Nanostructuring of a polymeric substrate with well-defined nanometer-scale topography and tailored surface wettability

This study demonstrates a simple and highly reproducible method for fabricating well-defined nanostructured polymeric surfaces with aligned nanoembosses or nanofibers of controllable aspect ratios, showing remarkable structural similarity with interesting natural biostructures such as the wing surface of Cicada orni and the leaf surface of Lotus. Our studies on the present biomimetic surfaces revealed that the wetting property of the nanostructured surface of a given chemical composition could be systematically controlled by rendering nanometer-scale roughness. The nanofabricating method we developed can be readily extended to other thermoplastic polymeric materials (e.g., light-emitting polymers, conducting polymers, block copolymers, liquid crystalline polymers), and it could be applied to developing a new generation of optical and electronic devices.     

Dynamic contact angle and spreading rate measurements for the characterization of the effect of dentin surface treatments

A new methodology capable of providing reliable and reproducible contact angle (theta) data has been employed to study the effect of clinical treatments grinding, acid etching, and deproteinization on medial dentin tissue. It is based on the application of the ADSA-CD algorithm to the determination of low-rate dynamic contact angles, obtained from slowly growing drops, and on contact angle measurement, as well as spreading behavior analysis, during the relaxation of the system (water on treated dentin) after initial drop growth. The theta data obtained were substantially more reproducible than those obtained with classical methods. A net effect of the treatment on theta was found, increasing dentin wettability: theta (polished) >theta (etched) >theta (deproteinized). The spreading rates correlate with the angles and are adequate for the dentin surface characterization. ANOVA and SNK tests show that for advancing contact angles the means corresponding to all treatments are significantly different. In the relaxing phase, mean angle and spreading rates on polished dentin differ significantly from those on etched and deproteinized dentin, but the latter do not differ significantly from each other.     

Eccentricity effect of micropatterned surface on contact angle

This article experimentally shows that the wetting property of a micropatterned surface is a function of the center-to-center offset distance between successive pillars in a column, referred to here as eccentricity. Studies were conducted on square micropatterns which were fabricated on a silicon wafer with pillar eccentricity ranging from 0 to 6 μm for two different pillar diameters and spacing. Measurement results of the static as well as the dynamic contact angles on these surfaces revealed that the contact angle decreases with increasing eccentricity and increasing relative spacing between the pillars. Furthermore, quantification of the contact angle hysteresis (CAH) shows that, for the case of lower pillar spacing, CAH could increase up to 41%, whereas for the case of higher pillar spacing, this increment was up to 35%, both corresponding to the maximum eccentricity of 6 μm. In general, the maximum obtainable hydrophobicity corresponds to micropillars with zero eccentricity. As the pillar relative spacing decreases, the effect of eccentricity on hydrophobicity becomes more pronounced. The dependence of the wettability conditions of the micropatterned surface on the pillar eccentricity is attributed to the contact line deformation resulting from the changed orientation of the pillars. This finding provides additional insights in design and fabrication of efficient micropatterned surfaces with controlled wetting properties.     

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.     

Determination of the surface anchoring potential of a nematic in contact with a substrate

We have developed a method for determining the surface anchoring potential for nematics in contact with a substrate that provides director alignment. Our main result is that the surface torque and hence the anchoring potential may be determined from either dielectric or optical phase response of a nematic undergoing a Freedericksz transition. The method is based on the Frank-Oseen continuum theory, and makes no assumptions about the functional form of the potential. We have measured the surface anchoring potential of two types of substrate in contact with the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl. The surfaces were ITO-coated float glass, coated either with obliquely evaporated SiO or a buffed polymer film. Comparison of the results obtained from capacitance and optical measurements provides an estimate of the goodness of the method.     

Determination of pK a of benzoic acid- and p-aminobenzoic acid-modified platinum surfaces by electrochemical and contact angle measurements

Acidity constant values of benzoic acid (BA)-modified platinum electrode (Pt-BA) and p-aminobenzoic acid (pABA)-modified platinum electrode (Pt-NHBA) surfaces were determined using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and contact angle measurements (CAM). Diazonium tetrafluoroborate salt reduction and pABA oxidation reactions were used to prepare (Pt-BA) and (Pt-NHBA) surfaces, respectively. Both surfaces exhibited pH dependence with [Fe(CN)₆]^3?/4? redox probe solutions at different pH; this allowed us to estimate the surface pK _a values. Acidity constants for Pt-BA surface were found to be pK _a (3.09 ± 0.25), (4.89 ± 0.11), and (3.91 ± 0.54) by CV, EIS, and CAM techniques, respectively, while the values for Pt-NHBA surface were pK _a (3.16 ± 0.45), (4.24 ± 0.40), and (5.64 ± 0.12). The Pt-BA surface pK _a values were lower in CV and CAM measurements relative to the bulk solution of BA, while a higher value was observed in EIS for Pt-BA surface. The pK _a values determined for Pt-NHBA surface via both CV and EIS were lower than the bulk value; however, the result obtained from CAM was one unit higher than pK _a of bulk pABA.     

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     

Determination of the dissociation constant and limiting equivalent conductance of a weak electrolyte from conductance measurements on the weak electrolyte

A method is described for computing the dissociation constant and equivalent conductance at infinite dilution, A_o , for a weak electrolyte from as few as two conductance measurements at different concentrations. The iterative procedure applied to acetic acid data yielded a dissociation constant of 1.80 X 10-⁵ and an average value (n = 4) of A_o of 389.5 compared with an expected value of 390.7.     

Determination of the anharmonicity constant of oxygen by raman measurements

The hot band ν = 1 → 2 of O₂, which could not be resolved in previous Raman measurements, has been observed by direct scanning of the purely isotropic part of Raman scattered light. A computer simulation of the spectrum confirmed the set of anharmonicity constants given by Babcock and Herzberg.     

Interpretation of contact angle measurements on two different fluoropolymers for the determination of solid surface tension

Contact angle measurements with a large number of liquids on the semi-fluorinated acryl polymer EGC-1700 films are reported. The surface tension was determined to be gammasv=13.84 mJ/m2 from contact angles of octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane (DMCPS). Inertness of these two liquids makes them ideal for determination of surface tension of low-energy fluoropolymers. On the other hand, contact angles of many other liquids deviated somewhat from a smooth contact angle pattern that represents the EGC-1700 surface tension. It is argued that noninertness of the molecules of these liquids gives rise to specific interactions with the polymer film, causing the deviations. Furthermore, contact angles of a series of n-alkanes (n-hexane to n-hexadecane) showed systematic deviations from this curve, similar to the trend observed for n-alkanes/Teflon AF 1600 systems studied earlier. Adsorption of vapor of short-chain liquids onto the polymer film caused their contact angles to fall above the gammasv=13.84 mJ/m2 curve, and a parallel alignment of molecules of the long-chain n-alkanes in the vicinity of the solid was the explanation for the deviation of their contact angles below it. It is found that vapor adsorption effect is more significant in the case of Teflon AF 1600, while the alignment of liquid molecules close to the surface is more pronounced for EGC-1700.     

Droplet compression and relaxation by a superhydrophobic surface: contact angle hysteresis

In this article, the contact angle hysteresis (CAH) of acrylic glass is experimentally and theoretically studied through the compression-relaxation process of droplets by using a superhydrophobic surface with negligible CAH effect. In contrast to the existing technique in which the volume of the droplet changes during the measurement of CAH, this procedure is carried out at a constant volume of the droplet. By observing the base diameter (BD) and the contact angle (CA) of the droplet during the compression-relaxation process, the wetting behavior of the droplet can be divided into two regimes, the contact line withdrawal and the contact line pinning regimes, depending on the gap thickness (H) at the end of the compression process. During the compression process, both regimes possess similar droplet behavior; the contact line will move outward and the BD will expand while the CA remains at the advancing angle. During the relaxation process, the two regimes are significantly different. In the contact line withdrawal regime, the contact line will withdraw with the CA remaining at the receding angle. In the contact line pinning regime, however, the contact line will be pinned at the final position and the CA will decline to a certain value higher than the receding angle. Furthermore, the advancing pinning behavior can also be realized through a successive compression-relaxation process. On the basis of the liquid-induced defects model, Surface Evolver simulations are performed to reproduce the behavior of the droplet during the compression-relaxation process; both contact line withdrawal and pinning regimes can also be identified. The results of the experiment and simulation agree with each other very well.     

Determination of acid dissociation constants by capillary electrophoresis

Capillary electrophoresis affords a simple, automated approach for the measurement of pKa values in the range 2-11 at a throughput of less than 1 h per sample per instrument. Agreement with literature values is usually within 0.20 log units with a precision better than 0.07 log units. The attractive features of capillary electrophoresis for pKa measurements are: (1) conventional instrumentation with a high level of automation are suitable for all measurements; (2) because it is a separation method samples need not be of high purity; (3) samples of low water solubility with suitable chromophores are easily handled (detection limits in the microM range); (4) sample consumption per measurement is in the microgram range; and (5) since only mobilities are measured, exact knowledge of concentrations is not needed. The general approach can be extended to pKa measurements in aqueous-organic solvent mixtures and non-aqueous solvents with suitable calibration. The widespread use of absorbance detection in capillary electrophoresis means that the sample must have a suitable chromophore for detection. The main source of controllable error is the accuracy of buffer standardization and their stability in use, and uncontrollable error, the retentive interactions of the sample with the column wall. The latter seems to be a rare problem in practice for typical operating conditions.     

Salt effect on the dissociation constant of acid-base indicators

The effect of the ionic environment on the dissociation constant of acid-base indicators is accounted for by using the specific interaction theory (SIT) of Broensted, Scatchard, and Guggenheim. It is shown that the SIT may be applied to evaluate the activity coefficients of organic ions, regardless of their size and structure, if suitable modifications are introduced.     

The determination of the second dissociation constant of sulfuric acid

The second dissociation constant of sulfuric acid is determined in 1M NaClO₄ at 25°C using an electrochemical cell without liquid junction consisting of a glass and a perchlorate electrode. By taking into account the association between the Na⁺ and SO ₄ ^2– ions an average value of 0.0184±0.0005 is found using three different methods. This corresponds with an apparent acidity constant KA ₂ ^* of 0.095±0.003     

Calculation of the surface potential and surface charge density by measurement of the three-phase contact angle

The silica/silicon wafer is widely used in the semiconductor industry in the manufacture of electronic devices, so it is essential to understand its physical chemistry and determine the surface potential at the silica wafer/water interface. However, it is difficult to measure the surface potential of a silica/silicon wafer directly due to its high electric resistance. In the present study, the three-phase contact angle (TPCA) on silica is measured as a function of the pH. The surface potential and surface charge density at the silica/water surface are calculated by a model based on the Young-Lippmann equation in conjunction with the Gouy-Chapman model for the electric double layer. In measurements of the TPCA on silica, two distinct regions were identified with a boundary at pH 9.5-showing a dominance of the surface ionization of silanol groups below pH 9.5 and a dominance of the dissolution of silica into the aqueous solution above pH 9.5. Since the surface chemistry changes above pH 9.5, the model is applied to solutions below pH 9.5 (ionization dominant) for the calculation of the surface potential and surface charge density at the silica/aqueous interface. In order to evaluate the model, a galvanic mica cell was made of a mica sheet and the surface potential was measured directly at the mica/water interface. The model results are also validated by experimental data from the literature, as well as the results obtained by the potentiometric titration method and the electro-kinetic measurements.     

Obtaining surface tension from contact angle data by the individual representation approach

Young equation is the fundamental equation of wetting theory in which the connection among the surface tensions, \(\gamma _{{\varphi \psi }} \) and the contact angle, θ _L, are given. The surface tension of solid surfaces, however, cannot be obtained directly from the Young equation. In this paper, the application of the individual representation theory is demonstrated for the determination of surface tensions of solids (or any phase pair) using experimentally obtained contact angle data. According to this approach, the state of the interfacial layers depends upon, by definition, the properties of the bulk phases in every heterogeneous system, and thus, it complements the traditional capillary theory.     

Analysis of the grafting process of PVP on a silicon surface by AFM and contact angle

Silicon wafers have been silylated with VTMS (vinyltrimethoxysilane) and hydrolyzed. Subsequently, PVP (polyvinyl pyrrolidone) was grafted onto the silylated surface by two different techniques: the grafting-through (GT) and the grafting-onto techniques (GO). The measurement of contact angles along with the topography analysis by atomic force microscopy (AFM) has allowed monitoring the different stages of the process and the temporal evolution of polymer grafting. The results have demonstrated the feasibility of both methods of grafting but have shown that the GT method gives a higher density of polymer-grafted chains. The AFM technique in adequate liquid environments has been proven to permit the surface density of chains to be distinguished by both methods and to estimate the length of the resulting PVP chains.     

The second dissociation constant of sulfuric acid at various temperatures by the conductometric method

The thermodynamic second dissociation constant K₂ for sulfuric acid has been determined by conductivity measurements of aqueous sulfuric acid solutions at various temperatures. The data are treated by using two different methods developed with two independent assumptions due to Noyes et al. and Shedlovsky. Both methods require the knowledge of relevant ionic conductivities, which may be calculated from the Onsager limiting law. The values for K₂ obtained with these two methods show excellent agreement. The value of 0.0103 mol-L^–1 at 25°C agrees with the best literature value of 0.0102 mol-L^–1 within the experimental uncertainty, as also does the enthalpy of dissociation which is derived from the derivative of the temperature coefficient of K₂.     

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.     

Determination of the acid dissociation constants of p-benzohydroquinone by the INDO method

The stepwise acid dissociation constants for p-benzohydroquinone (QH₂) in aqueous media have been explicitly calculated for the first time, with the INDO parametrized SCF –MO method. We have optimized the geometries of QH₂, QH^?, and Q^2? and of the QH₂ · 6H₂O, QH^? · (H₃O⁺) · 5H₂O, and Q^2? · (H₃O⁺)₂ · 4H₂O systems that model the solvated species. The presence of the associated water molecules (and hydronium ions) account for the stabilization due to hydrogen bonding as well as for a part of the effect of interaction of these molecules with the respective reaction fields in an aqueous medium. To simulate the first solvation shell in a more complete manner, four more water molecules have been considered to be placed above and below the quinonoid ring and the optimized geometries of the resulting hydrated species, QH₂ · 10H₂O, QH^? · (H₃O⁺) · 9H₂O, and QH^? · (H₃O⁺) · 8H₂O, have been determined. The standard free-energy changes calculated for the dissociation of QH₂ into QH^? and H⁺ is 0.0251 Hartree (65.9 kJ mol^?1) and that of QH^? into Q^2? and H⁺ is 0.0285 Hartree (74.8 kJ mol^?1). Experimentally observed dissociation constants for these two steps correspond to free-energy changes of 0.0214 Hartree (56.2 kJ mol^?1) and 0.0248 Hartree (65.1 kJ mol^?1), respectively. © 1995 John Wiley & Sons, Inc.