The surface tension of polymer liquids

A brief review of the surface tension of polymer liquids is presented. A strong emphasis is placed on recent measurements of surface tensions of homologous liquid series up to high-molecular-weight polymers, and the thermodynamic liquid properties of these same homologous series obtained from sources such as pressure-volume-temperature (PVT) data. The accuracy and limitations of the thermodynamic information which are used as input to many of the theories applied to the surface properties of polymer molecules are discussed. By scaling the surface tension data using a true measure of the cohesive energy density of the liquid state, we can clearly observe the entropic contribution to the surface tension caused by the conformational restriction of a large molecule at the liquid-vapor interface. The scaling implies the existence of a corresponding states principle for both polymer liquids and for low-molecular-weight liquids. The ramifications of the existence of a corresponding states principle for the surface tension of polymer melts are discussed. One consequence of the corresponding states principle is that it allows us to use surface tension measurements to compute the cohesive energy density of polymer melts using PVT data.     

Studies on the application of dynamic surface tensiometry of serum and cerebrospinal liquid for diagnostics and monitoring of treatment in patients who have rheumatic, neurological or oncological diseases

Human biological liquids comprise various surfactants, which adsorb at liquid interfaces and lead to a variation in surface tension. The adsorption processes involving low molecular weight surfactants, proteins and phospholipids play a vital role in the physiological functions of the human organism, especially if large surfaces are involved (e.g., gas exchange in lungs, metabolism of kidneys, liver and brain). Dynamic surface tensiometric studies of biological liquids like serum and cerebrospinal fluid provide surrogate parameters that reflect surface tension phenomena. We provide dynamic surface tension data of serum and cerebrospinal fluid that were collected from healthy volunteers and patients with rheumatic, neurological or oncological diseases. Our studies indicate that dynamic surface tension data are helpful for diagnostic purposes and for monitoring of therapeutic interventions.     

Dynamic surface tension measurements with submillisecond resolution using a capillary-jet instability technique

An oscillating capillary jet method is implemented to measure surface tension of aqueous nonionic surfactant solutions as a function of surface age from the jet orifice. The experimental technique captures the evolution of jet swells and necks continuously along the jet propagation axis and is used in conjunction with an existing linear, axisymmetric, constant-property model to determine surface tension of liquids. The method is first validated using deionized water and isopropyl alcohol (constant surface tension test fluids) and a procedure is described to identify the optimum wavelength from the breakup point, which produces the smallest error in surface tension measurements. Dynamic surface tension data of concentrated aqueous Tergitol NP-8 surfactant solutions is then presented. The measurements are performed over a spatial length of approximately 1.5 wavelengths, a span corresponding to 0.6-4.2 ms time window from the jet orifice. Submillisecond surface age measurements are made possible by decreasing the jet diameter. Increased surfactant concentrations make the liquid jet more stable and allow measurements at higher surface ages. The correlation of Hua and Rosen fits well the dynamic surface tension data, which includes submillisecond surface ages. Finally, the time required for surface tension to reach equilibrium levels is estimated using a simple adsorption kinetics theory of surfactant molecules on the liquid/air interface.     

Peculiar surface behavior of some ionic liquids based on active pharmaceutical ingredients

The ionic liquids based on biologically active cations and anions, commonly designated by ionic liquids based on active pharmaceutical ingredients (ILs-APIs), are interesting compounds for use in pharmaceutical applications. Lidocaine docusate, ranitidine docusate, and didecyldimethylammonium ibuprofen are examples of promising ILs-APIs that were recently synthesized. They were submitted to biological testing and calorimetric measurements, but nothing is known about their surface properties. In this work, we measured the surface tension and the contact angles on both hydrophilic and hydrophobic surfaces in a temperature range as wide as possible. Based on the wettability data, the polarity fractions were estimated using the Fowkes theory. The peculiar surface behavior observed was tentatively attributed to the presence of mesophases.     

Humidity-dependent surface tension measurements of individual inorganic and organic submicrometre liquid particles

Surface tension, an important property of liquids, is easily measured for bulk samples. However, for droplets smaller than one micron in size, there are currently no reported measurements. In this study, atomic force microscopy (AFM) and force spectroscopy have been utilized to measure surface tension of individual submicron sized droplets at ambient pressure and controlled relative humidity (RH). Since the surface tension of atmospheric aerosols is a key factor in understanding aerosol climate effects, three atmospherically relevant systems (NaCl, malonic and glutaric acids) were studied. Single particle AFM measurements were successfully implemented in measuring the surface tension of deliquesced particles on the order of 200 to 500 nm in diameter. Deliquesced particles continuously uptake water at high RH, which changes the concentration and surface tension of the droplets. Therefore, surface tension as a function of RH was measured. AFM based surface tension measurements are close to predicted values based on bulk measurements and activities of these three chemical systems. Non-ideal behaviour in concentrated organic acid droplets is thought to be important and the reason for differences observed between bulk solution predictions and AFM data. Consequently, these measurements are crucial in order to improve atmospheric climate models as direct measurements hitherto have been previously inaccessible due to instrument limitations.     

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.     

Importance of surface tension for physical paint properties

Surface tension is a parameter of decisive importance for characterizing painted and unpainted surfaces related to wetting and adhesion phenomena. This article presents measurements of the surface tension of solids by means of an automatic contact angle measurement device. The evaluation of the surface tension is based on a separation into polar and disperse components. In addition, this paper briefly touches on other, more far-reaching approaches (acid/base) and discusses a method for the determination of dynamic surface tension of liquids.     

Oscillating bubble SHG on surface elastic and surface viscoelastic systems: new insights in the dynamics of adsorption layers

Surface rheology governs a great variety of interfacial phenomena such as foams or emulsions and plays a dominant role in several technological processes such as high-speed coating. Its major difference with bulk rheology resides in the high compressibility of the surface phase, which is the direct consequence of the molecular exchange between adsorbed and dissolved species. In analogy to bulk rheology, a complex surface dilational modulus, epsilon, which captures surface tension changes upon defined area changes of the surface layer, can be defined. The module epsilon is complex, and the molecular interpretation of the dissipative process that gives rise to the imaginary part of the module is subject to some controversy. In this contribution, we used the oscillating bubble technique to study the surface dilational modulus in the mid-frequency range. The dynamic state of the surface layer was monitored by a pressure sensor and by surface second-harmonic generation (SHG). The pressure sensor measures the real and imaginary part of the modulus while SHG monitors independently the surface composition under dynamic conditions. The experiment allows the assessment of the contribution of the compositional term to the surface dilational modulus epsilon. Two aqueous surfactant solutions have been characterized: a surface elastic and a surface viscoelastic solution. The elastic surface layer can be described within the framework of the extended Lucassen-van den Tempel Hansen model. The change in surface concentration is in phase with the relative area change of the surface layer, which is in strong contrast with the results obtained from the surface viscoelastic solution. Here, surface tension, area change, and surface composition are phase-shifted, providing evidence for a nonequilibrium state within the surface phase. The data are used to assess existing surface rheology models.     

Adsorption and aggregation properties of amino acid-based N-alkyl cysteine monomeric and N,N'-dialkyl cystine gemini surfactants

An amino acid-based gemini surfactant derived from cystine (2CnCys, where n represents the hydrocarbon chain lengths of 8, 10, and 12) was synthesized by reacting cystine with n-alkyl bromide, and its adsorption and aggregation properties were characterized by measurements of equilibrium and dynamic surface tension and dynamic light scattering. The properties of 2CnCys were compared with those of an amino acid-based monomeric surfactant derived from cysteine (CnCys). For n=8 and 10, when compared to CnCys, 2CnCys exhibited excellent surface activities, such as a lower critical micelle concentration (cmc), greater efficiency in lowering the surface tension of water, and smaller area occupied per molecule. Adsorption rate at air/water interface decreased with an increase in hydrocarbon chain length, chain number, and concentration of respective compounds. Further, the kinetics were discussed using the monomer diffusion coefficient obtained from short and long time scales in dynamic surface tension plots. In addition, the aggregation properties of 2CnCys for n=8 and 10 differed from those in the case of n=12. In other words, relatively larger micelles with diameters of approximately 7 nm were formed by 2CnCys for n=8 and 10 in comparison to those formed by CnCys (2-3 nm). On the other hand, for a 0.832 mmol dm-3 2C12Cys solution, the aggregation structure investigated by cryogenic transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS) revealed the coexistence of small unilamellar vesicles and small rods.     

Solution/air interfaces I. An oscillating jet relative method for determining dynamic surface tensions

The oscillating jet method has been investigated for the determination of the surface tension of water using horizontal jets from elliptical orifices in bell-shaped and uniform-channel tubes. Improved techniques have been developed for measuring the wave parameters, the flow rate and for extending the range of investigations to include the initial 80–90 msec of jet surface age.The surface tension values, calculated using the Bohr equation from measurements on successive waves of the water jets, were dependent on the characteristics of the orifice, its position, the flow rate and the wave serial number, but were within ±2 mN/m of the equilibrium value if the initial wave values were disregarded. An extension of the Bohr equation developed for vertical jets was found to be invalid for horizontal jets.Calculated surface tension versus surface age relationships for surfactant solutions also varied with the experimental conditions, but by fixing the position of the orifice tube, and standardizing with water, a relative method was developed for determining dynamic tensions that were independent of the tube used and of the flow rate. The validity of the method was illustrated by results obtained with two surfactant solutions using seven tubes (bell-shaped and uniform-channel) over an age range from 0.6 to 75 msec. The surface tensions of deionized water samples have been determined by the relative method and compared with those obtained by a static method.The true surface age along the jet surface is concluded to be close to the value derived from the mean axial velocity.Evidence is given indicating that, within the millisecond age range, water does not have a dynamic tension above the equilibrium value.     

Dynamic surface tension analysis of dodecyl sulfate association complexes

First, a novel calibration method is used to expand the current understanding of spherical drop growth and elongation that occurs during on-line measurements of surface pressure using the dynamic surface tension detector (DSTD). Using a novel surface tension calibration method, the drop radius is calculated as a function of time from experimental drop pressure data and compared to the theoretical drop radius calculated from volumetric flow rate. From this comparison, the drop volume at which the drop shape starts to deviate ( approximately 4 mul) from a spherical shape is readily observed and deviates more significantly by approximately 6 mul drop volume (5% deviation in the ideal spherical drop radius) for the capillary sensing tip employed in the DSTD. From this assessment of drop shape, an experimental method for precise drop detachment referred to as pneumatic drop detachment is employed at a drop volume of 2 mul (two second drops at 60 mul/min) in order to provide rapid dynamic surface tension measurements via the novel on-line calibration methodology. Second, the DSTD is used to observe and study kinetic information for surface-active molecules and association complexes adsorbing to an air-liquid drop interface. Dynamic surface tension measurements are made for sodium dodecyl sulfate (SDS) in the absence and presence of either tetra butyl ammonium (TBA) or chromium (III). Sensitive, indirect detection of chromium and other multiply charged metals at low concentrations is also investigated. The DSTD is utilized in examining the dynamic nature of SDS: cation association at the air-liquid interface of a growing drop. Either TBA or Cr(III) were found to substantially enhance the surface tension lowering of dodecyl sulfate (DS), but the surface tension lowering is accompanied by a considerable kinetic dependence. Essentially, the surface tension lowering of these DS: cation complexes is found to be a fairly slow process in the context of the two second DSTD measurement. The limit of detection for both SDS and chromium (III) is in the 300-400 part-per-billion (by mass) range.     

Contact angle studies of the surface properties of covalently bonded poly-L-lysine to surfaces treated by glow-discharge

Contact angle data, measured by using a sessile drop arrangement in conjunction with Axisymmetric Drop Shape Analysis-contact Diameter (ADSA-CD), were used to quantify the effects of ammonia gas plasma treatment on the surface properties of previously untreated polystyrene surfaces. The surface tension of treated polystyrene samples is considerably higher than that of untreated samples. The increase in surface tension following plasma treatment is attributed to the addition of amine groups to the surface.Next, conformational changes following the attachment of poly-L-lysine to the untreated samples by simple adsorption and plasma treated samples by covalent bonding were investigated. Surface tension values obtained from contact angle data indicate that conformational changes to poly-L-lysine occur in both cases, because these values are lower than the surface tension of poly-L-lysine in solution. However, contact angle data show that covalently bonded poly-L-lysine undergoes less conformational changes than simply adsorbed poly-L-lysine.     

Hydrodynamic effects on the oscillations of supported bubbles: implications for accurate measurements of surface properties

Oscillating bubble techniques are commonly used to infer dynamic surface tensions (DST) from the measurements of the dynamic pressure differences across an interface. In inferring DST from such measurements, it is assumed that hydrodynamic efects either are negligible or can be approximated. In virtually all previous studies, the dynamic bubble shapes, which are set by the requisite balance of forces at the bubble surface, are taken to be nearly spherical and assumed to be governed by the static Young–Laplace (Y–L) equation alone. To examine these assumptions, the Navier–Stokes and continuity equations governing the flow of a liquid outside an axisymmetric bubble supported by a narrow capillary are solved simultaneously by a rigorous finite element method. Cases of constant surface tension (pure liquids or solutions with very fast adsorption) are tested to focus on understanding the effects of fluid motion on surface tension measurements. To test the capability of the computational algorithm on describing the hydrodynamics, computed and experimental velocity profiles are compared and found to be consistent, as are the apparent surface tensions. Parameters such as forcing frequency, forcing amplitude, chamber dimensions, and surface tension and viscosity of the liquid are varied to find the limits where pulsating bubbles depart from spherical and where hydrodynamic effects impact the determination of surface tension. In the commercial pulsating bubble surfactometer (PBS), the bubble shapes remain nearly spherical for low pulsation or oscillation rates (≤ 100 Hz) with moderate volume amplitudes. Under these conditions, however, two major hydrodynamic effects, due to the inertia of the bulk liquid, or to the liquid viscosity and the viscous forces acting on the chamber walls, are found to be important and can cause large errors in the surface tension measurements. For measurements of low surface tensions (≤ 5 mN/m) in a PBS, oscillating bubble methods that do not take into account shape deformations from the spherical are found to be inaccurate, and the results from dynamic bubble shape analysis (solving the Y–L equation for a nonspherical axisymmetric surface) are shown to provide another approach to obtain accurate results provided that hydrodynamic effects can be neglected. At higher frequencies (≥ 200 Hz), because of strong convection around the surface of the bubble, the bubble shapes become highly deformed. Further extensions of this algorithm are needed, to include the surfactant diffusion and adsorption effects by which the surface tension may change with time.     

Correlation between surface tension and void fraction in ionic liquids

An effort to systematize published and new data on the surface tension gamma of ionic liquids (ILs) is based on the hypothesis that the dimensionless surface tension parameter gamma V v (2/3)/ kT is a function of the void fraction x v = V v/ V m. The void volume V v is defined as the difference between the liquid volume V m occupied by an ion pair (known from cationic and anionic masses and liquid density measurements) and the sum V (+) + V (-) of the cationic and anionic volumes (known from crystal structures), while kT is the thermal energy. Our hypothesis that gamma V m (2/3)/ kT = G( x v) is initially based on cavity theory. It is then refined based on periodic lattice modeling, which reveals that the number N of voids per unit cell (hence the dimensionless surface tension) must depend on x v. Testing our hypothesis against data for the five ILs for which surface tension and density data are available over a wide range of temperatures collapses all of these data almost on a single curve G( x v), provided that slight (4%) self-consistent modifications are introduced on published crystallographic data for V (+) and V (-). An attempt to correlate the surface tension vs temperature data available for inorganic molten salts is similarly successful, but at the expense of larger shifts on the published ionic radii (8.8% for K; 3.3% for I). The collapsed G( x v) curves for ILs and inorganic salts do not overlap anywhere on x v space, and appear to be different from each other. The existence of a relation between gamma and x v is rationalized with a simple capillary model minimizing the energy. Our success in correlating surface tension to void fraction may apply also to other liquid properties.     

Surface tension of 1-alkyl-3-methylimidazolium based ionic liquids with trifluoromethanesulfonate and tetrafluoroborate anion

The amount of available accurate experimental data on the surface tension of ionic liquids is still limited; in many cases the data are rare or even absent. In the present study, air-liquid interfacial tension data were determined experimentally for five 1-C_n-3-methylimidazolium based ionic liquids (n = 2, 4, and 6), three with trifluoromethanesulfonate and two with tetrafluoroborate anion, at atmospheric pressure in the temperature range from 268 to 356 K. The resultant surface tension data are average values of the measurements repeated many times at each set point temperature. The accuracy of the results, was confirmed by employing the Wilhelmy plate and the du Noüy ring methods in parallel, using the Krüss K100MK2 tensiometer. For the Wilhelmy plate data the combined standard uncertainty is estimated to be about 0.05 mN m⁻¹. The data obtained by du Noüy method show about up to seven times greater scatter than those obtained by the Wilhelmy plate method. To the 50 up to now published surface tension values for the five studied ionic liquids the present study adds further 175 data points. In contrast to that of n-alkanes, the surface tension of 1-alkyl-3-methylimidazolium based ionic liquids decreases and their surface entropy increases with the cation alkyl chain length.     

Star-shaped trimeric quaternary ammonium bromide surfactants: adsorption and aggregation properties

Novel star-shaped trimeric surfactants consisting of three quaternary ammonium surfactants linked to a tris(2-aminoethyl)amine core were synthesized. Each ammonium had two methyls and a straight alkyl chain of 8, 10, 12, or 14 carbons. The adsorption and aggregation properties of these tris(N-alkyl-N,N-dimethyl-2-ammoniumethyl)amine bromides (3C(n)trisQ, in which n represents alkyl chain carbon number) were characterized by equilibrium and dynamic surface tension, rheology, small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM) techniques. 3C(n)trisQ showed critical micelle concentrations (CMC) 1 order of magnitude lower than that of the corresponding gemini surfactants with an ethylene spacer and the corresponding monomeric surfactants. The logarithm of the CMC decreased linearly with increasing hydrocarbon chain length for 3C(n)trisQ. The slope of the line, which is well-known as Klevens equation, was larger than those of the monomeric and gemini surfactants; however, considering the total carbon number in the chains, the slope was shallower than the monomeric and was close to the gemini. Through the results such as surface tensions at the CMC (32-34 mN m(-1)) and the parameters of standard free energy, CMC/C(20) and pC(20), it was found that 3C(n)trisQ could adsorb densely at the air/water interface despite the strong electrostatic repulsion between multiple quaternary ammonium headgroups. Moreover, dynamic surface tension measurements showed that the kinetics of adsorption for 3C(n)trisQ to the air/water interface was slow because of their bulky structures. Furthermore, the results of rheology, SANS, and cryo-TEM determined that 3C(n)trisQ with n = 10 and 12 formed ellipsoidal micelles at low concentrations in solution and the structures transformed to threadlike micelles with very few branches for n = 12 as the concentration increased, but for n = 14 threadlike micelles formed at relatively low concentrations.     

Capillary rise of liquids over a microstructured solid surface

The location of the triple line as a function of time has been recorded for a series of organic liquids, with various surface tension to viscosity ratios, wicking upward a rough Cu(6)Sn(5)/Cu intermetallic (IMC) substrate. The complex topographical features of such an IMC rough surface are characterized by surface porosity and surface roughness. A theoretical model for wicking upward a rough surface has been established by treating the rough IMC surface as a two-dimensional porous medium featuring a network of open microtriangular grooves. The model is verified against experimental data. The study confirms that the kinetics of capillary rise of organic liquids in a nonreactive flow regime over a porous surface having arbitrary but uniformly distributed topographical features involves (i) surface topography metrics (i.e., permeability, tortuosity/porosity, and geometry of the microchannel cross section); (ii) wicking features (i.e., contact angle and filling factor); and (iii) physical properties of liquids (i.e., surface tension and viscosity). An excellent agreement between theoretical predictions and experimentally obtained data proves, for a selected filling factor η, validity of the analytically established model. Scaled data sets show that, for a given rough surface topography, (i) wicking kinetics of considered liquids depend on properties of liquids, that is, surface tension to viscosity ratios and contact angles; (ii) the filling factor for all tested liquids is an invariant, offering good prediction within the range of ~0.9-1.0. The distance of the wicking front versus square root of time relationship was well established throughout the whole considered wicking evolution time.     

A first step towards a quantum mechanical description of surface energy and diffusivity in the bubble model of positronium annihilation

In the bubble model of positronium annihilation in liquids, the inward contractile force on the bubble surface is described through classical surface tension of the liquids. In the present calculation, we adopted a simple quantum mechanical approach to describe the bubble surface energy in terms of the motion of a representative quasi-free electron outside the bubble. The bubble parameters (radius, potential, etc.) for different liquids obtained using the prescribed model are consistent with the results obtained using classical surface tension.     

Surface phase transition of C12E1 at the air/water interface: a study by dynamic surface tension, external RA FT-IR, and 2D IR correlation methods

The surface conformational states of the Gibbs monolayer of ethylene glycol mono-n-dodecyl ether (C(12)E(1)) at the air/water interface was studied using dynamic surface tension, external reflection-absorption FT-IR spectroscopy (ERA FT-IR), and two-dimensional infrared (2D IR) correlation methods at constant temperature. The dynamic surface tensions were measured at different bulk concentrations of C(12)E(1), and it was observed that a constant surface tension region appears at approximately 38.5 mN m(-1) in a dynamic surface tension profile at concentrations higher than 11 micromol kg(-1). This constant surface tension region corresponds to the surface phase transition from liquid expanded (LE) to liquid condensed (LC). Two sets of ERA FT-IR spectra were collected, one at different bulk concentrations but after equilibrium time (equilibrium measurements) and another at constant bulk concentration (m = 16 micromol kg(-1)) but at different times (dynamic measurements). The first set of these measurements show that the peak area increases in the range of 11 < m < or = 16 micromol kg(-1), which means the increase in the number of surfactant molecules at the air/water interface. Also, the wavenumber of antisymmetric CH(2) stretching decreases gradually from approximately 2923 cm(-1) (for 10 and 11 micromol kg(-1)) to approximately 2918 cm(-1) (for m > or = 16 micromol kg(-1)) with increasing concentration. The wavenumbers of 2923 and 2918 cm(-1) were assigned to LE and LC phases, respectively, and the decrease of wavenumber in the concentration range of 11 < m < or = 16 micromol kg(-1) were correlated to the surface phase transition (LE --> LC), or in other words, in the mentioned concentration range, two phases coexist. The dynamic ERA FT-IR measurements at 16 micromol kg(-1) also confirm the surface phase transition from LE to LC. The 2D IR correlation method was applied to the both equilibrium and dynamic IR spectra of the C(12)E(1) monolayer. The synchronous correlation maps show two strong autopeaks at approximately 2922 and approximately 2851 cm(-1) and also show a strong correlation (cross-peaks) between antisymmetric CH(2) stretching (nu(a)) and symmetric CH(2) stretching (nu(s)). The asynchronous correlation maps show that both observed bands of nu(a) and nu(s) in one-dimensional IR split into two components with the characteristic of overlapped bands, which reveals the coexistence of two phases (LE and LC) at the interface at 11 < m < or = 16 micromol kg(-1). The synchronous and asynchronous maps that were obtained from dynamic IR spectra closely resembled the equilibrium map.     

Thermal healing of scratches and determination of solid surface tension of selenium

The healing of scratches on the surface of vitreous selenium was observed over a period of nine weeks, and from the data the solid surface tension of vitreous Se is estimated to be (100 ± 20) dyne/cm at 38.8°C, about the same as that of the liquid at the melting point. This value is three times as large as the critical surface tension determined from contact angle measurements, which indicates that for vitreous Se in contact with organic liquids, the solid—liquid interfacial tension is about two-thirds as much as the solid surface tension. The present method of measurement can probably be used to determine the solid surface tension of other polymers, and by measuring the healing of scratches on a solid immersed in a liquid the method could be used to determine the solid—liquid interfacial tension.