Ellipsometry and Infrared Reflection Absorption Spectroscopy of Adsorbed Layers of Soluble Surfactants at the Air-Water Interface

Sensing and structure analysis byin situIR spectroscopy: from mL flow cells to microfluidic applications

In situmid-infrared (MIR) spectroscopy in liquids is an emerging field for the analysis of functional surfaces and chemical reactions. Different basic geometries exist forin situMIR spectroscopy in milliliter (mL) and microfluidic flow cells, such as attenuated total reflection (ATR), simple reflection, transmission and fiber waveguides. After a general introduction of linear opticalin situMIR techniques, the methodology of ATR, ellipsometric and microfluidic applications in single-reflection geometries is presented. Selected examples focusing on thin layers relevant to optical, electronical, polymer, biomedical, sensing and silicon technology are discussed. The development of an optofluidic platform translates IR spectroscopy to the world of micro- and nanofluidics. With the implementation of SEIRA (surface enhanced infrared absorption) interfaces, the sensitivity of optofluidic analyses of biomolecules can be improved significantly. A large variety of enhancement surfaces ranging from tailored nanostructures to metal-island film substrates are promising for this purpose. Meanwhile, time-resolved studies, such as sub-monolayer formation of organic molecules in nL volumes, become available in microscopic or laser-based set-ups. With the adaption of modern brilliant IR sources, such as tunable and broadband IR lasers as well as frequency comb sources, possible applications of far-field IR spectroscopy inin situsensing with high lateral (sub-mm) and time (sub-s) resolution are considerably extended.

Orientation phase transitions of undissociated n-decanoic acid at the air/solution interface revealed by surface pressure and electric potential

The surface pressure (Π) and electric surface potential (ΔV) vs. concentration (c) isotherms of n-decanoic acid (DA) in 1 × 10^-3 mol/dm³ HCl were measured at the air/solution interface - the Π with a du Noüy ring, the ΔV with the vibrating plate (named also the dynamic condenser) method. The DA solutions fulfilled criterion of surface-chemical purity. The complementary Π-c and ΔV-c isotherms were jointly evaluated to obtain dependence of quotient of the effective dipole moment to the interface's permittivity, μ_⊥/ε_s, as a function of chosen adsorption ordinates such as the bulk concentration, c, partial molar area, A, and surface molar fraction of DA, X_DA^s. The crucial point for the analysis is knowledge of the surface excess (Γ) dependence on concentration (c). Since, experimental determination of a Γ-c course is problematic, so far, we used Γ-c courses calculated basing on different adsorption models (Gibbsian, the classical Frumkin' model and the Lunkenheimer' and Hirte' two state approach). Despite, the Γ-c courses determined basing on the different adsorption models differ significantly, the μ_⊥/ε_s dependences on different adsorption's ordinates (c, A or X_DA^s) revealed consistently three local μ_⊥/ε_s maxima of their height increasing with the adsorption coverage. The μ_⊥/ε_s change was recalculated into inclination angle (α_incl) of the total dipole moment vector (μ→) to the interface, assuming ε_s = 1. The μ_⊥/ε_s which reflects polarization orientation is an order parameter used by us for analysis of 2D phase transitions in the monolayer. The three μ_⊥/ε_s local maxima are ascribed by us to three 2D mono-phases, one transferring into the next one of the higher order in the sequence: liquid expanded (L1) → the liquid condensed tilted (L2) → the liquid condensed untilted (L2'). One inflection point appearing in the Π-A isotherm within the region between the μ_⊥/ε_s maxima 2 and 3 indicates that transition of the L2 into the L2' 2D phase is of the first order. Decrease of the μ_⊥/ε_s above the maximum 3 indicates transition into two phase regime by nucleation of aggregates (possibly in form of lamellas) within the L2' phase.

Effect of a PEGylated lipid on the dispersion stability and dynamic surface tension of aqueous DPPC and on the interactions with albumin

Dispersions of dipalmitoylphosphatidylcholine (DPPC) vesicles at 0.1 wt % (1000 ppm) in aqueous isotonic buffer solutions produced by extensive sonication were found to be colloidally stable for hours and days. They also had very low (<10 mN/m) dynamic surface tension minima (DSTM) under pulsating area conditions at 37 degrees C at 20 rpm area pulsation rate. When a 1000 ppm DPPC dispersion was mixed with a stable solution of 1000 ppm bovine serum albumin (BSA), it became colloidally unstable, aggregating within minutes, implying that heterocoagulation between lipid vesicles and albumin takes place. The heterocoagulated dispersion produced high DSTM because the lipid transport rate to the interface became slower. Moreover, the protein may have been transported to the surface faster and adsorbed more than the lipid at the surface. DPPC lipid vesicles were modified for reducing aggregation with other vesicles or with the protein with the addition of a small weight fraction of a neutral "PEGylated" lipid, with a covalently bonded poly(ethylene glycol) (PEG) group. The mixed vesicles were found to be quite more stable than the DPPC vesicles, remaining stable for months, apparently stabilized by steric forces. The colloidal stability at the initial stages of coagulation was evaluated quantitatively from the Fuchs-Smoluchowski stability ratio W. When the modified lipid vesicle dispersion was mixed with the albumin, the vesicles showed no tendency to aggregate with the albumin molecules for days, also probably because of steric repulsion between the PEGylated lipid and the protein. Finally, the mixed lipid dispersions maintained their low DSTM as did the DPPC vesicles without the albumin, and also in the presence of albumin. The results have implications on the use of DPPC or DPPC-based lipids in treating alveolar respiratory diseases without albumin inhibition of their surface tension lowering ability.

Implications of interfacial characteristics of food foaming agents in foam formulations

The manufacture of food dispersions (emulsions and foams) with specific quality attributes depends on the selection of the most appropriate raw materials and processing conditions. These dispersions being thermodynamically unstable require the use of emulsifiers (proteins, lipids, phospholipids, surfactants etc.). Emulsifiers typically coexist in the interfacial layer with specific functions in the processing and properties of the final product. The optimum use of emulsifiers depends on our knowledge of their interfacial physico-chemical characteristics - such as surface activity, amount adsorbed, structure, thickness, topography, ability to desorb (stability), lateral mobility, interactions between adsorbed molecules, ability to change conformation, interfacial rheological properties, etc. -, the kinetics of film formation and other associated physico-chemical properties at fluid interfaces. These monolayers constitute well defined systems for the analysis of food colloids at the micro- and nano-scale level, with several advantages for fundamental studies. In the present review we are concerned with the analysis of physico-chemical properties of emulsifier films at fluid interfaces in relation to foaming. Information about the above properties would be very helpful in the prediction of optimised formulations for food foams. We concluded that at surface pressures lower than that of monolayer saturation the foaming capacity is low, or even zero. A close relationship was observed between foaming capacity and the rate of diffusion of the foaming agent to the air-water interface. However, the foam stability correlates with the properties of the film at long-term adsorption.

Adsorption and direct probing of fibrinogen and sodium myristate at the air/water interface

Fibrinogen (FB), a serum protein, is considered a major inhibitor of lung surfactant function at the lining layer of the alveoli. In this study, the adsorption of aqueous bovine FB at the air/water interface was investigated with tensiometry and directly probed for the first time with ellipsometry and infrared reflection adsorption spectroscopy (IRRAS). The tension results show that FB has moderate surface activity. The surface densities of FB were calculated by using two different ellipsometry models to range from 3+/-0.2 to 17+/-2 mg/m2, for 7.5 to 750 ppm of FB in water at 25 degrees C. Although FB at concentrations from 75 to 750 ppm reached about the same steady surface tension value, the surface densities at 750 ppm FB were substantially larger. The same techniques were used for studying aqueous mixtures of 7.5 to 750 ppm FB with 2 mM of sodium myristate (SM) to investigate a possible interaction of the SM with the protein. The behavior of the FB/SM mixtures was found to be close to that of SM alone. The surface tension of the FB/SM mixtures reached values less than 10 mN/m under surface area oscillation at 20 or 80 rpm. These results and the ellipsometry and the IRRAS results indicate that at a concentration of 2 mM SM, FB, up to 750 ppm, does not inhibit the surfactant surface-tension-lowering function. In certain cases the results demonstrate that FB and SM may act cooperatively in lowering the surface tension.

Design and Interfacial Assembly of a New Series of Gemini Amphiphiles with Hydrophilic Poly(ethyleneamine) Spacers

A new series of gemini amphiphiles containing two Schiff base moieties linked by the poly(ethyleneamine) with different lengths were designed, and their interfacial assemblies were investigated. Condensed monolayers were obtained on nearly neutral subphase where the hydrophilic spacers were found to immerse into the subphase. On strong alkaline and acidic subphase, the headgroup and the spacer of the gemini amphiphiles underwent dissociation and protonation, respectively, resulting in the enlargement of the molecular areas. Flat and uniform domains were obtained for the monolayers from nearly neutral subphase; flower-like or dendritic domains were observed for the films transferred from strong acidic subphase. On the other hand, when an anionic tetrakis(4-sulfonatonphenyl)porphine (TPPS) was added into an acidic subphase, an in situ complex formation between the gemini amphiphiles and TPPS occurred. The complex monolayers were transferred onto solid substrate and TPPS existed predominantly as J-aggregate in the complex films. Due to the multisited positive charges in the spacer on acidic subphase, the complex films of gemini amphiphiles with TPPS appeared as short fiber or nanorod structures and formed two-dimensional (2D) conglomerate chiral domains.

Supramolecular assemblies of a new series of gemini-type schiff base amphiphiles at the air/water interface: in situ coordination, interfacial nanoarchitectures, and spacer effect

Great interest has been devoted to the gemini amphiphiles because of their unique properties. In this article, we report some interesting properties of the interfacial films formed by a series of newly designed gemini amphiphiles containing the Schiff base moiety. This novel series of gemini amphiphiles with their Schiff base headgroups linked by a hydrophobic alkyl spacer (BisSBC18Cn, n = 2, 4, 6, 8, 10) could be spread to form stable monolayers and coordinated with Cu(Ac)(2) in situ in the monolayer. The alkyl spacer in the amphiphiles has a great effect on the regulation of the properties of the Langmuir monolayers. A maximum limiting molecular area was observed for the monolayers of the gemini amphiphile with the spacer length of hexa- or octamethylene groups. Both the monolayers on water and on the aqueous Cu(Ac)(2) subphase were transferred onto solid substrates, and different morphologies were observed for films with different spacers. Nanonail and tapelike morphologies were observed for amphiphile films with shorter spacers (n = 2 and 4) on the water surface. Wormlike morphologies were observed for gemini films with longer spacers of C(8) and C(10) when coordinated with Cu(Ac)(2). An interdigitated layer structure was supposed to form in the multilayer films transferred from water or the aqueous Cu(Ac)(2) subphase.

Competitive adsorption of fibrinogen and dipalmitoylphosphatidylcholine at the air/aqueous interface

The competitive adsorption of fibrinogen (FB) and DPPC at the air/aqueous interface, in phosphate buffer saline at 25 degrees C, was studied with tensiometry, infrared reflection absorption spectroscopy (IRRAS), and ellipsometry. For FB/DPPC mixtures with 750 ppm (0.075 wt%) FB and 1000 ppm (0.10 wt%) DPPC, the tension behavior was found to be similar to that of FB when alone, even with DPPC and FB being at the interface. Thus, FB interferes with adsorption of DPPC and inhibits its surface tension lowering ability. When FB protein is introduced in the solution after a DPPC monolayer has formed, the adsorption of FB is inhibited by the DPPC monolayer. When a DPPC monolayer is spread onto a solution with a preadsorbed FB layer, the DPPC monolayer excludes FB from the surface and controls the tension behavior with little inhibition by FB. When a DPPC dispersion is introduced with the Trurnit method, or sprayed dropwise, onto an aqueous FB/DPPC surfaces, the DPPC layer formed on the surface prevents the adsorption of FB and dominates the surface tension behavior. These results have implications in controlling the inhibition of lung surfactant tension behavior by serum proteins, when they leak at the alveolar lining layer, and in developing surfactant replacement therapies for alveolar respiratory diseases.

Displacement of fibrinogen from the air/aqueous interface by dilauroylphosphatidylcholine lipid

Fibrinogen (FB) and other serum proteins leak into the aqueous alveolar lining layer due to lung injuries. The adsorption of these serum proteins at the air/aqueous interface can produce higher surface tensions than the pulmonary lipids, and acute respiratory distress syndrome (ARDS) can ensue. By having a molecular adsorption mechanism, as compared to a particulate adsorption mechanism of other longer chain lipids, dilauroylphosphatidylcholine (DLPC) lipid can expel FB from the air/aqueous interface at 25 degrees C, in water or in phosphate-buffered saline, as proven by tensiometry (also at 37 degrees C), ellipsometry, and infrared reflection-absorption spectroscopy. Moreover, before FB is displaced by DLPC at the interface, there is a substantial initial enhancement in the FB adsorption, consistent with some interaction or binding of DLPC with FB to produce a more hydrophobic protein surface. After the FB molecules have been displaced by DLPC, or when DLPC has already adsorbed at the interface, FB molecules are less favored to adsorb near the DLPC monolayer with the lecithin headgroups facing toward them. The results have implications for possible uses of DLPC lipid in potential lung surfactant formulations in treating patients with ARDS.

Ellipsometric characterization of ethylene oxide-butylene oxide diblock copolymer adsorption at the air-water interface

Ellipsometry was used to determine the adsorbed layer thickness (d) and the surface excess (adsorbed amount, Gamma) of a nonionic diblock copolymer, E(106)B(16), of poly(ethylene oxide) (E) and poly(butylene oxide) (B) at the air-water interface. The results were obtained (i) by the conventional ellipsometric evaluation procedure using the change of both ellipsometric angles Psi and Delta and (ii) by using the change of Delta only and assuming values of the layer thickness. It was demonstrated that the calculated surface excesses from the different methods were in close agreement, independent of the evaluation procedure, with a plateau adsorption of about 2.5 mg/m(2) (400 A(2)/molecule). Furthermore, the amount of E(106)B(16) adsorbed at the air-water interface was found to be almost identical to that adsorbed from aqueous solution onto a hydrophobic solid surface. In addition, the possibility to use combined measurements with H(2)O or D(2)O as substrates to calculate values of d and Gamma was investigated and discussed. We also briefly discuss within which limits the Gibbs equation can be used to determine the surface excess of polydisperse block copolymers.

Expulsion of bovine serum albumin from the air/water interface by a sparingly soluble lecithin lipid

Dynamic surface tensiometry, ellipsometry, and infrared reflection-absorption spectroscopy (IRRAS) were used to study the dynamic adsorption and surface tensions of dilauroylphosphatidylcholine (DLPC) in the presence of bovine serum albumin (BSA). Results show that the equilibrium adsorbed layers consist mostly of DLPC, which can produce dynamic surface tensions (1 mN/m) as low as the more successful lung surfactant replacement formulations. When the aqueous surface expands and contracts sinusoidally, BSA can coadsorb and lead to slightly higher dynamic surface tensions than when DLPC is alone. Similar results were obtained with BSA and sodium myristate [McClellan and Franses, Colloids Surf. B 30 (2003) 1]. Expulsion of the BSA in the layer by DLPC can take from 5 to 15 min, depending on relative concentrations and history of solute addition. This is shown by tensiometry measurements on mixtures, and also by injecting aqueous DLPC underneath adsorbed BSA layers and probing the surface layer with ellipsometry and IRRAS. Albumin layers from buffer solutions aged up to 30 h can be expelled by DLPC. In pure water, there is an initial enhancement in protein adsorption after the DLPC is injected. This can be explained by the hypothesis that DLPC molecules bind with BSA molecules to form a hydrophobic lipoprotein complex, which is more hydrophobic than the protein itself. Since DLPC produces lower surface energy than BSA and--being slightly soluble--adsorbs to the surface by a molecular mechanism, it fulfills the thermodynamic and dynamic requirements for expelling the BSA from the surface. The results have implications for minimizing lung surfactant inhibition by serum proteins, as it occurs in the cases of adult or acute respiratory distress syndrome.

Modeling of Equilibrium Adsorption and Surface Tension of Cationic Gemini Surfactants

A series of equilibrium tension models are used to evaluate the adsorption behavior of a novel class of lipoaminoacid gemini cationic surfactants, N(alpha),N(omega)-bis(long-chain N(alpha)-acylarginine)alpha,omega-dialkylamides or bis(Args). For purposes of comparison, the monomer LAM (the methyl ester of N(alpha)-lauroyl arginine) was also examined. These surfactants are of particular interest for both their low toxicity and biocompatibility. The tension models are based on the Gibbs adsorption isotherm and classified as "ionic" when the surface charge and the electric double layer are accounted for or as "pseudo-nonionic" when the surface charge is ignored. Both model predictions and fitted parameter values are evaluated with respect to physical plausibility and overall goodness of fit to the available tension and density data. In particular, the inferred values for the standard Gibbs free energy of adsorption DeltaG degrees, determined from an equilibrium constant defined on a nondimensional basis, without including artifacts due to an electrostatic contribution, are analyzed. The most reliable values of DeltaG degrees are found with the combined model to range from -110 to -120 kJ mol(-1) for the three dimers examined and -80 kJ mol(-1) for the monomer. For spacer chain lengths n=3, 6, or 9, the maximum surface area of surfactant adsorption and the maximum free energy of adsorption are observed for the surfactant with the spacer chain length of 6. Copyright 2001 Academic Press.