Gibbs Monolayers Research Articles

Adsorption of water-soluble polymers with surfactant character.: Adsorption kinetics and equilibrium properties

A comparative study between Langmuir and Gibbs monolayers of a hyperbranched polyol, poly(propylene glycol) homopolymers, and poly(propylene glycol)–poly(ethylene glycol) copolymers with different structure and molecular weight, is reported. Dynamic surface tension (DST) and surface pressure measurements have been carried out to characterize these amphiphilic water-soluble polymers. The adsorption kinetics results are consistent with a rapid diffusion stage followed by a slow reorganization at the air–water interface. The characteristic times of these steps, calculated by the Joos model, point out differences among the polymers in the diffusion rate and rearrangement mechanisms for diluted solutions. Short time analysis of DST data leads to diffusion coefficients in qualitative agreement with the diffusion times calculated with Joos' model. Spread monolayers remain stable for long periods of time. The desorption process seems quite inoperative. As a consequence, the surface pressure of the spread monolayers can be studied over a broad surface concentration range. 2D first-order phase transitions have been evidenced from plateaux observed in Langmuir and Gibbs isotherms. It has been found that Gibbs monolayers lead to lower surface tension states than the Langmuir ones.

Analysis of Gibbs Monolayer Adsorbed at the Toluene/Water Interface by UV−Visible Partial Internal Reflection Spectrometry

Gibbs monolayers of lipophilic tetraphenylporphyrinatomanganese(III) and hydrophilic diacid of meso-tetrakis(4-sulfonatopheny)porphyrin adsorbed at the liquid-liquid interface have been analyzed by UV-visible external reflection (ER) and partial internal reflection (PIR) spectra measured at different angles of incidence. The angle-dependent ER and PIR spectra over the Brewster angles (thetaERB and thetaIRB) have readily been measured at the toluene/water interface. As preliminarily expected in our previous study, the present study has first proved that the reflection-absorbance of UV-visible PIR spectra quantitatively agrees with the theoretical calculations for the Gibbs monolayer over thetaIRB. In addition, it has also been proved that the absorbance of the PIR spectra is greatly enhanced in comparison to that of the ATR spectra. The enhancement is caused by an optical effect in the monolayer sandwiched between two phases of toluene and water that have different but refractive indices close to each other. This optical enhancement requires an optically perfect contact between the phases, which is difficult to prepare for a solid-solid contact. At the liquid/liquid interface, however, an ideal optical contact is easily realized, which makes the enhancement as much as the theoretical expectation. The PIR spectrometry will be recognized to be a new high-sensitive analytical tool to study Gibbs monolayer at the liquid/liquid interface.

Surface Phase Transition of C12E1 at the Air/Water Interface: A Study by Dynamic Surface Tension, External RA FT-IR, and 2D IR Correlation Methods

The surface conformational states of the Gibbs monolayer of ethylene glycol mono-n-dodecyl ether (C(12)E(1)) at the air/water interface was studied using dynamic surface tension, external reflection-absorption FT-IR spectroscopy (ERA FT-IR), and two-dimensional infrared (2D IR) correlation methods at constant temperature. The dynamic surface tensions were measured at different bulk concentrations of C(12)E(1), and it was observed that a constant surface tension region appears at approximately 38.5 mN m(-1) in a dynamic surface tension profile at concentrations higher than 11 micromol kg(-1). This constant surface tension region corresponds to the surface phase transition from liquid expanded (LE) to liquid condensed (LC). Two sets of ERA FT-IR spectra were collected, one at different bulk concentrations but after equilibrium time (equilibrium measurements) and another at constant bulk concentration (m = 16 micromol kg(-1)) but at different times (dynamic measurements). The first set of these measurements show that the peak area increases in the range of 11 < m < or = 16 micromol kg(-1), which means the increase in the number of surfactant molecules at the air/water interface. Also, the wavenumber of antisymmetric CH(2) stretching decreases gradually from approximately 2923 cm(-1) (for 10 and 11 micromol kg(-1)) to approximately 2918 cm(-1) (for m > or = 16 micromol kg(-1)) with increasing concentration. The wavenumbers of 2923 and 2918 cm(-1) were assigned to LE and LC phases, respectively, and the decrease of wavenumber in the concentration range of 11 < m < or = 16 micromol kg(-1) were correlated to the surface phase transition (LE --> LC), or in other words, in the mentioned concentration range, two phases coexist. The dynamic ERA FT-IR measurements at 16 micromol kg(-1) also confirm the surface phase transition from LE to LC. The 2D IR correlation method was applied to the both equilibrium and dynamic IR spectra of the C(12)E(1) monolayer. The synchronous correlation maps show two strong autopeaks at approximately 2922 and approximately 2851 cm(-1) and also show a strong correlation (cross-peaks) between antisymmetric CH(2) stretching (nu(a)) and symmetric CH(2) stretching (nu(s)). The asynchronous correlation maps show that both observed bands of nu(a) and nu(s) in one-dimensional IR split into two components with the characteristic of overlapped bands, which reveals the coexistence of two phases (LE and LC) at the interface at 11 < m < or = 16 micromol kg(-1). The synchronous and asynchronous maps that were obtained from dynamic IR spectra closely resembled the equilibrium map.

Insertion of Lipidated Ras Proteins into Lipid Monolayers Studied by Infrared Reflection Absorption Spectroscopy (IRRAS)

Ras proteins have to be associated with the inner leaflet of the plasma membrane to perform their signaling functions. This membrane targeting and binding is controlled by post-translational covalent attachment of farnesyl and palmitoyl chains to cysteines in the membrane anchor region of the N- and H-Ras isoforms. Two N-Ras lipoproteins were investigated, namely a farnesylated and hexadecylated protein, presenting the natural hydrophobic modifications and a doubly hexadecylated construct, respectively. The proteins are surface active and form a Gibbs monolayer at the air-D 2O interface. The contours of the amide-I bands were analyzed using infrared reflection absorption spectroscopy (IRRAS). Langmuir monolayers of a mixture of POPC, brain sphingomyelin, and cholesterol were used as half of a model biomembrane to study the insertion of these N-Ras proteins. They insert with their hydrophobic anchors into lipid monolayers but at higher surface pressures (30 mN/m); the farnesylated and hexadecylated protein desorbs completely from the monolayer, whereas the doubly hexadecylated protein remains incorporated. During the insertion process, changes in the orientation of the protein secondary structure were detected by comparison with simulated IRRA spectra, based on the information on the relative orientation of the secondary structure elements from the protein crystal structure data.

Gibbs monolayer of ionic liquid + H 2O mixtures studied by surface tension measurement and sum-frequency generation spectroscopy

Room-temperature ionic liquid (RIL) + H 2O mixture system was investigated using surface tension measurement and surface specific IR–visible sum-frequency generation (SFG) spectroscopy. The surface tension measured as a function of a bulk mole fraction of the ionic liquid indicated that the liquid surface even at very low bulk concentration of the ionic liquid is covered mostly by the cations. A slight increase of surface tension from 0.016 to 0.05 bulk mole fraction suggested that the anions start to appear at the surface in this concentration range. After 0.05 bulk mole fraction, the cations and the anions both occupy the surface, and the chemical composition and the surface density are expected to change little with the further addition of the ionic liquid. The result from the SFG measured at different concentrations is in agreement with the above findings. This result and the similar studies on methanol + water mixtures suggested that SFG is mainly sensitive to the surfaces excess molecules at the air/mixture interface.

Phase diagram of n-tetradecyl phosphate in Gibbs monolayers

A phase diagram for Gibbs monolayers of n-tetradecyl phosphate ( n-TDP) has been constructed from the surface pressure–time isotherms and the direct observation of the monolayer morphology using Brewster angle microscopy (BAM). Three different phases, which are gas (G), liquid expanded (LE) and liquid condensed (LC) are observed during the formation of the monolayers under different conditions. All of these phases coexist at the triple point of 6.5 °C of n-TDP monolayers. Below this triple point, a first-order G-LC phase transition has been found to occur. Above the triple point, a first-order G-LE phase transition is followed by another first-order LE-LC phase transition. BAM images confirmed the order of phase transitions and the nature of phases. The critical temperature for the LC phase formation has been found to be 36.5 °C. All the findings are in line with the present understanding of the spread monolayers.

Kinetic Appearance of First-Order Gas−Liquid Expanded and Liquid Expanded−Liquid Condensed Phase Transitions below the Triple Point

Phase diagram of Gibbs monolayers of mixtures containing n-hexadecyl phosphate (n-HDP) and L-arginine (L-arg) at a molar ratio of 1:2 has been constructed by measuring surface-pressure-time (pi-t) isotherms with film balance and by observing monolayer morphology with Brewster angle microscopy (BAM). This phase diagram shows a triple point for gas (G), liquid expanded (LE), and liquid condensed (LC) phases at around 6.7 degrees C. Above this triple point, a first-order G-LE phase transition occurring at 0 surface pressure is followed by another first-order LE-LC phase transition taking place at a certain higher surface pressure that depends upon temperature. The BAM observation supports these results. Below the triple point, the pi-t measurements show only one first-order phase transition that should be G-LC. All of these findings are in agreement with the general phase diagram of the spread monolayers. However, the BAM observation at a temperature below the triple point shows that the thermodynamically allowed G-LC phase transition is, in fact, a combination of the G-LE and LE-LC phase transitions. The latter two-phase transitions are separated by time and not by the surface pressure, indicating that the G-LC phase transition is kinetically separated into these two-phase transitions. The position of the LE phase below the triple point in the phase diagram is along the phase boundary between the G and LC phases.

Autonomously Moving Nanorods at a Viscous Interface

We study the autonomous motion of catalytic nanorods in Gibbs monolayers. The catalytic activity of the rods on a hydrogen peroxide aqueous subphase gives rise to anomalous translational and rotational diffusion. The rods perform a Levy-walk superdiffusive motion that can be decomposed into thermal orientation fluctuations and an active motion of the rods with a constant velocity along their long axis. Since interfacial dissipation increases relative to bulk phase dissipation when miniaturizing the size of objects moving in the interface, the autonomous nanorods allow for precise measurements of surface shear viscosities as low as a few nN s/m. The cross over from active motion toward passive diffusion when increasing the surfactant concentration is explained by a loss of friction asymmetry of the rods.

Effect of an amino acid on the surface phase behavior of n-hexadecyl phosphate in Gibbs adsorption layers

Interaction of an amino acid, L-arginine (L-arg) with the Gibbs monolayers of n-hexadecyl phosphate ( n-HDP) has been studied by measuring surface pressure ( π)–time ( t) isotherms with a film balance and observing monolayer morphology with a Brewster angle microscopy (BAM). In the aqueous solution of the mixtures of the components, L-arg acts as a cation (L-arg +) and forms salt with n-HDP. Accordingly, the Gibbs monolayers of n-HDP experience a dramatic change in its phase behavior in the presence of L-arg. The monolayers of n-HDP show a first-order gas-intermediate (G-I) phase transition followed by an unusual second-order I-liquid condensed (LC) phase transition at 15 °C. In contrast to the pure system, the mixed monolayers show a first-order G-liquid expanded (LE) phase transition at zero surface pressure, which is followed by another first-order LE–LC phase transition at a certain higher surface pressure at the same temperature. With increasing the concentration of L-arg in the mixtures containing the same concentration of n-HDP, the critical surface pressure ( π c) necessary for the G-LE phase transition remains constant while that of the LE–LC ( π c(LE–LC)) phase transition increases linearly up to a composition of 1:2 of n-HDP and L-arg. A further increase in the concentration of L-arg allows the π c(LE–LC) to increase slowly. All of these results are explained by considering the formation of mono- and di-L-arg salts in the mixed systems.

Spectroscopic Study of Lysopalmitoylphosphatidylcholine Newton Black Films

Lysopalmitoylphosphatidylcholine (LPC) black films have been studied by confocal Raman spectroscopy and their spectra analyzed and compared to their counterparts obtained from LPC in the solid state and aqueous solution. It appears that LPC is able to form stable and highly ordered black films, despite the presence of only one hydrophobic chain in this molecule. A complementary infrared study of LPC Gibbs monolayers suggests that the whole LPC polar head is perpendicular to the air/water interface. Such an orientation could explain the high order and the close packing observed in black films.

Interaction of an organic cation with Gibbs monolayers of n-hexadecyl phosphate

Surface phase behavior of n-hexadecyl phosphate ( n-HDP) and its mixture with l-arginine (L-arg), which behaves as l-argininium cation (L-arg +) in aqueous solution, at a molar ratio 2:3 in Gibbs adsorption layers has been studied by film balance, Brewster angle microscopy (BAM) and surface tensiometry at ⩽20 °C. The monolayers of n-HDP show three phases that are gas (G), intermediate (I) and liquid condensed (LC), and two phase transitions. A first-order G-I phase transition that is followed by a second-order I-LC phase transition is found in these monolayers. Although the monolayers of the mixtures containing n-HDP and L-arg show three phases, the nature of the middle phase is different from that of the n-HDP monolayers. The three phases observed for the mixed systems are G, liquid expanded (LE) and LC phases. A first-order G-LE phase transition is found at a low surface pressure at ⩾10 °C. This transition is followed by another first-order LE-LC phase transition at a certain higher surface pressure. The first-order nature of the phase transitions for both the systems is confirmed by the presence of plateaus in the π – t curves, which are accompanied by two surface phases. A second-order phase transition in the monolayers of n-HDP is indicated by a gradual change in the surface morphology, from a uniformly bright isotropic to an anisotropic mosaic textured phase, which is accompanied by a continuous change in the surface pressure. The domains formed during the first-order phase transition in the adsorption layers of n-HDP are circular and remain unaffected by changing the temperature. Although the domains of an LE phase are circular, those of an LC phase at the latter transition are fractal in the mixed system. A further branching of the arms of the fractal domains is found to occur by an increase in the temperature. All the results are explained by considering salt formation between anion from n-HDP and L-arg +.

Surface Rheology of PEO−PPO−PEO Triblock Copolymers at the Air−Water Interface: Comparison of Spread and Adsorbed Layers

The dilatational rheological properties of monolayers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)-type block copolymers at the air-water interface have been investigated by employing an oscillating ring trough method. The properties of adsorbed monolayers were compared to spread layers over a range of surface concentrations. The studied polymers were PEO26-PPO39-PEO26 (P85), PEO103-PPO40-PEO103 (F88), and PEO99-PPO65-PEO99 (F127). Thus, two of the polymers have similar PPO block size and two of them have similar PEO block size, which allows us to draw conclusions about the relationship between molecular structure and surface dilatational rheology. The dilatational properties of adsorbed monolayers were investigated as a function of time and bulk solution concentration. The time dependence was found to be rather complex, reflecting structural changes in the layer. When the dilatational modulus measured at different concentrations was replotted as a function of surface pressure, one unique master curve was obtained for each polymer. It was found that the dilatational behavior of spread (Langmuir) and adsorbed (Gibbs) monolayers of the same polymer is close to identical up to surface concentrations of approximately 0.7 mg/m2. At higher coverage, the properties are qualitatively alike with respect to dilatational modulus, although some differences are noticeable. Relaxation processes take place mainly within the interfacial layers by a redistribution of polymer segments. Several conformational transitions were shown to occur as the area per molecule decreased. PEO desorbs significantly from the interface at segmental areas below 20 A(2), while at higher surface coverage, we propose that segments of PPO are forced to leave the interface to form a mixed sublayer in the aqueous region.

Phases and phase transitions in Gibbs monolayers of an alkyl phosphate surfactant

We present the adsorption kinetics and the surface phase behavior of water-soluble n-tetradecyl phosphate ( n-TDP) at the air–water interface by film balance and Brewster angle microscopy (BAM). The relaxation of the surface pressure at about zero value in the surface pressure ( π ) –time ( t ) adsorption isotherm is found to occur from 2 to 20 °C with appropriate concentrations of the amphiphile. These plateaus are accompanied by two surface phases, confirming that the relaxation of the surface pressure is caused by a first-order phase transition. Only this phase transition is observed at <6.5 °C and it is considered as a gas (G)–liquid condensed (LC) phase transition. Above 6.5 °C, the phase transition at zero surface pressure is followed by another phase transition, which is indicated by the presence of cusp points in the π– t curves at different temperatures. Each of the cusp points is followed by a plateau, which is accompanied by two surface phases, indicating that the latter transitions are also first-order in nature. At >6.5 °C, the former transition is classified as a first-order G–liquid expanded (LE) phase transition, while the latter transition is grouped into a first-order LE–LC phase transition. The critical surface pressure ( π c) necessary for the G–LC and G–LE phase transitions is zero and remains constant all over the studied temperatures, whereas that for the LE–LC transition increases linearly with increasing temperature. Based on these results, we construct a rather elaborated phase diagram that shows that the triple point for Gibbs monolayers of n-TDP is 6.5 °C. All the results are consistent with the present understanding of the Langmuir monolayers of insoluble amphiphiles at the air–water interface.

Resonance raman spectra and tautomeric structures of a surface-active azo dye, CI Acid Red 138, adsorbed at the aqueous solution–air interface

Raman spectra of Gibbs monolayers adsorbed from the aqueous solution to the solution–air interface were measured for the first time using resonance Raman technique against a surface-active azo dye, CI Acid Red 138 (AR138). The tautomeric structure change of AR138 from the azo form to the protonated hydrazone form occurred upon adsorption from a basic solution. This structural change was principally the same as those observed previously for the adsorption at the aqueous solution-CCl 4 interface and for micellization.

N-Decyl-glucopyranoside and n-Decyl-maltopyranoside Gibbs Monolayers. Phase Changes in the Dilute Liquid-Expanded Range

Surface tension isotherms were recorded for n-decyl-beta-d-glucopyranoside (Glu) and n-decyl-beta-D-maltopyranoside (Mal) solutions at temperatures of 8, 22, and 29 degrees C. Comparison was made with isotherms of n-decyl-beta-D-thiomaltopyranoside (S-Mal) at 22 degrees C. In addition to the transition from the gaseous to the liquid-expanded (LE) state, a second transition was observed in the early stages of the LE regime for Glu, Mal, and S-Mal at room temperature. The adsorption isotherm of Mal and Glu obtained at 22 degrees C shows the presence of an adsorption step at an average area/molecule of about 79 A2 between, approximately, 0.02 and 0.1 mM (the critical micelle concentration (cmc) is 2 mM) and 0.015 and 0.03 mM (the cmc is 2 mM), respectively. Similarly, for S-Mal an adsorption plateau is observed at 70 A2 between 0.01 and 0.03 mM (the cmc is 0.7 mM). From the temperature dependence of the surface tension, we have seen that there are considerable differences in the adsorption of Glu and Mal. For Mal, the adsorption plateau is also observed at 29 degrees C at around 79 A2, whereas Glu exhibits no adsorption plateau at this temperature. At 8 degrees C, both Mal and Glu exhibit saturation behavior in the dilute part of the liquid-expanded range, but at this temperature the average molecular areas are lower than at 22 degrees C: around 66 A2 for Glu and 75 A2 for Mal. Thus, the temperature sensitivity of Glu is considerably greater than for Mal in this range. The saturation regime coincides with a pronounced surface entropy minimum for Mal. The transition in the dilute liquid-expanded range supposedly occurs from a state with deformed surface micelles arranged in a hexagonal pattern, referred to as the granular range, to a true LE monolayer with a fluid hydrocarbon tail layer covering the entire surface.

Gibbs Monolayers at the Air/Water Interface: Surface Partitioning and Lateral Mobility of an Electrochemically Active Surfactant

Monolayer films of a water-soluble surfactant, 4-octaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy (C8-TEMPO) were investigated at the air/water interface. An electrochemical, horizontal touch method was developed to measure the equilibrium surface concentrations (gamma) of C8TEMPO. The dependence of gamma on the solution concentration followed a Langmuir isotherm and yielded the partition constant K = (2.3+/-0.2) x 10(4) M(-1). These results were verified by surface tension measurements and Brewster angle microscopy. Within experimental error, the same K values were obtained. The lateral diffusion constants vs surface concentration of this molecule were measured by 2D voltammetry. In these experiments, the component of the oxidation current due to C8TEMPO in the bulk of the solution was subtracted from the total measured current to obtain the component due to the lateral surface diffusion. In the ange of mean molecular areas from 120 to 400 A2/molecule, the lateral diffusion constant of C8TEMPO increased from 1.0 x 10(-6) to 1.0 x 10(-5) cm2/s. The latter value is about 2.5 times larger than the C8TEMPO diffusion constant in bulk water. Comparison of the lateral mobilities of C8TEMPO and two longer alkane chain, water-insoluble homologues, C14TEMPO and C18TEMPO, showed no statistically significant differences.

Interfacial properties of a silicone copolymer demulsifier at the air/water interface

Crude oil/water emulsion stability is one of the major obstacles to efficient off-shore separation of undesired aqueous phase. Chemical demulsifiers based on polysiloxanes have been successfully tested as accelerators of phase separation but the mechanism of coalescence is not fully understood. Better understanding of interfacial properties in the presence of demulsifier should give better insight in their mode of action. Comprehensive study of the interfacial properties of small water-soluble PEO–(PDMS)–PEO triblock copolymer is presented in this paper. Insoluble monolayers are formed by deposition of dissolved polymer at the air/water interface but also by adsorption from aqueous solution of the polymer. High affinity of the polymer molecule for the interface i.e. too high the energy needed to remove the molecules from the interface is at the origin of such behaviour. The elastic moduli are function of the surface pressure in the film and are very similar in both Gibbs and Langmuir monolayers.

Surface activity and interaction of StarD7 with phospholipid monolayers

StarD7 protein forms stable Gibbs and Langmuir monolayers at the air–buffer interface showing marked surface activity. The latter is enhanced by penetration into phospholipid films at an initial surface pressure above the protein’s own equilibrium adsorption surface pressure to a lipid-free interface. The protein–phospholipid stabilizing interactions at the interface depend on the lipid, with preference for phosphatidylserine, cholesterol, and phosphatidylglycerol, and the increases of lateral surface pressure generated are comparable to those of other membrane-active proteins. The surface activity of StarD7 is strong enough to thermodynamically drive and retain StarD7 at the lipid membrane interface where it may undergo lipid-dependent reorganization as indicated by changes of surface pressure and electrostatics.

Polymer Micelle Formation without Gibbs Monolayer Formation: Synthesis and Characteristic Behavior of an Amphiphilic Diblock Copolymer Having Strong Acid Groups

A novel amphiphilic diblock copolymer having sulfonic acid groups in its hydrophilic chain with poly(diethylsilacyclobutane) as a hydrophobic segment was synthesized by living anionic polymerization followed by treatment with 1,3-propane sultone. The polymer backbone with a degree of polymerization of 18 (hydrophobic):37 (hydrophilic) was synthesized, and three kinds of diblock copolymers with different degrees of sulfonation (α = 0, 0.50, 0.81) were prepared as sodium salts. The surface tension of aqueous solutions of highly sulfonated polymers as including the α = 0 (all carboxylic acid) polymer did not decrease with polymer concentration. The solutions did not show foam formation even after vigorous shaking, and almost no adsorption of polymer molecules was observed at the air/water interface by X-ray reflectivity measurement. However, micelle formation with a radius of 140−200 Å was confirmed by dynamic light scattering and small-angle X-ray scattering experiments. Hence, this polymer showed “micelle formation” without “Gibbs monolayer formation” in water. The surface tension decrease in 0.5 M NaCl aqueous solution was also small, but adsorption of polymer was observed by X-ray reflectivity in addition to formation of foam by shaking. The strong ionic character of this novel class of amphiphilic polymer is thought to be the origin of this curious behavior.

Anomalous Damping of the Capillary Waves at the Air−Water Interface of a Soluble Triblock Copolymer

The dilational viscoelasticity of Gibbs and Langmuir monolayers of pluronic F-68 (PEO76−PPO29−PEO76) has been studied by capillary wave techniques over a broad frequency range. The results show that the behaviors of Gibbs and Langmuir monolayers are equivalent in the low surface pressure range. For concentrated monolayers, the dynamic elasticity ε shows a noticeable frequency dependence, and the concentration dependence of ε (at constant frequency) is not compatible with the classical Lucassen−Van den Tempel model. Moreover, in this concentration range, the dilational viscosity takes apparent negative values. The spectra of light scattered by thermal capillary waves have been analyzed in terms of two recently proposed dispersion equations. The equation of Buzza et al. (J. Chem. Phys. 1998, 109, 5008) is able to fit the experimental spectra only if one assumes that the thickness of the monolayer takes unrealistic values of the order of 0.1 μm. The equation of Hennenberg et al. (J. Colloid Interface Sci. 2000, 230, 216) is able to fit the experimental spectra, although the physical meaning of its new coupling parameter remains obscure.