Surface Rheological Measurements Research Articles

Phase behavior and properties of salt-free cationic/anionic surfactant mixtures of oleic acid and stearic acid

Cationic base surfactant, tetradecyltrimethylammonium hydroxide (TTAOH), can be obtained through anion exchange from tetradecyltrimethylammonium bromide (TTABr). Salt-free cationic and anionic (catanionic) surfactant mixtures were studied by mixing TTAOH with oleic acid (OA) or stearic acid (SA) in water. The phase behavior of TTAOH/OA/H2O is compared with that of TTAOH/SA/H2O. It was found that the phase behavior of TTAOH/OA/H2O and TTAOH/SA/H2O system differs from each other due to the existence of the unsaturated double carbon bond (C=C) in OA. At fixed total surfactant concentration (25 mg/mL) of TTAOH/OA/H2O system at 25°C, one can observe an isotropic L1 phase, and a L1/Lα two-phase region with increasing OA content. The volume of top turbid Lα phase increases while the bottom phase changes gradually from transparently clear to a bit turbid until a single Lα-phase is reached. Finally at high OA concentration, excess OA is separated from the bulk aqueous solutions. TTAOH/SA/H2O system usually forms white precipitating at 25°C due to the high chain melting temperature of SA. When heated to 60°C, however, the state of samples changes. At fixed total surfactant concentration of 25 mg/mL, an isotropic L1 phase and a milk-white or bluish Lα-phase are observed with increasing SA concentration. Transparent thin layers which are strongly birefringent form at the tops of some samples within the Lα-phase region. Finally, at high SA concentration, excess SA is separated from the bulk aqueous solutions. In addition to phase behavior study, we also measured the conductivity of TTAOH/OA/H2O system at 25°C and TTAOH/SA/H2O system at 60°C, respectively. Surface tension and rheological measurements were also performed on typical samples.

Synthesis and Properties of Alkylbetaine Zwitterionic Gemini Surfactants

AbstractA series of alkylbetaine zwitterionic gemini surfactants, 1,2‐bis[N‐methyl‐N‐carboxymethyl‐alkyl‐ammonium]ethane (CnAb, n = 8, 10, 12, or 14), were synthesized by alkylation of N,N′‐dimethylethylenediamine with an alkyl bromide, followed by reaction with sodium 2‐bromoacetate. Their solution properties were characterized by surface tension, steady‐state fluorescence, and rheological measurements. Surface‐tension measurements showed CnAb had lower critical micelle concentration (CMC) and higher efficiency in lowering the surface tension than their corresponding monomeric surfactants. The logCMC of CnAb decreased linearly with increasing chain lengths up to 12. Viscosity measurements showed only C10Ab could enhance aqueous solution viscosity at high concentrations.

Interaction between Salt-Free Catanionic TTAOH−LA Mixtures and Nonionic Polymers: Surface Tension and Rheological Study

By mixing tetradecyltrimethylammonium hydroxide (TTAOH) and lauric acid (LA) in water, the salt-free cationic/anionic (catanionic) surfactant system TTAOH/LA/H2O can be constructed. Based on our recent presentation of the TTAOH/LA/H2O ternary phase diagram, we investigate the interaction between TTAOH-LA mixtures and two kinds of nonionic polymers, Pluronic F127 (PEO-PPO-PEO triblock copolymer, EO(97)PO(69)EO(97)) and polyvinylpyrrolidone (PVP), by means of surface tension and rheological measurements. It is found that the presence of F127 has an obvious influence on the surface activity of the TTAOH/LA/H2O system in the TTAOH-rich side. Upon addition of F127, a smaller critical micellar concentration (cmc) and a higher surface tension after cmc (gamma(cmc)) were observed, which should be due to the complex formation between F127 and TTAOH-LA mixtures at the air/water interface. F127 is also found to associate strongly with TTAOH-LA mixtures in bulk aqueous solutions, as revealed by rheological measurements. The association leads to a significant decrease in micelle length and makes the vesicular phase more fluid. The influence of PVP on the surface activity and rheological properties of the TTAOH/LA/H2O system, however, is much less obvious compared with that of F127. In the TTAOH-rich side, only a very limited effect has been observed, indicating a rather weak association between PVP and surfactants. As the LA content increases, the influence of PVP on rheological properties of the TTAOH/LA/H2O system becomes detectable in the wormlike micellar region and vesicular region. However, the influence of PVP remains weaker compared with that of F127.

Characterization of the Tetratricopeptide-Containing Domain of BUB1, BUBR1, and PP5 Proves That Domain Amphiphilicity over Amino Acid Sequence Specificity Governs Protein Adsorption and Interfacial Activity

The tetratricopeptide motif repeat (TPR) is an alpha-helix-turn-alpha-helix motif that typically mediates protein-protein and, in some cases, protein-lipid interactions. Because of its success, this motif has been preserved through evolution and can be identified in proteins of a wide range of functions in lower and higher organisms. The N-terminal region of BUB1, BUBR1, and protein phosphatase 5 (PP5) contains tandem arrangements of the TPR motif. BUB1 and BUBR1 are conserved multidomain protein kinases that play a key role in the mitotic checkpoint, the mechanism that ensures the synchrony of chromosome segregation. PP5 is an enzyme that targets a wide range of protein substrates including single transmembrane receptors and mammalian cryptochromes. The N-terminal TPR domain of PP5 regulates the activity of the C-terminal catalytic domain through direct interaction with protein and lipid molecules. We portray the biophysical and biochemical properties of the tandem arrangements of the TPR motif of BUB1, BUBR1, and PP5 using far-UV spectroscopy, solution X-ray scattering, null ellipsometry, surface rheology measurements, and Brewster angle microscopy (BAM) observations. We show that, despite the low amino acid sequence conservation and different function, the TPR motif repeats of the three proteins exhibit similar interfacial properties including adsorption kinetics, high surface activity, and the formation of stable, rigid films at the air/water interface. Our studies demonstrate that domain amphiphilicity is of higher importance than amino acid sequence specificity in the determination of protein adsorption and interfacial activity.

High Fluidity and Soft Elasticity of the Inner Membrane of Escherichia coli Revealed by the Surface Rheology of Model Langmuir Monolayers

We have studied the equilibrium and linear mechanical properties of model membranes of Escherichia coli built up as Langmuir monolayers of a native lipid extract using surface thermodynamics, fluorescence microscopy, and surface rheology measurements. The experimental study has been carried out at different temperatures across the physiological operative range 15-37 degrees C. Lipid phase coexistence has been revealed over a broad pressure range by fluorescence microscopy. The presence of ordered domains has been invoked to explain the emergence of shear elasticity accompanying the hydrostatic compression elasticity typical of fluid monolayers. The surface rheology measurements point out the soft character of E. coli membranes; i.e., upon deformation they react as a near-ideal compliant body with minimal energy dissipation, thus optimizing the effectiveness of external stresses in producing membrane deformations. These mechanical features appear to be independent of temperature, suggesting the existence of a passive thermoregulation mechanism.

Binding of RPE65 Fragments to Lipid Monolayers and Identification of Its Partners by Glutathione S-Transferase Pull-Down Assays

RPE65 is the major component of the retinal pigment epithelium (RPE) microsomal membrane, and it plays a critical role in the binding of retinoids involved in the visual cycle. To understand how RPE65 binds to membranes, we have expressed and purified soluble fragments of human RPE65 fused to glutathione S-transferase (GST). The interaction between two fragments of RPE65 (F1 and F2 which include residues 1-125 and 126-250, respectively) and lipid monolayers has been studied by surface pressure, ellipsometry, and surface rheology measurements. Surface pressure and ellipsometry clearly showed a rapid adsorption of F2 to lipid monolayers whereas the kinetics of binding of F1 was much slower. Furthermore, the data suggest that the F2 fragment inserts into the lipid monolayer. Surface rheology showed a clear increase in monolayer rigidity only in the presence of F2, thereby demonstrating high intermolecular interactions of this fragment. This observation is further supported by the GST pull-down assays which demonstrated that F2 cosediments with full-length RPE65, suggesting that RPE65 has the propensity to form clusters or oligomers. The structure homology modeling of RPE65 based on a related family member, apocarotene 15',15'-oxygenase, further suggests that a hydrophobic patch located in the F2 region might be responsible for membrane binding. The present work shows that F2 interacts much stronger with lipid monolayers than does F1, which suggests that the region of RPE65 located between residues 126-250 should be very important for its membrane binding. Moreover, given that these fragments are not acylated, these data also suggest that an effective binding of RPE65 to membranes can be achieved without palmitoylation. Furthermore, GST pull-down assays also indicated that F2 interacts with 11-cis-retinol dehydrogenase, which supports previous data suggesting that it could act as a partner of RPE65.

Surface Rheology of Two-Dimensional Percolating Networks: Langmuir Films of Polymer Pancakes

We report surface rheological measurements on Langmuir films of submicron-sized polymer coils (pancakes). The dynamics of the sol phase is found to be governed by free volume as in the simplest percolation problem. The observed rheology of the percolated phase (gel-like) is compatible with predictions for compressed arrangements of highly deformed soft particles interacting through their contact interfaces.

Adsorption Behavior and Dilational Rheology of the Cationic Alkyl Trimethylammonium Bromides at the Water/Air Interface

The dynamic and equilibrium surface tensions of C(n)TAB solutions for n = 12, 14, and 16 are studied using ring and bubble pressure tensiometry. Together with respective literature values, including neutron reflectivity and dilational surface rheology measurements, the experimental data are analyzed on the basis of two theoretical models, the Frumkin model and a modified reorientation model that takes into account an intrinsic compressibility of adsorbed surfactant molecules. It turns out that this new reorientation model, earlier applied to nonionic surfactant adsorption layers, is also applicable to ionic surfactants and superior to the Frumkin isotherm. All adsorption properties of one particular surfactant can be described by a single set of model parameters.

Rheology of mixed beta-casein/beta-lactoglobulin films at the air-water interface.

The adsorption of dilute mixtures of beta-casein/beta-lactoglobulin to the air-water interface was investigated using surface dilatation and surface shear rheology. The data were fitted to simple rheological models to try to gain further information regarding the composition and nature of the interface. The dilatational measurements suggested that the composition of the interface could be determined using these models and that the surface concentration was dominated by the beta-casein in the early stages of adsorption but that high levels of beta-lactoglobulin were present in the final stages. Surface shear rheological measurements showed a similar trend. However, the shear measurements appeared to be more sensitive to the strength of the network than to the composition of the interface. Fluorescence microscopy supported the findings and demonstrated that any "phase separation" capable of affecting the surface rheological measurements occurred at the sub-micrometer scale. The results also demonstrated that the heterogeneity of the interface, once formed, is kinetically trapped, and no further phase separation occurs over the time span of the experiments.

Erratum: “Surface-rheological measurements on glass forming polymers based on the surface tension driven decay of imprinted corrugation gratings” [J. Chem. Phys. 112, 4262 (2000)

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Surface-rheological measurements on glass forming polymers based on the surface tension driven decay of imprinted corrugation gratings

We report on surface-rheological experiments on glass forming polymers making use of the decay of imprinted surface corrugation gratings. Whereas the grating is frozen in at temperatures below the glass temperature Tg, surface tension drives its decay once the sample is heated to above Tg. From the balance between surface tension and viscous stress, the near-surface shear modulus G(ω) is derived. For polystyrene a marked dependence on chain length is found. While the near-surface moduli agree with the bulk values for highly entangled samples, an increase of the apparent stiffness by a factor of 10 compared to the bulk is found for chains shorter than the entanglement length.

Phase Behavior and Flow Properties of “Hairy-Rod” Monolayers

Brewster angle microscopy, interfacial stress rheometry, and Π−A isotherm measurements are employed to characterize spread monolayers of two new cellulose derivatives, one containing octadecyl side chains, the other p-nitroaniline substituents. Similar monolayer phase behavior is found in both cases. At high surface area, a liquid phase is observed to coexist with a dilute gaseous phase. Monolayer compression eventually leads to complete coalescence to yield a uniform liquid phase. Surprisingly, a homogeneous phase is observed only at surface pressures significantly higher than that characterizing the gas−liquid phase equilibrium. This observation is attributed to dipolar repulsion between neighboring liquid domains. Compression beyond surface areas at which a continuous monolayer is observed provokes a phase transition, appearing as either a constant pressure plateau or as changes in slope in the surface pressure area isotherms. BAM images reveal only uniform surface films at molecular areas within the transition region. Surface rheological measurements indicate significant changes in monolayer mechanical properties during compression. Importantly, these changes can be correlated with isotherm shape. In all cases, monolayers exhibit a predominantly viscous behavior at high surface areas where two phases coexist. Homogeneous films achieved upon compression exhibit rheological properties that depend on both temperature and the nature of the side chain substituents. In the case of long alkyl side chains (HPC-C18), the uniform film is found to be predominantly viscous at high temperatures, and more elastic at low temperatures. This observation is interpreted as an indication of the partial crystallization of interdigitated side chains, made possible by bilayer formation. Highly dipolar chromophores incorporated as side chain substituents increase interlayer interactions and interdomain repulsion.

Properties of β-Casein at the Air/Water Interface As Supported by Surface Rheological Measurements

The properties of air/water adsorbed and spread monolayers of native and dephosphorylated β-casein were monitored using surface pressure (Langmuir trough) and surface rheology (ring trough) techniques. Two stages of rearrangement are observed for native β-casein at surface areas of about 1.0-1.3 and 0.7 m 2 mg. The first accounts for distinct surface elasticity changes in the film, which are probably due to the expulsion of the most hydrophilic segments of the protein chain. The second accounts for the collapse of the monolayer. The experiments on dephosphorylated β-casein monolayers show that dephosphorylation changes the surface elasticity behavior of the monolayer, in particular between 1 and 1.3 m 2 mg -l . We calculated a two-dimensional Flory exponent, v, for both proteins. This exponent is constant over a (semi-) dilute range of surface tensions, maximally up to a surface area of around 1.3 m 2 mg. The adsorption of native β-casein is shown to be diffusion-limited up to a surface area of around 1 m 2 mg -1 . Experiments at high ionic strength show the importance of charge on the typical surface elasticity behavior of β-casein. Experiments with enzymatically treated β-casein show the importance of the presence of hydrophilic section in the molecule on the surface elasticity behavior. It is assumed and shown that, at surface concentrations below monolayer collapse and at given solvent conditions, native /Lcasein and dephosphorylated β-casein show irreversible (air/water) adsorption behavior. Furthermore, the proteins in the monolayer are very flexible (i.e. quick relaxations).

CHARACTERISATION OF FOAM PROPERTIES USING IMAGE ANALYSIS

ABSTRACTThe physical properties of a series of model protein, surfactant and protein/surfactant foams were enumerated and investigated using foam conductivity, surface tension, preliminary surface rheology and bulk viscosity measurements. Images of various predetermined stages in the evolution of foam samples were obtained. Foam images were recorded, digitised and analysed using mathematical analysis of image texture. A textural appreciation was achieved by counting bubbles in selected images at key points during the evolution of the foam. Statistical treatment allowed delineation of real and mathematical artefacts generated in textural descriptions of foams. A principal component analysis parameter was identified, which not only corresponded with bubble size but also described bubble shape and occurrence in the texture. Analysis of the data showed correlations between sample images and changes in the structure and the composition of the adsorbed interfacial layer, highlighted using physical measurements.

Interfacial Properties of the Aqueous Phases of Wheat Flour Doughs

Aqueous phases were isolated from wheat flour doughs by ultracentrifugation, using a selection of wheats differing in bread-making quality. The interfacial properties of the aqueous phases of doughs were measured using surface tensiometry and surface rheological measurements. It was found that the material in the aqueous fraction from the cultivar Brock produced the greatest reduction in the surface tension of water and those from the CWRS class of wheat produced the least. Avalon and Mercia cultivars produced intermediate values. The elastic modulus, Gδ, of the adsorbed surface films was highest for CWRS and least for Brock. Avalon and Mercia again produced intermediate values. Lipid and protein measurements of the aqueous phases of doughs and the parent flours showed CWRS flour to contain the highest level of lipid material, whereas its aqueous phase contained the least of the flours studied. Brock flour, which was found to contain the lowest amount of lipid material, had the highest level of lipid in its aqueous phase. Avalon and Mercia were found to contain intermediate values in the flour and their aqueous phases. The influence of endogenous surface-active materials of flour on the interfacial properties of the air-aqueous interface in doughs are discussed.

Surface Rheology of Surfactant Solutions Close to Equilibrium

In this paper we present surface rheological measurements of various surfactant solutions close to equilibrium in a Langmuir trough. We find that the storage modulus is, in the systems investigated, higher than the loss modulus. The rheological behavior depends strongly on the surfactant concentration, even at concentrations exceeding the CMC. Films with quite different surface rheological properties are found to show similar drainage rates. This supports earlier work (Baets and Stein, Langmuir 8, 3099 (1992)), in which the velocity of film drainage was found to be determined by bulk viscosity effects. A number of possible explanations for the cause of the surface rheological effects found in our solutions are examined. The rheological effects at concentrations exceeding the CMC can best be ascribed to two-dimensional ordering of surfactant molecules at the surface combined with interaction of these molecules with micelles in the nearby solution.

Characterization of the interaction between gelatin and a spread monolayer of octadecanoic acid

Surface rheological measurements of octandecanoic acid monolayers spread on gelatin solutions were performed. The interaction of the spread monolayer with the gelatin involves a strong rise of the solidity of the surface structure with distinct viscoelastic properties and is comparable with the experimental findings looking into the interaction between gelatin and an ionic surfactant. The results found by the addition of sodium-dodecylsulfate to the gelatin solution are discussed, assuming the formation of a multilayer consisting of octadecanoic acid molecules interacting to a negligible degree with the gelatin-surfactant complexes displaced from the surface. The surface rheological parameters are influenced by both layers in opposition to the dynamic behaviour at compression and dilation described in the first part of this paper.

An improved apparatus and analysis for surface rheological measurements

An improved surface rheological technique, based on the viscous-traction concept, is described. In the proposed geometry, an annular canal is formed by two concentric cylinders. A moving floor supplied the motive force for flow without a pressure gradient, and provisions are made for maintaining an almost perfectly flat gas/liquid interface. Equations are derived for the flow of two immiscible fluids separated by an interface that may possess the properties of Newtonian surface viscosity or rigidity. Measurements with different solvents were used to verify the basic hydrodynamical assumptions, and very close agreement (98.7% ± 0.4%) was obtained between the measured and calculated surface flow of uncontaminated liquids. The residual error of approximately 1% has been accounted for by the small gap between the canal walls and the floor. These results justify the technique of making surface rheological measurements of a solution relative to the pure solvent; however due consideration must be given to the basic assumptions of the analysis in order to avoid anomalous results.

Shear-dependent surface rheological measurements of foam stabilizers in nonaqueous liquids

Theoretical relations are discussed for the flow of a two-dimensional Bingham plastic in a viscous-traction canal viscometer. Furthermore, this relatively simple model has been found to correlate the observed shear dependency of the system studied over a wide range of experimental conditions and has made it possible to interpret complex effects of temperature and relative humidity in terms of their respective effects on the surface viscosity and surface yield value.