Amphiphilic Layer Research Articles

Branched Fluorinated Nonionic Y-Shaped Surfactants. Effect of Molecular Geometry on Liquid Crystalline Phase Behavior

The synthesis of branched partially fluorinated nonionic Y-shaped surfactants, with the general formula CFnC2N(EmM)2, where CFn denotes fluorinated alkyl chain F(CF2)n with n = 4−10, C2 is C2H2, and EmM is oligooxyethylene monomethyl ether with m = 2, 3, and their hydrocarbon analogs Cn+2N(EmM)2 (Cn equals hydrocarbon chain) is described. These surfactants are synthesized to analyze the influence of the fluorinated alkyl chain on the packing effects to amphiphilic layers in a binary surfactant/water system. The lyotropic phase behavior of these surfactants is investigated by polarizing microscopy. The liquid crystalline (LC) phase polymorphism of the fluorinated CFnC2N(E3M)2 surfactants strongly changes with n from hexagonal H1 phase (n = 6) to lamellar Lα phase (n = 10). In contrast to other Y-shaped hydrocarbon oligooxyethylene surfactants CnG(EmM)2 (where G denotes a glycerol unit) which form only cubic I1 and H1 phases for n = 12−16 and m = 3−5, the phase behavior of the CFnC2N(EmM)2 surfactant/water ...

Electric-Field-Induced Transitions of Amphiphilic Layers on Mercury Electrodes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTElectric-Field-Induced Transitions of Amphiphilic Layers on Mercury ElectrodesXiaoping Gao, Henry S. White, Shaowei Chen, and Hector D. AbrunaCite this: Langmuir 1995, 11, 11, 4554–4563Publication Date (Print):November 1, 1995Publication History Published online1 May 2002Published inissue 1 November 1995https://doi.org/10.1021/la00011a060RIGHTS & PERMISSIONSArticle Views90Altmetric-Citations13LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

Spontaneous curvature and thermodynamic stability of mixed amphiphilic layers

The elastic behavior of mixed bilayers composed of two amphiphilic components with different chain length (and identical head groups) is studied using two molecular level models. In both, the bilayer free energy is expressed as a sum of chain, head group and interfacial contributions as well as a mixing entropy term. The head group and interfacial terms are modeled using simple phenomenological but general expressions. The models differ in their treatment of the chain conformational free energy. In one it is calculated using a detailed mean-field molecular theory. The other is based on a simple ‘‘compression’’ model. Both models lead to similar conclusions. Expressing the bilayer free energy as a sum of its two monolayer contributions, a thermodynamic stability analysis is performed to examine the possibility of spontaneous vesicle formation. To this end, we expand the bilayer free energy as a power series (up to second order) in terms of the monolayer curvatures, their amphiphilic compositions and the average cross sectional areas per molecule; all variables are coupled, with the molecular composition and areas treated as degrees of freedom which are allowed to relax during bending. Using reasonable molecular interaction parameters we find that a second order transition from a planar to a curved (vesicle) geometry in a randomly mixed bilayer is unlikely. Most of our analysis is devoted to calculating the spontaneous curvature and the bending rigidity of the bilayer as a function of its amphiphile chain composition. We find that adding short chain amphiphiles to a layer of long chain molecules reduces considerably its bending rigidity, as already known from calculations involving only the chain contributions. However, we find that inclusion of head group and interfacial interactions moderates the effect of the added short chains. We also find that the bending rigidity of pure monolayers is approximately linear in chain length, as compared to the nearly cubic dependence implied by the chain free energy alone (at constant head group area). Our main result involves the calculation of the spontaneous curvature as a function of composition. We find, for different chain mixtures, that upon adding short chains to long chain monolayers, the spontaneous curvature first increases nearly linearly with composition and then (beyond mole fraction of about 0.5) begins to saturate towards the spontaneous curvature of a pure short chain layer. Qualitative arguments are provided to explain this behavior.

From monolayers to bilayers: Effective rigidity and compressibility in asymmetric ternary amphiphilic systems.

Fluctuation effects in lamellar phases of ternary asymmetric amphiphilic systems are considered. These systems are characterized by two length scales corresponding to the thickness of the oil and water layers. For large separations between the amphiphilic layers, the dominant contribution to the free energy can be written in the Helfrich form if an effective bending rigidity is introduced that is allowed to depend on the water and oil layer thicknesses. The scaling form of this effective rigidity is determined from Monte Carlo simulations of an asymmetric three-layer system. An expression for the effective layer compressibility is derived, which can be expressed in terms of the effective bending rigidity. These effects lead to pronounced corrections of the scattering exponents ${\mathrm{\ensuremath{\eta}}}_{\mathit{m}}$, which can be measured in scattering experiments.

Structure and tension of the boundary line between amphiphilic layers

The structure and tension of the region of contact where an oil phase, a water phase and a microemulsion phase meet, is theoretically investigated. The analysis has as a starting point a Landau-theory-like expression for the free energy in which, besides the usual gradient of the density squared term, a term proportional to the second derivative of the density squared is present. The coefficient of the squared gradient term is taken to be negative in the microemulsion phase (cf. G. Gompper and M. Schick, Phys. Rev. Lett. 65 (1990) 1116). In a closely related model, we show that a first order wetting transition exists at which point infinitely many ( n = 0, 1, 2,…) surface phases can coexist, each described by the presence of a different thickness (ℓ n = (2 n + 1)ℓ 0) of the microemulsion phase between the oil and water phase. This situation physically describes the coexistence of layers of amphiphilic molecules with different thicknesses between a water and an oil bulk phase, the thickness of one amphiphilic layer being equal to ℓ 0. We present the calculation of the density profile and the boundary tension of the contact region of a coexisting n = 0 and n = 1 surface phase, e.g. between the oil-water interface with a single amphiphilic layer present and the oil-water interface where three amphiphilic layers are present. Also, an approximate analysis is made for the calculation of the boundary tension between surface phases consisting of three or more amphiphilic layers.

Disjoining pressure and stratification in asymmetric thin-liquid films

We directly measure, for the first time, disjoining pressure isotherms for asymmetric oil/aqueous surfactant/gas (i.e., pseudoemulsion) films using a modified version of the porous-plate technique first developed by Mysels in conjunction with thin-film interferometry. Dynamic film-thinning experiments are also performed on individual foam and pseudoemulsion films. At SDS surfactant concentrations above the critical micelle concentration (CMC) (0.1 M SDS), the pesudoemulsion films exhibit the same step-wise layer thinning observed in foam films under similar conditions. Further, we conduct dynamic thinning experiments on solid/liquid/gas systems and show that aqueous 0.2 M CTAB films sandwiched between glass and air also display discrete thinning transitions. All of these stratification transitions arise from oscillations in the disjoining pressure isotherm, generated by amphiphilic structuring within the film. For 0.1 M SDS dedecane/air pesudoemuslion films, the slope and peak height of the disjoining-pressure oscillations increase with each subsequent amphiphilic layer as film thickness decreases. Magnitudes of the structural forces are low (<100 Pa) but the length scale of the oscillations is large (∼10 nm) and rathe far reaching (∼50 nm). Moreover, for 0.1 M SDS solutions, the capillary pressures associated with film rupture are significantly lower for pseudoemulsion films (∼0.1 kPa) when compared to foam films (∼15 kPa) at equivalent conditions. Taken together, our dynamic thinning and equilibrium disjoining pressure measurements indicate that stratification in 0.1 M SDS films has little effect on both kinetic and thermodynamic films stability.

Boundary tension between amphiphilic layers

The structure and tension of the region of contact where an oil phase, a water phase, and a microemulsion phase meet, are theoretically investigated. The analysis has as a starting point a Landau theory-like expression for the free energy in which, besides the usual gradient of the density squared term, a term proportional to the second derivative of the density squared is present, while the coefficient of the squared gradient tern is taken to be negative in the microemulsion phase. It is shown that a first-order wetting transition exists at which point infinitely many (n=0,1,2,...) surface phases can coexist, each described by the presence of a diferent thickness of the microemulsion phase between the oil and the water phase. This situation physically describes the coexistence of layers of amphiphilic molecules with different thicknesses between a water and an oil bulk phase, the thickness of one amphiphilic layer being equal tol0. We present the calculation of the density profile and the boundary tension of the contact region of a coexistingn = 0 andn = 1 surface phase, e.g., between the oil-water interface with a single amphiphilic layer present and the oil-water interface where three amphiphilic layers are present.

Self-assembly of photopolymerizable bolaform amphiphile mono- and multilayers

Monoand multilayers of photopolymerizable bolaform amphiphiles and polyelectrolytes self-assembled onto mica from aqueous solution were studied by surface force measurement and atomic force microscopy. Multiple layers of positively charged amphiphiles self-assemble if a negatively charged polyelectrolyte is inserted between them. The layers can be photopolymerized with a high-pressure UV lamp. The atomic force microscopy images indicate that the photopolymerized amphiphile monolayer has a higher degree of order than the unpolymerized monolayer, although the latter possesses some long-range order. Mechanical properties of the system measured by the surface force apparatus indicate that a photopolymerized layer is more robust and demonstrate that the rigidity and integrity of the first anchoring layer greatly influence the stability of the multilayer. The interaction between polyelectrolyte layers in water can be either attractive or repulsive depending on the coverage of the polyelectrolyte on the amphiphile layer. The mica separation of a pair of opposed monolayers in contact in the presence of water is surprisingly small (25 A) compared to the thickness (78 A) of a bilayer composed of fully extended amphiphiles.

Adsorption of alkyl triphenylphosphonium amphiphiles on Nafion membranes. X-ray photoelectron spectroscopy and static secondary ion mass spectrometry analysis

Conductivity, UV, and attenuated total reflectance IR measurements show that n-alkyltriphenylphosphonium amphiphiles adsorb on a Ndion 117 membrane. Approximately 20% of the Ndion protons are exchanged for a cationic amphiphile (n-hexadecyltriphenylphoephonium). Diffusion of amphiphile through the membrane was not observed. Once adsorbed, the amphiphiles did not leach from the membrane. Surface-sensitive techniques (x-ray photoelectrion spectroscopy, static secondary ion mass spectrometry) were used to investigate the presence, concentration, and distribution of the amphiphile in the Ndion membrane. Our experiments point to an incomplete monolayer coverage of the membrane, the molar ratio of amphiphile to sulfonate groups being only slightly less than 1 in the uppermost 2-5 nm. The amphiphile is bonded to the membrane, most likely via an ionic bond with the sulfonate groups. X-ray fluorescence measurements show that the amphiphile is also present in the bulk of the membrane, at least in the uppermost micrometer. However, in the bulk the concentration of amphiphile is significantly lower than the sulfonate groups. These results show that thin and stable amphiphilic layers can be made on a solid support material using adsorption of an amphiphile and coupling via an ionic bond.

Catanionic amphiphilic layers: a Monte Carlo simulation study of surface forces

Monte Carlo simulations have been performed to study the interaction between catanionic amphiphilic surfaces in an aqueous solution. Experimentally these systems show a phase behavior similar to phospholipid systems, where a lamellar phase is in equilibrium with almost pure water, and the swelling of the lamellar phase cannot be explained by the DLVO theory. It has recently been proposed that the short-range repulsive force between bilayers has its origin in the confinements of the protrusional motions of the amphiphilic molecules as a second bilayer approach. A model has been developed which allows for the amphiphiles to move in a direction perpendicular to the hydrocarbon-water interface and for the correlations between these motions

A small angle neutron scattering study of the lamellar and nematic phases of caesium pentadecafluoro-octanoate (CsPFO)/2H2O and CsPFO/CsCl/2H2O

The binary lyotropic liquid crystalline system caesium pentadecafluoro-octanoate (CsPFO)/water ( 2 H 2 O) exhibits an isotropic micellar phase, a lyotropic nematic phase and a positionally ordered, lamellar phase whose planes contain substantial water filled defects. It has been suggested that the structural unit in all three phases is a disk shaped micelle which, as temperature is lowered, first become orientationally ordered and then positionally ordered onto planes. Increasing the surfactant concentration apparently causes the micelles to first increase in size and then decrease, explained by invoking an attractive inter-micellar interaction. In this small angle neutron scattering study, we present further results on the strucutre of the binary and the ternary systems, the latter containing added electrolyte. Our results call into question the previously prposed model of the lamellar phase and show that the form of the temperature dependence of the aggregate dimensions depends on the structural model that has been adopted. We show that a much more satisfactory picture of the lamellar phase is obtained if the shape of the aggregates is assumed to consist of continuous amphiphile layers pierced by irregular water filled holes; increasing the CsPFO concentration then causes the amphiphile aggregates to grow. This structure is confirmed by the effect of adding electrolyte (CsCl) to the lamellar phase. Here, scattering shows explicitly that the water filled holes anneal out. The addition of electrolyte is also shown to cause changes of aggregate shape in the nematic phase. These changes in both binary and ternary systems are shown to be attributable to modifications of the intra-aggregate electrostatic head-group interactions

Light scattering study of spherical latex particles: Measurement of surfactant adsorption and of intrinsic anisotropy

A light scattering study is performed on dispersions of spherical latex particles in mixed water-urea solvent. A nonionic amphiphile is added to the dispersion for stabilization purpose. The particles are made of a fluorinated polymer (PFA), and possess a partially crystalline internal structure which makes them optically anisotropic. We show that from the light scattering data it is possible to derive not only the size of the particles, but also the thickness of the adsorbed amphiphile layer. Information is also obtained about the intrinsic optical anisotropy of the particles.

A novel biosensor elaboration by electropolymerization of an adsorbed amphiphilic pyrrole-tyrosinase enzyme layer

A novel biosensor elaboration by electropolymerization of an adsorbed amphiphilic pyrrole-tyrosinase enzyme layer

Emulsification and Interfacial Tension of Lipid in Brine in the Presence of Cholates and Monoglyceride

The metabolism of lipids in the intestines was studied in vitro on their surface. Interfacial tension between lipids and water is depressed by adsorption of bile salts at the lipid-water interface. During triglyceride hydrolysis, the fraction of monoglyceride at this interface increases, leading to additional depression of interfacial tension and the formation of a fine lipid dispersion. The interfacial tension of lipid-water is minimum at optimum HLB (hydrophile lipophile balance) of mixed emulsifiers. With further hydrolysis, the monoglyceride fraction at the interface becomes larger, leading in turn to greater interfacial tension. Fine emulsion droplets produced on decreasing the interfacial tension to 0.50.3mN·m-1, may be adsorbed on the intestinal wall when the HLB of the adsorbed amphiphile layer becomes lipophilic. This is because an amphiphile aggregate disperses water when it is hydrophilic, but forms an amphiphile phase when the hydrophile lipophile property is balanced or lipophilic. After absorption of emulsion droplets, triglyceride may form by esterification of glycerides. Thus, the HLB of mixed amphiphiles may become hydrophilic, and bile salts may dissolve into the blood and recirculate.

Solutions of alkali soaps and water in fatty acids XIII. Circumstances that regulate the extension of theL 2-phase and the role of the acid-soaps rich in fatty acid for the occurrence of the phase

A basic requirement for that type ofL 2-phase which exists in the system sodium octanoate-octanoic acid-water is the formation of acid-soaps. In order for the phase to be formed at all, the temperature must lie above the melting point of the fatty acid so that a reaction in non-aqueous milieu between neutral soap and fatty acid is possible. In order to obtain the characteristic shape and complete extension of the phase in direction of high water content the temperature must be so high that also the hydrated acid-soaps occur in fluid state. On the other hand the temperature cannot be so high that the acid-soaps become unstable. At temperatures at which the phase has obtained its full extension those circumstances differs which in different regions regulate the location of the phase borders; they depend on the composition of the acid soaps and on their amounts. In that part of the phase where the molar ratio between octanoic acid and sodium octanoate lies between 2 and 3 and where one has a continuous transition from “reversed” to “normal” structure only the two acid octanoates 1 NaC8 ∶ 2 HC8 ∶ x H2O and 1 NaC8 ∶ 3 HC8 ∶ x H2O occur and both are at 20 °C in fluid state. At water contents from about 22 % to 40 % the hydrate-water molecules belonging to the first mentioned soap are capable of contributing actively to the formation of large aggregates of acid-soap, a process which however is counteracted by the inmixing of the latter acid-soap. This mixture of the two acid-soaps decides in this region where the border of the phase will lie in direction towards an increased content of sodium octanoate; the result is that in spite of the fact that the hydration is increased, the border is only slowly displaced towards a higher content of fatty acid. As soon as the hydration of the acid octanoates has been completed and the additional water occurs as unbound bulkwater, the location of the phase boundary will no longer be influenced by the water content — now it will be the amphiphilic composition of the acid-soaps that determines the location of the border and it remains at the molar ratio 2.5 between octanoic acid and sodium octanoate at water contents from about 40% and up to 82%. In the direction of decreasing content of neutral sodium octanoate and increased content of water theL 2-phase both at the highest content of fatty acid and the highest contents of water will be in equilibrium with the water-richL 1-phase; in the first mentioned region with theL 1-phase below the lac where at the border it is saturated with octanoic acid and in the latter region with theL 1-phase just above the lac, where the dilute sodium octanoate solution contains dissolved 1 NaC8 ∶ 1HC8 ∶ x H2O. In the large central part of theL 2-phase, from about 20 % to about 86 % of water, the location of the border is dominated by the acid octanoate 1 NaC8 ∶ 3 HC8 ∶ x H2O and that makes an equilibrium with theL 1-phase impossible; instead one has an equilibrium via a two-phase zone between the amphiphile-rich region of theL 2-phase and its water-rich region. In the first region the location of the border is regulated by the decreasing capability of the hydrated acid octanoate 1 NaC8 ∶ 3 HC8 ∶ x H2O to dissolve octanoic acid; in the latter it is regulated by the fact that 1 NaC8 ∶ 3 HC8 ∶ x H2O is the most fatty acid-rich acid-soap that is formed and that the octanoic acid is very little soluble in water and in the aqueous solution of this acidsoap. The middle part of theL 2-phase, especially the region between about 55 % and 82 % of water, constitutes a direct continuation of the liquid crystalline lamellarD-phase. The liquid crystalline character of theD-phase is lost at the transition, but the lamellar organization is retained. That the molecules at least up to a water content of about 40 % are of the original “reversed” type and have an elongated shape with a central part of hydrated polar groups, from which core the hydrocarbon chains extend in two opposite directions, is the reason to that they, at crowding, form transient layer-like agglomerates of tightly packed more or less parallel molecules; this facilitates the transformation to coherent double amphiphilic layers, in which all molecules lie with the hydrated polar groups outwards toward coherent domains of bulk-water, without another liquid phase occurs.

Lamellar liquid crystal and other phases in the system sodium dodecyl sulfate, hexylamine, water, and heptane

Phase equilibria have been studied for the system composed of sodium dodecyl sulfate, hexylamine, heptane, and water. Greatest emphasis has been placed on the lamellar liquid crystal region and X-ray diffraction measurements. Equations are developed or modified to allow determination of the degree of penetration of water into the amphiphilic double layer as a function of water or amphiphile concentration. It is found that the interfacial area occupied by the polar or ionic head groups of amphiphiles increases with increasing water content.

Phase behavior and solubilization of 1-hexanol in the water-continuous phases containing surface-active compounds

For ternary systems of 1-hexanol, water, and surface-active compounds, the phase diagrams and the solubilizations of alcohol in the micellar water-continuous phases were determined by chromalographic analysis and by measuring conductivities and turbidities. Near the region of the water-1-hexanol binary system, a narrow three-phase region including two water-continuous phases appeared as surfactant added. Depending on the levels of surfactant concentration, the transition of the micelles to the liquid crystallines occurred in the first order type or in the continuous type. The maxima of the electrical conductivities and the onset of the turbidities were consistent with the approximated phase boundaries. The change of the solubilities of alcohol in the aqueous micellar solutions indicated that on or near the phase transition regions there might exist cylinderically-organized flexible micelles, having high conductivities and high turbidities but small solubilization efficiencies of alcohol in the amphiphilic layer.

Calculations on the electric conductivity of a lyotropic mesophase with perforated lamellae

We calculate the electric conductivity of a system consisting of parallel layers of insulator, embedded into a uniform conductor. Each layer is perforated by holes, which are arranged on a two-dimensional hexagonal lattice. We investigate various stacking patterns of the layers, in volume fractions of 40% to 50%. The results are used to interpret conductivity measurements on the lamellar lyotropic mesophase. We find that on the basis of conductivity anisotropy, a layered structure of disk-shaped aggregates, and a structure of perforated layers are not readily distinguishable. The analysis of experimental data indicates that the amphiphile layers in a lamellar phase can be perforated to about 20% of the area. Also, the interpretation of the conductivity data permits evaluation of the periodicity of the layered structure.

Investigation of water-containing inverted micelles by fluorescence polarization. Determination of size and internal fluidity

Water-containing inverted micelles of sodium di(2-ethylhexyl) sulfosuccinate (AOT) have been investigated by fluorescence polarization using fluorescent hydrophilic probes localized in the aqueous core of the micelles. Measurements of the stationary polarization in two apolar solvents of different viscosity but of the same chemical nature permit rapid determination of both micellar hydrodynamic volume and water pool fluidity as a function of water content up to [H2o]/[Aot]= 11. The characteristics of AOT micelles appear to be unchanged in the n-alkane series from hexane to dodecane and slightly affected in various apolar solvents. Solvents of high polarizability such as benzene, toluene, and carbon tetrachloride penetrate into the amphiphile layer, presumably up to the water core boundary. No significant effect of sodium chloride was observed up to a concentration of 0.4 M. The inverse micelle size is independent of surfactant concentration below 0.3 M.

Effects of Solubilized Hydrocarbons on the Structure and the Dislocation Pattern in a Lamellar Liquid Crystal

Abstract The structure and dislocation pattern in a lamellar liquid crystal from water and polyoxyethylene dodecyl ether were studied for the effects of solubilization of large amounts of hydrocarbon using low angle X-ray diffraction and optical microscopy. The X-ray data showed extremely small changes of interlayer spacings with added aromatic hydrocarbons indicating them to be located within the amphiphilic layer of surfactant and water. Addition of the aliphatic hydrocarbon gave an increase of the lamellar distance equal to or even in excess of the one for non-penetrative addition indicating a change in the direction of the amphiphilic substances. The lamellar liquid crystal containing only water and surfactant showed a pattern of enhanced presence of focal conic dislocations with increased surfactant concentration for tetraoxyethylene dodecyl ether. For pentaoxyethylene dodecyl ether with a hydrophilic character this trend was observed after enhancing the lipophilic part by addition of hydrocarbon.