Use Of Contact Angle Measurements Research Articles

In pursuit of key features for constructing electrochemical biosensors – electrochemical and acid-base characteristic of self-assembled monolayers on gold

ABSTRACT Electrochemical biosensors are a well-known group of tools used widely in a variety of industries. Due to interactions between analytes and a surface, a measurable signal occurs which can then be processed to quantitative and/or qualitative data. The knowledge of surfaces’ electrochemical properties as well as their pKa is of great importance when it comes to the construction of biosensors based on an electrochemical signal. In our work, we focused on determining electrochemical properties as well as pKa of thiol-based SAMs – commonly used linker layers in biosensors. By applying cyclic voltammetry and electrochemical spectroscopy we were able to describe the overall resistive behaviour of SAMs synthesised with both alkanethiols and thiols with carboxyl and amine groups. Coherent use of contact angle measurements let us also describe the overall polarity of the investigated surfaces. The obtained data of pKa values differed from the ones described for bulk solutions of modifying agents, which yields important information for further modification of the surface.

Surface Analysis of Titanium Cleaning and Activation Processes: Non-thermal Plasma Versus Other Techniques

A medium pressure (5 kPa) dielectric barrier discharge operating in different atmospheres (air and argon) is used to develop a fast and easy way to remove adsorbed carbon contamination. Chemical and physical changes at the sample surface after plasma treatment are studied, making use of contact angle measurements, X-ray photoelectron spectroscopy analysis and atomic force microscopy measurements. The obtained results are compared with other chemical and thermal treatments typically used. This comparison shows that plasma treatment at medium pressure is able to remove up to 20 % more of the adsorbed carbon compared to the classical cleaning methods, while at the same time being less aggressive, leaving the sub-surface chemistry unchanged. Moreover, the analysis techniques give a fundamental insight in the reactions processes at the titanium surface when exposed to a medium pressure plasma.

Implications of wettability in biological materials science

Understanding liquid-solid interactions through the behavior of the liquid-solid interface is of paramount interest in many applications ranging from plant surfaces to fabrics, metal casting and biomedical implants. Liquid-solid interactions may or may not include chemical reactions, and the degree of liquid spreading over the solid surface (wetting) may vary based on the chemical properties of the materials involved (surface free energy) and the topography of the solid surface (roughness). The wetting of solid surfaces by biological fluids is often necessary for a chain of biological events to unfurl so that a foreign material may be accepted in vivo and thus become bioactive. We discuss the fundamentals of wettability, and how it pertains to the biological environment. The widespread use of contact angle measurements for the determination of surface free energy is also discussed. The use of contact angle as a general test for biocompatibility has inherent pitfalls as the effects of roughness on contact angle may be significant and misleading about the true chemical nature of the surface. Techniques to characterize the surface free energy are much more reliable, but are not as easily implemented as contact angles.

The use of contact angle measurements to estimate the adhesion propensity of calcium carbonate to solid substrates in water

We have studied a series of solids using contact angle measurements; stainless steel, gold, aluminium, titanium nitride and PTFE that are frequently used in domestic water environments. It was found the influence of electron-donor ( γ −) and electron-acceptor ( γ +) free energies on material scaling rate was dominated by water wetting angles, providing materials exhibit an average roughness below 100 nm. The γ − component had the greatest influence on theoretical adhesion, while γ LW, (Lifshitz-van der Waals) γ + and γ AB (acid-base) had little effect. From the materials analysed, amorphous carbon coatings were least adhesive, while ‘kettle coating’ and highly roughened steel the most adhesive. The size and distribution of asperities also influenced the polar free energies and subsequent adhesion due to fluctuations in the wetting angle. The results obtained indicate works of adhesion can be used as a complementary technique with Lewis acid–base theory to deliver useful information about the propensity of scale to deposit on solids.

A reversibly switching surface.

We report the design of surfaces that exhibit dynamic changes in interfacial properties, such as wettability, in response to an electrical potential. The change in wetting behavior was caused by surface-confined, single-layered molecules undergoing conformational transitions between a hydrophilic and a moderately hydrophobic state. Reversible conformational transitions were confirmed at a molecular level with the use of sum-frequency generation spectroscopy and at a macroscopic level with the use of contact angle measurements. This type of surface design enables amplification of molecular-level conformational transitions to macroscopic changes in surface properties without altering the chemical identity of the surface. Such reversibly switching surfaces may open previously unknown opportunities in interfacial engineering.

Reinforcement of Amorphous and Semicrystalline Polymer Interfaces via in-Situ Reactive Compatibilization

The interfacial reinforcement between polypropylene (PP) and amorphous polyamide (aPA) has been investigated by in-situ reactive compatibilization. PP reactivity with aPA was introduced by melt blending a certain amount of maleic anhydride grafted polypropylene (mPP) with pure PP. The effects of the mPP content, bonding time, and temperature on the interfacial fracture toughness, GC, of the aPA/(PP + mPP) adhesive joint were studied. The GC values were measured by means of a wedge test in an asymmetric double cantilever beam geometry. The GC enhancement was proved to be due to the in-situ formation of copolymers at the interface by the use of contact angle measurements and X-ray photoelectron spectroscopy (XPS) analysis. Incorporation of 3 wt % mPP in the mixed PP would be enough for effective interfacial reinforcement in this particular system. At a certain bonding temperature, the interfacial fracture toughness reaches the saturation value GC* above the critical bonding time. The value of GC* increases, but not monotonically, with critical areal density of copolymers at the interface ΣC as a result of increased bonding temperature. The effect of bonding temperature on the GC* can be divided into three distinct temperature regions: (i) a region below the glass transition temperature of aPA and the beginning temperature of melting of mixed PP where there is virtually no adhesion; (ii) a region across the melting range of mixed PP crystals where the interfacial fracture toughness steadily increases with temperature; (iii) a region well above the melting temperature of mixed PP where GC becomes relatively low. Scanning electron microscopy (SEM) and XPS results show that fibrillation of PP induced by the interfacial adhesion and subsequent breakage of the fibrils are the characteristics of fracture mechanism. Due to different crystalline morphologies of PP at different bonding temperatures, fracture mechanisms phenomenologically vary with bonding temperature at the microscale. Analysis on the locus of failure by microscopy and XPS reveals that the interfacial fracture toughness GC of amorphous and semicrystalline polymer pairs in the reactive system is influenced not only by the areal density of copolymers formed at the interface but also by the crystalline morphology of the system.

Use of surface insulation resistance and contact angle measurements to characterize the interactions of three water soluble fluxes with FR-4 substrates

Soldering flux chemistry and its interaction with the printed wiring board have been important reliability concerns for a number of years. Post Vietnam investigation of military hardware revealed corrosion in some areas. The test method most frequently used to assess the corrosion potential of flux residues is surface insulation resistance (SIR) testing. This paper gives some background on surface insulation resistance testing and reports on its application to three different water soluble fluxes. The appearance of surface dendrites is linked to test procedures that allowed water condensation on the board surface. Subsurface conductive anodic filament formation is associated with the use of fluxes which contained polyglycols. The use of contact angle measurements to assess the effect of the soldering flux residues on the board is demonstrated.

The use of contact angle measurements to quantify the adsorption density and thickness of organic molecules on hydrophilic surfaces

A relationship between the alkyl chain length of molecules separating hydrophilic surfaces from hydrophilic liquids and the resulting contact angle is established by considering both the molecular packing of the alkane chains and equations developed to describe contact angles on heterogeneous surfaces. A model is developed for the ideal packing of alkane molecules with particular reference to alcohol- and silane-coated silica surfaces. The model works reasonably well in describing the contact angle for alkylthioether-coated gold surfaces and alkyltrichlorosilane-treated, oxidised silicon surfaces. However, it does not adequately describe the contact angles for n-alcohol-coated, alkyldimethylsilane-coated or branched-alkylsilane-coated silica surfaces. The results are not a failing of the model; rather these molecules do not form close packed monolayers on silica surfaces. The limitations of this simple approach are recognised, but the approach does offer an acceptable explanation of much of the experimental contact angle data.

Morphological, physical and chemical evaluation of the Vascugraft® arterial prosthesis: comparison of a novel polyurethane device with other microporous structures

In this study the morphology, physical properties, surface chemical characteristics and microstructure of the Vascugraft ® arterial prosthesis have been investigated. This is a novel microporous polyurethane device, recently developed by the company Braun-Melsungen AG in Germany for use as a small calibre arterial substitute. This comparative study included two other synthetic grafts: the Mitrathane ® prosthesis, a hydrophilic prototype polyetherurethane urea graft with closed internal pores, and the commercially successful expanded polytetrafluoroethylene reinforced Goretex ® prosthesis with an open microporous structure. The Vascugraft ® prosthesis contains a network of fused microfibres of varying thickness and orientation which provide open and communicating pores similar in size to those in the Goretex ® material. In addition, they extend from one side of the graft wall to the other. As well as having superior longitudinal and radial compliance to the reinforced Goretex ® device, the Vascugraft ® prosthesis has more than adequate bursting and suture retention strengths. Through the use of contact angle measurements, electron spectroscopy for chemical analysis, Fourier transform infrared spectroscopy, differential scanning calorimetry and molecular weight analysis by size exclusion chromatography, the surface of the Vascugraft ® prosthesis has been shown to be uniquely hydrophobic, as well as containing carbonate groups within an aliphatic polyesterurethane polymer. In addition, variations in micro-phase separation structure of hard and soft segment domains between different sizes and batches of product are marginal. Because of the interesting physical and chemical properties, it is recommended that in vitro biocompatibility and biostability studies be undertaken prior to using the prosthesis in animal or clinical trials.

Characterization of water vapor plasma‐modified polyimide

AbstractTo enhance polyimide‐to‐polyimide adhesion, we have investigated the effect of surface modification in water vapor plasma. The use of a water vapor plasma to treat a fully cured polyimide (PMDA–ODA) surface before subsequent layers of polyimide are applied results in dramatically enhanced interfacial adhesion. The polyimide‐to‐polyimide interfacial adhesion strength attained following water vapor plasma treatment exceeds the cohesive strength of the applied polyimide layer. The effect of surface modification in water vapor plasma on metal‐to‐polyimide adhesion has also been investigated. The use of a water vapor plasma to treat a fully cured polyimide (PMDA–ODA) surface prior to metallization results in increased metal‐to‐polymer interfacial adhesion. A study of both electroless and vacuum‐deposited metal was conducted. The use of contact‐angle measurements, peel tests, Fourier transform infrared spectroscopy, optical emission spectroscopy, nuclear forward scattering, and X‐ray photoelectron spectroscopy has led us to a preliminary understanding of the resulting surface modification and the subsequent effect of adhesion promotion. © 1992 John Wiley & Sons, Inc.

Optical, electrical, and electrochemical characteristics of ultrathin poly(phenylene oxide) films: organic dielectrics less than 10 nm thick

Passive, ultra-thin dielectric films (5-7 nm) of poly(phenylene oxide), PPO, are formed by the anodic electropolymerization of tetramethylammonium phenoxide from acetonitrile onto Pt or Au electrodes. These films can have exceptionally low permeabilities to solution redox species, depending on electropolymerization potential, the time of deposition, and cross-linking reactions. The latter becomes evident at +0.4 V vs. SSCE and in the presence of electrogenerated mediator-oxidant. PPO blocks the cyclic voltammetry of ferrocene carboxylic acid and Cl − oxidations in acetonitrile by > 99% and 92%, respectively, and reduction of [Fe(CN) 6] −3 and oxidation of the sodium salt of ferrocene carboxylic acid in water by ⩾ 99%. By rotating disk voltammetry, the permeability PD S,POL of the film to FeCp 2 * in CH 3CN is as low as 8 × 10 −11 cm 2 s . Films prepared from alkyl substituted phenolates are thicker, more hydrophobic, but also more permeable owing to less efficient cross-linking. Film thicknesses were obtained by profilometry, ellipsometry, XPS, and high frequency electrical capacitance measurements, and physical and structural information by use of contact angle measurements and attenuated total reflectance FTIR.

Hydrophobic monolayers adsorbed from aqueous solutions

Experimental methods are described for the preparation and examination of hydrophobic monomolecular films adsorbed, and isolated from aqueous solutions of certain types of polar-nonpolar organic compounds. The use of contact angle measurements for the quantitative characterizations of the hydrophobic and oleophobic properties of such films permits the evaluation of the various factors governing their formation, structure, and repellency characteristics. The present paper reports the results of studies on several members of a homologous family of primary amines of the normal alkyl type adsorbed onto platinum surfaces. The critical limits to hydrophobic film formation and the degree of hydrophobicity are found to be functions of the amine concentration and the pH of the generating solution. A definite correlation is established for the relative effectiveness of these factors and the length of the hydrocarbon chain in the aliphatic portion of the amine molecule. The marked response of hydrophobic film formation to pH and concentration is interpreted as an adsorption-desorption phenomenon in which the equilibrium conditions are determined by the solubility and degree of ionization of the individual amines. The short chain amines (up through dodecyl) are found to be most adsorptive in the alkaline region, whereas film formation of the hexadecylamine is limited to the highly acid regions. The intermediate tetradecylamine evinces a transitional behavior, having two separate pH regions favorable to film formation. The identity of adsorbed monomolecular films isolated hydrophobically from aqueous solution and oleophobically from the molten amine or from solution in hexadecane is established. The use of water as solvent increases the number of molecular types available for study as reproducible, repellent films. The method is proposed as a possible approach to the examination of films important in the fields of selective adsorption, mineral flotation, corrosion inhibition, and “pickling” of metals.