Surface force mercury

The mercury penetration approach is based on the fact that liquid mercury has a very high surface tension and the observation that mercury does not wet most catalyst surfaces. This situation holds true for oxide catalysts and supported metal catalysts that make up by far the overwhelming majority of the porous commercial materials of interest. Since mercury does not wet such surfaces, the pressure required to force mercury into the pores will depend on the pore radius. This provides a basis for measuring pore size distributions through measurements of the... 

Figure 1.2 A barometer is a device for measuring pressures. A vacuum-filled glass tube (sealed at one end) is placed in a trough of mercury with its open end beneath the surface of the liquid metal. When the tube is erected, the pressure of the external air presses on the surface and forces mercury up the tube. The height of the mercury column li is directly proportional to the external pressure p...

Using low-pressure porosimetry, Winslow measured contact angles by determining the breakthrough pressure required to force mercury into numerous holes drilled into the surface of solid discs. 

In this section we discuss five different materials as examples with different charging mechanisms mercury, silver iodide, oxides, mica, and semiconductors. Mercury is one example of an inert metal. Silver iodide is an example of a weakly soluble salt. Oxides are an important class of minerals. For most biological substances like proteins or lipids a similar charging process dominates. Mica is an example for a clay mineral. In addition, it is widely used as a substrate in surface force measurements and microscopy. We also included a general discussion of semiconductors because the potential in the semiconductor can be described similarly to the diffuse layer in electrolytes and there is an increasing effort to make a direct contact between a liquid or a living cell and a semiconductor. 

In the classical electrocapillary electrometer the configuration is inverted. Mercury is placed in a glass tube that ends with a fine capillary, as shown in Fig. 3H. Since we need pressure to force mercury into a fine capillary, there will be a certain height of mercury column supported by the capillary in this configuration. This is the exact equivalent of the capillary depression shown in Fig. 2H(b), and the height of the column is also given by Eq. 53H. In this equation we note that h depends on y, and the surface tension depends on potential hence, the height of the mercury column above the capillary is a function of potential. 

The increased rapidity of solution of a solid at the surface of a liquid solvent, resulting in the etching away of that part of a rod of solid at the place where it is surrounded by the liquid surface, was noticed with camphor in water it is a real effect and is not due to convection currents of denser solution falling away from the surface. It seems to be caused by surface forces causing a direct passage of molecules from the solid to the liquid surface, since it is shown by benzo-phenone in contact with mercury, in which benzophenone is insoluble. ... 

Mercury porosimetry is governed in each pore by an equilibrium force/surface tension balance (the Washburn equation) that relates the diameter of a cylindrical pore to the pressure needed to force mercury into it. The pressured step-by-step invasion of a pore network is then controlled by a pattern of pone accessibly at each given pressure. Systematic penetration, starting from an empty network surrounded by mercury, can be readily performed. Results for the network in Fig. 5 are given in Fig. 6, showing both the penetration curve and the retraction curve. Stochastic pore networks implicitly predict hysteresis between penetration and retraction as well as a residual final entrapment of mercury. In Fig. 6, the final entrapment is about 45%, with much of the retained mercury entrapped in the larger pores [11]. More details of the pore-by-pore calculation have been published [4]. 

There are two established methods for measuring the distribution of pore volumes. The mercury-penetration method depends on the fact that mercury has a significant surface tension and does not wet most catalytic surfaces. This means that the pressure required to force mercury into the pores depends on the pore radius. The pressure varies inversely with a 100 psi (approximately) is required to fill pores for which a = 10,000 A, and 10,000 psi is needed for a — 100 A. Simple techniques and equipment are satisfactory for evaluating the porervolume distribution down to 100 to 200 A, but special high-pressure apparatus is necessary to go below a = 100 A, where much of the surface resides. In the second method, the nitrogen-adsorption experiment (described in Sec. 8-5 for surface area measurement) is continued until the nitrogen pressure approaches the... 

Liquids other than water exhibit surface tension. Mercury is a good example of a liquid that has a high surface tension and strong interparticle attractive forces. When mercury is spilled, it forms droplets, much like the water beads that you see when it rains on a freshly waxed car. 

Another technique for measurement of pore-size distributions is mercury poro-simetry [9]. Because mercury does not wet the surface of oxides (the contact angle varies from 135 to 143 °), pressure is required to force mercury into the pores. The pressure at which mercury is taken up indicates the diameter of the pores, and the volume of mercury intruding gives the volume of the pores. Modem equipment enables the use of very high pressures, and thus measurement of pore diameters of ca 4 nm. It can therefore be concluded that mercury porosimetry and nitrogen adsorption can both be used to measure pores down to a diameter of about 4 nm mercury porosimetry can, however, be used to determine pore of diameters as large as 200 pm. Modern equipment employs computer programs that enable ready calculation of the pore-size distribution from experimental data. 

Mercury is a non-wetting fluid for most materials. Because the contact angle (0) is 180°, cos = -1, and pressure is required to force mercury into the pores-see equation (1). We speak of mercury "intrusion pressures" these are quite high due to the high surface tension of mercury (476 dynes/cm). Thus, for a given pore size, the pressure required to force mercury into the pores is almost seven times greater than the pressure required to expel water from the pores. 

Direct observation of an ordered phase of NA bases on sohd electrodes by techniques, such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM), may help determine the orientation of the molecules in the compact film [83, 92-98], These techniques were also recently apphed to the surface of mercury [99-101], It was found that cationic detergent benzalkonium chloride (BAG), used for DNA spreading on mica in scanning force microscopy, forms a condensed film at the mercury electrode surface. The corresponding pit on C-E curves resembled the pits of bases and... 

Mercury Penetration. The pressure, p, required to force mercury into a pore is inversely related to its diameter, and the volume of mercury, v, which penetrates at that pressure measures the volume and thus the length of the pore. From this the internal area of the pore can be calculated. However, in practice, the pores are widely different in size and the surface area is determined by an integral ... 

In agreement with Equation 1.19, the adhesive force between the solid surfaces in mercury should be equal to the sum of the adhesive force in air, and the capillary contraction force, Ap. For this... 

Pressure is measured in several different units. A common unit of pressure, the millimeter of mercury (mmHg), originates from how pressure is measured with a barometer (Figure 5.4 ). A barometer is an evacuated glass tube, the tip of which is submerged in a pool of mercury. The liquid mercury is forced upward into the evacuated tube by atmospheric pressure on the hquid s surface. Because mercury is so dense (13.5 times more dense than water), attnospheric pressure can support a column of Hg that is only about 0.760 m or 760 mm (about 30 in) tall. (By contrast, atmospheric pressure can support a column of water that is about 10.3 m taU.) This makes a column of mercury a convenient way to measure pressure. 

The second method to be discussed here is the determination of surface tension by measuring the weight of falling mercury drops. To a first approximation the weight of the drop (corrected for the weight of solution displaced by it) is proportional to the surface forces retaining it... 

Rootare and Penslow [66] obtained surface areas from mercury intrusion data using no assumption of any specific pore geometry. The problem was approached from the point of view that work is required to force mercury into the pores, the work, dW, required to immerse an area S5 of powder being ... 

In the mercury method, for example, the sample is placed in a container and evacuated. Mercury is then admitted by applying pressure. The pressure necessary to force mercury into the capillary depends on the contact angle and surface tension, and is given by ... 

Experimentally, electrostatic double-layer forces versus distance were first quantitatively measured in foam films [444—446]. Aqueous foam films with adsorbed charged surfactant at air-liquid interfaces are stabilized by double-layer forces, at least for some time. Voropaeva ef al. measured the height of the repulsive barrier between two platinum wires at different applied potentials and in different electrolyte solutions [447]. U sui et al. [448] observed that the coalescence of two mercury drops in aqueous electrolyte depends on the applied potential and the salt concentration. Accurate measurements between solid-liquid interfaces were first carried out between rubber and glass with a special setup [449]. In the late 1970s, DLVO force could be studied systematically with the surface forces apparatus [424,450,451]. With the introduction of the atomic force microscope, DLVO forces between dissimilar surfaces could be measured [198, 199, 452, 453]. 

In molecular distillation, the permanent gas pressure is so low (less than 0 001 mm. of mercury) that it has very little influence upon the speed of the distillation. The distillation velocity at such low pressures is determined by the speed at which the vapour from the liquid being distilled can flow through the enclosed space connecting the still and condenser under the driving force of its own saturation pressure. If the distance from the surface of the evaporating liquid to the condenser is less than (or of the order of) the mean free path of a molecule of distillate vapour in the residual gas at the same density and pressure, most of the molecules which leave the surface will not return. The mean free path of air at various pressures is as follows —... 

Important physical properties of catalysts include the particle size and shape, surface area, pore volume, pore size distribution, and strength to resist cmshing and abrasion. Measurements of catalyst physical properties (43) are routine and often automated. Pores with diameters <2.0 nm are called micropores those with diameters between 2.0 and 5.0 nm are called mesopores and those with diameters >5.0 nm are called macropores. Pore volumes and pore size distributions are measured by mercury penetration and by N2 adsorption. Mercury is forced into the pores under pressure entry into a pore is opposed by surface tension. For example, a pressure of about 71 MPa (700 atm) is required to fill a pore with a diameter of 10 nm. The amount of uptake as a function of pressure determines the pore size distribution of the larger pores (44). In complementary experiments, the sizes of the smallest pores (those 1 to 20 nm in diameter) are deterrnined by measurements characterizing desorption of N2 from the catalyst. The basis for the measurement is the capillary condensation that occurs in small pores at pressures less than the vapor pressure of the adsorbed nitrogen. The smaller the diameter of the pore, the greater the lowering of the vapor pressure of the Hquid in it. 

Porosity and pore-size distribution usually are measured by mercury porosimetry, which also can provide a good estimate of the surface area (17). In this technique, the sample is placed under vacuum and mercury is forced into the pore stmcture by the appHcation of external pressure. By recording the extent of mercury intmsion as a function of the pressure appHed, it is possible to calculate the total pore volume and obtain the population of the various pore sizes in the range 2 nm to 10 nm. 

Some emphasis has been placed inthis Section on the nature of theel trified interface since it is apparent that adsorption at the interface between the metal and solution is a precursor to the electrochemical reactions that constitute corrosion in aqueous solution. The majority of studies of adsorption have been carried out using a mercury electrode (determination of surface tension us. potential, impedance us. potential, etc.) and this has lead to a grater understanding of the nature of the electrihed interface and of the forces that are responsible for adsorption of anions and cations from solution. Unfortunately, it is more difficult to study adsorption on clean solid metal surfaces (e.g. platinum), and the situation is even more complicated when the surface of the metal is filmed with solid oxide. Nevertheless, information obtained with the mercury electrode can be used to provide a qualitative interpretation of adsorption phenomenon in the corrosion of metals, and in order to emphasise the importance of adsorption phenomena some examples are outlined below. 

The more highly charged the interface becomes, the more the charges repel each other, thereby decreasing the cohesive forces, lowering the surface tension, and flattening the mercury drop. The second differential of the electrocapillary plot gives directly the differential capacitance of the double layer ... 

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