Wetting, mixing, and phase transitions in Langmuir-Gibbs films

Non-Classical Euler Buckling and Brazier Instability in Cylindrical Liquid Droplets

Crystalline monolayers prevalent in nature and technology possess elusive elastic properties with important implications in fundamental physics, biology, and nanotechnology. Leveraging the recently discovered shape transitions of oil-in-water emulsion droplets, upon which these droplets adopt cylindrical shapes and elongate, we investigate the elastic characteristics of the crystalline monolayers covering their interfaces. To unravel the conditions governing Euler buckling and Brazier kink formation in these cylindrical tubular interfacial crystals, we strain the elongating cylindrical droplets within confining microfluidic wells. Our experiments unveil a nonclassical relation between the Young's modulus and the bending modulus of these crystals. Intriguingly, this relation varies with the radius of the cylindrical crystal, presenting a nonclassical mechanism for tuning of elasticity in nanotechnology applications.

"Magic Numbers" in Self-Faceting of Alcohol-Doped Emulsion Droplets

Oil-in-water emulsion droplets spontaneously adopt, below some temperature Td , counterintuitive faceted and complex non-spherical shapes while remaining liquid. This transition is driven by a crystalline monolayer formed at the droplets' surface. Here, we show that ppm-level doping of the droplet's bulk by long-chain alcohols allows tuning Td by >50 °C, implying formation of drastically different interfacial structures. Furthermore, "magic" alcohol chain lengths maximize Td . This we show to arise from self-assembly of mixed alcohol:alkane interfacial structures of stacked alkane layers, co-crystallized with hydrogen-bonded alcohol dimers. These structures are accounted for theoretically and resolved by direct cryogenic transmission electron microscopy (cryoTEM), confirming the proposed structures. The discovered tunability of key properties of commonly-used emulsions by minute concentrations of specific bulk additives should benefit these emulsions' technological applicability.

Effect of Surface Freezing of a Cationic Surfactant and n-Alkane Mixed Adsorbed Film on Counterion Distribution and Surface Dilational Viscoelasticity Studied by Total Reflection XAFS and Surface Quasi-Elastic Light Scattering

When liquid alkane droplets are placed on a surfactant solution surface having a proper surface density, alkane molecules penetrated into the surfactant-adsorbed film to form a mixed monolayer. Such a mixed monolayer undergoes a thermal phase transition from two-dimensional liquid to solid monolayers upon cooling when surfactant tail and alkane have similar chain lengths. We applied the total-reflection XAFS spectroscopy and surface quasi-elastic light scattering to the mixed adsorbed film of cetyltrimethylammonium bromide and hexadecane to elucidate the impact on the surface phase transition on the counterion distribution of the mixed monolayer. The EXAFS analysis verified that a higher percentage of counter Br^- ions were localized in the Stern layer than in the diffuse double layer in the surface solid film compared to the surface liquid film, which resulted in a reduction in the surface elasticity measured by the SQELS. The finding that the surface phase transition accompanies the change in the counterion distribution will be important to consider the future applications of the colloidal systems, in which the coexistence of a surfactant and alkane molecules is essential, such as foams and emulsions.

Salt-induced stability and modified interfacial energetics in self-faceting emulsion droplets

HYPOTHESIS

The counterintuitive temperature-controlled self-faceting of water-suspended, surfactant-stabilized, liquid oil droplets provides new opportunities in engineering of smart liquids, the properties of which are controllable by external stimuli. However, many emulsions exhibiting self-faceting phenomena have limited stability due to surfactant precipitation. The emulsions' stability may be enhanced, and their inter-droplet electrostatic repulsion tuned, through controlled charge screening driven by varying-concentration added salts. Moreover, in many technologically-relevant situations, salts may already exist in the emulsion's aqueous phase. Yet, salts' impact on self-faceting effects has never been explored. We hypothesize that the self-faceting transitions' temperatures, and stability against surfactant precipitation, of ionic-surfactants-stabilized emulsions are significantly modified by salt introduction.

EXPERIMENTS

We explore the temperature-dependent impact of NaCl and CsCl salt concentration on the emulsions' phase diagrams, employing optical microscopy of emulsion droplet shapes and interfacial tension measurements, both sensitive to interfacial phase transitions.

FINDINGS

A salt concentration dependent increase in the self-faceting transition temperatures is found, and its mechanism elucidated. Our findings allow for a significant enhancement of the emulsions' stability, and provide the physical understanding necessary for future progress in research and applications of self-faceting phenomena in salt-containing emulsions.

Nucleation of Surfactant-Alkane Mixed Solid Monolayer and Bilayer Domains at the Air-Water Interface

We investigated the wetting transitions of tetradecane and hexadecane droplets in dodecyltrimethylammonium bromide (C12TAB), tetradecyltrimethylammonium bromide (C14TAB), and hexadecyltrimethylammonium bromide (C16TAB) aqueous solutions. By varying the surfactant concentration, the formation of mixed monolayers of a surfactant and an alkane was observed at the air–water interface. Depending on the combination of surfactant and alkane, these wetting monolayers underwent another thermal phase transition upon cooling either to a frozen mixed monolayer (S1) or a bilayer structure composed of a solid monolayer of a pure alkane rested on a liquid-like mixed monolayer (S2). Based on the phase diagrams determined by phase modulation ellipsometry, the difference in the morphology of the nucleated S1 and S2 phase domains was also investigated using Brewster angle microscopy. Domains of the S1 phase were relatively small and highly branched, whereas those of the S2 phase were large and circular. The difference in domain morphology was explained by the competition of the domain line tension and electrostatic dipole interactions between surfactant molecules in the domains.

Anomalous Temperature-Controlled Concave-Convex Switching of Curved Oil-Water Menisci

While the curvature of the classical liquid surfaces exhibits only a weak temperature dependence, we demonstrate here a reversible temperature-tunable concave-convex shape switching in capillary-contained, surfactant-decorated, oil-water interfaces. The observed switching gives rise to a concave-convex shape transition, which takes place as a function of the width of the containing capillary. This apparent violation of Young's equation results from a hitherto-unreported sharp reversible hydrophobic-hydrophilic transition of the glass capillary walls. The transition is driven by the interfacial freezing effect, which controls the balance between the competing surfactants' adsorption on, and consequent hydrophobization of, the capillary walls and their incorporation into the interfacially frozen monolayer. Since capillary wetting by surfactant solutions is fundamental for a wide range of technologies and natural phenomena, the present observations have important implications in many fields, from fluid engineering to biology, and beyond.

Nucleation of melt: From fundamentals to dispersed systems

The most evident aspects of a first order transition of a system from an old to a new phase, are the presence of a discontinuity at the interface between both phases and the thermal effects related to the latent heat exchanged with the surrounding environment. These effects are the result of a sequence of events promoted by thermodynamic conditions persisting over the equilibrium in a metastable state. The breakdown of metastability is promoted by infinitesimal energy fluctuations resulting in the germination of clusters of the new phase that can grow to a critical size (nucleus) and then develop or vanish. Examples of these sequences are common in various technological fields such as combustion, food processing, pharmaceutical manufacturing, condensation, and phase change heat transfer, etc. This work aims to highlight a logical path that leads the readers from the fundamental phenomenology to the most intricated aspects of the nucleation within dispersed systems such as oil-in-water emulsions. Differences between the homogeneous and heterogeneous mechanisms are, under the light of the Classical Nucleation Theory (CNT), presented in bulk and confined systems until defining a minimum confinement size. By collecting insights coming from a rich scientific literature mostly focused on the stability of emulsified systems, the discussion is then on the aspects related to the surface related mechanisms. Two main aspects are then considered: a) the wettability of the nucleating cluster by the surrounding melt; b) the affinity between the adsorbed layer, where a surfactant is located, and the oil melt phase (mainly n-alkanes and triacylglycerols with different moieties). In cases where nucleation is dominating over the dewetting of the nucleus, the contact angle can be considered as a constant value. The affinity in terms of molecular features between the surfactant and the oil phase can promote the template effect. Several factors seem to play a role in this interaction such as the thermal characteristics of the surfactant and comparable dimensions between the molecule (or fractions) of the dispersed compound and the tail of the surfactant.

Surface Freezing of Cetyltrimethylammonium Chloride-Hexadecanol Mixed Adsorbed Film at Dodecane-Water Interface

The surface freezing transition of a mixed adsorbed film containing cetyltrimethylammonium chloride (CTAC) and n-hexadecanol (C16OH) was utilized at the dodecane-water interface to control the stability of oil-in-water (O/W) emulsions. The corresponding surface frozen and surface liquid mixed adsorbed films were characterized using interfacial tensiometry and X-ray reflectometry. The emulsion samples prepared in the temperature range of the surface frozen and surface liquid phases showed a clear difference in their stability: the emulsion volume decreased continuously right after the emulsification in the surface liquid region, while it remained constant or decreased at a much slower rate in the surface frozen region. Compared to the previously examined CTAC-tetradecane mixed adsorbed film, the surface freezing temperature increased from 9.5 to 25.0 °C due to the better chain matching between CTAC and C16OH and higher surface activity of C16OH. This then renders such systems much more attractive for practical applications.

Condensed Film Formation and Molecular Packing in Cationic Surfactant-Cholesterol and Zwitterionic Surfactant-Cholesterol Systems at the Hexane/Water Interface

A condensed film formation of surfactants with a charged head group at the oil/water interface was achieved by mixing surfactants of different geometric shapes to control molecular packing at the interface. The adsorbed films of mixed tetradecyltrimethylammonium bromide (C14TAB)-cholesterol (Chol) and tetradecylphosphocholine (C14PC)-Chol systems at the hexane/water interface were examined by interfacial tension and X-ray reflectivity measurements. The interfacial tension versus Chol concentration curves have break points because of the expanded-condensed phase transition of the adsorbed film. A two dimensional (2D) phase diagram, phase diagram of adsorption, indicated that 1:1 mixing in the condensed film is energetically favorable because of stronger mutual interaction between different molecules than between the same ones. The electron density profile normal to the interface manifested that the packing of C14TAB (or C14PC) and Chol molecules is like a 2D solid in the condensed state. As C14TAB and C14PC molecules take a corn shape with a large head group (critical packing parameter: CPP ≈ 1/3) and Chol takes an inverted corn shape with a bulky sterol ring (CPP > 1), the mixing of corn shape and inverted corn shape molecules produces well-ordered packing to promote solid-like molecular packing at the interface by energy gain because of vdW interaction between hydrophobic chains in addition to attractive ion-dipole interaction between head groups. Furthermore, the heterogeneous feature in the adsorbed film of the C14TAB-Chol system is explained by an interplay between contact energy and dipole interaction, which contribute to line tension at the domain boundary.

Impact of Surfactants on the Formation and Properties of Droplet Clusters

A levitating cluster of condensed microdroplets can form over the heated area of a water layer. The thermocapillary (TC) flow at the surface of the water layer combined with the convective flow in the layer often prevents a cluster's stability due to disturbances that it creates in the gas flow over the water surface. The TC flow can be suppressed by introducing small amounts of surfactants into the water layer. We conduct a systematic study of the effect of a surfactant on the cluster. We show experimentally that the introduction of the surfactant sodium laureth sulfate with concentrations of 0.05-0.5 g/L can suppress the TC convection. It is shown that the amount of surfactant does not affect the condensational growth of droplets and the structure of the cluster. In the absence of the surfactant, a ring-shaped cluster is formed, which is reported in this paper for the first time.

Thinning and thickening transitions of foam film induced by 2D liquid-solid phase transitions in surfactant-alkane mixed adsorbed films

Mixed adsorbed film of cationic surfactant and linear alkane at the air-water interface shows two-dimensional phase transition from surface liquid to surface frozen states upon cooling. This surface phase transition is accompanying with the compression of electrical double layer due to the enhancement of counterion adsorption onto the adsorbed surfactant cation and therefore induces the thinning of the foam film at fixed disjoining pressures. However, by increasing the disjoining pressure, surfactant ions desorb from the surface to reduce the electric repulsion between the adsorbed films on the both sides of the foam film. As a result, the foam film stabilized by the surfactant-alkane mixed adsorbed films showed unique thickening transition on the disjoining pressure isotherm due to the back reaction to the surface liquid films. In this review, we will summarize all these features based on the previously published papers and newly obtained results.

Role of the Counterions in the Surface Tension of Aqueous Surfactant Solutions. A Computer Simulation Study of Alkali Dodecyl Sulfate Systems

We have investigated the surface tension contributions of the counterions, surfactant headgroups and tails, and water molecules in aqueous alkali dodecyl sulfate (DS) solutions close to the saturated surface concentration by analyzing the lateral pressure profile contribution of these components using molecular dynamics simulations. For this purpose, we have used the combination of two popular force fields, namely KBFF for the counterions and GROMOS96 for the surfactant, which are both parameterized for the SPC/E water model. Except for the system containing Na+ counterions, the surface tension of the surfactant solutions has turned out to be larger rather than smaller than that of neat water, showing a severe shortcoming of the combination of the two force fields. We have traced back this failure of the potential model combination to the unphysically strong attraction of the KBFF counterions, except for Na+, to the anionic head of the surfactants. Despite this failure of the model, we have observed a clear relation between the soft/hard character (in the sense of the Hofmeister series) and the surface tension contribution of the counterions, which, given the above limitations of the model, can only be regarded as an indicative result. We emphasize that the obtained results, although in a twisted way, clearly stress the crucial role the counterions of ionic surfactants play in determining the surface tension of the aqueous surfactant solutions.

Precise Self-Positioning of Colloidal Particles on Liquid Emulsion Droplets

Decorating emulsion droplets by particles stabilizes foodstuff and pharmaceuticals. Interfacial particles also influence aerosol formation, thus impacting atmospheric CO₂ exchange. While studies of particles at disordered droplet interfaces abound in the literature, such studies for ubiquitous ordered interfaces are not available. Here, we report such an experimental study, showing that particles residing at crystalline interfaces of liquid droplets spontaneously self-position to specific surface locations, identified as structural topological defects in the crystalline surface monolayer. This monolayer forms at temperature T = T_s, leaving the droplet liquid and driving at T_d < T_s a spontaneous shape-change transition of the droplet from spherical to icosahedral. The particle's surface position remains unchanged in the transition, demonstrating these positions to coincide with the vertices of the sphere-inscribed icosahedron. Upon further cooling, droplet shape-changes to other polyhedra occur, with the particles remaining invariably at the polyhedra's vertices. At still lower temperatures, the particles are spontaneously expelled from the droplets. Our results probe the molecular-scale elasticity of quasi-two-dimensional curved crystals, impacting also other fields, such as protein positioning on cell membranes, controlling essential biological functions. Using ligand-decorated particles, and the precise temperature-tunable surface position control found here, may also allow using these droplets for directed supra-droplet self-assembly into larger structures, with a possible post-assembly structure fixation by UV polymerization of the droplet's liquid.

Rotator phases in alkane systems: In bulk, surface layers and micro/nano-confinements

Medium- and long-chain alkanes and their mixtures possess a remarkable physical property - they form intermediate structured phases between their isotropic liquid phase and their fully ordered crystal phase. These intermediate phases are called "rotator phases" or "plastic phases" (soft solids) because the incorporated alkane molecules possess a long-range positional order while preserving certain mobility to rotate, which results in complex visco-plastic rheological behaviour. The current article presents a brief overview of our current understanding of the main phenomena involved in the formation of rotator phases from single alkanes and their mixtures. In bulk, five rotator phases with different structures were identified and studied in detail. Along with the thermodynamically stable rotator phases, metastable and transient (short living) rotator phases were observed. Bulk rotator phases provided important information about several interfacial phenomena of high scientific interest, such as the energy of crystal nucleation, entropy and enthalpy of alkane freezing, interfacial energy between a crystal and its melt, etc. In alkane mixtures, the region of existence of rotator phases increases significantly, reflecting the disturbed packing of different molecules. All these phenomena are very important in the context of alkane applications as lubricants, in cosmetics, as phase-change materials for energy storage, etc. Significant expansion of the domain of rotator phases was observed also in confinements - in the pores of solid materials impregnated with alkanes, in polymeric microcapsules containing alkanes, and in micrometer sized emulsion droplets. The rotator phases were invoked to explain the mechanisms of two recently discovered phenomena in cooled alkane-in-water emulsions - the spontaneous "self-shaping" and the spontaneous "self-bursting" (fragmentation) of emulsion drops. The so-called "α-phases" formed by fatty acids and alcohols, and the "gel phase" formed in phospholipid and soap systems exhibit structural characteristics similar to those in the alkane rotator phases. The subtle connections between all these diverse systems are outlined, providing a unified outlook of the main phenomena related to the formation of such soft solid materials. The occurrence of alkane rotator phases in natural materials and in several technological applications is also reviewed to illustrate the general importance of these unique materials and the related phenomena.

The interfacial structure of nano- and micron-sized oil and water droplets stabilized with SDS and Span80

In this work, we provide a comparison between the stability and the interfacial structure of micrometer-sized and nanometer-sized droplets by employing a multi-instrumental approach comprised of the surface-sensitive technique of sum frequency scattering as well as dynamic light scattering and microscopy. We monitor the stability of oil-in-water and water-in-oil emulsions and the structure of surfactants at the oil/water nano-interface, when stabilized with an oil-soluble neutral surfactant (Span80), a water-soluble anionic surfactant (sodium dodecyl sulfate, SDS), or with a combination of the two. Micron-sized droplets are found to be stabilized only when a surfactant soluble in the continuous phase is present in the system, in agreement with what is traditionally observed empirically. Surprisingly, the nanodroplets behave differently. Both oil and water nanodroplets can be stabilized by the same (neutral Span80) surfactant but with different surface structures. A combination of SDS and Span80 also suffices, but for the case of water droplets, the strongly amphiphilic SDS molecules are not detected at the interface. For the case of oil droplets, both surfactants are at the interface but do not structurally affect one another. Thus, it appears that, in this study, empirical rules such as the Bancroft rule, the hydrophile-lipophile-balance scale, and the surfactant affinity difference predict the stability of the micrometer-sized droplets better than the nanometer-sized ones, probably due to a different balance of interactions on different length scales.

Nanostructures, Faceting, and Splitting in Nanoliter to Yoctoliter Liquid Droplets

Contrary to everyday experience, where all liquid droplets assume rounded, near-spherical shapes, the temperature-tuning of liquid droplets to faceted polyhedral shapes and to spontaneous splitting has been recently demonstrated in oil-in-water emulsions. However, the elucidation of the mechanism driving these surprising effects, as well as their many potential applications, ranging from faceted nanoparticle synthesis through new industrial emulsification routes to controlled-release drug delivery within the human body, have been severely hampered by the micron-scale resolution of the light microscopy employed to date in all in situ studies. Thus, the thickness of the interfacially frozen crystalline monolayer, suggested to drive these effects, could not be directly measured, and the low limit on the droplet size still showing these effects remained unknown. In this study, we employ a combination of super-resolution stimulated emission depletion microscopy, cryogenic transmission and freeze-fracture electron microscopy, to study these effects well into the nanometer length scale. We demonstrate the occurrence of the faceting transition in droplets spanning an incredible 12 decades in volume from nanoliters to yoctoliters and directly visualize the interfacially frozen, few nanometer thick, crystalline monolayer suggested to drive these effects. Furthermore, our measurements allow placing an upper-limit estimate on the two-dimensional Young modulus of the interfacial nanometer-thick surface crystal in the smallest droplets, providing insights into the virtually unexplored domain of nanoelasticity.

Unique Interfacial Phenomena on Macroscopic and Colloidal Scales Induced by Two-Dimensional Phase Transitions

This feature article addresses a variety of unique macroscopic-scale and colloidal-scale interfacial phenomena, such as wetting transitions of oil droplets into molecularly thin films, spontaneous merging and splitting of oil droplets at air-water interfaces, solid monolayer and bilayer formation in mixed cationic surfactant/alkane adsorbed films, switching of foam-film thickness, and oil-in-water emulsion stability. All of these phenomena can be observed using commercial cationic surfactants, liquid alkanes, and water.

Hydration mediated interfacial transitions on mixed hydrophobic/hydrophilic nanodroplet interfaces

Interfacial phase transitions are of fundamental importance for climate, industry, and biological processes. In this work, we observe a hydration mediated surface transition in supercooled oil nanodroplets in aqueous solutions using second harmonic and sum frequency scattering techniques. Hexadecane nanodroplets dispersed in water freeze at a temperature of ∼15 °C below the melting point of the bulk alkane liquid. Addition of a trimethylammonium bromide (C_XTA⁺) type surfactant with chain length equal to or longer than that of the alkane causes the bulk oil droplet freezing transition to be preceded by a structural interfacial transition that involves water, oil, and the surfactant. Upon cooling, the water loses some of its orientational order with respect to the surface normal, presumably by reorienting more parallel to the oil interface. This is followed by the surface oil and surfactant alkyl chains losing some of their flexibility, and this chain stretching induces alkyl chain ordering in the bulk of the alkane phase, which is then followed by the bulk transition occurring at a 3 °C lower temperature. This behavior is reminiscent of surface freezing observed in planar tertiary alkane/surfactant/water systems but differs distinctively in that it appears to be induced by the interfacial water and requires only a very small amount of surfactant.

Effect of Surface Freezing on Stability of Oil-in-Water Emulsions

Penetration of alkane molecules into the adsorbed film of a cationic surfactant gives rise to a surface freezing transition at the alkane-water interface upon cooling. In this paper, we show that surface freezing of hexadecyltrimethylammonium chloride (CTAC) at the tetradecane-water interface stabilizes oil-in-water (OW) emulsions. For concentrations of CTAC near the critical micelle concentration, an OW emulsion coalesced readily above the surface freezing transition whereas the OW emulsion was stable in the surface frozen state. There was a discontinuous change in the stability of the OW emulsion at a temperature very close to the surface phase transition temperature as determined by interfacial tensiometry and ellipsometry on a planar oil-water interface. The mechanical elasticity of the surface frozen layer opposes film drainage and density fluctuations that could lead to rupture and is the most likely cause of the enhanced emulsion stability.

The Molecular Mechanism of Nanodroplet Stability

Mixtures of nano- and microscopic oil droplets in water have recently been rediscovered as miniature reaction vessels in microfluidic environments and are important constituents of many environmental systems, food, personal care, and medical products. The oil nanodroplet/water interface stabilized by surfactants determines the physicochemical properties of the droplets. Surfactants are thought to stabilize nanodroplets by forming densely packed monolayers that shield the oil phase from the water. This idea has been inferred from droplet stability measurements in combination with molecular structural data obtained from extended planar interfaces. Here, we present a molecular level investigation of the surface structure and stability of nanodroplets and show that the surface structure of nanodroplets is significantly different from that of extended planar interfaces. Charged surfactants form monolayers that are more than 1 order of magnitude more dilute than geometrically packed ones, and there is no experimental correlation between stability and surfactant surface density. Moreover, dilute negatively charged surfactant monolayers produce more stable nanodroplets than dilute positively charged and dense geometrically packed neutral surfactant monolayers. Droplet stability is found to depend on the relative cooperativity between charge-charge, charge-dipole, and hydrogen-bonding interactions. The difference between extended planar interfaces and nanoscale interfaces stems from a difference in the thermally averaged total charge-charge interactions in the two systems. Low dielectric oil droplets with a size smaller than the Debye length in oil permit repulsive interactions between like charges from opposing interfaces in small droplets. This behavior is generic and extends up to the micrometer length scale.

Temperature-Tuned Faceting and Shape Changes in Liquid Alkane Droplets

Recent extensive studies reveal that surfactant-stabilized spherical alkane emulsion droplets spontaneously adopt polyhedral shapes upon cooling below a temperature T_d while remaining liquid. Further cooling induces the growth of tails and spontaneous droplet splitting. Two mechanisms were offered to account for these intriguing effects. One assigns the effects to the formation of an intradroplet frame of tubules consisting of crystalline rotator phases with cylindrically curved lattice planes. The second assigns the sphere-to-polyhedron transition to the buckling of defects in a crystalline interfacial monolayer, known to form in these systems at some T_s > T_d. The buckling reduces the extensional energy of the crystalline monolayer's defects, unavoidably formed when wrapping a spherical droplet by a hexagonally packed interfacial monolayer. The tail growth, shape changes, and droplet splitting were assigned to the decrease and vanishing of surface tension, γ. Here we present temperature-dependent γ(T), optical microscopy measurements, and interfacial entropy determinations for several alkane/surfactant combinations. We demonstrate the advantages and accuracy of the in situ γ(T) measurements made simultaneously with the microscopy measurements on the same droplet. The in situ and coinciding ex situ Wilhelmy plate γ(T) measurements confirm the low interfacial tension, ≲0.1 mN/m, observed at T_d. Our results provide strong quantitative support validating the crystalline monolayer buckling mechanism.

Imidazolium-Based Lipid Analogues and Their Interaction with Phosphatidylcholine Membranes

4,5-Dialkylated imidazolium lipid salts are a new class of lipid analogues showing distinct biological activities. The potential effects of the imidazolium lipids on artificial lipid membranes and the corresponding membrane interactions was analyzed. Therefore, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was employed to create an established lipid monolayer model and a bilayer membrane. Mixed monolayers of DPPC and 4,5-dialkylimidazolium lipids differing by their alkyl chain length (C₇, C₁₁, and C₁₅) were characterized by surface pressure-area (π-A) isotherms using a Wilhelmy film balance in combination with epifluorescence microscopy. Monolayer hysteresis for binary mixtures was examined by recording triplicate consecutive compression-expansion cycles. The lipid miscibility and membrane stability of DPPC/imidazolium lipids were subsequently evaluated by the excess mean molecular area (ΔA^ex) and the excess Gibbs free energy (ΔG^ex) of mixing. Furthermore, the thermotropic behavior of mixed liposomes of DPPC/imidazolium lipids was investigated by differential scanning calorimetry (DSC). The C₁₅-imidazolium lipid (C₁₅-IMe·HI) forms a thermodynamically favored and kinetically reversible Langmuir monolayer with DPPC and exhibits a rigidification effect on both DPPC monolayer and bilayer structures at low molar fractions (X ≤ 0.3). However, the incorporation of the C₁₁-imidazolium lipid (C₁₁-IMe·HI) causes the formation of an unstable and irreversible Langmuir-Gibbs monolayer with DPPC and disordered DPPC liposomes. The C₇-imidazolium lipid (C₇-IMe·HI) displays negligible membrane activity. To better understand these results on a molecular level, all-atom molecular dynamics (MD) simulations were performed. The simulations yield two opposing molecular mechanisms governing the different behavior of the three imidazolium lipids: a lateral ordering effect and a free volume/stretching effect. Overall, our study provides the first evidence that the membrane interaction of the C₁₅ and C₁₁ derivatives modulates the structural organization of lipid membranes. On the contrary, for the C₇ derivative its membrane activity is too low to contribute to its earlier reported potent cytotoxicity.

How faceted liquid droplets grow tails

Liquid droplets, widely encountered in everyday life, have no flat facets. Here we show that water-dispersed oil droplets can be reversibly temperature-tuned to icosahedral and other faceted shapes, hitherto unreported for liquid droplets. These shape changes are shown to originate in the interplay between interfacial tension and the elasticity of the droplet's 2-nm-thick interfacial monolayer, which crystallizes at some T = Ts above the oil's melting point, with the droplet's bulk remaining liquid. Strikingly, at still-lower temperatures, this interfacial freezing (IF) effect also causes droplets to deform, split, and grow tails. Our findings provide deep insights into molecular-scale elasticity and allow formation of emulsions of tunable stability for directed self-assembly of complex-shaped particles and other future technologies.

Surfactant-induced phases in water-supported alkane monolayers: II. Structure

The structure of the Langmuir-Gibbs films of normal alkanes C(n) of length n = 12-21 formed at the surface of aqueous solutions of C(m)TAB surfactants, m = 14, 16, and 18, was studied by surface-specific synchrotron X-ray methods. At high temperatures, a laterally disordered monolayer of mixed alkane molecules and surface-adsorbed surfactant tails is found, having thicknesses well below those of the alkanes' and surfactant tails' extended length. The mixed monolayer undergoes a freezing transition at a temperature T(s)(n,m), which forms, for n ≤ m + 1, a crystalline monolayer of mixed alkane molecules and surfactant tails. For n ≥ m + 2, a bilayer forms, consisting of an upper pure-alkane, crystalline monolayer and a lower liquidlike monolayer. The crystalline monolayer in both cases consists of hexagonally packed extended, surface-normal-aligned chains. The hexagonal lattice constant is found to decrease with increasing n. The films' structure is discussed in conjunction with their thermodynamic properties presented in an accompanying paper.

Surfactant-induced phases in water-supported alkane monolayers: I. Thermodynamics

Alkanes longer than n = 6 carbons do not spread on the water surface, but condense in a macroscopic lens. However, adding trimethylammonium-based surfactants, C(m)TAB, in submillimolar concentrations causes the alkanes to spread and form a single Langmuir-Gibbs (LG) monolayer of mixed alkanes and surfactant tails, which coexists with the alkane lenses. Upon cooling, this LG film surface-freezes at a temperature T(s) above the bulk freezing temperature T(b). The thermodynamics of surface freezing (SF) of these LG films is studied by surface tension measurements for a range of alkanes (n = 12-21) and surfactant alkyl lengths (m = 14, 16, 18), at several concentrations c. The surface freezing range T(s)-T(b) observed is up to 25 °C, an order of magnitude larger than the temperature range of SF monolayers on the surface of pure alkane melts. The measured (n,T) surface phase diagram is accounted for well by a model based on mixtures' theory, which includes an interchange energy term ω. ω is found to be negative, implying attraction between unlike species, rather than the repulsion found for SF of binary alkane mixtures. Thus, the surfactant/alkane mixing is a necessary condition for the occurrence of SF in these LG films. The X-ray derived structure of the films is presented in an accompanying paper.

Line tension of alkane lenses on aqueous surfactant solutions at phase transitions of coexisting interfaces

Alkane droplets on aqueous solutions of surfactants exhibit a first-order wetting transition as the concentration of surfactant is increased. The low-concentration or "partial wetting" state corresponds to an oil lens in equilibrium with a two-dimensional dilute gas of oil and surfactant molecules. The high-concentration or "pseudo-partial wetting" state consists of an oil lens in equilibrium with a mixed monolayer of surfactant and oil. Depending on the combination of surfactant and oil, these mixed monolayers undergo a thermal phase transition upon cooling, either to a frozen mixed monolayer or to an unusual bilayer structure in which the upper leaflet is a solid layer of pure alkane with hexagonal packing and upright chains while the lower leaflet remains a disordered liquid-like mixed monolayer. Additionally, certain long-chain alkanes exhibit a surface freezing transition at the air-oil interface where the top monolayer of oil freezes above its melting point. In this review, we summarize our previous studies and discuss how these wetting and surface freezing transitions influence the line tension of oil lenses from both an experimental and theoretical perspective.

Morphological Transformations in Solid Domains of Alkanes on Surfactant Solutions

Alkanes on surfactant solutions can form three distinct phases at the air-solution interface, a liquid phase (L), a solid monolayer phase (S1), and a hybrid bilayer phase (S2). Phase coexistence between any two, or all three, of these phases has been observed by Brewster angle microscopy of tetradecane, hexadecane, and their mixtures on solutions of tetradecyltrimethylammonium bromide. The morphologies of the domains depend on the competition between line tension and electrostatic interactions, which are essentially different depending on the pair of phases in contact. Domains of S1 in the L phase are long and thin; however, long, thin domains of L in an S1 phase are not stable but break up into a string of small circular domains. The bilayer S2 domains are always circular, owing to the dominance of line tension on the morphology.

Checkerboard self-patterning of an ionic liquid film on mercury

Å-resolution studies of room temperature ionic liquid (RTIL) interfaces are scarce, in spite of their long-recognized importance for the science and many applications of RTILs. We present an Å-resolution x-ray study of a Langmuir film of an RTIL on mercury. At low (high) coverage [90 (50) Å2/molecule] a mono-(bi)layer of surface-parallel molecules is found. The molecules self-assemble in a lateral ionic checkerboard pattern, unlike the uniform-charge, alternate-ion layers of this RTIL at its bulk-solid interface. A 2D-smectic order is found, with molecules packed in parallel stripes, forming long-range order normal to, but none along, the stripes.

Modification of deeply buried hydrophobic interfaces by ionic surfactants

Hydrophobicity, the spontaneous segregation of oil and water, can be modified by surfactants. The way this modification occurs is studied at the oil-water interface for a range of alkanes and two ionic surfactants. A liquid interfacial monolayer, consisting of a mixture of alkane molecules and surfactant tails, is found. Upon cooling, it freezes at T(s), well above the alkane's bulk freezing temperature, T(b). The monolayer's phase diagram, derived by surface tensiometry, is accounted for by a mixtures-based theory. The monolayer's structure is measured by high-energy X-ray reflectivity above and below T(s). A solid-solid transition in the frozen monolayer, occurring approximately 3 °C below T(s), is discovered and tentatively suggested to be a rotator-to-crystal transition.

Surface chemistry of Alzheimer's disease: a Langmuir monolayer approach

Amyloid beta (1-40) and (1-42) peptides are the major constituents of hallmark senile plaques found in Alzheimer's disease (AD) patients. Study of aggregation of Abeta (1-40) and (1-42) peptides and the truncated Abeta fragments could lead towards the mechanism of AD. Langmuir monolayer approach is one of the excellent methods to investigate the mechanism and origin of AD. Particularly, to study the steps involved in the formation and assembly of beta-sheet structures leading to formation of amyloid fibrils. Surface pressure- and surface potential-area isotherms provide information regarding the nature of short-range and long-range interactions between the molecules especially the lipids and the Abeta peptides. Spectroscopic methods like IRRAS, PM-IRRAS, FTIR-ATR, and GIXD at the air-water interface provide insight into the structural characterization, and orientation of the molecules in the Langmuir monolayer.

Nano- and microstructure of air/oil/water interfaces

We report the creation of air/oil/water interfaces with variable-thickness oil films using polyisobutylene-based (PIB) surfactants cospread with long-chain paraffinic alkanes on clean water surfaces. The resultant stable oil layers are readily measurable with simple surface techniques, exhibit physical densities the same as expected for bulk oils, and are up to approximately 100 A thick above the water surface as determined using X-ray reflectometry. This provides a ready system for studying the competition of surfactants at the oil/water interface. Results from the competition of a nonionic polyamide surfactant or an anionic sodium dodecyl sulfate with the PIB surfactant are reported. However, this smooth oil layer does not account for the total volume of spread oil nor is the increase in thickness proportional to the film compression. Brewster angle microscopy (BAM) reveals surfactant and oil structures on the scale of 1 to 10 microm at the interface. At low surface pressure (pi < 24 mN m(-1)) large, approximately 10 microm inhomogeneities are observed. Beyond a phase transition observed at pi approximately = 24 mN m(-1), a structure with a spongy appearance and a microscale texture develops. These structures have implications for understanding the microstructure at the oil/water interface in emulsions.

Wetting behaviour and wetting transitions of alkanes on aqueous surfaces

The nature of the surface forces, acting in the wetting films of pentane, hexane and heptane on water and on aqueous NaCl solutions is discussed. It is shown that an important contribution to the surface forces originates from the low solubility of water in short-chain alkanes. The isotherms of water adsorption, the disjoining pressure and the excess free energy were calculated for wide range of temperatures and salinities of underlying aqueous solution. To account for the screening action of ions in the aqueous solution on the image-charge interactions in alkane films we have solved the set of Poisson equations with the appropriate boundary conditions and within the Debye-Hückel approximation applied for the electrolyte phase. The analysis of the influence of temperature and salinity on different components of the disjoining pressure allows the conclusion that both factors shift the total isotherm onto the region of positive values resulting in the wetting transition. Such transitions for the range of temperatures and salinities studied were detected for pentane and hexane films only. Achievement of the wetting transition in heptane requires higher salinities and temperatures. The nature of wetting transitions from partial to complete wetting and from thin alpha- to thick beta-films was analyzed on the basis of Derjaguin -Frumkin theory taking into account the behaviour of the disjoining pressure and the excess free energy isotherms. It was shown that the surface forces, considered here, provide in the range of vapour undersaturation two sequential (partial-to-complete followed by thin-to-thick films) wetting transitions which combine at saturation into a single transition. For the oversaturated vapours only partial-to-complete wetting transition may be described.