Equilibrium Spreading Pressure Research Articles

Fluorination of the Hydrophilic Head Accelerates the Collapse of the Monolayer but Stabilizes the Bilayer of a Long-Chain Trifluoroethyl Ether on Water

A comparison of the collapse of Langmuir monolayers of docosyl trifluoroethyl ether (DFEE) and docosyl ethyl ether (DEE) on water shows that in both films the 3D phase is formed layer-by-layer. The substitution of CH3 by a CF3 group in the hydrophilic head yields a more stable bilayer exhibiting lower equilibrium spreading pressure, pi(esp)(DFEE) < pi(esp)(DEE). Upon lateral compression, the DFEE bilayer fractures abruptly as a compact solid body whereas the DEE bilayer breaks down gradually as a polycrystalline material. A comparison of the collapse kinetics of the two films at the same constant supersaturation pi-pi(esp) = 7 mN/m shows that the fluorinated DFEE monolayer transforms more quickly, yielding a stable bilayer of closely packed upright molecules, whereas the DEE film undergoes a continuous monolayer-bilayer-multilayer transition. Brewster angle microscopy allows us to visualize different collapse mechanisms of the DFEE and DEE films; the domains of the fluorinated DFEE bilayer grow laterally at constant thickness and density, and the collapse of the nonfluorinated DEE monolayer occurs through a sequence of disordered stripelike and broken elongated textures. The characteristic molecular areas of the monolayer and bilayer collapse suggest that the 2D-3D transition in the DFEE and DEE films is accompanied by at least partial dehydration of their headgroups. The faster collapse of the fluorinated monolayer could result from a lower energy barrier due to the more hydrophobic CF3 group in the heads. The increased stability of the DFEE bilayer could be associated with the electrostatic attraction between the -C(F delta-)3 versus (H delta+)3C- terminals at the heads-to-tails contact plane of the top and the bottom layer, contrasting with the repulsion between the -C(H delta+)3 versus (H delta+)3C- terminals of the top-layer heads and the bottom-layer tails in the DEE bilayer.

Potential-Driven Dynamic Behavior of Surface Modified Gold Nanoparticles at a Au(111) Electrode Surface

Potential-driven movements of a surface-modified Au nanoparticle at a Au(111) electrode/aqueous solution interface were examined. An interfacial capacity peak due to the reorientation of terminal 4-pyridyl groups of the surface modifier layer was observed when the Au(111) electrode was touched to the spread film of the particles on the surface of an aqueous solution at lower surface pressures than equilibrium spreading pressure. When the spread amount was increased, appearance of a capacity response presumably due to partial adsorption-desorption of the particles was observed. Implications of the results of electrochemical and electroreflectance measurements were discussed in regards to the reversible movement of the particles. [DOI: 10.1380/ejssnt.2005.141]

Highly fluorinated C18 fatty acids: synthesis and interfacial properties

A fluorinated oleic acid 1- Z containing a perfluorooctyl group and its analogues ( E-isomer 1- E , alkyne type 2 and saturated type 3) were synthesized in good yields. In these syntheses, it was found that a key compound 5 could be converted to each of 1- Z , 1- E and 2. Furthermore, equilibrium spreading pressures of their monolayers at the air–water interface were measured in order to demonstrate how the degree of unsaturation in the hydrophobic chain, the geometric isomerization, and the presence of F-atoms influence the monolayer stability. Irrespective of the structural alteration in the hydrophobic chains, the fluorinated fatty acids formed more stable monolayers with high spreading pressures as compared to their hydrocarbon counterparts.

Transformation diagrams for the collapse of a phospholipid monolayer.

The kinetics of phase transitions in three-dimensional bulk materials are commonly presented in transformation diagrams. Time-temperature transformation (TTT) and continuous-cooling-transformation (CCT) diagrams plot the time required to transform specific fractions of the material to the new phase by cooling below a transition temperature. Transformation occurs isothermally for the TTT diagrams and during continuous cooling through a range of temperatures for CCT curves. Here we present analogous transformation diagrams for two-dimensional monolayers, which collapse at the equilibrium spreading pressure (pi e) to form a three-dimensional bulk phase. Time-surface pressure-transformation (TpiT) diagrams give the time required for specific fractions of the film to collapse when surface pressure is constant, and continuous-compression-transformation diagrams give the same information when surface pressure varies continuously. The diagrams, constructed here from previously reported data for 1-palmitoyl-2-oleoyl phosphatidylcholine, provide insights into the behavior of the films. The TpiT diagrams successfully predict the existence and approximate magnitude of a threshold rate for compressing the films to high surface pressures above pi e and the approximate shape of isotherms obtained with different rates of interfacial compression. The diagrams also caution that the behavior of mixed monolayers, explained previously in terms of compositional changes, can instead result from collapse that varies with surface pressure. Finally, the similarity between the shapes of the TTT and TpiT diagrams, with the time for transformation passing through a minimum and then increasing as the systems deviate further from equilibrium, suggests that analogous mechanisms determine the behavior of both systems.

Persistence of Metastability after Expansion of a Supercompressed Fluid Monolayer

Fluid monolayers of 1-palmitoyl-2-oleoyl-phosphatidylcholine collapse from an air/water interface to form a three-dimensional bulk phase at the equilibrium spreading pressure (pie) of approximately 47 mN/m. This phase transition limits access to higher surface pressures under equilibrium conditions or during slow continuous compressions. We have shown previously that these films avoid collapse and become metastable when compressed on a captive bubble to surface pressures above 60 mN/m and that the metastability persists during expansion at least to pie. Here, we first documented the extent of this persistent metastability. Rates of isobaric collapse during expansion of the metastable films were up to 3 orders of magnitude slower than those during the initial compression to high surface pressures. Recovery of the ability to collapse depended on the surface pressure to which the films were expanded and how long they were held there. Films reverted after brief exposure to 20 mN/m and after 1 h at 35 mN/m. At pie, films remained capable of reaching high surface pressures during slow compressions after 65 h, although an increase in compressibility above 55 mN/m suggested somewhat increased rates of collapse. We also determined if the films remained metastable when they acquired sufficient free area to allow reinsertion of collapsed material. Faster isobaric expansion in the presence of more collapsed material and with further deviation below pie supported the existence of reinsertion. The persistence of metastability to pie shows that films with sufficient free area to allow reinsertion remain resistant to collapse. Observations that suggest heterogeneous reinsertion, however, argue that free area may be distributed heterogeneously and leave open the possibility that metastability persists because significant regions retain a restricted free area.

Study on water evaporation through 1-alkanol monolayers by the thermogravimetry method

The influence of 1-alkanol monolayers on the rate of water evaporation has been studied by measuring water loss per unit time using thermogravimetry. The evaporation rate of water from the surface covered by an insoluble monolayer for each of four saturated 1-alkanols (C 13OH, C 15OH, C 17OH, and C 19OH) was measured as a function of temperature and alkyl chain length, where the monolayer was under equilibrium spreading pressure. The evaporation rate decreased with increasing alkyl chain length or increasing molecular interaction among 1-alkanol molecules in the insoluble monolayer. Using the Arrhenius equation, the activation energy for the water evaporation was calculated from the temperature dependence of the evaporation rate, which showed that the activation energy decreased with increasing temperature. On the other hand, the activation energy increased with increasing alkyl chain length, which indicates that the activation energy includes the energy to cross the insoluble monolayer at the air/water interface. This energy increased almost linearly with alkyl chain length, when the length is longer than a dodecyl group. This means that water molecules need more energy to escape from the liquid to the gaseous phase across a membrane of longer 1-alkanols, which becomes more evident at lower temperatures. The temperature dependence of the activation energy was slightly larger for longer 1-alkanols than for shorter ones.

Metastability of a Supercompressed Fluid Monolayer

Previous studies showed that monomolecular films of extracted calf surfactant collapse at the equilibrium spreading pressure during quasi-static compressions but become metastable at much higher surface pressures when compressed faster than a threshold rate. To determine the mechanism by which the films become metastable, we studied single-component films of 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC). Initial experiments confirmed similar metastability of POPC if compressed above a threshold rate. Measurements at different surface pressures then showed that rates of collapse, although initially increasing above the equilibrium spreading pressure, reached a sharply defined maximum and then slowed considerably. When heated, rapidly compressed films recovered their ability to collapse with no discontinuous change in area, arguing that the metastability does not reflect transition of the POPC film to a new phase. These observations indicate that in several respects, the supercompression of POPC monolayers resembles the supercooling of three-dimensional liquids toward a glass transition.

Structural characterization and surface activity of spread and adsorbed soy globulin films at equilibrium

In this paper we studied the β-conglycinin (fraction 7S) and glycinin (fraction 11S) structural characterization, hydrophobicity, and solubility, and the surface activity at equilibrium of adsorbed and spread films at the air–water interface. These properties were analyzed as a function of the aqueous phase pH and the protein concentration in the bulk phase, including the chemical reduction of glycinin with dithiothreitol (DTT). The surface activity of 7S, 11S, and 11S+10 mM DTT in aqueous solutions has been investigated over a range of protein concentrations (5–1×10 −6% wt/wt) at pH 2.0, 5.0, and 8.0. The temperature and the ionic strength were maintained constant at 20 °C and 0.05 M, respectively. The surface activity at equilibrium was quantified by the surface pressure, which was determined by the Wilhelmy plate method. The fractions 7S and 11S would be under native conditions at pH 8.0, and they would be progressively unfolded when the pH diminishes towards the acidic region. These fractions are found to be partially denatured at pH 5.0 near the isoelectric point. The conformational change at the highest acidic pH takes place together with a greater exposure of the hydrophilic residues in a more polar region. Reduction of glycinin resulted in molecular conformational changes, which enhanced molecular hydrophobicity and increase molecule flexibility. Surface pressure data at low protein concentration indicated a low surface activity that rises to a plateau as the monolayer is saturated at higher protein concentrations. The protein concentration and the surface pressure at the plateau depend on the pH, but not on the type of protein in the aqueous phase and on the chemical reduction of glycinin with DTT neither. Poor surface activity was observed for pH 5.0. However, the equilibrium spreading pressure depends on the protein and the aqueous phase pH. Some differences were observed between surface pressure at the plateau for adsorbed protein and equilibrium spreading pressure for 7S, 11S, and 11S+10 mM DTT.

Surface behavior, microheterogeneity and adsorption equilibrium of myelin at the air–water interface

Interfacial films of whole myelin membrane adsorb at the air–water interface from myelin vesicles. The films show a liquid state and their equilibrium spreading pressure is equal to the collapse pressure (about 47 mN/m). The films appear microheterogeneous as seen by epifluorescence microscopy, consisting in two liquid phases over all the adsorption isotherm, starting with rounded liquid expanded domains (low surface pressure) immersed in a cholesterol enriched phase and reaching a fractal pattern at high surface pressure similar to those previously observed by compressing the film. Vesicles adsorb to the interfacial film mainly at the lateral interfaces. The high surface pressure at equilibrium (almost equal to the collapse pressure) indicates the formation of surface multilayers, also shown by fluorescence microscopy.

Filler–elastomer interactions: influence of oxygen plasma treatment on surface and mechanical properties of carbon black/rubber composites

In this work, the influence of oxygen plasma treatment on the surface and adsorption properties of carbon blacks was investigated using X-ray photoelectron spectroscopy (XPS), ζ-potential, and BET isotherms. Then the mechanical properties [tensile strength and tearing energy ( G IIIC)] of carbon black/acrylonitrile butadiene rubber (NBR) composites were measured. As a result, it was found that oxygen plasma treatment generated oxygen-containing functional groups, such as, carboxyl, hydroxyl, lactone, and carbonyl groups, on the carbon black surfaces, resulting in a decrease in the equilibrium spreading pressure or London dispersive component of surface free energy. The tearing energy of the carbon black/NBR composites improved as the oxygen-containing functional groups on the carbon black surfaces increased. This revealed that there is a relatively high degree of interaction between the polar NBR and the oxygen-functional groups of carbon blacks.

Effect of Acidic Surface Treatment of Red Mud on Mechanical Interfacial Properties of Epoxy/Red Mud Nanocomposites

In this work, red mud (RM) was chemically modified by 0.1, 1, and 5 M H3PO4 solution to prepare epoxy/RM nanocomposites. The effect of chemical treatment on a RM surface was studied in terms of pH, acid–base values, N2/77 K gas adsorption, equilibrium spreading pressure (πe), and surface free energy. The mechanical interfacial properties of epoxy/RM nanocomposites was measured in the context of critical stress intensity factor (KIC) and critical strain energy release rate (GIC) measurements. From the experimental results, the acidic surface treatment led to a modification of the RM surface properties, such as the surface acid values, porosity, specific surface area, πe, and London dispersive component (γSL) of surface free energy of the treated RM as an increase of the treatment concentration. In the fracture toughness (KIC and GIC) measurements, the mechanical interfacial properties of epoxy/treated RM nanocomposites were intimately correlated with the improvement of interfacial adhesion between the RM surface and a matrix and the increase of γSL of surface free energy of the RM due to the development of SBET or porosity of RM surfaces.

Effect of Temperature on Surfactant-Driven Water Movement in Wet “Unsaturated” Sand

The effect of temperature on the surfactant-driven movement of water in wet “unsaturated” sand was investigated and correlated with the equilibrium spreading pressure of the surfactant and the character of the surfactant monolayer at πe. The previously determined criteria for surfactant effectiveness were confirmed. An approximately linear relationship between πe and the amount of water moved by effective surfactants was found. Temperature changes affect water movement by influencing the surfactant πe and the monolayer state at πe.

Protein and lipid films at equilibrium at air‐water interface

AbstractThe surface activity of β‐casein, caseinate, and a whey protein isolate in aqueous solutions has been investigated over a range of protein concentrations (1·10−5 to 5% w/w) at pH 5 and 7. The surface pressure data were determined by the Wilhelmy plate method. Surface pressure data at low protein concentration indicate a low surface activity that rises to a plateau as the monolayer is saturated at higher protein concentrations. The protein concentration and the surface pressure at the plateau depend on the pH and the type of protein in the aqueous phase. Protein‐monoglyceride interactions were investigated by spreading an insoluble monoglyceride (monopalmitin, monoolein, or monolaurin) on a film of protein previously adsorbed on the interface at equilibrium. The existence of protein‐monoglyceride interactions depends on the interfacial composition and on the protein/monoglyceride ratio. The surface activity of mixed protein‐monopalmitin and protein‐monoolein films is determined by the lipid as the surface pressure of the mixed film is the same as the monoglyceride equilibrium spreading pressure, and the monolayer is not saturated by the protein. However, the protein determines the surface activity of mixed protein‐monopalmitin and protein‐monoolein films as the protein saturates the monolayer. For protein and monolaurin mixed films, protein determines the surface activity over the range of protein‐monolaurin compositions due to monolaurin dissolution in the bulk aqueous phase.

The Unified Lewis Acid - Base Approach to Adhesion and Solvation at the Liquid-Polymer Interface

We present our unified Lewis acid–base approach to adhesion and solvation at the liquid-polymer interface. This approach is to complement the original methodologies proposed by Fowkes and by van Oss, Chaudhury and Good (VCG). Intermolecular interactions are primarily dominated by dispersion, d, hydrogen bonding, h, and secondarily affected by orientation, o, and induction, i. Generally, the polarization component, p, represents both i and o interactions. Fowkes suggested that the acid–base component, γab, of the surface tension should consist of both h and p interactions. However, VCG proposed that the acid–base components, γab, result solely from hydrogen bonding, γh, that is equivalent to 2(γ+ γ−)1/2, where γ+ and γ− are the two hydrogen bonding parameters. VCG defined γLW as the Lifshitz-van der Waals component consisting of d, o and i contributions, thus, surface tension, γ, equals γab(VCG)+γLW. Both Fowkes and VCG assumed that the polar interactions for a liquid on a low energy surface are negligible. Now, we assume otherwise, and we treat the specific acid-base interaction to be hydrogen bonding. In addition, we also take into account the nonspecific polarization, p, interaction in terms of the equilibrium spreading pressure, πe, resulting from the adsorption of a liquid vapor on the polymer surface. Thus, our unified approach uses the dispersion component, γd, of Fowkes, the hydrogen bonding, h, of VCG and the polarization, p, in terms of πe. The difference between the initial (theoretical) and equilibrium (experimental) surface tensions is πe, and others have observed that πe on some polymers is substantial. The determination of several initial surface tensions of polymers by considering the effect of polarization is discussed. In the Appendix, we shall illustrate that this polar component, πe, is equivalent to the LESR polarity-dipolarity parameter, πe, (represented by the same symbol but in different context) for the solvatochromic treatment. Furthermore, the surface tension components, πd, γ+, γ− and πe, are now somewhat comparable with the four parameters in the original Taft-Kamlet relationship, δ, α, B, and πe. Thus, our proposed unified approach may finally help elucidate the long-debated Lewis acid–base theories pertaining to adhesion and solvation of polymers.

Rapid Compression Transforms Interfacial Monolayers of Pulmonary Surfactant

Films of pulmonary surfactant in the lung are metastable at surface pressures well above the equilibrium spreading pressure of 45 mN/m but commonly collapse at that pressure when compressed in vitro. The studies reported here determined the effect of compression rate on the ability of monolayers containing extracted calf surfactant at 37°C to maintain very high surface pressures on the continuous interface of a captive bubble. Increasing the rate from 2 Å 2/phospholipid/min (i.e., 3% of (initial area at 40 mN/m)/min) to 23%/s produced only transient increases to 48 mN/m. Above a threshold rate of 32%/s, however, surface pressures reached >68 mN/m. After the rapid compression, static films maintained surface pressures within ±1 mN/m both at these maximum values and at lower pressures following expansion at <5%/min to ≥45 mN/m. Experiments with dimyristoyl phosphatidylcholine at 37°C produced similar results. These findings indicate that compression at rates comparable to values in the lungs can transform at least some phospholipid monolayers from a form that collapses readily at the equilibrium spreading pressure to one that is metastable for prolonged periods at higher pressures. Our results also suggest that transformation of surfactant films can occur without refinement of their composition.

Two-dimensional Thermodynamic Properties of Sterols.

5α-cholestan-3β-ol (II) is similar to 5-cholesten-3β-ol (cholesterol) (I) except the absence of the Δ5 double bond, whereas 5β-cholestan-3α-ol (III) is remarkably different from I in its skeletal structure and in the direction of its OH moiety. Two-dimensional thermodynamic properties such as surface pressure-molecular area (π-A) relationship, equilibrium spreading pressure (ESP) and enthalpy of compression of monolayer at air-water interface were studied for these sterols. II shows similar π-A relationships to I, whereas III gives the different ones. ESP value, increasing with increasing temperature, followed the order of II>I>III. Enthalpy of elution, ΔHESP, from the crystals onto water surface at ESP and also enthalpy of compression, ΔHcomp, of the monolayers from π=0 to ESP were obtained from the temperature dependence of ESP and of π-A relationship, respectively. The sum of ΔHESP and −ΔHcomp, which means the two-dimensional enthalpy of fusion, is approximately proportional to the ordinary enthalpy of fusion. II and III molecules gave a 1:1 compound in the mixed monolayer on water surface as well as in solid state. The ESP value for the 1:1 compound was also determined.

Role of Chemically Modified Carbon Black Surfaces in Enhancing Interfacial Adhesion between Carbon Black and Rubber in a Composite System

The surface and adsorption characteristics of carbon blacks treated with H3PO4, KOH, and C6H6 were investigated. The equilibrium spreading pressure (πe), surface energy (γs), and specific surface area (SBET) were studied by the BET method with N2 adsorption. In this work, an interpretation based on the nitrogen amount adsorbed for filling a monolayer (a0) was proposed for the determination of the Gibbs free energy of nitrogen adsorption, allowing evaluation of the equilibrium spreading pressure or London dispersive component of the surface free energy of the carbon blacks studied. Also, the microstructures of the carbon blacks treated were investigated by transmission electron microscopy. Acidic treatment led to significant decreases in adsorption amount, SBET, and surface free energy of the carbon blacks, due to aggregation of the microstructures and increasing weight of the swollen specimen in an equilibrium state. Polar basic and nonpolar chemical treatments resulted in an increase of the equilibrium spreading pressure or London dispersive component of surface free energy of the carbon blacks without significantly changing the surface and adsorption properties and microstructures. Results from the surface energetics and parameter of polymer–filler interaction (χ) showed that the tearing energy of the composites is greatly dependent on the carbon blacks studied in the treatment.

Equilibrium Spreading Pressure of Steroids

Equilibrium spreading pressure (ESP) for the three kinds of steroids, i.e., (I) 5-cholesten-3β-ol, (II) 5α-cholestan-3β-ol, and (III) 5β-cholestan-3α-ol, was measured as a function of temperature and discussed thermodynamically in connection with their π-A relationships and the thermal results such as the melting point and the heat of fusion. It became clear that the direction of the OH moiety positioned at the 3rd-carbon atom of the steroid molecule gives a considerably large effect on the various thermodynamic properties in the solid states or in the monolayers on the water surface.

Relevance of Film Pressures to Interfacial Tension, Miscibility of Liquids, and Lewis Acid–Base Approach

In this paper, we demonstrate the spreading pressure and interfacial film pressure to be profoundly relevant to interfacial tension, miscibility of liquids, and the Lewis acid–base approach. For immiscible liquid–solid and liquid–liquid systems, we prefer to employ Harkins' spreading model containing the equilibrium spreading pressure πe. With the inclusion of πe, we can also improve the Lewis acid–base approach for hydrogen-bonding, proposed by van Oss, Chaudhury, and Good. We establish an acidity–basicity scale for the initial surface tension by taking πeinto account, and we further calculate interfacial tensions for liquid pairs containing formamide or dimethyl sulfoxide (DMSO) with dispersion components cited in Fowkeset al.'s later publication. However, for initially immiscible liquid–liquid systems, the Harkins model does not apply, and we propose, instead, an adsorption model, which requires the interfacial tension to be varying and surface tensions of the bulk liquids at a distance from the interface to remain unchanged. Thus, the difference between the initial and equilibrium interfacial spreading coefficients (Si) equals the equilibrium interfacial film pressure (πi)e, and that difference also equals that of the two interfacial tensions. For liquid–liquid systems, we can choose either of these two models depending on the miscibility of liquids. According to our adsorption model, there should be two interfacial tensions. The initial (or calculated) interfacial tension can be positive or negative, while the equilibrium (experimental) interfacial tension can reach zero. The former has more predictive value than the latter. A negative, initial interfacial tension is delineated to favor miscibility or spontaneous emulsification, but it tends to revert to zero instantaneously. Furthermore, a slightly positive, initial interfacial tension can also lead to miscibility, if (πi)ecan help reduce it to a zero equilibrium interfacial tension. We have also found a new important relationship between (πi)eand πeon the basis of the Laplace equation. We also attempt to calculate with our model the (πi)e's for at least 34 liquid pairs, by assuming the published interfacial tensions to be reasonable equilibrium values. Finally, the equilibrium spreading pressure πefor the liquid–solid interface, or the (πi)efor the liquid–liquid interface, appears to be the missing link between the wetting thermodynamics and linear free energy solvatochromic relationship (LFER). We have shown the πeor (πi)eof the former to be the equivalent of the Π*, polarity/dipolarity parameter, of the latter. Our findings about the correlation between the surface tension component (STC) concept and LFER have provided a new support for the STC theory.

Surface active properties of amphiphilic sequential isopeptides: Comparison between α‐helical and β‐sheet conformations

Poly(Leu–Lys–Lys–Leu) and poly(Leu–Lys) are sequential amphiphilic peptide isomers that adopt respectively an α-helical conformation and a β-sheet structure in saline solutions and at the air/water interface. The surface active properties of LKKL and LK sequential isopeptides containing 16, 20, and n residues have been compared in order to evaluate the contributions of the α-helical and β-sheet conformations. Both have a natural tendency to spread at the surface of a saline solution and the values of the equilibrium spreading pressure πe lie in the same range. When dissolved in a saline solution, α-helical peptides diffuse faster and adsorb faster at the interface than the β-sheet isomers. From the compression isotherms of LKKL and LK peptide monolayers it is possible to extract parameters that characterize the behavior of α-helical and β-sheet conformations: β-sheet peptide monolayers are more stable and less compressible than the monolayers formed with the α-helical isomers. The LK peptides differ also by their high degree of self-association at the air/water interface. © 1999 John Wiley & Sons, Inc. Biopoly 49: 415–423, 1999