Wilhelmy plate technique, surface tension measurement

Surface tension measurements. Solutions of the betaines were prepared with quartz-condensed, distilled water, specific conductance, 1.1 X 10" mho cm" at 25°C. All surface tension measurements were made by Wilhelmy vertical plate technique. Solutions to be tested were immersed in a constant-temperature bath at the desired temperature 0.02°C and aged for at least 0.5 h before measurements were made. The pH of all solutions was > 5.0 (usually, in the range 5.5-5.9), where surface properties show no change with pH. 

Each term on the right hand-side of Eq. (2.9) is measured independently providing a direct determination of n, also under low pressures [30]. It is necessary as well to eliminate the fluctuations in the atmospheric pressure and to measure more precisely the differential pressure pg - p, (Fig. 2.12). This is done with a sensitive draft-range differential pressure transducer from Omega (Model PX750-06DI) with an accuracy of 0.5 Pa. The capillary radius r is determined by a Kondon micrometer calliper while the surface tension of the surfactant solution - by the Wilhelmy-plate technique, and the height hc in the capillary tube - with a Wild cathetometer (Model KM-274). 

These equations may be compared with those for cylinders, see for instance [1.3.21. For flat plates one does not have to worry about complications of the details of the profile, but this advantage is offset by the much lower rise. Typically, h is of order i.e. h = O (mm) and y is proportional to whereas it scales with ah in capillaries. Over the last few decades laser-optical techniques for scanning the meniscus and establishing h down to about 10" mm have become available In a modem variant of the Wilhelmy plate technique, to be described in sec. 1.8a, the force needed to pull the plate out of the liquid is measured as a function of the height above the zero level. In this way the surface tension and contact angle can be determined simultaneously. Alternatively, the method can be used to obtain contact angles, i.e. from [1.3.161 after y has been measured by some other technique. 

The Wilhelmy plate technique has also been modified to measure surface or interfacial tensions under special conditions. By way of illustration we mention the application to the measurement of electrocapillanj curves for the mercuiy-aqueous electrolyte solution interface by Montgomery and Anson ), extending earlier work by Smith ). Electrocapillary curves are plots of y as a function of the potential applied across the interface, which should be polarizable. In fig. II.3.48 we already gave a few such curves. Montgomery and Anson found that their curve in 10 2 M NaF agreed within 0.005 mN m- with data obtained using the maximum bubble pressure. 

The force at the barrier may be measured directly by a calibrated torsion wire that is mechanically attached to the barrier. However, nowadays the surface tensions at both sides of the barrier are measured Independently using one of the appropriate methods in sec. 1.8, mostly by the static Wilhelmy plate technique. The latter method has the advantage that einy leakage of monolayer material across the barrier can be easUy detected. 

Monolayer Techniques. Surface tension and surface potential were measured by a torsion balance with the Wilhelmy plate and a radioactive (226Ra) air electrode (5, 20). 

Steady State Expansion Measurements. The dynamic surface tension in steady state expansion was determined using a modified Langmuir trough equipped with six barriers fixed to an endless belt which were moved caterpillar-wise one after another over the liquid surface.10,24 Surface tension was determined using the Wilhelmy plate technique. Measurements were performed going from the highest to the lowest expansion rate. 

The Wilhelmy plate technique was used. The decrease of the surface tension with time due to the adsorption of protein molecules at the air/protein solution interface was recorded. The values after 40 minutes were taken. To set up a new pH we used an automatic micropump which introduced 10 M NaOH to the measuring cell equipped with a magnetic stirrer. The same surface tension values for each pH were obtained with albumin initially dissolved in 10 NaOH and... 

In order to determine the infants lung maturity and the necessity of surfactant therapy it is of great importance to substantiate the functionality of the alveolar surfactant, derived via invasive techniques [13], Several techniques and models have been largely used to investigate inteifacial physicochemical properties in vitro and to assess clinical efficiency of ES in vivo the Langmuir monolayer technique in combination with Wilhelmy plate method for surface tension measurements and black foam film method for determination of the ability of ES for stable film formation [14]. The pendant drop method combined with the Axisym-metric Drop Shape Analysis (ADSA) has been also used for similar purposes [4,15-18]. 

The surface tension measurement techniques can be divided into the following three categories (i) Force Methods, which include the truly static methods of the capillary rise and Wilhelmy plate methods, as well as the dynamic detachment methods of the Du Nouy ring and drop weight, (ii) Shape Methods, which include the pendant or sessile drop or bubble, as well as the spinning drop methods, and (iii) Pressure Methods, which are represented by the maximum bubble pressure method. These techniques are summarized in the following sections of this chapter. 

The most common methods used to measure surface tension of surfactant solutions using commercial instruments are the du Noiiy ring and Wilhelmy plate techniques (Fig. 4.7c and d). In the former, the force necessary to detach a ring or wire loop from a liquid surface is measured (for example using... 

Monolayers of PDA were prepared by spreading a solution of PDA in petroleum ether or hexane on a thoroughly cleaned water surface (pH = 2)in a glass trough with paraffin coated edges. The surface density of molecules was controlled by a movable Teflon barrier and the surface tension was measured by a Wilhelmy plate. For the ellipsometry measurements we used a weak He-Ne laser at 632.8 nm. The phase shift was measured by a standard high resolution ellipsometry technique.5 The accuracy in (j> was 10 4 rad. 

The du Nouy ring and Wiiheimy plate methods (Fig. 3.4) are two most frequently used techniques of measuring surface tension at the liquid—air interface or mterfacial tension at a liquid—liquid interface. Only the du Nouy method can be applied to measure interfacial tension. Both of these techniques are based on pulling an object with a well-defined geometry off the surface of liquids and measuring the pull force. These techniques are also known as pull-force methods. In the Wiiheimy method, a plate is the pull object, whereas in the du Nouy technique a ring is used. These techniques are ascribed to two scientists who conducted some of the earliest research in the area of surface tension measurement. In 1863, Wilhelmy described measurement of capillary constants in a paper, without a detailed calculation of surface tension. Lecomte du Nouy illustrated the... 

Figure 6 shows polystyrene (PS) particles of an average diameter of 11.9 xm retained on a freshly excised piece of sheep trachea and on a surfactant film of dipalmitoylphosphatidylcholine (DPPC) in a modified Langmuir-Wilhelmy balance. The surface tension in both cases is approximately 30 mN/m, measured by a drop-spreading technique on the trachea and by a Wilhelmy plate in the surface balance. The aqueous substrate for the film in the balance was 0.9% NaCl with 55% sucrose. The density of the substrate was 1.26 g/mL, considerably higher than that of the particle (1.05 g/cm ). Most of the particles retained on the tracheal surface appear to be submerged in the substrate, and only a small segment of the spheres is exposed to air above the three-phase line. This is equivalent to the situation in the surface balance. 

Dynamic surface tension has also been measured by quasielastic light scattering (QELS) from interfacial capillary waves [30]. It was shown that QELS gives the same result for the surface tension as the traditional Wilhelmy plate method down to the molecular area of 70 A. QELS has recently utilized in the study of adsorption dynamics of phospholipids on water-1,2-DCE, water-nitrobenzene and water-tetrachloromethane interfaces [31]. This technique is still in its infancy in liquid-liquid systems and its true power is to be shown in the near future. 

The Wilhelmy hanging plate method (13) has been used for many years to measure interfacial and surface tensions, but with the advent of computer data collection and computer control of dynamic test conditions, its utility has been greatly increased. The dynamic version of the Wilhelmy plate device, in which the liquid phases are in motion relative to a solid phase, has been used in several surface chemistry studies not directly related to the oil industry (14- 16). Fleureau and Dupeyrat (17) have used this technique to study the effects of an electric field on the formation of surfactants at oil/water/rock interfaces. The work presented here is concerned with reservoir wettability. 

The situation shown in Figure 6.2b is one in which surface tension and contact angle considerations pull a liquid upward in opposition to gravity. A mass of liquid is drawn up as if it were suspended by the surface from the supporting walls. At equilibrium the upward pull of the surface and the downward pull of gravity on the elevated mass must balance. This elementary statement of force balance applies to two techniques by which 7 can be measured if 6 is known the Wilhelmy plate and capillary rise. 

Apparatus and Procedure. Surface Isotherms. The technique for determining the n-A and AV-A curves of the lipid films has been described (6). Briefly, the Wilhelmy plate method was used to measure surface tension, from which the surface pressure was calculated (n = 7h2o—yfiim) The surface potential was measured by means of a radioactive (226Ra) air electrode and a saturated calomel electrode connected to a high impedance model 610 B Keithley electrometer (Keithley Instruments, Cleveland, Ohio). 

Provides measuring techniques of contact angle, surface tension, interfacial tension, and bubble pressure. Suitable methods for both static and dynamic inteifacial tension of liquids include du Nous ring, Wilhelmy plate, spinning drop, pendant drop, bubble pressure, and drop volume techniques. Methods for solids include sessile drop, dynamic Wilhelmy, single fiber, and powder contact angle techniques. 

Important techniques to measure the surface tension of liquids are the sessile drop method, the pendant or sessile bubble method, the Du-Notiy ring tensiometer, and the Wilhelmy-plate method. 

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