Insoluble Monomolecular Films Research Articles

Alignment of Inverse Boojums in a Langmuir Monolayer

Insoluble monomolecular films at the air–water interface (Langmuir monolayers) exhibit a variety of textures in tilted condensed domains in both one phase and two coexisting phase regions. They include mosaics, stripes, stars, and boojums. The appearance of these striking textures are a manifestation of a long ranged orientational order of the constituent amphiphilc molecules in mm length scales. It is well known that the textures in monolayers can be understood in terms of a Landau–de Gennes theory of tilted hexatic phases. Besides the above mentioned textures, there are inverse boojum, i.e., liquid droplets (in the L1 phase) or gas bubbles (in the G phase) surrounded by an anisotropic tilted condensed (L2) phase. They have been observed in polarized fluorescence microscopy and atomic force microscopy. Inverse boojums are virtual topological defects the texture of tilt director, since they are created and located inside the isotropic substance if we extend the cdirector (the projecton of the molecular axis on the water surface) pattern in the monolayer into the isotropic gas or liquid domain. The liquid condensed domains with no internal textures in a liquid expanded (or gas) phase form a hexagonal superstructure and the formation was explained by the line tension between the two coexisting phases and long range electrostatic repulsion between domains. There has been, however, less studied on the interactions between isotropic inclusions surrounded by a tilted liquid condensed phase. In ref. 4, Fang et al. observed an interesting alignment of inverse boojums in Langmuir monolayers of pentadecanoic acid. While this ovservation seems to be closely associated with the problem that is posed by colloidal particles suspended in an anisotropic fluid, it has not been examined theoretically before. In this note we present a simplified model for the alignment of inverse boojums. It is shown that an inverse boojum constitutes a topological dipole (a pair of point disclinations with opposite topological charges (winding numbers)) and that the dipole– dipole interaction arising from the topological charges leads to the alignment of the inverse boojums. First, we follow the the approach of Fang et al. for the minimization of the total energy for a two-dimensional gas cavity of an isotropic phase surrounded by an orientationally ordered liquid condensed phase (Fig. 1):

Gemini surfactants with a disaccharide spacer.

A gemini surfactant is an amphiphile possessing (in sequence) the following: hydrocarbon tail/polar group/spacer/polar group/hydrocarbon tail. Widespread interest in geminis has emerged recently from both industrial and academic laboratories. In the present contribution, two related families of geminis have been synthesized, both with trehalose, a disaccharide, as a polar spacer. One family, Series-A, is nonionic and has amide groups separating the long chains from the trehalose spacer. The other family, Series-B, has quaternary ammonium ions connecting the long chains to the trehalose spacer. It was found that Series-A geminis are water insoluble despite the two amides and multiple hydroxyls. When hydrated or extruded, these geminis form microscopically visible vesicular and tubular structures above their transition temperatures (which were determined calorimetrically). Insoluble monomolecular films, constructed from these geminis, have interfacial areas that are dominated by the sugar spacer although intermolecular chain/chain interactions seem to stabilize the films. Thus, the behavior of Series-A geminis in many ways parallels that of phospholipids and simple double-chain surfactants. It is as if the trehalose is less of a spacer than a large but conventional headgroup. In contrast, cationic Series-B geminis are water soluble and form micelles with critical micelle concentrations an order of magnitude lower than that of corresponding conventional surfactants. Molecular modeling using the Amber force field explains the difference in properties between the two families of geminis. Series-A are tubular in shape and thus prefer bilayer packing as do other amphiphiles in which the headgroups are similar in width to the sum of the tail diameters. Series-B geminis are conical-shaped and pack more readily into spherical micelles. This work entails synthesis, tensiometry, conductance, microscopy, surface balance studies, calorimetry, light scattering, and molecular modeling. In colloid chemistry, a balanced perspective cannot be achieved by one methodology alone but only through the pursuit of consilience among multiple approaches.

Effects of surfactants on Faraday-wave dynamics

The damping rates, natural frequencies and amplitudes of parametrically excited, standing, water waves in a partially filled, right circular cylinder are measured and compared to existing theoretical models that assume wave slopes are small. The water surfaces were covered by insoluble monomolecular (surfactant) films of oleyl alcohol, lecithin, diolein, cholesterol, and arachidyl alcohol whose concentrations were varied from zero (clean) to saturation; wave slopes were varied from about 0.1 to 1.2. Measured damping rates increased with increasing film concentration as predicted using films of oleyl alcohol, lecithin, and diolein, even when wave slopes were about one. Measured damping rates increased with increasing film concentration as predicted, using films of cholesterol and arachidyl alcohol when wave slopes were small, but not when wave slopes were large. In fact, the measured damping rates for large-slope waves on these films were equivalent to those of waves on a clean surface. Measured natural frequencies varied as predicted for all films, but were about 5% larger. Contact-line effects are incorporated, using an empirical value for contact-line speed, to account for discrepancies between measurements and predictions of damping rates and natural frequencies. Measured steady-state amplitudes agreed well with predictions that used measured damping rates and natural frequencies in the calculations for all films except lecithin and arachidyl alcohol for which there was significant disagreement.

Interactions between β-cyclodextrin and insoluble glyceride monomolecular films at the argon/water interface: application to lipase kinetics

A study of the desorption rate of insoluble monomolecular films of oleic acid (OA), monoolein (MO), 1,2-diolein (1,2-DO), 1,3-diolein (1,3-DO) and triolein (TO) at the argon/water interface by water-soluble β-cyclodextrin (β-CD) is reported. The desorption of OA and MO involves probably the complexation of the single acyl chain with β-CD and sequestering of the formed soluble OA/β-CD and MO/β-CD complexes from the argon/water interface. In the case of monolayers of multiple acyl chain molecules such as DO and TO, no detectable change in the surface pressure occurred after β-CD injection. The surface rheological dilatational properties of the monolayers of DO and TO in the presence of β-CD in the subphase were studied. The elasticity of the DO monolayer remained unchanged, whereas the decrease in the surface elasticity of the TO film was attributed to the formation of a water-insoluble TO/β-CD complex. With the ‘tuning fork’ model, one acyl chain of TO can be included in the β-CD cavity. The formed TO/β-CD complex present at the interface retarded the propagation of the dilatational deformation along the plane of the monolayer. Schematic models have been proposed in an attempt to explain the different complexation of these lipids by β-CD at the argon/water interface. In addition to the above results, the presence of β-CD in the water subphase makes it possible for the first time to perform kinetic measurements of the lipase hydrolysis rates of long-chain glycerides forming monomolecular films.

Calcium ion interactions with insoluble phospholipid monolayer films at the A/W interface. External reflection-absorption IR studies

External reflection Fourier transform infrared (FT-IR) experiments are reported for insoluble monomolecular films of an equimolar mixture of 1,2-dipalmitoylphosphatidylcholine (DPPC) and 1,2-dipalmitoylphosphatidylserine (DPPS) at the A/W interface as a function of surface pressure and Ca2+ ion presence. The separate components showed a surface pressure-induced conformational ordering of the acyl chains. The conformational ordering occurred more cooperatively for the DPPS. Acyl chain perdeuteration of the DPPC permitted the observation of the response of the individual components in the binary mixture to changes in surface tension and to the presence of Ca2+. Plots of surface pressure versus CH2 or CD2 stretching frequencies were analyzed with a two-state model. At each surface pressure within the two-state region, the fraction of disordered form was the same for each lipid component, suggesting that they are well mixed on the surface. Calcium ion (5 mM in the subphase) produces almost no effect on the pressure-induced acyl chain ordering of the DPPC in a single component film, whereas the same levels of Ca2+ induce acyl chain ordering at all surface pressures in both components of the binary mixture. Thus, unlike the bulk phase mixture of DPPC/DPPS, the binary lipids in this mixed monolayer film appear to retain their miscibility in the presence of Ca2+. Finally, Ca(2+)-induced dehydration of the phosphate group was observed through characteristic frequency shifts in the asymmetric PO2- stretching mode.

Viscoelastic relaxation of insoluble monomolecular films

Etude des monocouches de mono-oleate de glycerol a l'interface eau-air en utilisant les ondes capillaires excitees thermiquement qu'on observe dans une grande plage de nombres d'onde par diffusion quasielastique de la lumiere. Par une nouvelle procedure d'analyse, quatre nouvelles proprietes viscoelastiques de la surface sont obtenues

Molecular accommodation and molecular interactions in mixed insoluble monomolecular films

Molecular accommodation and molecular interactions in mixed insoluble monomolecular films

Surface activity of prostaglandins E2, F2α, A1, and B1 in presence of insoluble monomolecular films

Surface activities of four naturally occurring prostaglandins, PGE2, PGF2α, PGA1, and PGB1, were investigated by examining the π-A compression isotherms obtained for insoluble monomolecular films of stearic acid spread on subphases containing the prostaglandins. All four compounds were observed to bring about increased instability of stearic acid monolayers, as evidenced by reductions in both transition and collapse surface pressures. Prostaglandin E2 did not readily penetrate the monolayer but appeared capable of associating with the polar groups of the film, bringing about monolayer instability. PGB1 exhibited a disruptive effect upon monolayer structure, causing some expansion of the π-A isotherm and increased instability. Prostaglandin F2α penetrated into the stearic acid film, giving rise to increased surface pressure development in the initial regions of the π-A curve, resulting in a significantly expanded isotherm. This same effect, in the case of PGF2α, was observed using cholesterol and distearoyl phosphatidic acid monolayers. PGA1 also appeared to penetrate the stearic acid monolayer, forming a mixed monolayer system and causing considerable rearrangement in monolayer structure as evidenced by an extended plateau region occurring in the π-A plot. The order of surface activities observed for the prostaglandins was PGA1 > PGB1 > PGF2α > PGE2. The relative importance of polar functions, hydrophobic interactions, and hydrogen bonding are discussed with respect to the observed effects on monolayer stability. Some biological implications of the data are presented.

Time-dependent behavior of insoluble monomolecular films: Fatty acids and some derivatives

Saturated fatty acids ranging from 15 to 20 carbons in alkyl chain length and some stearic acid derivatives were spread as insoluble monomolecular films and compressed at a constant velocity. As these surface films assume close packing, they exhibit a loss in surface pressure when held at constant area below an apparent collapse pressure which is not due to dissolution into the subphase solution. The higher the surface pressure to which the film is compressed, the greater the number of repeated compressions to a given surface pressure after periods of noncompression, and the longer the time period between successive compressions, the greater the rate and extent of instability. Increasing chain length increases film stability, but different behavior is noted for odd- and even-carbon acids in a manner analogous to their melting behavior. From these studies it is concluded that three-dimensional phase separation occurs when the film is compressed to surface pressures approaching the liquid condensed-solid condensed transition point. The relative rates of molecular expulsion from the surface, and of lens formation and growth determine the rate of surface pressure loss when compression is stopped. Similar studies with methyl stearate, 2-hydroxystearic acid, methyl 2-OH stearate and methyl 9-OH stearate are consistent with these conclusions.

Dynamic properties of fatty acid monomolecular films

Saturated fatty acids ranging from fifteen to twenty carbons in alkyl chain length were spread as insoluble monomolecular films and compressed continually at constant velocities. As these surface films assume close-packing they exhibit a spontaneous loss in surface pressure when held at constant area per molecule or when compressed beyond an apparent collapse area, the greater the surface pressure compressed to the greater the rate of instability. Increasing chain length increases film stability but different behavior for odd and even carbon compounds has been noted for collapse pressures and for the rate of surface pressure loss. Comparison of saturated fatty acids with their methyl esters and their corresponding α-hydroxy acids indicates a change in stability in the order: α-hydroxy acids > fatty acids > methyl esters. From the evidence obtained it is concluded that three-dimensional lens formation occurs when the film enters the region of instability before an apparent collapse pressure is reached. The rate of molecular expulsion from the surface and the rate of lens formation determine the apparent collapse pressure and the rate of surface pressure loss at any area per molecule. This helps to explain the well-known effect of compression rate on the apparent collapse pressure of surface films.

Monomolecular films of poly [methyl(n‐alkyl)siloxanes] on organic liquids

AbstractA series of poly [methyl(n‐alkyl)siloxanes] were studied as insoluble monomolecular films on the following organic substrates: tricresyl phosphate, propylene carbonate, diethyl phthalate, and a highly chlorinated polyphenyl (Aroclor 1242). The siloxanes were discovered to form stable, insoluble, reproducible monomolecular films at each organic liquid/air interface. Film pressure (F) vs. area/molecule (A) isotherms were determined in each instance and were compared with previously determined F‐vs.‐A isotherms for polydi‐methylsiloxanes on the same organic substrates as well as on water. By combining the film balance data with molecular dimensions calculated from ball models, it was possible to make several tentative conclusions regarding the conformation of the adsorbed siloxane molecules as a function of film pressure, n‐alkyl chain length and substrate composition. The F‐vs.‐A curves for the poly(methylethylsiloxane) and polydimethylsiloxane films were consistent with the assumption that they assume a helical configuration when compressed at organic liquid/air interfaces; however, there is little evidence to indicate whether the polymer chains become fully extended at higher areas/molecule or remain in a somewhat coiled configuration.

Monomolecular films of polydimethylsiloxanes on organic liquids

A homologous series of polydimethylsiloxanes were spread as insoluble monomolecular films on water and on a number of organic liquids. The organic liquids studied as substrates were tricresyl phosphate, propylene carbonate, diethylphthalate, bis(2-ethylhexyl)adipate, and hexadecane. Monolayer stability and insolubility were found to vary with the molecular weight of the polydimethylsiloxane and the polarity of the substrate liquid. Stable, reversible monomolecular films were formed on all substrates by the higher molecular weight polymers (molecular weight > 6,000); however, film stability decreased with decreasing polymer molecular weight and decreasing substrate polarity. The orientations of the polydimethylsiloxane films at the organic liquid/air interfaces, as inferred from these studies, appeared to be functions of the surface composition of the substrate liquids. On tricresyl phosphate and propylene carbonate the high molecular weight polymers adsorbed in a fully extended configuration at large areas/molecule, analogous to their behavior on water. Upon compression the film pressure vs. area/molecule curves suggested they assumed a helical configuration. On the less polar substrates the higher molecular weight polydimethylsiloxanes apparently remained in the helical configuration regardless of the state of compression. Owing to the instability and/or solubility of the lower molecular weight polydimethylsiloxane monolayers on the less polar substrates, no conclusions were made regarding changes in molecular orientation as a function of film pressure.

The influence of monomolecular surface films on the production of condensation nuclei from bubbled sea water

Salt condensation nuclei were produced in large quantities (105 particles per cm3) from bubbled sea water and sodium chloride solutions. The addition of pure and mixed insoluble monomolecular films to the sea-water surface increased the concentration of salt nuclei by as much as three-fold. The greatest increases were caused by the binary monolayers that were synergistic in their behavior. No increases in nuclei concentration resulted when monolayers were spread onto the surface of aqueous systems that did not contain surface-active foam-forming materials. The increase in the number of salt particles measured is not due to a surface-chemical modification of either the sea-water droplets or the surface of the salt particles but it is a consequence of the alteration of the mechanics of the bubble-bursting process. The insoluble film decreases the degree of foaming at the sea-water surface and enhances the immediate breaking of small bubbles. This leads to a greater rate of surface fragmentation of the air-water interface. The meteorological and oceanographic implications of these data are discussed.

Surface Activity of Chlorpromazine and Chlorpromazine Sulfoxide in the Presence of Insoluble Monomolecular Films

The surface activity of chlorpromazine and its sulfoxide has been investigated at interfaces covered with insoluble monomolecular films which simulate the oriented structure of biological membranes. These compounds have been observed to form mixed monomolecular films with cholesterol, l-<i>α</i>-dipalmitoyl phosphatidylethanolamine, and l-<i>α</i>-dipalmitoyl phosphatidylcholine at concentrations where the drugs alone exhibit no surface activity. In all cases chlorpromazine was more surface active than its sulfoxide. The penetration of the drugs into the films and their subsequent ejection at higher surface pressures were found to depend on the surface characteristics of the lipid, the pH, and ionic strength of the underlying solution.

New Compounds for the Control of Water Evaporation

CONSIDERABLE attention has lately been directed towards effective suppression of water evaporation from lakes and reservoirs by spreading insoluble monomolecular films over the water surfaces1,2. In order to be effective, the film-forming compound should have several desirable properties3 such as : (i) low melting point (∼ 50° C.), which will enable it to spread rapidly on the water surface so that its monomolecular film, if damaged, is easily healed ; (ii) large equilibrium film pressure at the temperatures of water surfaces (15–40° C.) normally met with in practice ; (iii) the film formed should be stable, incompressible and condensed.

The electrical conductivity caused by insoluble monomolecular films of fatty acid on water

The well-known researches on thin films of insoluble fatty substances on water have brilliantly demonstrated that such films are monomolecular and that the molecules are oriented with their polar groups in contact with the water. Thus stearic acid spreads on water with its carboxyl groups on the water surface. No one, however, has as yet investigated the possible dissociation of such fatty acids with formation of free hydrogen ions in the water, leaving the fatty ions behind in a monomolecular film on the surface. Such dissociation need not and probably should not involve more than a small fraction of the molecules on the surface. One of us has pointed out the enormous voltages which would be caused between a complete monomolecular layer of fresh ions and the charges of opposite sign, vastly exceeding the electromotive forces of vigorous chemical reactions. This and many other lines of evidence have led to the theory of sparse surface dissociation," according to which a surface such as that of a colloidal particle may be largely covered or consist of undissociated molecules or accorded ion pairs, forming a double layer in the strict Helmholtz sense, but with a further portion of the surface covered only with unbalanced or singly adsorbed ions whose partner remain free and mobile somewhere in the solution.