Critical Surface Tension Values Research Articles

Oscillatory mode of the operation of an asymmetric poly (vinyltrimethylsilane) membrane

The oscillatory mode of permeability and selectivity of an asymmetric poly (vinyltrimethylsilane) (PVTMS) membrane during the pervaporation of binary solutions containing a surface-active component is revealed. A mechanism for the process, including the periodic appearance and collapse of micropores in the membrane skin, is suggested. The appearance of pores is determined by the existence of internal tensile stresses in the membrane skin and the adsorption-induced decrease of the polymer strength. The collapse of pores is caused by desorption of the surfactant taking place due to predominant transport of the surface-inactive component. The experimental confirmation of the suggested mechanism is presented. It is shown that the critical value of polymer-solution surface tension at which auto-oscillations start is 9 mN/m. The value of internal stresses in the membrane skin is estimated as 100 kg/cm 2.

Hydrophobicity measurements of microfiltration and ultrafiltration membranes

A method for the determination of the hydrophobicity of membrane materials is developed. The advantage of this method over existing methods is that it is not influenced by the presence of the pores. A piece of the membrane material is submerged horizontally in a liquid with surface tension γ L. Hydrophobicity is expressed in terms of γ d, the surface tension at which an air bubble brought into contact with the top surface of the membrane has a 50% chance of detaching from the surface. Values of γ d are expected to be 2-4 mN/m higher than critical surface tension (γ c) values found in the literature. For polypropylene, PTFE and polydimethylsiloxane membranes, a good agreement was found between γ d and γ c values. Poly (vinylidene fluoride), polysulfone and polyethersulfone membranes appeared to be more hydrophilic than was expected on the basis of the literature γ c values for the polymers. Using X-ray photoelectron spectroscopy, constituents that are not present in the pure polymer have been found in the surface of some membranes. These constituents and the production techniques are shown to influence the hydrophobicity of the membranes investigated.

Surface characterization of a series of polyurethanes by X-ray photoelectron spectroscopy and contact angle methods

X-ray photoelectron spectroscopy (XPS) and contact angle methods were used to examine the surfaces of an homologous series of poly(ether urethane) (PEU) samples before and after cleaning treatments. Four PEU films with Shore hardnesses varying from 45 to 75 D were studied as well as two commercially available intravenous catheters of related PEUs. The four as received PEU films have similar surface compositions (approximately 79% C, approximately 17% O, approximately 2% N, and approximately 2% Si) although they differ in bulk composition. Critical surface tension (gamma c) values are all similar and high (45-46 dynes/cm). The similarity in the surfaces of the four PEUs, despite the differences in their mechanical properties, demonstrates that surface properties do not necessarily reflect bulk properties. Soap washing and methanol-acetone extraction of the PEU films resulted in surfaces more representative of the bulk compositions of the PEUs. Analysis of the intravenous catheters confirmed that they are lubricated with PDMS, a common practice in the medical device industry. This study documents the value of detailed surface analysis for an enhanced understanding of the surface zone of PEUs. It also illustrates how cleaning protocols can remove labile surface species.

Slip effect for thin liquid film on a rotating disk

A flow for thin liquid films on rotating disks has been theoretically and experimentally studied. Liquid depletion behavior during a spin-coating process is calculated by solving the Navier–Stokes equation, taking into account interface slip between liquid and disk. Excellent agreement is seen between the model prediction and experimental data. According to observed depletion behavior on thin liquid films for various spin-coating parameters, half life falls off at the inverse square of rotational speed, and increases when viscosity increases, although the increasing rate falls off. The interface slip, represented as an external friction coefficient, is thermodynamically explained by the different (Δrc) in critical surface tension (rc) values between the liquid and the disk, which will be proportional to the solubility parameter. An infinite external friction coefficient, representing nonslip flow, may be given, when Δrc is zero. Spin-off experiments for liquids of various rc values, prepared by differing surface treatments, support this consideration.

Characterization of particle wettability by the measurement of floatability

The wettability of particles has been characterized by floatability experiments with model systems of monosized spheres of various materials and surface coatings. Equilibrium floatability experiments have been employed, where each individual particle was tested for flotation on the interface of various aqueous ethanol or methanol solutions. Results are expressed in terms of the percentage of floating particles (out of 40 tested for each experiment) vs the concentration of the solution. The highest concentration of ethanol (methanol) at which all the particles float is termed the “total floating concentration” (TFC), and the lowest ethanol (methanol) concentration at which all the particles sink is called the “total sinking concentration” (TSC The reasons for the difference between the TFC and TSC are discussed, and the general trends followed by these values are compared with the theoretical condition for floatability equilibrium. The values of the surface tension of the various solid surfaces, as calculated from the present data, closely resemble values of critical surface tension for flat surfaces. The TFC and TSC are shown to be a sensitive measure of variations in the surface properties of the particles.

Preparation and properties of plasma-deposited films with surface energies varying over a wide range

Polymer surfaces have been modified by chemical reaction or thin film deposition using a capacitatively coupled plasma RF-discharge system. This has allowed the production of a graded, reproducible series of surfaces with surface energies ranging from low to high. Three gases, hexafluoroethane (C2F6), perfluoropropane (C3F8), and hexafluoropropene (CF3CFCF2), were used to prepare the low-energy surfaces. Ethylene oxide (C2H4O) was used to prepare the higher energy surfaces. Intermediate values of surface energy were obtained by using gas mixtures of perfluoropropane and ethylene oxide. The plasma-deposited films were characterized by ESCA and contact-angle studies. In the low-energy range, the critical surface tension (γc) values of the films prepared were found to be lower than those for Teflon. For the higher energy surfaces, values up to about 45 dyn/cm were obtained. Addition of oxygen to the plasma permitted the production of films with still higher γc values. ESCA studies indicate that, under the reaction conditions used, C3F8 and CF3CFCF2 form a fluoropolymer deposit on glass that is at least 100 Å thick. For the same reaction parameters, C2F6 resulted in more etching and deposition of a much thinner film.

Surface free energy characteristics of mixtures

The surface free energies of two series of mixtures have been determined indirectly from contact angle measurements and compared with values of the critical surface tension. The values of critical surface tension vary with the liquids used for the determination. For an ideal mixture of spherical particles, the surface free energy values obtained change in a regular manner with mixture composition. For a pharmaceutical system, this was not the case and may be a reflection of the preferential adsorption of one component on the other. The values should, however, represent a value for the mixture as it is used.

Organic surface film contamination of Vitallium implants.

Conventional finishing and polishing techniques used to prepare Vitallium subperiosteal dental implant castings were found to produce low energy surfaces as measured by critical surface tension. Standard metallographic preparation gave slightly higher values. Glow discharge cleaning of both types of polished surface gave much higher critical surface tension values. This suggests the presence of an organic film after surface polishing of the implant which may later affect tissue reaction, in particular attachment, as has been noticed in related animal studies.

Surfactant Solutions as Test Liquids for Measurement of Critical Surface Tension

Contact angles of various liquids and surfactant solutions on polytef and paraffin were measured. Critical surface tension values were obtained by extrapolation of plots of cosine of the contact angles versus corresponding surface tension values. Contact angles measured using polyoxyethylene octylphenols produced linear Zisman plots and yielded critical surface tensions that agreed with accepted values. This liquid series provides a reasonable approach to the measurement of critical surface tension for solid drugs that are soluble in organic liquids but relatively insoluble in water.

Wetting properties of bis(trifluoromethyl)carbinyl acrylate polymers

AbstractA series of bis(trifluoromethyl)carbinyl acrylate monomers [Y‐C(CF3)2 OCOCHCH2] in which Y is CH3, CH3CH2, CH3CH2CH2, CH3CH2CH2CH2, C6H5, H, F, CF3, N3, CN, and CH3OCH2CH2O, was prepared. Polymers were easily prepared from all of these monomers except where Y = CN, wherein a variety of initiation methods failed to produce high molecular polymer. Wettabilities of the polymer films were examined by means of contact angle measurements by using n‐alkane test liquids and water. Values of the dispersion force contribution (γsd) and the polar force contribution (γsp) to the solid surface energy were calculated by employing both geometric and harmonic mean approximations. Values of γsd calculated by either method agreed well with γc (critical surface tension) values determined graphically from contact angle data employing n‐alkane test liquids, confirming the suggestion that γc is an approximate measure of the dispersion force contribution to solid surface energy. Values of γsd ranged from 15 dyne/cm (Y = F or CF3) to 25 dyne/cm (Y = C6H5). Values of the polar force contribution to solid surface energy (γsp) varied from 0.6 dyne/cm (Y = CH3CH2CH2CH2) to 3.4 dyne/cm (Y = CH3OCH2CH2O) when calculated by the geometric mean equation. The values of γsp obtained from the harmonic mean equation followed the same trend upon varying substituents, but were larger in value, ranging from 2.9 dyne/cm (Y = CH3CH2CH2CH2) to 7.5 dyne/cm (Y γ CH3OCH2CH2O).

Effect of the length of branches on the critical surface tension of poly(n-alkyl methacrylates) and copolymers of stearyl methacrylate with methacrylonitrile

Critical surface tension values γc were measured for poly(n-alkyl methacrylates) and copolymers of stearyl methacrylate with methacrylonitrile. Surface tension values γL of n-alkyl methacrylate increase with increasing side chain length: γL = 23.2 dynes/cm for methyl methacrylate to 33.2 dynes/cm for stearyl methacrylate, but γc values of poly(n-alkyl methacrylate) decrease with increasing side chain length: γc = 36.3 dynes/cm for poly(methyl methacrylate) to 20.8 dynes/cm for poly(stearyl methacrylate). The decrease in γc is attributed to a tighter packing of the alkyl chain with a greater concentration of the pendent CH3 group at the air/solid interface. Values of γc of copolymers hardly depended on the methacrylonitrile content in copolymers and did not satisfy the equation γc = N1γc1 + N2γc2 proposed by Lee. The difference in γc values for casting, annealing, and quenching films of poly(stearyl methacrylate) and the surface structure of copolymers were discussed using electron microscopy and measurement of melting point, heat of melting, and γc.

Solid surface tensions of amorphous and crystalline selenium

The surface tension of selenium affects the adhesion of particles as well as organic polymers on its surface. However, owing to the difficulties in measuring solid surface tension directly, no data have been published. In 1967, the critical surface tension of hexagonal selenium was determined to be 32 dyne/cm at room temperature. That result has led others to believe that selenium is a low-surface-energy inorganic polymer. We redetermined the critical surface tension of amorphous selenium and found the value to fall within the same range. We then apply our recent theory on the effect of phase transition on liquid surface tension to calculate solid surface tensions of amorphous and crystalline selenium. The values so obtained are all higher than the critical surface tension value. For example, the surface tension of amorphous selenium is 125.6 dyne/cm at 20°C, while that of the extended-chain hexagonal selenium is 175.3 dyne/cm. Our results lead us to conclude that selenium is not a low-surface-energy material. Therefore, we should hereafter treat selenium accordingly when adhesion and other surface phenomena are concerned.

Wettability and wetting of corneal epithelium

The wettability of cleansed corneal epithelium by aqueous solutions has been studied. The critical surface tension of corneal epithelium in situ and that of epithelial monolayers cultured from cornea has been determined with pure hydrophobic liquids. Results indicate that corneal epithelium free of adsorbed mucins exhibits a low energy surface as indicated by its critical surface tension value; 28 dyn/cm. The epithelial surface has a certain affinity toward aqueous phases, but it is insufficient for complete wetting. The possible wetting action of mucous glycoproteins has been investigated by using a commercially available glycoprotein mixture, bovine submaxillary mucin. The activity of this mucin at water-air and water-oil interfaces was measured as a function of concentration. The mucin was studied in the form of a surface film spread over water from an aqueous solution. The wettability of mucin adsorbed on various solid surfaces was also studied. Bovine submaxillary mucin lowers the surface tension considerably at both water-air and water-oil interfaces. The average area occupied by one molecule was calculated from the concentration dependence of surface tension to be between 50 and 100 square A. The area occupied by one mucin molecule in a monolayer spread over water was measured to be about 90,000 square A. The mucin adsorbed on low energy solids like polyethylene and corneal epithelium yields a surface with higher critical surface tension and with considerably higher affinity toward water. It is suggested that the primary role of the conjunctival glycoproteins in the eye is to transform the low-energy corneal surface into a higher energy surface via adsorption. This effect combined with increased affinity toward water and with the lowering of the surface tension of the tears is sufficient to achieve the complete wetting of the corneal surface.

Characterization of Tablet Surfaces by Their Critical Surface-Tension Values

Experiments were conducted to determine the critical surface-tension (γe) values of a range of acetylsalicylic acid tablets in combination with various adjuvants.A telemicroscope was used to measure the contact angle (θ) as test liquids with known surface tensions (γL,) advanced across the tablet surface.The cosine of the contact angles (cos θ) were plotted as a function of the γL, of the test liquids.Straight-line extrapolation to cos θ = 1 resulted in γ evalues for each particular type of tablet.These values were determined for each group of tablets, using different sets of test liquids.The γevalues obtained by this method indicated that pure acetylsalicylic acid tablets presented a surface with intermediate activity, having a γevalue of about 31 dynes/cm.The addition of a lubricant like magnesium stearate presented a surface richer in —CH3and —CH2— groups, resulting in a lower value, whereas the addition of adjuvants such as starch, cellulose, and talc resulted in surfaces richer in =O and —OH, causing an increase in the γevalues.Increased γevalues result in increased wetting by the coating solution and in an increased bonding force between the tablet surface and the polymer film coating after the solvent has evaporated.Characterization of the tablet surface should eliminate some of the guesswork involved in developing an adequate film coating for a particular type of medicinal tablet.

Forces involved in the adhesive process: I. Critical surface tensions of polymeric solids as determined with polar liquids

Critical surface tensions γ e of nine representative polymer surfaces with four series of polar liquids differed considerably from commonly accepted values. The Good-Girafalco-Fowkes-Young equation is used to explain the results, and it is shown that if certain precautions are observed, the equation may be used to predict γ c of solid polymers for “standardized” series of liquids. The theoretical concepts of Fowkes and Good are shown to be compatible with Zisman's approach to the determination of γ c . Serious errors may result, however, in the evaluation of contact angle data from misuse of the theoretical concepts of Fowkes or from misinterpretation of critical surface tension values as determined by the Zisman technique. Curve of cos θ vs. γ L are straight lines only for one particular series of liquids and normal curves are of power form. It is suggested that many of the experimental contact angle data in the literature may be reinterpreted, including those for poly-(styrene), human skin, nylon 11, poly(ethylene), and monolayers of perfluorolauric acid.

HYDRODYNAMIC DETERMINATION OF CRITICAL SURFACE TENSION OF FIBERS

The critical surface tension (CST) pointed out by Zisman is a practical parameter related to complete wetting of fibers. In the form of fiber, in general, the value of CST is estimated with some difficulties because of complex contact of fiber with liquid or unevenness of fibersIt was found that the value of CST could also be estimated, using the previously reported hydrodynamic apparatus on estimation of wettability of fiber assemblies. This study is mainely concerned with the principles and methods of determination of CST in the fiber assembly.The measurement of CST was based on the calculations of penetrating velosity (dh/dt) of dipsolution through capillaries in fiber assembly under forced flowing of dip-solution. Continious record of water pressure during the wetting process permitted the calculations of average water level in sample chamber and two curves of pressure and water level on the same time-scale permitted the comparision of amounts of pressure occured naturally in capillaries and of pressure loss due to forced flowing. The recorded pressure, when the two kinds of pressure just compensated, gave the value of dh/dt. By determination of the relation between dh/dt and water level in chamber, the product of radius of capillary by cosine of contact angle was obtained as an average value on numerous capillaries. Further, the plotting of these products vs surface tension of dip-solutions gave the value of CST of sample fibers in accordance with Zisman's method.Using above described method, value of CST of merino '64 wool fiber was determined at about 30 dyne/cm. This value was much smaller than the values reported in literatures, but it might be considered that this represented the more real value on undamaged clean wool fiber.Consequently, the merits of this hydrodynamic estimation were summarized as follows; 1) CST was determined as an average value of numerous fibers, 2) not only the CST, but the average radius of capillaries in fiber assembly could be estimated, 3) convenient determination of CST was also possible, by comparision of the shapes of pressure-time curves obtained on a series of solutions having various surface tensions.

Modification of metal surfaces with difluorocarbene and heptyl nitrene

Difluorocarbene and heptyl nitrene have been shown to irreversibly modify the surfaces of several metals as established by wettability measurements. The critical surface tension values thus obtained are shown to have a strong dependence upon the atomic radius of the underlying metal substrate. Several possible surface structures are discussed; however, the results suggest bonding between the difluorocarbene (or heptylnitrene) biradical and the metal substrate.

Surface reactions of carbenes and nitrenes on stainless steel

Three substituted carbenes, nitrene, and two substituted nitrenes generated by pyrolysis of the appropriate precursors are shown to irreversibly modify the surface of stainless steel. Critical surface tension values obtained from wettability measurements are used to establish the presence of the various reacted species on the stainless steel surface. The infrared frustrated multiple internal reflection spectrum of the difluorocarbene modified stainless steel surface is used to establish that a sigma-diadsorbed-olefin complex may be formed on the metal surface.

Sulfur modification of polyethylene surfaces. II. Modification of polyethylene surfaces with fuming sulfuric acid

AbstractPrevious work has shown that atomic sulfur irreversibility modifies polyethylene, presumably through an insertion reaction into carbon—hydrogen bonds with formation of surface thiol groups. The thiol groups were then oxidized to sulfonic acid surface groups, which were further reacted chemically as shown by wettability measurements. In this work the thiol group was bypassed and the surface sulfonic acid groups were obtained by exposing the polyethylene surface directly to fuming sulfuric acid. The sulfonic acid groups were reacted further. Critical surface tension values identical with those in the previous work with atomic sulfur were obtained, thus substantiating the previous work.

Difluorocarbene modification of polymer and fiber surfaces

AbstractDifluorocarbene (difluoromethylene, :CF2) generated by the pyrolysis of sodium chlorodifluoroacetate has been shown to irreversibly modify the surfaces of polymers and fibers. The critical surface tension values of several polymers are shown to decrease with increasing exposure to difluorocarbene vapors and ultimately approach the critical surface tension values of partially fluorinated polyethylenes. The rate‐controlling step appears to be the diffusion of difluorocarbene from the precursor to the polymer substrate. The nature of the interaction between difluorocarbene and the polymer surface is unknown; however, the absence of infrared bands due to CF bonds in attenuated total reflection measurements tends to establish the mono‐molecular‐like character of the modified surface.