Number Of Surfactant Molecules Research Articles

Surfactant-Like Behavior for the Adsorption of Mixtures of a Polycation and Two Different Zwitterionic Surfactants at the Water/Vapor Interface.

The bulk and interfacial properties of solutions formed by a polycation (i.e., poly(diallyl-dimethylammonium chloride), PDADMAC) and two different zwitterionic surfactants (i.e., coco-betaine (CB) and cocoamidopropyl-betaine (CAPB)) have been studied. The bulk aggregation of the polyelectrolyte and the two surfactants was analyzed by turbidity and electrophoretic mobility measurements, and the adsorption of the solutions at the fluid interface was studied by surface tension and interfacial dilational rheology measurements. Evidence of polymer–surfactant complex formation in bulk was only found when the number of surfactant molecules was closer to the number of charged monomers in solutions, which suggests that the electrostatic repulsion associated with the presence of a positively charged group in the surfactant hinders the association between PDADMAC and the zwitterionic surfactant for concentrations in which there are no micelles in solution. This lack of interaction in bulk is reflected in the absence of an influence of the polyelectrolyte in the interfacial properties of the mixtures, with the behavior being controlled by the presence of surfactant. This work has evidenced the significant importance of the different interactions involved in the system for controlling the interaction and complexation mechanisms of in polyelectrolyte–surfactant mixtures.

Myoglobin and α-Lactalbumin Form Smaller Complexes with the Biosurfactant Rhamnolipid Than with SDS

Biosurfactants (BSs) attract increasing attention as sustainable alternatives to petroleum-derived surfactants. This necessitates structural insight into how BSs interact with proteins encountered by current chemical surfactants. Thus, small-angle x-ray scattering (SAXS) has been used for studying the structures of complexes made of the proteins α-Lactalbumin (αLA) and myoglobin (Mb) with the biosurfactant rhamnolipid (RL). For comparison, complexes between αLA and the chemical surfactant sodium dodecyl sulfate (SDS) were also investigated. The SAXS data for pure RL micelles can be described by prolate core-shell structures with a core radius of 7.7 Å and a shell thickness of 12 Å, giving an aggregation number of 11. The small core radius is attributed to RL’s complex hydrophobic tail. Data for the αLA-RL complex agree with a 12-molecule micelle with a single protein molecule in the shell. For Mb-RL, the analysis gives complexes of two connected micelles, each containing 10 RL and one protein in the shells. αLA-RL and Mb-RL form surfactant-saturated complexes above 5.6 and 4.7 mM RL, respectively, leaving the remaining RL in free micelles. The SAXS data for SDS agree with oblate-shaped micelles with a core of 20 Å, core eccentricity 0.7, and shell thickness of 5.45 Å, with an aggregation number of 74. The αLA-SDS complexes contain a prolate micelle with a core radius of 11−14 Å and a shell of 8−12 Å with up to 3 αLA per particle and up to 43 SDS per αLA, both considerably larger than for RL. Unlike the RL-protein complexes, the number of surfactant molecules in αLA-SDS complexes increases with surfactant concentration, and saturate at higher surfactant concentrations than αLA-RL complexes. The results highlight how RL and SDS follow similar overall rules of self-assembly and interactions with proteins, but that differences in the strength of protein-surfactant interactions affect the formed structures.

Origin of the Surfactant‐Dependent Redox Chemistry of Single‐Wall Carbon Nanotubes

AbstractThe redox chemistry of single‐walled carbon nanotubes (SWCNTs) was examined by using optical absorption spectroscopy measurements in the presence of sodium dodecyl sulfate (SDS) and its analog surfactants, that is, sodium 1‐undecanesulfonate (SUS) and sodium dodecylbenzenesulfonate (SDBS). Redox reactions of the SWCNTs were observed in the SDS and SUS solutions, whereas such reactions were suppressed in the SDBS solution. Molecular dynamics simulations were performed to investigate the assembly of the surfactants around an SWCNT. SDS and SUS were shown to primarily form single layers on the SWCNT and certain solvent‐exposed areas, whereas SDBS predictably densely coated the SWCNTs. The results suggest that the SDBS layer on the SWCNT surfaces prevents charge‐transfer reactions; in contrast, the sparse layers of SDS and SUS allow the redox chemistry through the charge‐transfer reactions. Interestingly, a positively charged SWCNT as a model for the oxidized SWCNT showed an increase in the number of surfactant molecules around the SWCNT in the SDS and SUS solutions owing to electrostatic interaction, which is related to the chirality‐dependence of the SWCNT colloidal stability.

Polymer brushes modified by photosensitive azobenzene containing polyamines

Abstract This paper describes a strategy for preparing photosensitive polymeric grafts on flat solid surfaces by loading diblock-copolymer or homopolymer brushes with cationic azobenzene-containing surfactants. In contrast to previous work, we utilize photosensitive surfactants that bear positively-charged polyamine head groups whose charge varies between 1+ and 3+. Poly(methylmethacrylate-b-methacrylic acid) (PMMA-b-PMAA) brushes were prepared by employing atom transfer radical polymerization, where the bottom poly(methyl methacrylate) block was grown first followed by the synthesis of t-butyl methacrylate block that after de-protection yielded poly(methacrylic acid). We used PMMA-b-PMAA brushes with constant grafting density and length of the PMMA block, and three different lengths of the PMAA block. The azobenzene-based surfactants attached only to the PMAA block. The degree of binding (i.e., the number of surfactant molecules per binding site on the brush backbone) of the surfactants to the brush depends strongly on the valence of the surfactant head-group; within the brushes the concentration of the surfactant carrying unit charge is larger than that of multivalent surfactants. We detect pronounced response of the brush topography on irradiation with UV interference pattern even at very low degree of binding (as small as 0.08) of multi-valence surfactant. Areas on the sample that receive the highest UV dose exhibit chain scission. By removing the ruptured chains from the substrate via good solvent, one uncovers a surface topographical relief grating, whose spatial arrangement follows the intensity distribution of the UV light on the sample during irradiation. Due to strong coupling of the multi-valence surfactants to the polymer brush, it was possible in some cases to completely remove the polyelectrolyte block from the PMMA layer. The application of multi-valence azobenzene surfactants for triggering brush photosensitive has important advantage over usage of surfactant with unit charge because relative to single-valence surfactants much lower concentrations of the multivalent surfactant are needed to achieve comparable response upon UV irradiation.

Dispersing Carbon Nanotubes with Ionic Surfactants under Controlled Conditions: Comparisons and Insight

A fundamental understanding of the mechanisms involved in the surfactant-assisted exfoliation and dispersion of carbon nanotubes (CNTs) in water calls for well-controlled experimental methodologies and reliable comparative metrics. We have assessed the ability of several ionic surfactants to disperse single and multiwalled carbon nanotubes, resorting to a stringently controlled sonication-centrifugation method for the preparation of the dispersions. The CNT concentration was accurately measured for a wide range of surfactant concentration, using combined thermogravimetric analysis and UV-vis spectroscopy. The obtained dispersibility curves yield several quantitative parameters, which in turn allow for the effects of nanotube morphology and surfactant properties (aromatic rings, chain length, headgroup charge, and cmc) to be assessed and rationalized, both in terms of dispersed nanotube mass and surface area. The data also indicate that the CNT-surfactant association follows patterns that are markedly different from other equilibrium processes governed by hydrophobicity (such as micellization); in particular, the surfactant concentration needed for maximum dispersibility, c(s,max), and the number of surfactant molecules per unit CNT area at c(s,max) are shown to depend linearly on chain length. The results further suggest that the presence of micelles in the exfoliation process is not a key factor either for starting CNT dispersibility or attaining its saturation value.

Polar Solvents Trigger Formation of Reverse Micelles.

We use molecular dynamics simulations and molecular thermodynamics to investigate the formation of reverse micelles in a system of surfactants and nonpolar solvents. Since the early observation of reverse micelles, the question has been whether the existence of polar solvent molecules such as water is the driving force for the formation of reverse micelles in nonpolar solvents. In this work, we use a simple coarse-grained model of surfactants and solvents to show that a small number of polar solvent molecules triggers the formation of large permanent aggregates. In the absence of polar molecules, both the thermodynamic model and molecular simulations show that small aggregates are more populated in the solution and larger ones are less frequent as the system evolves over time. The size and shape of reverse micelles depend on the size of the polar core: the shape is spherical for a large core and ellipsoidal for a smaller one. Using the coarse-grained model, we also investigate the effect of temperature and surfactant tail length. Our results reveal that the number of surfactant molecules in the micelle decreases as the temperature increases, but the average diameter does not change because the size of the polar core remains invariant. A reverse micelle with small polar core attracts fewer surfactants when the tail is long. The uptake of solvent particles by a micelle of longer surfactant tail is less than shorter ones when the polar solvent particles are initially distributed randomly.

Monolayer adsorption of dodecylamine Surfactants at the mica/water interface

Monolayer adsorption of aqueous dodecylamine surfactants on the muscovite (001) surface was investigated by means of molecular dynamics simulations. Three systems including different number of surfactant molecules have been designed to examine the effect of surface coverage on the microscopic structure of the mica/water interface. In all surface coverage, hemimicelle structures formed at the muscovite/water interface, with their head groups directed to the surface and the hydrocarbon tails oriented toward the solution. The modified mica surface obtained a hydrophobic character, the hydrophobicity became stronger with the increasing surface coverage. Results from density field showed adsorption sites for water molecules in the first two layers near the muscovite surface. It is found that fraction of gauche conformation decreased with the increasing surface coverage, which is in agreement with IR measurements. Increased chain–chain interactions between alkyl chains and a closer packing of the alkyl chains may account for the relatively lower gauche probability for high surface coverage. Hydrogen bonds between bridging oxygen atoms and surfactant head groups were also investigated, and we observed that they were influenced by interfacial water molecules. Furthermore, surfactants formed hemimicelle structure on the mica surface with a thickness roughly less than that of the molecule chain length, which is in good agreement with experimental AFM data.

Structure and Rheology of Charge-Free Reverse Micelles in Aromatic Liquid Phenyloctane

Reverse micelles formulation requires an inclusion of water or other polar molecules in the binary mixture of ionic surfactant and oil and generally exhibit spheroid geometry with a small aggregation number. Here, we report structure and rheology of charge-free (nonionic) reverse micelles in surfactant/oil systems. We have systematically investigated intrinsic parameters for the shape, size, and internal cross section structure control of such micelles using small-angle X-ray scattering (SAXS) and the rheometry. We found that diglycerol monomyristate (C14G2) when added into an aromatic organic liquid phenyloctane, spontaneously self-assembles into spheroid micelles with maximum diameter ca. 6.7 nm. Decrease in surfactant chain length favors globular-to-rod type transition and micellar aggregation number (N(agg)) increases significantly. On the other hand, increase in surfactant weight fraction induces one-dimensional (1-D) micellar growth; N(agg) increases in parallel to the surfactant concentration. Reverse micelles shrink with the rise of temperature, which is close to the rod-to-sphere type transition. However, water causes a significant micellar growth; N(agg) increases drastically, which shows that water not only increase reverse micellar size but also increases the number of surfactant molecules per micelle. All these microstructure transitions could be understood in terms of the modification of the critical packing parameter (cpp). The SAXS results are very well supported by the geometrical model fittings and rheometry.

Surface Phase Transition of C12E1 at the Air/Water Interface: A Study by Dynamic Surface Tension, External RA FT-IR, and 2D IR Correlation Methods

The surface conformational states of the Gibbs monolayer of ethylene glycol mono-n-dodecyl ether (C(12)E(1)) at the air/water interface was studied using dynamic surface tension, external reflection-absorption FT-IR spectroscopy (ERA FT-IR), and two-dimensional infrared (2D IR) correlation methods at constant temperature. The dynamic surface tensions were measured at different bulk concentrations of C(12)E(1), and it was observed that a constant surface tension region appears at approximately 38.5 mN m(-1) in a dynamic surface tension profile at concentrations higher than 11 micromol kg(-1). This constant surface tension region corresponds to the surface phase transition from liquid expanded (LE) to liquid condensed (LC). Two sets of ERA FT-IR spectra were collected, one at different bulk concentrations but after equilibrium time (equilibrium measurements) and another at constant bulk concentration (m = 16 micromol kg(-1)) but at different times (dynamic measurements). The first set of these measurements show that the peak area increases in the range of 11 < m < or = 16 micromol kg(-1), which means the increase in the number of surfactant molecules at the air/water interface. Also, the wavenumber of antisymmetric CH(2) stretching decreases gradually from approximately 2923 cm(-1) (for 10 and 11 micromol kg(-1)) to approximately 2918 cm(-1) (for m > or = 16 micromol kg(-1)) with increasing concentration. The wavenumbers of 2923 and 2918 cm(-1) were assigned to LE and LC phases, respectively, and the decrease of wavenumber in the concentration range of 11 < m < or = 16 micromol kg(-1) were correlated to the surface phase transition (LE --> LC), or in other words, in the mentioned concentration range, two phases coexist. The dynamic ERA FT-IR measurements at 16 micromol kg(-1) also confirm the surface phase transition from LE to LC. The 2D IR correlation method was applied to the both equilibrium and dynamic IR spectra of the C(12)E(1) monolayer. The synchronous correlation maps show two strong autopeaks at approximately 2922 and approximately 2851 cm(-1) and also show a strong correlation (cross-peaks) between antisymmetric CH(2) stretching (nu(a)) and symmetric CH(2) stretching (nu(s)). The asynchronous correlation maps show that both observed bands of nu(a) and nu(s) in one-dimensional IR split into two components with the characteristic of overlapped bands, which reveals the coexistence of two phases (LE and LC) at the interface at 11 < m < or = 16 micromol kg(-1). The synchronous and asynchronous maps that were obtained from dynamic IR spectra closely resembled the equilibrium map.

Concentration effects on the grafting of magnetic nanoparticles by Monte Carlo simulations

In this work, we report a systematic comparison between three-dimensional Monte Carlo simulations and magnetic resonance measurements of polydisperse Fe3O4 based nanoparticles. Magnetization curves clearly show the validity of the simple model employed by us. Our theoretical results are then compared to real ferrofluids samples that have been experimentally characterized by transmission electron microscopy and magnetic resonance measurements. Our data indicates that the number of surfactant molecules and its physical distribution (grafting) decisively affects the surface-surface particle distance within an agglomerate. Two different grafting regimes are clearly identified from our theoretical data.

A theoretical study of model lipid monolayers

Studies of pure phospholipid monolayers or various well defined lipid mixtures have greatly contributed to the current knowledge of the relationship between monolayer composition and its properties and to understand how their physicochemical properties, e.g. refraction, polarization, are controlled by structural variations at the molecular level. Therefore, an attempt was made to investigate model lipid molecules adsorbed on the air/water interface. Semi-empirical (AM1) quantum chemical calculations were performed for several clusters and were compared with experimental data. The optimized acidic molecules show a marked tendency to group forming domains. An increase in the number of surfactant molecules in the cluster leads to a decrease in the effective headgroup area. On the other hand, the area grows with the increase of the degree of ionization. Both relations are in accordance with the experimental trends and the electrostatic theory. The employed theoretical approach proves to be applicable to the study of monolayers, thus providing a reliable description at molecular level of the traditionally phenomenological investigations in the field.

Tuning Microstructure of Cationic Micelles on Multiple Length Scales: The Role of Electrostatics and Specific Ion Binding

Small-angle neutron scattering has been used to obtain detailed information on local structure, including micellar cross-sectional radii, areas per headgroup, and number of surfactant molecules per unit length for mixed micelles of cetyltrimethylammonium 2,6-dichlorobenzoate and cetyltrimethylammonium chloride in water as a function of surfactant and salt concentrations and the relative amounts of the two counterions (the counterion inventory) at the micellar surface. The differing effects of penetrating (2,6-dichlorobenzoate) and nonpenetrating (chloride) counterions on micellar surface charge density and on the relative energies of surfactant monomers in cylindrical vs end-cap micellar environments are elucidated.

Transbilayer Movement of Sodium Dodecyl Sulfate in Large Unilamellar Phospholid Vesicles

The ability of sodium dodecyl sulfate (SDS) to traverse phosphatidylcholine bilayers was examined by fluorescence spectroscopy. To this end, we measured the interaction of the anionic fluorescent probe 2-(p-toluidinyl)naphthalene-6-sodium sulfonate present in the outer vesicle leaflet with the SDS monomers incorporated in this structure. The surfactant transbilayer movement, or “flip-flop”, was measured from the fluorescence intensity changes due to the interaction of the liposome/probe with SDS versus incubation time. This effect was quantified as the resulting variations in the surface potential (ψo) of liposomes. When the SDS concentration increased, ψo rose due to the electronegative contribution of the sulfate group incorporated in the bilayer surface. Increased periods of incubation resulted in a decreased ψo and, consequently, in a fall in the number of surfactant molecules in the outer vesicle leaflet. This variation was associated to the SDS “flip-flop”. The maximum “flip-flop” (of about 50%) was always detected at a very low surfactant concentration, and the effective molar ratio of surfactant to PC for this maximum was always a constant value (0.02 mol/mol). Although the incorporation of SDS monomers to the bilayer surface was a very rapid process, the “flip-flop” rate of these monomers across lipid bilayer was very slow and time dependent with an enhanced kinetics between 10 and 90 min after mixing.

Interaction of Rhizomucor miehei Lipase with an Amphoteric Surfactant at Different pH Values

The interactions of surfactants with the enzyme Rhizomucor miehei lipase were studied using surface tension measurements and ellipsometry. From earlier surface tension studies it is known that the cationic surfactant didodecyldimethylammonium bromide forms an aggregate with R. miehei lipase, while anionic and nonionic surfactants do not form such a complex. In this work the influence of surfactant charge on complex formation with the lipase is systematically investigated, using the amphoteric surfactant sodium N-(2-hydroxydodecyl)sarcosinate. The charge of the amphoteric surfactant was varied from anionic via zwitterionic to cationic by adjusting the pH. By this procedure the influence of the surfactant charge on the interaction with the enzyme could be studied while the molecular structure of the surfactant hydrophobe was kept unchanged. It was confirmed that only when the head group is positively charged does the enzyme bind with the surfactant. However, the number of surfactant molecules that bind to e...

Microemulsions formed by water, aliphatic hydrocarbons, and pentaethylene glycol dodecyl ether: The temperature dependence of aggregate size

The aggregation of water and pentaethylene glycol dodecyl ether (C12E5) in aliphatic hydrocarbons at different temperatures in the concentration range 0–10% surfactant and water has been studied by dynamic light scattering. Maximum solubilization of water in the different solvents (homologous aliphatic hydrocarbons) occurs at temperatures for which the molar free volumes of the solvents are equal. Micelle formation occurs only in the presence of water and is cooperative; i.e., rather well-defined amounts of water as well as C12E5 must be added before micelles can be detected. At a given temperature, the size of the micellar aggregates is directly proportional to the water concentration, and the total number of aggregates is independent of the concentration. At constant concentration of water, the size of the aggregates decreases with increasing temperature. The reason for the cooperativity of surfactant/water association probably is that a minimum number of surfactant molecules are required to shield water that is bound between the polyethylene oxide chains from extensive contact with the hydrocarbon.

Quantitative treatment of the effect of counterions on ctab-catalyzed reactions of stabilized carbonium ions with cyanide ion

The reactions of stabilized carbonium ions of setoglaucin, methyl violet, and ethyl violet with cyanide ions are largely catalyzed by the cationic micelles of cetyltrimethylammonium bromide (CTAB) in aqueous media. Added counterions (anions in this case) have strong inhibitory effects on the CTAB-catalyzed reactions in the following order: N > NO > Br− > Cl− > F− > no salt. The inhibitory effects of the counterions have been attributed to the exchange between added anions and reagent (CN−) in the micellar media. The data have been analyzed by the model schemes, and mathematical formulations were developed. Various parameters associated with the exchange process, such as equilibrium exchange constant, number of surfactant molecules per substrate molecule, number of added anions, and a factor related to the binding of additives to the catalytic micellar aggregates, have been evaluated.