle with a radius of gyration of 14.2 ? and that the larger micelles are more ellipsoidal. The surfactant tail groups are hidden away from the solvent to form a central dry hydrophobic core. This is surrounded by a disordered corona containing the surfactant headgroups, counterions, water, and some alkyl groups from the hydrophobic tails. We find a Stern layer of 0.7 bromide counterion per surfactant molecule, in which the bromide counterions maintain their hydration shells. The atomistic resolution of this technique provides us with unprecedented detail of the physicochemical properties of the micelle in its solvent.     

Coarse-grained molecular dynamics simulation of self-assembly of polyacrylamide and sodium dodecylsulfate in aqueous solution

Coarse-grained molecular dynamics simulations have been performed to study the self-assembly of polymer, polyacrylamide (PAM) and surfactant, sodium dodecylsulfate (SDS) in aqueous solution. Our simulations revealed that PAM curled into clusters in the absence of SDS, while it was stretched if SDS was added. For the SDS-PAM complexes, the aggregate formation process can be divided into three stages: firstly, PAM quickly absorbs some SDS monomers until the radius of gyration (Rg) of polymer reaches a minimum; then, PAM stretches and the Rg of PAM increases due to more and more adsorbed SDS; ultimately, the commonly accepted "necklace" structure is formed with PAM located at the interface of the hydrophobic and hydrophilic regions of the SDS micelle. The main driving force for the association was hydrophobic interactions between the polymer backbone and the surfactant hydrophobic tails. As the concentration of SDS increased, the Rg of PAM increased up to a maximum, indicating the polymer was saturated with surfactant.     

New insights into the interactions between dendrimers and surfactants by two dimensional NOE NMR spectroscopy

The interactions between polyamidoamine dendrimers and different surfactants including sodium dodecyl sulfate (SDS) and dodecyltrimethyl ammonium bromide in aqueous solutions have been investigated by a NOESY NMR technique. Strong NOE cross-peaks between hydrophobic chain protons of SDS and methylene protons of cationic dendrimers were found, suggesting a strong tendency for the long hydrophobic tails of SDS to associate with the hydrophobic pockets of dendrimers. The hydrophilic head of SDS localizes near the core or the boundary of each generation of dendrimers, and the hydrophobic chain of SDS localizes in the relative nonpolar pockets of dendrimers. The encapsulation of surfactant monomers by dendrimers is dependent on the charge type of the surfactants, the surface functionality, and the generation of dendrimers. The NOESY analysis provides a new insight into interactions between dendrimers and surfactants in comparison with previous investigations.     

In situ AFM studies on self-assembled monolayers of adsorbed surfactant molecules on well-defined H-terminated Si(111) surfaces in aqueous solutions

The formation of self-assembled monolayers (SAMs) of adsorbed cationic or anionic surfactant molecules on atomically flat H-terminated Si(111) surfaces in aqueous solutions was investigated by in situ AFM measurements, using octyl trimethylammonium chloride (C8TAC), dodecyl trimethylammonium chloride (C12TAC), octadecyl trimethylammonium chloride (C18TAC)) sodium dodecyl sulfate (STS), and sodium tetradecyl sulfate (SDS). The adsorbed surfactant layer with well-ordered molecular arrangement was formed when the Si(111) surface was in contact with 1.0x10(-4) M C18TAC, whereas a slightly roughened layer was formed for 1.0x10(-4) M C8TAC and C12TAC. On the other hand, the addition of alcohols to solutions of 1.0x10(-4) M C8TAC, C12TAC, or SDS improved the molecular arrangement in the adsorbed surfactant layer. Similarly, the addition of a salt, KCl, also improved the molecular arrangement for both the cationic and anionic surfactant layers. Moreover, the adsorbed surfactant layer with a well-ordered structure was formed in a solution of mixed cationic (C12TAC) and anionic (SDS) surfactants, though each surfactant alone did not form the well-ordered layer. These results were all explained by taking into account electrostatic repulsion between ionic head groups of adsorbed surfactant molecules as well as hydrophobic interaction between their alkyl chains, which increases with the increasing chain length, together with the increase in the hydrophobic interaction or the decrease in the electrostatic repulsion by incorporating alcohol molecules into the adsorbed surfactant layer, the decrease in the electrostatic repulsion by increasing the concentration of counterions, and the decrease in the electrostatic repulsion by alternate arrangement of cationic and anionic surfactant molecules. The present results have revealed various factors to form the well-ordered adsorbed surfactant layers on the H-Si(111) surface, which have a possibility of realizing the third generation surfaces with flexible structures and functions easily adaptable to circumstances.     

抗衡离子对硫酸盐表面活性剂气/液界面性质影响的分子动力学模拟

采用全原子分子动力学方法研究了抗衡离子为第一主族离子(Li⁺、Na⁺、K⁺、Rb⁺和Cs⁺)的十二烷基硫酸盐表面活性剂的气/液界面性质. 通过分析体系中各组分的密度分布曲线, 考察表面活性剂单分子层在界面的聚集形态, 并利用径向分布函数分析了表面活性剂极性头基与抗衡离子间的相互作用. 研究结果表明: 随着抗衡离子半径的增大, 不同体系的界面水层厚度依次增加, 表面活性剂极性头基与抗衡离子形成的Stern和扩散层厚度也相应增加. 但表面活性剂吸附层的抗衡离子缔合度以及体系表面张力却随抗衡离子半径的增大而减小. 研究表明抗衡离子的差异对十二烷基硫酸盐表面活性剂气/液界面性质有很大影响.     

Adsorption of model surfactantlike copolymers on nanopatterned surfaces

The adsorption of polymers, copolymers, surfactants, and biopolymers is often used to engineer surfaces. Towards improving our understanding of polymer adsorption we report simulation results for the adsorption of model copolymers, resembling surfactants, on nanoscale patterned hydrophobic surfaces at infinitely dilute concentrations. The surfactants are composed by a hydrophobic tail and a hydrophilic head. Surfactant adsorption on the hydrophobic surface occurs in the tail-down configuration in which the tail segments are in contact with the surface. We investigate how the presence of a solid hard mask, used to create the nanoscale pattern on the underlying hydrophobic surface, affects the surfactant adsorption. We find that surfactant adsorption on the underlying hydrophobic surface is prevented when the characteristic dimensions of the solid hard mask are less than twice the radius of gyration. We also show that details about mask-surfactant head effective interactions have the potential to alter the characteristics of adsorption. When the mask repels the head segments, the surfactants hardly adsorb on the underlying hydrophobic surface. When the mask strongly attracts the surfactant heads, the surfactants may preferentially adsorb on the mask rather than on the underlying hydrophobic surface. Under these latter circumstances the adsorbed surfactants in some cases assume a head-down configuration in which the head segments are in contact with the mask and the tail segments extend towards the bulk solution. We explain our results in terms of enthalpy and entropy of adsorption and discuss practical implications.     

Monte Carlo simulations of Lennard-Jones nonionic surfactant adsorption at the liquid/vapor interface

We present Monte Carlo simulations of nonionic surfactant adsorption at the liquid/vapor interface of a monatomic solvent. All molecules in the system, solvent and surfactant, are characterized by the Lennard-Jones (LJ) potential using differing interaction parameters. Surfactant molecules consist of an amphiphilic chain with a solvophilic head and a solvophobic tail. Adjacent atoms along the surfactant chain are connected by finitely extensible harmonic springs. Solvent molecules move via the Metropolis random-walk algorithm, whereas surfactant molecules move according to the continuum configurational bias Monte Carlo (CBMC) method. We generate quantitative thermodynamic adsorption and surface tension isotherms in addition to surfactant radius of gyration, tilt angles, and potentials of mean force. Surface tension simulations compared to those calculated from the simulated adsorbed amounts and the Gibbs adsorption isotherm agree confirming equilibrium in our simulations. We find that the classical Langmuir isotherm is obeyed for our LJ surfactants over the range of head and tail lengths studied. Although simulated surfactant chains in the bulk solution exhibit random orientations, surfactant chains at the interface orient roughly perpendicular and the tails elongate compared to bulk chains even in the submonolayer adsorption regime. At a critical surfactant concentration, designated as the critical aggregation concentration (CAC), we find aggregates in the solution away from the interface. At higher concentrations, simulated surface tensions remain practically constant. Using the simulated potential of mean force in the submonolayer regime and an estimate of the surfactant footprint at the CAC, we predict a priori the Langmuir adsorption constant, KL, and the maximum monolayer adsorption, Gammam. Adsorption is driven not by proclivity of the surfactant for the interface, but by the dislike of the surfactant tails for the solvent, that is by a "solvophobic" effect. Accordingly, we establish that a coarse-grained LJ surfactant system mimics well the expected equilibrium behavior of aqueous nonionic surfactants adsorbing at the air/water interface.     

Comparison of morphology and mechanical properties of surfactant aggregates at water-silica and water-graphite interfaces from molecular dynamics simulations

Cationic surfactants are important for a wide range of applications, including controlled drug delivery systems, emulsifiers, and chemical mechanical polishing. It is therefore important to better understand surfactant structure and properties at the solid-liquid interface. Here, classical molecular dynamics simulations with empirical potentials are used to compare the structures and mechanical properties of cationic surfactant micelles at hydrophobic (graphite) and hydrophilic (silica) surface-water interfaces. In particular, the morphology of monolayers and bilayers of C12TAB (n-dodecyltrimethylammoniumbromide) at these interfaces, and their responses to atomic force microscopy indentation, are examined. The simulations predict that surfactant monolayers and bilayers on silica evolve into a spherical micelle structure, in agreement with theoretical models of surfactant morphology. In contrast, surfactant monolayers on graphite evolve into a hemi-cylindrical structure, in agreement with experimental findings. In the simulated indentation of the micelle/silica system, the spherical micelle breaks apart and forms a surfactant monolayer. The indentation force curve has a maximum value of 2.25 nN. On the other hand, the simulated indentation of the micelle/graphite system causes the hemi-cylindrical micelle structure to break apart and the surfactant tails to wrap around the graphite indenter. The indentation force curve has a maximum value of 13 nN.     

Effects of Charged Surfactants on Interfacial Water Structure and Macroscopic Properties of the Air-Water Interface

Surfactants are used to control the macroscopic properties of the air-water interface. However, the link between the surfactant molecular structure and the macroscopic properties remains unclear. Using sum-frequency generation spectroscopy and molecular dynamics simulations, two ionic surfactants (dodecyl trimethylammonium bromide, DTAB, and sodium dodecyl sulphate, SDS) with the same carbon chain lengths and charge magnitude (but different signs) of head groups interact and reorient interfacial water molecules differently. DTAB forms a thicker but sparser interfacial layer than SDS. It is due to the deep penetration into the adsorption zone of Br⁻ counterions compared to smaller Na⁺ ones, and also due to the flip-flop orientation of water molecules. SDS alters two distinctive interfacial water layers into a layer where H⁺ points to the air, forming strong hydrogen bonding with the sulphate headgroup. In contrast, only weaker dipole-dipole interactions with the DTAB headgroup are formed as they reorient water molecules with H⁺ point down to the aqueous phase. Hence, with more molecules adsorbed at the interface, SDS builds up a higher interfacial pressure than DTAB, producing lower surface tension and higher foam stability at a similar bulk concentration. Our findings offer improved knowledge for understanding various processes in the industry and nature.     

The influence of hydrophobic counterions on micellar growth of ionic surfactants

This review covers the effects of hydrophobic counterions on the phase behavior of ionic surfactants and the properties of the phases. Mixing hydrophobic counterions with ionic surfactant micellar solutions may initiate the micellar growth and transform the micellar microstructure into different morphologies. This behavior may also be achieved by mixing ionic surfactants with hydrophilic counterions, although higher counterionic concentrations are then required. First, the role of hydrophilic and hydrophobic counterions in regards to micelle growth is discussed. Second, the effect of the hydrophobic counterion on the self-assembly of cationic and anionic surfactants and their viscoelastic behavior are presented. Third, the relationships between geometry, hydrophobicity and their consequences on micellar growth for different hydrophobic counterions are reviewed. Forth, the influence of hydrophobic counterion substituents (substitution pattern) on the phase behavior is discussed. Some results we previously obtained for different isomers of hydroxy naphthaoic acids and the cationic surfactant cetyltrimethylammonium hydroxide are included. With these systems the effect that the hydrophobic counterion microenvironment has on the phase behavior, rheological behavior and the micellar microstructure is discussed. The results from other research groups are also discussed.     

Surface properties of mixtures of surface-active sugar derivatives with ionic surfactants: theoretical and experimental investigations

Surface properties of systems that are mixtures of ionic surfactants and sugar derivatives-anionic surfactant sodium dodecyl sulfate and n-dodecyl-beta-D-maltoside (SDS/DM) and cationic surfactant dodecyltrimethylammonium bromide and n-dodecyl-beta-D-glucoside (DTABr/DG)-were investigated. The experimental results obtained from measurements of surface tension of mixtures with various ratio of ionic to nonionic components were analyzed by two independent theories. First is Motomura theory, derived from the Gibbs-Duhem equation, allowing for indirect evaluation of the composition of mixed monolayers and the Gibbs energies of adsorption, corresponding to mutual interaction between surfactants in mixed adsorbed film. As second theory we used our newly developed theoretical model of adsorption of ionic-nonionic surfactant mixtures. Using this approach, we were able to describe the experimental surface tension isotherms for mixtures of surface-active sugar derivatives and ionic surfactants. We obtained a good agreement with experimental data using the same set of model parameters for a whole range of studied compositions of a given surfactant mixture. The values of surface excess calculated from both theories agreed with each other with a reasonable accuracy. However, the newly developed model of adsorption of ionic-nonionic surfactant mixtures has the advantage of straightforward determination of surface layer composition. By the solution of equations of adsorption, one can obtain directly the values of surface excess of all components (surfactant ions, counterions, and nonionic surfactants molecules), which are present in the investigated system.     

Interaction of surfactants with hydrophobic surfaces in nanopores

Surfactant-induced wetting of hydrophobic nanopores is investigated. SDS micelles interact with the C18 layer on the nanopore walls with their hydrophobic tails, creating a charged wall lining with their head groups and inducing a breakthrough of the aqueous solution to wet the pores. The surface coverage of the surfactant molecules is evaluated electrophoretically. A surprising discovery is that pore wetting is achieved with 0.73 μmol/m(2) coverage of SDS surfactant, corresponding to only 18% of a monolayer on the walls of the nanopores. Clearly, the surfactant molecules cannot organize as a compact uninterrupted monolayer. Instead, formation of hemimicelles is thermodynamically favored. Modeling shows that, to be consistent with the experimental observations, the aggregation number of hemimicelles is lower than 25 and the size of hemimicelle is limited to a maximum radius of 11.7 ?. The hydrophobic tails of SDS thus penetrate into and intercalate with the C18 layer. The insight gained in the C18-surfactant interactions is essential in the surfactant-induced solubilization of hydrophobic nanoporous particles. The results have bearing on the understanding of the nature of hydrophobic interactions.     

Surface self-assembly of fluorosurfactants during film formation of MMA/nBA colloidal dispersions

These studies focus on the behavior of fluorosurfactants (FS) containing hydrophobic and ionic entities in the presence of methyl methacrylate/n-butyl acrylate (MMA/nBA) colloidal dispersions stabilized by sodium dodecyl sulfate (SDS). The presence of FS significantly not only alters the mobility of SDS in MMA/nBA films, but their hydrophobic and ionic nature results in self-assembly near the film-air (F-A) interface leading to different surface morphologies. Spherical islands and rodlike morphologies are formed which diminish the kinetic coefficient of friction of films by at least 3 orders of magnitude, and the presence of dual hydrophobic tails and an anionic head appears to have the largest effect on the surface friction. Using internal reflection IR imaging, these studies show that structural and chemical features of FS are directly related to their ability to migrate to the F-A interface and self-assemble to form specific morphological features. While the anionic nature of FS allows for SDS migration to the F-A interface and the formation of stable domains across the surface, intermolecular cohesion of nonionic FS allows for the formation of rodlike structures due to inability to form mixed micelles with SDS. These studies also establish the relationship between surface morphologies, kinetic coefficient of friction, and structural features of surfactants in the complex environments.     

Molecular dynamics simulation of interactions between a sodium dodecyl sulfate micelle and a poly(ethylene oxide) polymer

We have performed atomistic molecular dynamics simulations of an anionic sodium dodecyl sulfate (SDS) micelle and a nonionic poly(ethylene oxide) (PEO) polymer in aqueous solution. The micelle consisted of 60 surfactant molecules, and the polymer chain lengths varied from 20 to 40 monomers. The force field parameters for PEO were adjusted by using 1,2-dimethoxymethane (DME) as a model compound and matching its hydration enthalpy and conformational behavior to experiment. Excellent agreement with previous experimental and simulation work was obtained through these modifications. The simulated scaling behavior of the PEO radius of gyration was also in close agreement with experimental results. The SDS-PEO simulations show that the polymer resides on the micelle surface and at the hydrocarbon-water interface, leading to a selective reduction in the hydrophobic contribution to the solvent-accessible surface area of the micelle. The association is mainly driven by hydrophobic interactions between the polymer and surfactant tails, while the interaction between the polymer and sulfate headgroups on the micelle surface is weak. The 40-monomer chain is mostly wrapped around the micelle, and nearly 90% of the monomers are adsorbed at low PEO concentration. Simulations were also performed with multiple 20-monomer chains, and gradual addition of polymer indicates that about 120 monomers are required to saturate the micelle surface. The stoichiometry of the resulting complex is in close agreement with experimental results, and the commonly accepted "beaded necklace" structure of the SDS-PEO complex is recovered by our simulations.