Following protein kinase acivity by electrochemical means and contact angle measurements

The electrochemical analysis of the protein kinase, casein kinase, is accomplished by the voltammetric response of Ag(+) ions associated with the phosphorylated product; the sensing surface is regenerated by the cleavage of the phosphorylated product with alkaline phosphatase, and the phosphorylation/de-phosphorylation processes are monitored by XPS and contact angle measurements.     

Determination of pK(a) of acetic and benzoic acids in pyridine

The pK(a)'s of acetic acid and benzoic acid in pyridine as solvent are found to be 10.1 and 9.8, respectively, at 25 degrees . These results are based on measurements of hydrogen ion activities in mixtures of the acids and their tetrabutylammonium salts. Supplementary studies of differential vapour pressure characteristics of solutions of the acids and the acid-salt mixtures, and conductance of tetrabutylammonium benzoate solutions are also incorporated.     

Assessing the accuracy of contact angle measurements for sessile drops on liquid-repellent surfaces

Gravity-induced sagging can amplify variations in goniometric measurements of the contact angles of sessile drops on super-liquid-repellent surfaces. The very large value of the effective contact angle leads to increased optical noise in the drop profile near the solid-liquid free surface and the progressive failure of simple geometric approximations. We demonstrate a systematic approach to determining the effective contact angle of drops on super-repellent surfaces. We use a perturbation solution of the Bashforth-Adams equation to estimate the contact angles of sessile drops of water, ethylene glycol, and diiodomethane on an omniphobic surface using direct measurements of the maximum drop width and height. The results and analysis can be represented in terms of a dimensionless Bond number that depends on the maximum drop width and the capillary length of the liquid to quantify the extent of gravity-induced sagging. Finally, we illustrate the inherent sensitivity of goniometric contact angle measurement techniques to drop dimensions as the apparent contact angle approaches 180°.     

Solvatochromism as an alternative for contact angle measurements in the characterisation of modified polymer surfaces

Polyolefine surfaces have been pre-treated by UV/ozone and UV/water methods. The increase in polar groups on a polymer surface and the contribution of these polar groups to the adhesion of the polymer has been investigated by using contact angles and tensile strength tests. Because contact angle measurements do not give information about the specific polar groups on the surface we have used solvatochromic analyses to identify specific polar groups. The results showed that solvatochromic analyses is a promising method to discriminate between different kinds of polar groups and therefore solvatochromic surface characterisation may become an important surface analytical tool in adhesive technology.     

Surface chemical properties of biopolymers determined by contact angle measurements

Summary The surface chemical properties of various biopolymer solids, e.g. fibrinogen, BSA, Lysozyme and denatured BSA, were studied by measuring the contact angles of the liquid-solid interface. The magnitude of the solid surface tension, _S ^D , determined from Fowkes' relations for the various biopolymers was found to be 37.0 dyne/cm. These results show that the dispersion force component of s is of the same order of magnitude for proteins with different polarapolar character. The films of denatured BSA also show that the dispersion force component ofys does not change on denaturation, even though the polarapolar character of the protein changes on denaturation.With 6 figures and 1 table     

Phospholipid foam films studied by contact angle measurements and fluorescence microscopy

 Foam films drawn from suspensions of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) in water/ethanol mixtures were used for the investigation of the relation between the properties of the monolayers and the interaction between the film surfaces. The film thickness and the contact angle between the film and the meniscus were measured as a function of the temperature in a range around the temperature of the main phase transition for the lipid. Additionally, fluorescence microscopy was applied to investigate the distribution of a fluorescent lipidlike dye in the surface of the film and the meniscus. From the contact angle the free energy of film formation was calculated. At the temperature of the chain-melting phase transition the film thickness decreases by 0.7 nm. This can be related to a decrease in the thickness of the hydrocarbon layers of the lipid monolayers at this temperature. The decrease in the film thickness leads to a reduction in the free energy by increasing the van der Waals attraction between the film surfaces. No structures were observed in the monolayers of the film in the fluorescence investigation. However, on formation of the very thin equilibrium film the dye was expelled from the film area, indicating an increase in the packing density of the lipid, if the monolayers are in adhesive contact in the film. Received: 31 January 2000 Accepted: 25 February 2000     

Rapid identification and high sensitive detection of cancer cells on the gold nanoparticle interface by combined contact angle and electrochemical measurements

In this study, we have proposed a novel strategy for the rapid identification and high sensitive detection of different kinds of cancer cells by means of electrochemical and contact angle measurements. A simple, unlabeled method based on the functionalized Au nanoparticles (GNPs) modified interface has been utilized to distinguish the different cancer cells, including lung cancer cells, liver cancer cells, drug sensitive leukemia K562/B.W cells and drug resistant leukemia K562/ADM cells. The relevant results indicate that under optimal conditions, this method can provide the quantitative determination of cancer cells, with a detection limit of ∼10³ cells mL⁻¹. Our observations demonstrate that the difference in the hydrophilic properties for target cellular surfaces and in the uptake efficiency of the anticancer drug daunorubicin for different cancer cells could be readily chosen as the elements of cancer identification and sensitive detection. This raises the possibility to advance the promising clinic diagnosis and monitoring of tumors with the aim of successful chemotherapy of human cancers.     

Determining the contact angle between liquids and cylindrical surfaces

One of the simplest methods of measuring the quantities for estimating the adhesion properties of materials (i.e., the adhesion work, the surface energy, and the interfacial tension between certain liquids and a surface) requires the determination of the contact angle between the liquid and the surface. In the case of plane surfaces the determination of the drop dimensions makes it possible to calculate the contact angle by the sessile drop method, but in the case of cylindrical surfaces (such as the monofilaments), several methods were developed to improve the accuracy of the contact angle measurements. This paper presents a comprehensive method for precise evaluation of the contact angle between liquid drops and monofilaments by establishing a differential equation describing the drop contour. This equation makes it possible to accurately compute the contact angle using the dimensions of the drop. A comparison of the values of the contact angle calculated by our method and those obtained by other approaches is made. We applied our method in the case of polyamide-6 monofilaments treated using dielectric barrier discharge, knowing their medical applications in surgical sutures.     

Contact angle hysteresis: study by dynamic cycling contact angle measurements and variable angle spectroscopic ellipsometry on polyimide

The phenomenon of contact angle hysteresis was studied on smooth films of polyimide, a polymer type used in the microelectronic industry, by dynamic cycling contact angle measurements based on axisymmetric drop shape analysis-profile in combination with variable angle spectroscopic ellipsometry (VASE). It was found that both advancing and receding contact angles became smaller with increasing the number of cycles and are, therefore, not a property of the dry solid alone. The changes of the wetting behavior during these dynamic cycling contact angle measurements are attributed mainly to swelling and/or liquid retention. To reveal the water-induced changes of the polymer film, the polyimide surface was studied before and after the contact with a water droplet by VASE. Both the experimental ellipsometric spectrum for Delta and that for Psi as well as the corresponding simulations show characteristic shifts due to the contact with water. The so-called effective medium approximation was applied to recover information about the thickness and effective optical constants of the polymer layer from the ellipsometrically measured values of Delta and Psi. On the basis of these results, the swelling and retention behavior of the polyimide films in contact with water droplets were discussed.     

Electrokinetic and contact angle measurements of grafted carbon fibers

 The grafting method that has successfully been applied to methacrylic acid and liquid crystalline monomers was expanded to prepare amphoteric carbon fiber surfaces using 2-(N,N-dimethylamino)ethyl methacrylate as monomer. The obtained carbon fiber surfaces were characterized by contact angle and ζ-potential measurements. The expected basic behavior was not observed, instead an amphoteric character of the modified carbon fiber surface was found. The fiber surfaces display a basic character in the acidic pH-range, while they are acidic in the alkaline part of the pH-scale. An important influence is derived from the amount of initiator used to graft the monomers onto the fibers. The smaller the initiator concen-tration used during polymerization, the larger the amount of amino functionalities introduced to the carbon fiber surface. The wetting behavior versus water depends on the overall conformation of the immobilized polymer. During immersion into water the polymer acts hydrophobic, while during emersion, a hydrophilic character is observed, probably derived from conformational changes and swelling during the contact angle measure-ments in water. Received: 9 June 1998 Accepted: 13 August 1998     

Factors influencing the contact angle value. The contact angle,as a characteristic of the properties of solid surfaces

A survey of publications concerning the properties of solids in relation to wetting phenomena is presented. Factors influencing the contact angle value as well as problems of objective approach to research into wetting phenomena are discussed. Peculiarities of the direct and reverse processes during the formation of the solid—liquid—vapor three-phase contact and the inevitability of contact angle hysteresis for polar solids and liquids are analyzed. It is suggested that contact angle hysteresis is due to high energy of the interaction between the liquid and the solid and hence a long relaxation time of the three-phase contact system. Specific features of the response of a solid surface to all surface processes (“chemomechanics”) is discussed. Cleaning of solid surfaces as well as surface preparation for repeated measurements is considered. It is shown that good reproducibility of results is possible if conditions for sample preparation are met. The results of determination of the activation energy for wetting of glass surface with water are presented. The influence of the structure of solids (their hardness) on the contact angle values is demonstrated. Inevitability of the presence of different-type active sites characterized by different dissociation constants (pK_a) on the surface of solids is discussed. The pK_a values and content of these surface sites obtained from potentiometric titration and wetting data are estimated. The estimates thus obtained are in reasonable agreement with each other and can thus be used in practical applications. However, potentiometric titration is currently inappropriate for evaluating the content of individual surface sites as well as the surface charge.     

Modification of the wettability of a polymeric substrate by pH effect. Determination of the surface acid dissociation constant by contact angle measurements

The wetting properties of a substrate can be changed by chemical reaction. Here, we studied simple materials with acid-base properties, by preparing poly(vinyl chloride) films containing lauric acid. These substrates constitute simple polymeric surfaces the wettability of which can be easily controlled by the acid-base equilibrium. The roughness of the material was then varied by adding Aerosil (hydrophobic fumed silica). We then studied the wettability of these materials toward aqueous buffer solutions between pH 2 and 12 from contact angle measurements. The variation of the contact angle of a droplet of buffer solution with the pH of the solution was described by a simple thermodynamic model requiring only two parameters. Thus, we could characterize the acid polymer by an effective surface acid dissociation constant the value of which was consistent with those obtained with a similar surface. We showed that the behavior of any substrate could be described even if the surface geometry was not well-known.     

Surface esterification of cellulose fibers: characterization by DRIFT and contact angle measurements

The surface chemical modification of microcrystalline cellulose and cellulose fibers obtained from different sugar cane bagasse pulping processes, viz. Kraft, organosolv ethanol/water and organosolv/supercritical carbon dioxide, were studied in heterogeneous conditions using modest amounts of octadecanoyl and dodecanoyl chloride. The ensuing surfaces acquired a non-polar character, suitable for incorporating these fibers as reinforcing agents in composite materials based on polymeric matrices. The success of these chemical modifications was assessed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, elemental analysis, scanning electron microscopy (SEM) and contact angle measurements. In particular, the dynamic and equilibrium contact angle measurements, before and after the treatments, revealed that the value of the polar component (gamma(s)p) of the surface energy had decreased very considerably following the modification.     

Why surfaces modified by flexible polymers often have a finite contact angle for good solvents

Polymers adsorbing from a dilute solution onto the solvent-vapor interface generate a nonzero surface pressure. When the same polymers are end-grafted onto a surface such that a so-called polymer brush is formed, one will find that the solvent wets this compound interface partially. The partial wetting and the finite surface pressure are intimately linked properties of the polymer-solvent-vapor combination. It is shown that the spreading parameter in the wetting problem is proportional to the surface pressure in the adsorption case. Complete wetting is only possible when this surface pressure is nonpositive. The wetting characteristics are hardly influenced by the grafting density and chain length characterizing the brush. We argue that the grafted polymer chains can bridge to the solvent-vapor interface, thereby preventing the wetting film to become macroscopically thick. We present experimental data underpinning our self-consistent field analysis. Indeed, finite contact angles should be expected in various systems in which bridging attraction contributes to the disjoining pressure in wetting films.     

Modeling contact angle hysteresis on chemically patterned and superhydrophobic surfaces

We investigate contact angle hysteresis on chemically patterned and superhydrophobic surfaces, as the drop volume is quasistatically increased and decreased. We consider both two (cylindrical drops) and three (spherical drops) dimensions using analytical and numerical approaches to minimize the free energy of the drop. In two dimensions, we find, in agreement with other authors, a slip, jump, stick motion of the contact line. In three dimensions, this behavior persists, but the position and magnitude of the contact line jumps are sensitive to the details of the surface patterning. In two dimensions, we identify analytically the advancing and receding contact angles on the different surfaces, and we use numerical insights to argue that these provide bounds for the three-dimensional cases. We present explicit simulations to show that a simple average over the disorder is not sufficient to predict the details of the contact angle hysteresis and to support an explanation for the low contact angle hysteresis of suspended drops on superhydrophobic surfaces.     

Slip-stick wetting and large contact angle hysteresis on wrinkled surfaces

Wetting on a corrugated surface that is formed via wrinkling of a hard skin layer formed by UV oxidation (UVO) of a poly(dimethylsiloxane) (PDMS) slab is studied using advancing and receding water contact angle measurements. The amplitude of the wrinkled pattern can be tuned through the pre-strain of the PDMS prior to surface oxidation. These valleys and peaks in the surface topography lead to anisotropic wetting by water droplets. As the droplet advances, the fluid is free to move along the direction parallel to the wrinkles, but the droplet moving orthogonal to the wrinkles encounters energy barriers due to the topography and slip-stick behavior is observed. As the wrinkle amplitude increases, anisotropy in the sessile droplet increases between parallel and perpendicular directions. For the drops receding perpendicular to the wrinkles formed at high strains, the contact angle tends to decrease steadily towards zero as the drop volume decreases, which can result in apparent hysteresis in the contact angle of over 100°. The wrinkled surfaces can exhibit high sessile and advancing contact angles (>115°), but the receding angle in these cases is generally vanishing as the drop is removed. This effect results in micrometer sized drops remaining in the grooves for these highly wrinkled surfaces, while the flat analogous UVO-treated PDMS shows complete removal of all macroscopic water drops under similar conditions. These wetting characteristics should be considered if these wrinkled surfaces are to be utilized in or as microfluidic devices.     

Drop shape visualization and contact angle measurement on curved surfaces

The shape and contact angles of drops on curved surfaces is experimentally investigated. Image processing, spline fitting and numerical integration are used to extract the drop contour in a number of cross-sections. The three-dimensional surfaces which describe the surface-air and drop-air interfaces can be visualized and a simple procedure to determine the equilibrium contact angle starting from measurements on curved surfaces is proposed. Contact angles on flat surfaces serve as a reference term and a procedure to measure them is proposed. Such procedure is not as accurate as the axisymmetric drop shape analysis algorithms, but it has the advantage of requiring only a side view of the drop-surface couple and no further information. It can therefore be used also for fluids with unknown surface tension and there is no need to measure the drop volume. Examples of application of the proposed techniques for distilled water drops on gemstones confirm that they can be useful for drop shape analysis and contact angle measurement on three-dimensional sculptured surfaces.     

Investigating the thermodynamic causes behind the anomalously large shifts in pKa values of benzoic acid-modified graphite and glassy carbon surfaces

The difference between the values of 4-carboxyphenyl groups, covalently attached to either graphite (BAcarbon) or glassy carbon (BA-GC) surfaces, and benzoic acid in solution is explored using potentiometric titration and cyclic voltammetry. In solution, benzoic acid has a pKa of 4.20 at 25 degrees C. However, the observed pKa value on the graphitic surfaces shows significant deviations, with BAcarbon exhibiting a large shift to higher pKa values (pKa = 6.45) in contrast to BA-GC, which is shifted to lower pKa values (pKa = 3.25). Potentiometric titrations at temperatures between 25 and 50 degrees C allowed us to determine the surface pKa of these materials at each temperature studied and hence to determine the enthalpy, entropy, and Gibbs' energy changes associated with the ionization of the carboxylic acid groups. It was found that the enthalpic contribution is negligible and that the changes in surface pKa values are entropically controlled. This suggests that solvent ordering/disordering around the interface strongly influences the observed pKa value, which then reflects the relative hydrophobicity/hydrophilicity of the different graphitic surfaces.     

Determination of accurate surface tensions of maleimide copolymers containing fluorinated side chain from contact angle measurements

Surface energetics of two fluorinated maleimide copolymers containing fluorinated side chain, i.e., poly(ethene-alt-N-(4-(perfluoroheptylcarbonyl)aminobutyl)maleimide) (ETMF) and poly(octadecene-alt-N-(4-(perfluoroheptylcarbonyl)aminobutyl)maleimide) (ODMF), are studied by contact angle measurements with 10 liquids consisting of fairly bulky molecules. Because of the inertness of octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane (DMCPS) molecules, their contact angles are used to determine the surface tension of the two polymers. It is found that other liquids show specific interactions with the ETMF films, and their contact angles deviate from a smooth curve that represents the surface tension of ETMF, i.e., 11.00 mJ/m². On ODMF surfaces, only OMCTS and DMCPS yield useful contact angles. Other liquids either dissolve the polymer film or show a slip-stick pattern. This finding is discussed in terms of interactions between segments of the polymer chains and the test liquids. OMCTS and DMCPS are suggested as the appropriate probe liquids, meeting specific criteria necessary for the determination of accurate surface tension of fluoropolymers.     

Stabilizing contact angle hysteresis of paraffin wax surfaces with nanoclay

The addition of montmorillonite clay modified with an alkylammonium salt surfactant (i.e., organoclay) to paraffin wax is found to reduce the decay in wetting properties associated with its heating in the melt. It was previously shown that holding wax in its molten form prior to characterization reduces crystallinity when the solid forms. This results in the development of microscale amorphous regions at wax surfaces, which appear to be more polar given the abundance of methylene linkages versus methyl groups. These regions are believed to impact the receding angles for more polar liquids almost exclusively. It is known that the introduction and exfoliation of a small amount of the organoclay greatly enhances the stiffness, strength, and toughness of paraffin wax. Here, it is shown that the organoclay also promotes the formation of coatings possessing fewer thermal cracks and helps maintain higher crystallinity levels. Fresh wax surfaces containing the clay are slightly rougher than those without, which produces a slight increase in hysteresis. However, the significant drops in receding angles found for paraffin wax samples cast from the melt subsequent to heating are absent.