Drop weight method, surface tension measurement

The surface tension of mercury in the presence of the vapour at various partial pressures was measured by the drop weight method. The following values were obtained for the surface tensions of mercury in the presence of vapours of methyl acetate, water and benzene at various partial pressures at 26 —27° C. 

There are numerous other methods for measuring surface tension that we do not discuss here. These include (a) the measurement of the maximum pressure beyond which an inert gas bubble formed at the tip of a capillary immersed in a liquid breaks away from the tip (the so-called maximum bubble-pressure method) (b) the so-called drop-weight method, in which drops of a liquid (in a gas or in another liquid) formed at the tip of a capillary are collected and weighed and (c) the ring method, in which the force required to detach a ring or a loop of wire is measured. In all these cases, the measured quantities can be related to the surface tension of the liquid through simple equations. The basic concepts involved in these methods do not differ significantly from what we cover in this chapter. The experimental details may be obtained from Adamson (1990). 

Figure 2.9 Maximal bubble pressure and drop-weight method to measure the surface tension of liquids.

Harkins and his colleagues1 have extended these measurements of the work of adhesion to water, and also to mercury.2 The measurements of surface tension were made by the drop-weight method, using the corrections necessary for accurate results as three separate measurements of surface tension are required, considerable accuracy is desirable for trustworthy results in the work of adhesion. 

There are static and dynamic methods. The static methods measure the tension of practically stationary surfaces which have been formed for an appreciable time, and depend on one of two principles. The most accurate depend on the pressure difference set up on the two sides of a curved surface possessing surface tension (Chap. I, 10), and are often only devices for the determination of hydrostatic pressure at a prescribed curvature of the liquid these include the capillary height method, with its numerous variants, the maximum bubble pressure method, the drop-weight method, and the method of sessile drops. The second principle, less accurate, but very often convenient because of its rapidity, is the formation of a film of the liquid and its extension by means of a support caused to adhere to the liquid temporarily methods in this class include the detachment of a ring or plate from the surface of any liquid, and the measurement of the tension of soap solutions by extending a film. 

Ring method — Method to determine the - interfacial tension in liquid-gas systems introduced by Lecomte du Noiiy [i]. It is based on measuring the force to detach a ring or loop of a wire from the surface of a liquid. The method is similar to the -> Wilhelmyplate method when used in the detachment mode [ii]. See also -> electrocapillarity, -r electrocapillary curve, -> Gibbs-Lippmann equation, - Wilhelmy plate (slide) method, - drop weight method, - Lippmann capillary electrometer. 

For this reason, the surface tension has units of either energy per unit area or force per unit length. The surface tension of a liquid is measured in one of several ways capillary rise, ring detachment, or drop weight. Each method of surface tension measurement is outlined in Figure 9.2. With capillary rise, the fluid is suspended by the surface... 

Surface tension measurement. Adsorption titration, also called soap titration, (2.3) was carried out by the drop volume method at different polymer concentrations. The equivalent concentration of salt was held constant. The amount of emulsifier necessary to reach the critical micelle concentration (CMC) in the latex was determined by each titration. The total weight of emulsifier present in the latex is the weight of emulsifier in the water plus the weight of emulsifier adsorbed. The linear plot of emulsifier concentration (total amount of emulsifier corresponding to the end-point of each titration) versus polymer concentration gives the CMC as the intercept and the slope determines the amount of emulsifier adsorbed on the polymer surface in equilibrium with emulsifier in solution at the CMC (E ). 

Various experimental methods for dynamic surface tension measurements are available. Their operational timescales cover different time intervals. - Methods with a shorter characteristic operational time are the oscillating jet method, the oscillating bubble method, the fast-formed drop technique,the surface wave techniques, and the maximum bubble pressure method. Methods of longer characteristic operational time are the inclined plate method, the drop-weight/volume techniques, the funnel and overflowing cylinder methods, and the axisym-metric drop shape analysis (ADSA) " see References 54, 55, and 85 for a more detailed review. 

In order to measure the surface tension of solutions containing surfactants, the maximum bubble pressure, pendant drop and Wilhelmy plate (immersed at a constant depth) methods are suitable capillary rise, ring, mobile Wilhelmy plate, sessile drop and drop weight methods are not very suitable. These methods are not recommended because surfactants preferably adsorb onto the solid surfaces of capillaries, substrates, rings, or plates used during the measurement. In a liquid-liquid system, if an interfacially active surfactant is present, the freshly created interface is not generally in equilibrium with the two immiscible liquids it separates. This interface will achieve its equilibrium state after the redistribution of solute molecules in both phases. Only then can dynamic methods be applied to measure the interfacial tension of these freshly created interfaces. 

The threshold of the equilibrium state can generally be reached slowly, and thus the surface tension values obtained by semi-static methods closely resemble those obtained by static ones. The rate of approaching the equilibrium state should be optimized in each system, in order to avoid lengthy measurements and to obtain surface tension values as close to the equilibrium ones as possible. Among the most common semi-static methods are the method of maximum pressure, the du Noiiy ring method and the drop-weight method. 

Figure 18.4 The drop-weight method for measuring surface tension. (Adapted from Experimental Physical Chemistry, 5th ed., by F. Daniels, J. H. Matthews, P. Bender, R. A. Alberty. Copyright (c) 1956 McGraw-Hill Book Co. Used with the permission of McGraw-Hill Book Co.)...

Drop-weight method of measuring surface tension. 

Bubble Pressure or Drop Weight Method In this approach, surface tension is calculated from the measurement of pressure inside the bubble or from the weight of the drop when it detaches from an orifice. The gas bubble can be introduced into a liquid through a capillary with known radius. The maximum pressure during this process is recorded and used to calculate the surface tension. At the maximum pressure, the gas bubble radius is the same as the capillary with the gas bubble forming an exact hemisphere. If the hydrostatic pressure is po and the inner radius... 

To determine the onset of surfactant aggregation, necessary for the calculations of the surfactant aggregation numbers, cac was determined for 0.1% PEO at various NaCl concentrations by surface tension measurements using the drop-weight method. For this puipose, an apparatus built at the department s workshop was us. ... 

Surface tension of liquids can be measured by either of the two methods static and dynamic. The static methods are based on the assumption that the liquid has attained surface equilibrium. For pure liquids and solutions of crystalloids the process of attainment of equilibrium is very fast and the static methods are best suitable. But for colloidal solutions a considerable time is required to reach the equilibrium state and therefore the dynamic methods of measuring surfacf tension are preferred. The dynanJc methods measure the tension of a liquid before the surface film has had time to form. TTiere are other methods too which fall between the static and the dynamic methods. Among the static methods, the most commonly used ones are (0 the capillary rise method, (ip the du Nouy ring method, (Up the Wilhelmy balance method, and (iv) the drop-weight method.,... 

Drop Weight Method The drop weight, or Tate s, method is yet another technique for determining surface tension. This approach relies on dripping of a liquid in a gas from a capillary and measuring the combined vol-... 

One of the simplest techniques that can be used to measure surface tension is the so-called drop weight method (Figure 1.17). In this method, the weight of a droplet separating from a pipette tip with a radius Tq is determined by direct weighing of 50-100 droplets, and the surface tension is estimated from the relationship mg = 2%r o. This method provides only a crude estimate of the... 

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. 

Most techniques stretch the liquid-air surface at the moment of measurement. For example, the drop weight method [318] and the ring method [319-322] stretch the surface during detachment. However, instruments are now available which measure surface tension without detaching the ring from the liquid (e.g., the Ki-iiss Tensiometer K12). 

The drop weight method measures the weight of a drop (or several drops) emerging from a capillary of known dimensions [318,336,344-347]. Slight vacuum is applied to the apparatus through a tubing until the drop, forming at the outlet of the capillary, assumes almost its full size. The drop is then allowed to detach itself from the capillary. Surface tension is calculated from the equation... 

Several convenient ways to measure surface tension involve the detachment of a solid from the liquid surface. These include the measurement of the weight in a drop falling from a capillary and the force to detach a ring, wire, or thin plate from the surface of a liquid. In this section we briefly describe these methods and their use. 

This is a fairly accurate and convenient method for measuring the surface tension of a liquid-vapor or liquid-liquid interface. The procedure, in its simpli-est form, is to form drops of the liquid at the end of a tube, allowing them to fall into a container until enough have been collected to accurately determine the weight per drop. Recently developed computer-controlled devices track individual drop volumes to = 0.1 p [32]. 

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