Equilibrium Spreading Research Articles

Measuring the Equilibrium Spreading Pressure-A Tale of Three Amphiphiles.

A surfactant's equilibrium spreading pressure (ESP) is the maximum decrease in surface tension achievable at equilibrium below the Krafft point. Difficulties in measuring the ESP have been noted previously but no well-established experimental protocols to overcome them exist. We present a case study of three solid amphiphiles with different propensities to spread on the air-water interface. Starting with the partially water soluble n-dodecanol (C12H25OH), which spreads instantaneously. The strong Marangoni flows associated with the spreading result in the dislocating of the Wilhelmy plate or crystals attaching to it. A temporary mechanical barrier in front of the spreading crystals mitigates the flows disturbing the plate. Presaturating the subphase with the amphiphile prevents the establishment of dynamic steady states, reduces the standard error by a factor of three and causes faster equilibration. The perfluoroalkylated analog of dodecanol (11:1 fluorotelomer alcohol, C11F23CH2OH) is slow spreading. With surfactant crystals on the interface, the surface pressure reaches a pre-equilibrium plateau within an hour, followed by equilibration on day-long timescales. We show that it is better to estimate the ESP by averaging the values of multiple pre-equilibrium plateaus rather than waiting for equilibrium to be established. Finally, the nonspreading amphiphile DPPC exhibits a large barrier for the mass transfer from the DPPC crystal to the aqueous surface. This was overcome by introducing a volatile, water-immiscible solvent deposited on the surface next to the crystals to facilitate the spreading process and leave behind a monolayer.

Electrolytes at Uncharged Liquid Interfaces: Adsorption, Potentials, Surface Tension, and the Role of the Surfactant Monolayer.

The article summarizes the results of our research on the behavior of ions at uncharged fluid interfaces, with a focus on moderately to highly concentrated aqueous electrolytes. The ion-specific properties of such interfaces have been analyzed. The ion-specificity series are different for water|air and water|oil; different for surface tension σ, surface Δχ potential and electrolyte adsorption, and they change with concentration. A methodology has been developed that allows to disentangle the multiple factors controlling the ion order. The direct ion-surface interactions are not always the most significant factor behind the observed ion sequences: indirect effects stemming from conjugate bulk properties are often more important. For example, the order of the surface tension with the nature of the anion (σKOH > σKCl > σKNO3 for potassium salts) is often the result of bulk nonideality and follows the order of the bulk activity coefficients (γKOH > γKCl > γKNO3) rather than that of a specific ion-surface interaction potential. The surface Δχ potential of aqueous solutions is, in many cases, insensitive to the ion distribution in the electric double layer but reflects the orientation of water at the surface, through the ion-specific dielectric permittivity ε of the solution. Even the sign of Δχ is often the result of the decrement of ε in the presence of electrolyte. A whole new level of complexity appears when the ions interact with an uncharged surfactant monolayer. A method has been developed to measure the electrolyte adsorption isotherms on monolayers of varying area per surfactant molecule via a combination of experiments-compression isotherms and surface pressure of equilibrium spread monolayers. The obtained isotherms demonstrate that the ions exhibit a maximum in their adsorption on monolayers of intermediate density. The maximum is explained with the interplay between ion-surfactant complexation, volume exclusion and osmotic effects.

Structural determinants of collapse by a monomolecular mimic of pulmonary surfactant at the physiological temperature.

Pulmonary surfactant forms a thin film on the liquid that lines the alveolar air-sacks. When compressed by the decreasing alveolar surface area during exhalation, the films avoid collapse from the air/water interface and reduce surface tension to exceptionally low levels. To define better the structure of compressed films that determines their susceptibility to collapse, we measured how cholesterol affects the structure and collapse of dipalmitoyl phosphatidylcholine (DPPC) monolayers at physiological temperatures. Grazing incidence X-ray diffraction (GIXD) and grazing incidence X-ray off-specular scattering (GIXOS) established the lateral and transverse structures of films on a Langmuir trough at a surface pressure of 45 mN m-1, just below the equilibrium spreading pressure at which collapse begins. Experiments with captive bubbles at a surface pressure of 51 mN m-1 measured how the steroid affects isobaric collapse. Mol fractions of the steroid (Xchol) at 0.05 removed the tilt by the acyl chains of DPPC, shifted the unit cell from centered rectangular to hexagonal, and dramatically decreased the long-range order. Higher Xchol produced no further change in diffraction, suggesting that cholesterol partitions into a coexisting disordered phase. Cholesterol had minimal effect on rates of collapse until Xchol reached 0.20. Our results demonstrate that the decreased coherence length, indicating conversion of positional order to short-range, is insufficient to make a condensed monolayer susceptible to collapse. Our findings suggest a two-step process by which cholesterol induces disorder. The steroid would first convert the film with crystalline chains to a hexatic phase before generating a fully disordered structure that is susceptible to collapse. These results lead to far-reaching consequences for formulation of animal-derived therapeutic surfactants. Our results suggest that removal of cholesterol from these preparations should be unnecessary below Xchol = 0.20.

OFFSET COALESCENCE BEHAVIOR OF IMPACTING LOW-SURFACE TENSION DROPLET ON HIGH-SURFACE-TENSION DROPLET

The impact of droplets of varying surface tension and subsequent spreading over a solid surface are inherent features in printing applications. In this regard, an experimental study of the impact of two drops of varied surface tension is carried out where the sessile water droplet on a hydrophilic substrate is impacted upon by another droplet of sequentially lowered surface tension. The impacts are studied for different impact velocities and offsets with respect to the mid-plane of the two colliding droplets. Sodium dodecyl sulfate is used to: (i) alter the surface tension without altering the viscosity, (ii) study the various parameters affecting the spreading length viz. the surface tension, (iii) offset between the drops, and (iv) impact velocity. The spreading lengths are obtained through image processing of the captured footage of the impact dynamics by a high-speed camera. It is found out that upon lowering the surface tension, the maximum and equilibrium spreading length varies to a significant extent, and the nature of the spreading dynamics changes. Both side- and top-view imaging are performed to understand the overall hydrodynamics. There is also a substantial change in "drawback" when dissimilarity in surface tension between the impacting droplets exists. Finally, a fit model is obtained to predict the maximum spread length of the various cases.

A universal rescaling law for the maximum spreading factor of non-Newtonian droplets with power-law fluids

The maximum spreading diameter of non-Newtonian fluid droplets impacting on the solid surface is a key concern in a variety of industrial and medical applications. In this work, we focus on the effect of the shear-thinning, one of the most important non-Newtonian properties, on the spreading dynamics of impacting droplets. A finite element scheme combined with a phase field method and dynamic contact angle model has been employed to perform extensive studies on the spreading process of power-law fluid droplets on solid surfaces with various rheological parameters, impact conditions and surface wettability. The simulation results show that on both hydrophilic and hydrophobic surfaces, impacting droplets exhibit two typical morphologies at the maximum spreading state: a spherical cap in the low-Weber-number range (capillary regime) and a thin-film form in the high-Weber-number range (viscous regime). The maximum spreading factor βmax, of droplets with different degrees of shear-thinning converges to the equilibrium spreading state for a droplet with U0=0 at the low-Weber-number limit. Furthermore, a theoretical relationship of βmax∼We1/2 has been derived in the capillary regime. In contrast, the effect of the shear-thinning property becomes significant in the high-Weber-number regime. We discussed the influence of the power-law coefficients K and n on the spreading process and βmax independently. Specifically, as the power-law index n decreases, the morphology of the shear-thinning droplet at the maximum spreading state tends to change from a spherical cap to a thin-film form. Considering the non-uniform distribution of shear rates in the spreading shear-thinning droplet, a new scaling relationship of βmax∼ln(Ren1/(2n+3)) has been proposed based on theoretical derivation and numerical simulations. By introducing an interpolation function on the scaling relationships between the capillary and viscous regimes, we obtained a universal rescaling model that agrees well with numerical and experimental results of non-Newtonian droplets with shear-thinning fluid over a wide range of We numbers, surface wettability and rheological parameters.

Phase transitions of the pulmonary surfactant film at the perfluorocarbon-water interface

Pulmonary surfactant is a lipid-protein complex that forms a thin film at the air-water surface of the lungs. This surfactant film defines the elastic recoil and respiratory mechanics of the lungs. One generally accepted rationale of using oxygenated perfluorocarbon (PFC) as a respiratory medium in liquid ventilation is to take advantage of its low surface tensions (14–18 mN/m), which was believed to make PFC an ideal replacement of the exogenous surfactant. Compared with the extensive studies of the phospholipid phase behavior of the pulmonary surfactant film at the air-water surface, its phase behavior at the PFC-water interface is essentially unknown. Here, we reported the first detailed biophysical study of phospholipid phase transitions in two animal-derived natural pulmonary surfactant films, Infasurf and Survanta, at the PFC-water interface using constrained drop surfactometry. Constrained drop surfactometry allows in situ Langmuir-Blodgett transfer from the PFC-water interface, thus permitting direct visualization of lipid polymorphism in pulmonary surfactant films using atomic force microscopy. Our data suggested that regardless of its low surface tension, the PFC cannot be used as a replacement of pulmonary surfactant in liquid ventilation where the air-water surface of the lungs is replaced with the PFC-water interface that features an intrinsically high interfacial tension. The pulmonary surfactant film at the PFC-water interface undergoes continuous phase transitions at surface pressures less than the equilibrium spreading pressure of 50 mN/m and a monolayer-to-multilayer transition above this critical pressure. These results provided not only novel biophysical insight into the phase behavior of natural pulmonary surfactant at the oil-water interface but also translational implications into the further development of liquid ventilation and liquid breathing techniques.

Interactions between Small Inorganic Ions and Uncharged Monolayers on the Water/Air Interface.

The interaction of several simple electrolytes with uncharged insoluble monolayers is studied on the basis of tensiometric and potentiometric data for the surface electrolyte solution|air. The induced adsorption of electrolyte on the monolayer is determined via a combination of data for equilibrium spreading pressure and surface pressure versus area isotherms. We show that the monolayer-induced adsorption of electrolyte is not only strongly ion-specific but also surfactant-specific. The comparison between the ion-specific effects on a carboxylic acid monolayer at low pH and an ester monolayer shows that the anion series follows the same order while the cation series reverses. The effect of the electrolyte on the chemical potential of the monolayer shows attraction between the surfactant and the ions at low monolayer densities, but at high surface densities, repulsion seems to come into play. In nearly all investigated cases, a maximum of monolayer-induced electrolyte adsorption is observed at intermediate monolayer densities. This suggests specific interactions between the surfactant headgroup and the ions. The Volta potential data for the monolayers are analyzed on the basis of the equations of quadrupolar electrostatics. The analysis suggests that the ion-specific effect on the Volta potential is due to the ion-specific decrement of the bulk dielectric constant of the electrolyte solution. Moreover, we present evidence that in most cases the effect of the electrolyte on the orientation of the adsorbed dipoles cannot be neglected. Instead, the change in the ion distribution in the electric double layer seems to have a small effect on the Volta potential.

Oil droplet spread on an immiscible surface of a vertically falling liquid film

Droplet spread over a vertically falling liquid film is studied in this paper. A simulation model is built and verified by experiment. Following this, a unique phenomenon that emerges in this context, namely, a strong inertial oscillation in an early stage of spreading, is analyzed. Finally, the equilibrium features of an oil droplet in this circumstance are discussed. The results show that the maximum spreading length in a strong inertial oscillation is much longer than the equilibrium length, being 152% the length of the latter in the base case. Furthermore, the equilibrium spreading length increases nearly linearly with the initial diameter of the droplet. The paper provides data to understand the effects of an oil droplet on a vertically falling film absorber to promote energy saving in a cold storage refrigeration system with low-grade heat.

Phase State and Thermodynamic Properties of Proxy Sea Spray Aerosol Interfaces Derived from Temperature-Dependent Equilibrium Spreading Pressure

Phase State and Thermodynamic Properties of Proxy Sea Spray Aerosol Interfaces Derived from Temperature-Dependent Equilibrium Spreading Pressure

Nonswelling and Hydrolytically Stable Hydrogels Uncover Cellular Mechanosensing in 3D.

While matrix stiffness regulates cell behavior on 2D substrates, recent studies using synthetic hydrogels have suggested that in 3D environments, cell behavior is primarily impacted by matrix degradability, independent of stiffness. However, these studies did not consider the potential impact of other confounding matrix parameters that typically covary with changes in stiffness, particularly, hydrogel swelling and hydrolytic stability, which may explain the previously observed distinctions in cell response in 2D versus 3D settings. To investigate how cells sense matrix stiffness in 3D environments, a nonswelling, hydrolytically stable, linearly elastic synthetic hydrogel model is developed in which matrix stiffness and degradability can be tuned independently. It is found that matrix degradability regulates cell spreading kinetics, while matrix stiffness dictates the final spread area once cells achieve equilibrium spreading. Importantly, the differentiation of human mesenchymal stromal cells toward adipocytes or osteoblasts is regulated by the spread state of progenitor cells upon initiating differentiation. These studies uncover matrix stiffness as a major regulator of cell function not just in 2D, but also in 3D environments, and identify matrix degradability as a critical microenvironmental feature in 3D that in conjunction with matrix stiffness dictates cell spreading, cytoskeletal state, and stem cell differentiation outcomes.

Wetting Dynamics on Solvophilic, Soft, Porous, and Responsive Surfaces

We employ molecular dynamics (MD) simulations to study the spreading and imbibition of a liquid drop on a porous, soft, solvophilic, and responsive surface represented by a layer of polymer molecules grafted on a solvophilic solid. These polymer molecules are in a crumpled and collapsed globule-like state before the interaction with the drop but transition to a “brush”-like state as they get wetted by the liquid drop. We hypothesize that for a wide range of densities of polymer grafting (σg), the drop spreading is dictated by the balance of the driving inertial pressure and balancing viscoelastic dissipation (associated with the spreading of the liquid drop on the polymer layer that undergoes globule-to-brush transition and serves as the viscoelastic solid). Using the well-known idea that the viscoelastic resisting force exerted by the viscoelastic solid on a spreading drop scales as un (where n is the index of the power-law-like rheology of the polymer layer serving as the viscoelastic solid and u is the spreading velocity of the drop on this viscoelastic solid) and considering n = 2/3, we show that the scaling calculation recovers the MD simulation prediction of r ∼ t1/4 and req ∼ σg–1/3 (where r and req are the instantaneous and equilibrium spreading radii, respectively). We further describe the wicking behavior of the drop through the polymer layer by appropriately accounting for the manner in which the progressive time-dependent swelling of the grafted polymer molecules provides larger space for the wicking. Third, we quantify, possibly for the first time, the temporal dynamics of the “brush”-forming process (i.e., capture the dynamics of wetting-mediated globule-to-brush transition). We show that the dynamics of the polymer chain swelling depends on σg and is faster for sparser grafting. Most importantly, we confirm that the height of the relaxed polymer chains approximately scales as σg1/3, confirming the attainment of brush-like configuration by the polymer molecules as they are wetted by the liquid drop. Finally, we argue that our simulations raise the possibility of designing soft, “responsive”, and widely deployable liquid-infused surfaces where the polymer grafted solid, with the polymer undergoing a globule-to-brush transition, serves as the responsive “surface”.

COVID-19 Pandemic: Social Distancing, Public Policy, and Market Response

The outbreak of COVID-19 has become a worldwide concern for consumers, businesses, and public policy makers. Social distancing is now a top consideration for consumers when deciding where and how to shop, which reduces their willingness to shop in physical stores with others who may carry the virus. In this paper, we use a game-theoretic model to analyze how consumers' preference for social distancing affects store profits, consumer surplus, social welfare, and the equilibrium spread of COVID-19 among consumers when they, along with stores and online delivery platform, strategically respond to social distancing. We find that the preference for social distancing 1) decreases store profits in a monopoly but can increase them in a duopoly, 2) strictly benefits the delivery platform, and 3) weakly decreases consumer surplus and social welfare in both a monopoly and a duopoly. Meanwhile, the preference for social distancing reduces the spread of COVID-19 by reducing store traffic, but this positive effect only arises when stores sell online through their own channels. When stores sell online through a third-party platform, consumers' preference for social distancing does not affect the spread of COVID-19 amongst shoppers. Moreover, we find that, while facilitating delivery services can be ineffective in reducing the spread of COVID-19, subsidizing online shopping and regulating third-party platforms can help contain the virus.

Establishment of a Synthetic In Vitro Lung Surfactant Model for Particle Interaction Studies on a Langmuir Film Balance.

With the intention to provide a robust and economical model that can be used for predicting particle interactions with the pulmonary surfactant, this study was aimed to find an artificial surfactant model that perfectly mimics the properties of the natural pulmonary surfactant. A surfactant model should be reproducible, robust, and able to predict interactions between the pulmonary surfactant and exogenous influences from air and the aqueous site. We compared three synthetic models with the natural bovine surfactant Alveofact. The lung conditions were simulated by spreading the surfactants at the air/aqueous interface on a Langmuir trough with movable barriers. All three artificial surfactant models showed properties very similar to that of Alveofact. Visualization of the monolayers by atomic force microscopy revealed very similar structures with domain formation. The Tanaka lipid mixture has already shown good results in vitro and in vivo in previous studies. The 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)-1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) model has large conformations in the surface pressure isotherms and showed a biomimetic exclusion plateau, indicative of an effective lung surfactant formulation. Also, the equilibrium spreading pressure was similar. DPPC-1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-1'-rac-glycerol (POPG) had the greatest similarities with Alveofact in the hysteresis areas. The kinetic constants of the relaxation experiments during desorption showed that the PCPG model (at 30 mN/m) had almost identical diffusion and dissolution values as Alveofact. As a proof of concept, the interaction of the models with PLGA nanoparticles showed promising results in all experiments for all the three surfactant models. The results show that the choice of components in a model play a crucial role in obtaining reproducible results. The selected models can be used for further studies as synthetic in vitro lung models.

Intermolecular Forces Model for Lipid Microbubble Shells.

Lipid-coated microbubbles are currently used clinically as ultrasound contrast agents for echocardiography and radiology and are being developed for many new diagnostic and therapeutic applications. Accordingly, there is a growing need to engineer specific formulations by employing rational design to guide lipid selection and processing. This approach requires a quantitative relationship between lipid chemistry and interfacial properties of the microbubble shell. Just such a model is proposed here on the basis of lateral Coulomb and van der Waals interactions between lipid head- and tailgroups, using previous coarse graining and force fields developed for molecular dynamics simulations. The model predicts with sufficient accuracy the monolayer permeability, the elasticity as a function of either lipid composition or temperature, and the equilibrium spreading surface tension of the lipid onto an air/water interface. In the future, the intermolecular forces model could be employed to elucidate more complex phenomena and to engineer novel microbubble formulations.

Establishing a model organic film of low volatile compound mixture on aqueous aerosol surface

Long chain fatty acids and alcohols are low volatile species in continental and marine aerosols. We established a surface film model by Langmuir trough to investigate the interfacial properties of sea salts droplets coated by surface active molecules—stearic acid (SA), oleic acid (OA), 1-octadecanol (C18OH) and their mixtures. The aim of this work was to verify the impact of the head group, saturation degree, mixing ratio of different surfactants on miscibility and stability between these compounds in monolayers at the air–water interface. Compared to the organic-coated water droplets, the surface properties of mixed fatty acid and alcohol-coated aqueous sea salt particles are substantially different. Mixed SA/C18OH monolayers are less stable than pure SA or C18OH monolayer at high surface pressure. From the point of view of the geometry, steric hindrance of unsaturated chain is the crucial factor in loose packing of OA monolayer. The negative values of excess mixing areas (ΔAex) for OA/C18OH monolayer on artificial seawater result from the attractive interaction between tail groups. The maximum negative value of excess Gibbs free energies (ΔGex) appears at the equimolecular OA and C18OH. Surface pressure−area (π–A) isotherm combined with equilibrium spreading pressure (ESP) confirms that the tightly packed monolayer formed by C18OH molecules can minimize water evaporation rate of the potential droplet. Different surface properties of organic films coated on aqueous aerosol must have a significant impact on interaction droplet growth. The interactions between fatty acid and alcohol at the air-water interface provide an insight into the nucleation and growth mechanism of droplets covered by film-forming species.

Wetting and interfacial behavior of molten Al–Si alloys on SiC monocrystal substrates: effects of Cu or Zn addition and Pd ion implantation

C-terminated 6H-SiC (0001) single crystal substrates implanted with Pd ions with an energy of 20 keV and three fluences of 5 × 1015, 5 × 1016 and 5 × 1017 ions/cm2 at room temperature. The wetting experiments of SiC by molten Al–10Si, Al–10Si–4Cu and Al–10Si–10Zn were performed by using the sessile drop method in a high vacuum at 1323 K to determine the effects of the third element (Cu and Zn) addition and Pd ion implantation on the wettability of Al–10Si/SiC system. The experimental results showed that the wettability of Al–10Si/SiC system can be improved significantly by adding the 4% Cu with the final contact angle decreasing from ~ 60° to ~ 40°. The final contact angle of Al–10Si/SiC system decreased sharply from ~ 60° to ~ 26° after the addition of 10% Zn, which can be mainly derived from the evaporation behavior of Zn at the interface. The spreading time for reaching the equilibrium or slow spreading stage of the three Al–10Si(– 4Cu, – 10Zn)/SiC systems was prolonged more or less with the increase of Pd implantation dose, which can be mainly attributed to the variation of interfacial interactions between drops and Pd-implanted SiC substrates. Moreover, the final contact angles of Al–10Si/SiC and Al–10Si–4Cu/SiC systems decreased while that of Al–10Si–10Zn/SiC system almost kept constant with increasing the Pd implantation dose.

Tuning reversible cell adhesion to methacrylate-based thermoresponsive polymers: Effects of composition on substrate hydrophobicity and cellular responses.

Thermoresponsive polymer (TRP) cell culture substrates are widely utilized for nonenzymatic, temperature-triggered release of adherent cells. Increasingly, multicomponent TRPs are being developed to facilitate refined control of cell adhesion and detachment, which requires an understanding of the relationships between composition-dependent substrate physicochemical properties and cellular responses. Here, we utilize a homologous series of poly(MEO2 MAx -co-OEGMAy ) brushes with variable copolymer ratio (x/y) to explore the effects of substrate hydrophobicity on L-929 fibroblast adhesion, morphology, and temperature-triggered cell detachment. Substrate hydrophobicity is reported in terms of the equilibrium spreading coefficient (S), and variations in copolymer ratio reveal differential hydrophobicity that is correlated to serum protein adsorption and initial cell attachment at 37°C. Furthermore, quantitative metrics of cell morphology show that cell spreading is enhanced on more hydrophobic surfaces with increased (x/y) ratio, which is further supported by gene expression analysis of biomarkers of cell spreading (e.g., RhoA, Dusp2). Temperature-dependent cell detachment is limited for pure poly(MEO2 MA); however, rapid cell rounding and detachment (<20 min) are evident for all poly(MEO2 MAx -co-OEGMAy ) substrates. These results suggest that increased MEO2 MA content in poly(MEO2 MAx -co-OEGMAy ) substrates elicits enhanced protein adsorption, cell adhesion, and cell spreading; however, integration of small amounts of the more hydrophilic OEGMA unit facilitates both cell attachment/spreading and detachment. This study demonstrates an important role for the composition-dependent control of surface hydrophobicity in the design of multicomponent TRPs for desired biological outcomes. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2416-2428, 2017.

Modeling of adsorption isotherms of probe vapors on aggregates for accurate determination of aggregate surface energy components

The vapor adsorption method is a commonly-used method to determine the surface energy components of aggregates for paving asphalt mixtures. It is crucial to appropriately model the adsorption isotherms of the probe vapors on the aggregates. However, the traditionally-used five-parameter modified Toth model was limited to monolayer adsorption and its fitting parameters were highly dependent of seed values.This study performed the adsorption and desorption of nitrogen on four types of aggregates to characterize the surface pores of the selected aggregates. Micropores, mesopores and macropores in a wide size range were identified at the aggregate surface. Adsorption tests of three probe vapors were then performed on the selected aggregates using the Gravimetric Sorption Analyzer. The measured adsorption isotherms were modeled using multiple isotherm models, including the five-parameter modified Toth model, Dubinin-Astakhov (DA) model and the modified Brunauer-Emmett-Teller (BET) model, for the purpose of comparison. The DA model was identified to be the most appropriate option for modeling the adsorption isotherms of the probe vapors on the aggregates in terms of both physical significance and goodness of model fit. The DA model was able to address the volume filling of micropores and the adsorption in mesopores and macropores; this model also provided the best model fit while its model parameters were independent of the seed values.It was proved that the appropriate modeling of the adsorption isotherms was critical for the accurate determination of the equilibrium spreading pressure of the probe vapor on the aggregate surface, which then directly affected the accuracy of the calculated surface energy components of the aggregates. A deviation in the determined spreading pressure would cause significantly larger variations in the calculated surface energy components. This fact further demonstrated the importance of the accurate modeling of the adsorption isotherms of the probe vapors on the aggregates.

First observation of 3D aggregates in a single-component Langmuir film below the equilibrium spreading pressure

Single component monolayers from Dipalamitoyl Phosphatidyl Ethanolamine head labelled with the fluorescent chromophore NitroBenzoxaDiazole (DP-NBD-PE) were investigated at the air-water interface as Langmuir films and deposited on silicon wafers or glass plates as Langmuir-Blodgett (LB) films. A step compression and monitoring of the pressure relaxation together with domain formation as observed with Brewster angle microscopy suggests that main transition from liquid expanded to liquid compressed state start at around 6.9 mN/m though on the isotherm it starts around 9 mN/m at 25° C. Brewster angle microscopy also reveals a nonuniform structure in the monolayer. 3D aggregates - cylinders with 50 – 150 nm diameter and bilayer height were observed with Atomic Force Microscopy when deposition was carried at 7 mN/m, above the main phase transition but considerably lower than the equilibrium spreading pressure of 19.6 mN/m for this molecule. When LB film deposition is carried just below the main phase transition a uniform height layer of film in a liquid phase is observed with AFM with no structures. Both compression and deposition were carried at very low speeds with large time to relax in ordered to avoid kinetic effects. These 3D aggregates are not due to the transfer process or interaction with the substrate. These aggregates provide a highly developed area combined with monolayer thick structure which can produce very fast and highly sensitive biosensors.

Adsorption of Ions at Uncharged Insoluble Monolayers.

A method is proposed for the experimental determination of the adsorption of inorganic electrolytes at a surface covered with insoluble surfactant monolayer. This task is complicated by the fact that the change of the salt concentration alters both chemical potentials of the electrolyte and the surfactant. Our method resolves the question by combining data for the surface pressure versus area of the monolayer at several salt concentrations with data for the equilibrium spreading pressure of crystals of the surfactant (used to fix a standard state). We applied the method to alcohols spread at the surface of concentrated halide solutions. The measured salt adsorption is positive and has nonmonotonic dependence on the area per surfactant molecule. For the liquid expanded film, depending on the concentration, there is one couple of ions adsorbed per each 3-30 surfactant molecules. We analyzed which ion, the positive or the negative, stands closer to the surface, by measuring the effect of NaCl on the Volta potential of the monolayer. The potentiometric data suggest that Na(+) is specifically adsorbed, while Cl(-) remains in the diffuse layer, i.e., the surface is positively charged. The observed reverse Hofmeister series of the adsorptions of NaF, NaCl, and NaBr suggests the same conclusion holds for all these salts. The force that causes the adsorption of Na(+) seems to be the interaction of the ion with the dipole moment of the monolayer.