Gemini surfactants at the air/water interface: a fully atomistic molecular dynamics study

Gemini surfactants typically consist of two single-chain surfactants chemically linked by a spacer molecule. We report herein the results of fully atomistic molecular dynamics (MD) simulations of a series of Gemini surfactants: CsH2s-alpha,omega-bis(C12H25N+(CH3)2Cl-), at the air/water interface with s = 3, 4, 6, 12, 14, and 16, at values of the initial surface area per surfactant AS = 70 A2, 77 A2, 95 A2, 151 A2, 133 A2, and 103 A2, respectively. The AS values employed were obtained from surface tension and neutron reflection experiments at the respective cmc of each surfactant. The Gemini surfactant corresponding to s = 3 was also simulated at AS = 105 A2, which is the experimentally derived value of surface area per surfactant at 1/10th of cmc. Only the surfactants with s = 12 and 14 and the surfactant with s = 3 at AS = 105 A2 gave a stable monolayer at the air/water interface. In other cases, we observe movement of some surfactant molecules from the air/water interface into the aqueous phase, resulting in a stable primary monolayer of surfactants at the air/water interface and a small concentration of surfactant molecules below it. The latter form aggregates, with their hydrophobic chains in the core. The density profiles along the normal to the interface are compared with the ones obtained from neutron reflection experiments. The MD simulations confirm the bending of the spacer toward the hydrophobic chains as the spacer length is increased and the spacer becomes more hydrophobic. The simulations have helped to shed light on the low-resolution picture which emerges from experimental analyses.     

Adsorption of sugar surfactants at the air/water interface

The adsorption isotherms of n-decyl-β-D-glucoside (β-C(10)G(1)) as well as various n-alkyl-β-D-maltosides (β-C(n)G(2)) with n=8, 10, 12 and 14 were determined from surface tension measurements. Based on the analysis of the adsorption isotherms, the total free energy change of adsorption was determined and a novel method was proposed to determine the maximum adsorbed amount of surfactant. It can be concluded that the driving force for adsorption first increases with increasing adsorbed amount of the sugar surfactants and then levels off in a plateau. This peculiar behaviour is interpreted as formation of a thin liquid-like alkane film of overlapping alkyl chains at the air/water interface once a certain adsorbed amount is exceeded. The driving force of adsorption depends on the alkyl chain length only and is not affected by the type of the head group. The hydrophobic contribution to the standard free energy change of adsorption was compared with the values of sodium alkylsulfate and alkyltrimethylammonium bromide surfactants. This comparison reveals that the hydrophobic driving force of adsorption is the largest for the sodium alkylsulfates, whereas it is the same for the sugar surfactants and the alkyltrimethylammonium bromides.     

Computer simulations of catanionic surfactants adsorbed at air/water interfaces

Structural properties pertaining to the solvation of mixtures of dodecytrimethylammonium/dodecylsulfate adsorbed at water/air interfaces were studied using molecular dynamics techniques. Two different surfactant coverages, both in the submonolayer regime, were considered: an infinite-diluted catanionic pair and an equimolar mixture, at a surface concentration of 78.7 A2/headgroup. The most stable solvated structures for the single surfactant pair correspond to contact-head-ion-pairs (CHIP) at a distance close to 5 A. In addition, marginally stable solvent-separated-head-ion-pairs (SSHIP) at distances approximately 7 A were also observed. The mean free energy for the dissociation of CHIP was estimated to be approximately 1 kcal/mol. At finite surfactant concentrations, one observes a considerable degree of clustering between the amphiphiles, due to the strong Coulomb coupling between headgroups. The resulting spatial domains show asymmetric structures with linear dimensions comparable to the simulation box, suggesting the onset of percolative structures. The connectivity pattern of these domains was interpreted in terms of a simplified model consisting of two-dimensional charged Lennard-Jones spheres.     

Adsorption behavior of oxyethylated surfactants at the air/water interface

The low-shear viscosity eta(0) of colloidal suspensions of acrylic latex or silica in aqueous gelatin has been measured at a temperature above the sol-gel transition. Measurements were made on dilution of a concentrated suspension with water or a gelatin solution. Thus, either the gelatin : colloid ratio was maintained or it was varied at constant aqueous gelatin concentration. Systems were studied with four lime-processed gelatins of different molecular weights at two concentrations of added salt. In addition, the latex particle size and the thickness of the adsorbed gelatin layer were measured by photon correlation spectroscopy (PCS) under dilute conditions. The dependence of the low-shear viscosity eta(0) on particle concentration was exponential and did not follow the well-established Krieger-Dougherty model for simple hard-sphere suspensions over the concentration range studied. A simple phenomenological model, eta(0)=eta(o)10(phi(e)/phi(s)), was found to predict the behavior well. Here, eta(o) is the viscosity of a gelatin solution of the corresponding solution concentration, phi(e) is proportional to the volume fraction of the particles, and phi(s) is a scaling factor, which was determined to have a value of 0.85. With this value of phi(s), the dimensions determined from PCS could be used to predict the viscosity values.     

Pulsed-flow microcalorimetric study of the template-monolayer region of nonionic surfactants adsorbed at the graphite/water interface

The formation of half-cylindrical surfactant aggregates at the graphite/aqueous solution interface is templated by an ordered monolayer of molecules disposed parallel to the graphite basal plane. Beyond a critical alkyl chain length, monolayer formation is effectively irreversible. Since enthalpic interactions in this template-monolayer region cannot be resolved with adequate accuracy by the traditional adsorption calorimetric methods, we applied a novel method, pulsed-flow calorimetry, for simultaneous measurement of the material balance and the enthalpy balance in this high-affinity region. For the three nonionic surfactants studied, n-octyl beta-D-glucoside (C(8)G(1)), dimethyl-n-decylamine oxide (C(10)DAO), and n-octyl tetraethylene glycol monoether (C(8)E(4)), the adsorption was found to be strongly exothermic and effectively irreversible at low adsorbate densities, and the differential heat of adsorption markedly decreased with increasing surface coverage in this region. This deviation from the ideal adsorption behavior was attributed to intermolecular interactions within the adsorption layer rather than to surface heterogeneity of the graphite basal planes. A thermodynamic consistency test clearly demonstrated that pulsed-flow calorimetry is a unique experimental method for the study of nonreversible adsorption phenomena at solid/solution interfaces, representing an excellent tool to complement traditional methods, e.g., frontal-flow and titration adsorption calorimetry. Studies by the frontal-flow method revealed that aggregation on top of the surfactant monolayer was endothermic and reversible.     

Structure of adsorbed layers of nitrophenoxy-tailed quaternary ammonium surfactants at the air/water interface studied by neutron reflection

The adsorbed layers of N,N,N-trimethyl-10-(4-nitrophenoxy)decylammonium bromide (PhiC(10)TAB) and N,N,N('),N(')-tetramethyl-N,N(')-bis[10-(4-nitrophenoxy)decyl]-1,6-hexanediammonium dibromide [(PhiC(10))(2)C(6)] at the air/water interface have been studied by neutron reflection. The coverage of the surfactants was obtained over the concentration range from critical micelle concentration (CMC) to CMC/100. The area per PhiC(10)TAB molecule changes from 50+/-3 to 390+/-60 A(2) over this concentration range and the area per (PhiC(10))(2)C(6) molecule changes from 139+/-3 to 288+/-10 A(2). The overall thicknesses (single uniform layer) of the surfactant layers at CMC are about 19 and 16 A for PhiC(10)TAB and (PhiC(10))(2)C(6) respectively. The distributions of the C(10) chains show that the chains of both surfactants are tilted away from surface normal, with the tilt increasing in the outer part of the layer. The distribution of C(10) chains in (PhiC(10))(2)C(6) is narrower than that in PhiC(10)TAB, indicating that the alkyl chains of (PhiC(10))(2)C(6) are more tilted. For both surfactants, the broad nitrophenoxy distribution may indicate significant positional disorder of the nitrophenoxy groups along the surface normal direction and their intermixing with alkyl chains in the adsorbed layer.     

The adsorbed conformation of globular proteins at the air/water interface

External reflection FTIR spectroscopy and surface pressure measurements were used to compare conformational changes in the adsorbed structures of three globular proteins at the air/water interface. Of the three proteins studied, lysozyme, bovine serum albumin and beta-lactoglobulin, lysozyme was unique in its behaviour. Lysozyme adsorption was slow, taking approximately 2.5 h to reach a surface pressure plateau (from a 0.07 mM solution), and led to significant structural change. The FTIR spectra revealed that lysozyme formed a highly networked adsorbed layer of unfolded protein with high antiparallel beta-sheet content and that these changes occurred rapidly (within 10 min). This non-native secondary structure is analogous to that of a 3D heat-set protein gel, suggesting that the adsorbed protein formed a highly networked interfacial layer. Albumin and beta-lactoglobulin adsorbed rapidly (reaching a plateau within 10 min) and with little change to their native secondary structure.     

Anomalous behavior of amine oxide surfactants at the air/water interface

A commonly stated requirement for the preparation of stable Langmuir monolayers of amphiphilic molecules at an air/water interface is that the surfactant must be insoluble in the subphase solution; however, a few prior studies have reported that some soluble surfactants can, under certain conditions, be compressed. The anomalous compression of soluble amphiphiles is extremely interesting and important, as it presents the possibility of greatly increasing the number of candidate compounds suitable for Langmuir monolayer studies and Langmuir-Blodgett deposition. The aim of this work was to obtain a better understanding of the factors that determine whether monolayers of a given water-soluble surfactant can be compressed. A series of amine oxide surfactants, including a novel gemini surfactant, were studied to explore the relationship between molecular structure and behavior at the air/water interface. Amine oxides are an especially interesting class of surfactants because their self-assembly in solution and at interfaces is pH-sensitive. Surface pressure-area isotherms show that the solubility of a surfactant in the subphase solution is not, in and of itself, a useful parameter in predicting whether the monolayer is compressible. Molecular modeling calculations suggest that the tendency of molecules to self-assemble plays a much more important role than solubility in this regard. The effect of pH was also investigated. We present a hypothesis that formation of dimers or small clusters of molecules at the interface inhibits the dissolution of these species into the subphase, and as a consequence the monolayer can be compressed.     

Micellization and adsorption of zwitterionic surfactants at the air/water interface

Zwitterionic surfactants are formally neutral but with headgroups containing both a positive charge center and a negative charge center separated from each other by a spacer group, with a long hydrophobic tail attached to one of the charge centers, usually but not always the positive charge center. The micellization and adsorption properties of zwitterionic surfactants depend on specifics of the surfactant structure such as the length m of the hydrophobic alkyl chain, the length n of the intercharge spacer and the nature of the headgroup charge centers. Micellization is favored by an increase in the hydrophobic tail length m, but goes through a maximum for interchange spacings of n = 3–4 methylene groups. There are additional effects from the presence of additional hydrophilic substituent groups in the spacer. Specific binding of anions and the cation valence of added electrolyte are factors that also modulate the micellization and adsorption properties of zwitterionic surfactants in the presence of added electrolyte. Anions in particular bind preferentially to zwitterionic micelles independent of the relative order of the charge centers in the headgroup. The anion binding affinities follow a Hofmeister series and impart a net negative charge to the micelles. Micellization is temperature-dependent and exhibits enthalpy-entropy compensation, with entropy dominant at lower temperatures and enthalpy more important at higher temperatures. The judicious manipulation of these factors permits control of the interfacial properties of zwitterionic surfactants, responsible for a wide range of applications in chromatography, electrophoresis, cloud point extraction, solubilization, stabilization of biomolecules and nanomaterials and catalysis.     

Monolayer characteristics of mixed octadecylamine and stearic acid at the air/water interface

Mixed monolayers of stearic acid (SA) and octadecylamine (ODA) at the air/water interface were investigated in this article. The miscibility of the two compounds was evaluated by the measurement of surface pressure-area per molecule (pi-A) isothems and the direct observation of Brewster angle microscopy (BAM) on the water surface. The two compounds were spread individually on the subphase (method 1) or premixed first in the spreading solvent and then cospread (method 2). The effect of spreading method on the miscibility of the two compounds was also studied. The results show that the mixed monolayers prepared by method 1 cannot get a well-mixed state. The isotherms of mixed monolayers preserve both characteristics of SA and ODA and exhibit two collapse points. The calculated excess surface area is very small. Besides, distinguished domains corresponding to those of pure SA and ODA can be inspected from the BAM images. Such results indicate that SA and ODA cannot get a well-mixed phase via 2-dimensional mixing. On the contrary, in the mixed monolayer prepared by cospreading, the two compounds exhibit high miscibility. In the pi-A isotherms, the individual characteristics of SA and ODA disappear. The calculated excess area exhibits a highly positive deviation which indicates the existence of special interaction between the two compounds. The low compressibility of isotherm implies the highly rigid characteristic of the mixed monolayer. which was also sustained by the striplike collapse morphology observed from the BAM. The rigid characteristic of SA/ODA mixed monolayer was attributed to the formation of "catanionic surfactant" by electrostatic adsorption of headgroups of SA and ODA or to the formation of salt by acid-base reaction.     

Monolayer behaviors of poly(hexamethylene adipamide) at the air/water interface

Monolayer films on the neutral water substrate were obtained by spreadingN-trifluoroacetic anhydride (NTF)-modified nylon 66 or nylon 612 in chloroform solutions. Alternatively, monolayer films were obtained by spreading from nylon 66 solutions in the 31 mixture of benzene (B) and phenol (P). The temperatures studied are 10.3°, 14.7°C, and 19.4°C. The isothermss of surface pressure (), and surface moment () against surface area per residue (A) were determined. The -A isotherms of the NTF-modified nylon 66/chloroform and the nylon 66/BP were found to be an expanded type, while that of NTF-modified nylon 612/chloroform was of a condensed type. The NTF-modified nylon 66/chloroform solutions could yield well-spread films even higher concentrations than nylon 66/BP solutions. In the -A isotherms at 10.3° and 14.7°C, the surface moments are constant at 143 mD/residue for NTF-modified nylon 66/chloroform, and 340 mD/residue for nylon 66/BP until the surface area reaches where the -A isotherms show a transition point. After the transition point, the surface moments for both systems drop steadily. However, the surface moment at 19.4°C shows a maximum at the transition point. Possible configuration of the nylon 66 residue in monolayer is discussed.     

Molecular dynamics simulation of four typical surfactants at oil/water interface

In the present study, we have performed molecular dynamics simulations to describe the microscopic behaviors of the anionic, nonionic, zwitterion, and gemini surfactants at oil/water interface. The abilities of reducing the interfacial tension and forming the stable interfacial film of the four surfactants have been investigated through evaluating interfacial thickness, interface formation energy and radial distribution function. The results show that the four kinds of surfactants can form in stable oil/water interface of monolayer, and the gemini surfactant can form the more stable monolayer. The results of the above three parameters demonstrate that the gemini surfactant has the best simulation effect in the four surfactants. From the calculated interfacial tension values, the gemini surfactant possess the more powerful ability of reducing the interfacial tension than others, so it is more suitable to be used as the surfactant for flooding. In addition, the effects of different electric field intensities on surfactants were calculated, through the radial distribution function of the hydrophilic group in the surfactant and the oxygen atom in the water. We have found that the adding of the periodic electric field can significantly affect the diffusion behavior of the molecules, and nonionic surfactant has stronger demulsification capability than others.     

Mixed monolayers of Gemini surfactants and stearic acid at the air/water interface

The properties of mixed monolayers composed of the cationic Gemini surfactant ([C(18)H(37)(CH(3))(2)N(+)(CH(2))(3)N(+)(CH(3))(2)C(18)H(37)],2Br(+), abbreviated as 18-3-18,2Br(-1)) and stearic acid (SA) at the air/water interface were investigated by using a Langmuir film balance. The excess areas at the different mixed monolayer compositions were obtained and used to evaluate the miscibility and nonideality of mixing. Due to the electrostatic attractive interactions between 18-3-18,2Br(-1) and SA, the excess areas indicated negative deviations from ideal mixing. Moreover, 18-3-18,2Br(-1) and SA were miscible at the air/water interface, as was confirmed by atomic force microscopy (AFM) images of the LB films transferred onto mica substrates. The attenuated total reflectance (ATR) infrared spectra showed that SA in the mixed monolayers was ionized completely at a composition X(SA)=0.67 and formed a "cationic-anionic surfactant," i.e., the carboxylate, with 18-3-18,2Br(-1) owing to the electrostatic interaction between the head groups.     

A computer simulation study of water drying at the interface of protein chains

This study investigated the water drying (cavitation) in the interfacial region of two chains of a dimeric protein by nanosecond molecular dynamics simulations using explicit water representation. Separation-induced cavity of water was directly observed in the region. We evaluated the separation length scale of two chains on which the drying transition occurs, and the average number of water molecules that are expelled from the interfacial region during the transition. The obtained values can be rationalized by Kelvin equation for finite lateral size of confinement [K. Lum and A. Luzar, Phys. Rev. E 56, R6283 (1997)]. Also, we found that the drying transition is accompanied by an exponential reduction in the average hydrogen-bond number per interfacial water molecule. The results of this study may deepen the understanding of how hydrophobic interaction drives the assembly of protein chains.     

The structure of adsorbed films of some aliphatic compounds at the water/air interface

Electric surface potential (V) and surface tension measurements of aqueous solutions of some aliphatic compounds were used to determine the surface activity, orientation of molecules at the water/air interface, effective dipole moments (connected with water molecules, hydrophilic and hydrophobic groups), and local dielectric permittivities of the surface layer.     

The properties of a binary mixture of nonionic surfactants in water at the water/air interface

The behavior of mixed nonionic/nonionic surfactant solutions, that is, p-(1,1,3,3-tetramethylbutyl)phenoxy poly(ethylene glycol)s Triton X-100 (TX100) and Triton X-165 (TX165) have been studied by surface tension and density measurements. The obtained results of the surface tension measurements were compared with those calculated from the relations derived by Joos, Miller, and co-workers. From the comparison, it appeared that by using these two approaches the adsorption behavior of TX100 and TX165 mixtures at different mole fractions can be predicted. The negative deviation from the linear relationship between the surface tension and composition of TX100 and TX165 mixtures in the concentration range corresponding to that of the saturated monolayer at the interface, the values of the parameters of molecular interaction, the activity coefficients, as well as the excess Gibbs energy of mixed monolayer formation calculated on the basis of Rosen and Motomura approaches proved that there is synergism in the reduction of the surface tension of aqueous solutions of TX100 and TX165 mixture when saturation of the monolayer is achieved. The negative parameters of intermolecular interaction in the mixed micelle and calculations based on MT theory of Blankschtein indicate that there is also synergism in the micelle formation for TX100 and TX165 mixture. It was also found that the values of the standard Gibbs energy of adsorption and micellization for the mixture of these two surfactants, which confirm the synergetic effect, can be predicted on the basis of the proposed equations, which include the values of the mole fraction of surfactant and excess Gibbs energy TX100 and TX165 in the monolayer and micelle.     

Monolayer characteristics of a long-chain N,O-diacyl substituted ethanolamine at the air/water interface

The N- and/or O-acylation of amphiphilic ethanolamine attracts particular attention because of its interesting biological, pharmaceutical, and medicinal properties. Tetradecanoic acid-2-[(1-oxotetradecyl)amino]ethyl ester (TAOAE) as the selected N,O-diacyl derivative of ethanolamine has been synthesized in order to obtain first information about its main interfacial characteristics, such as the surface pressure-area (π-A) isotherms, the morphology of the condensed phase domains, the lattice structure of the condensed phase, and information about the existence of interfacial hydrogen bonds (-NH···O═C-). The π-A isotherms of TAOAE, similar to those of the most usual monolayers of amphiphiles, show a sharp break point (A(c)) indicating the first-order phase transition from the fluid (liquid-expanded (LE), gaseous (G)) to the condensed (liquid-condensed (LC)) phase. On the mesoscopic scale, the dendritic domains homogeneously reflecting suggest an orientation of the alkyl chains perpendicular to the aqueous surface. The grazing incidence X-ray diffraction (GIXD) studies reveal hexagonal packing of the TAOAE molecules oriented perpendicular to the surface in an LS phase. The existence of a hydrogen-bonding network in the monolayer is supported by infrared reflection absorption spectroscopy (IRRAS) experiments.     

Ions at the air/water interface

In a recent review of this topic [B.C. Garett, Science 303 (2004) 1146] the emphasis was on some recent experiments, in which it was found that some anions accumulate at the air/water interface and not in the bulk, as usually happens to the cations, and on some simulations which explained those positive surface adsorption excesses. Because a large number of these experiments could be explained on the basis of some simple physical models proposed by the authors for the interaction between the ions and the air/water interface [M. Manciu, E. Ruckenstein, Adv. Colloid Interface Sci. 105 (2003) 63; Adv. Colloid Interface Sci. 112 (2004) 109; Langmuir 21 (2005) 11312], those models are reviewed in the present note, the goal being to draw attention to them.