Surface tension drop weight method

Figure 3.9 Illustration of the drop weight/volume surface tension method (shown here for a wetting contact angle).

The surface tension of a liquid is determined by the drop weight method. Using a tip whose outside diameter is 5 x 10 m and whose inside diameter is 2.5 x 10 m, it is found that the weight of 20 drops is 7 x 10 kg. The density of the liquid is 982.4 kg/m, and it wets the tip. Using r/V /, determine the appropriate correction factor and calculate the surface tension of this liquid. 

The following values for the surface tension of a 10 Af solution of sodium oleate at 25°C are reported by various authors (a) by the capillary rise method, y - 43 mN/m (b) by the drop weight method, 7 = 50 mN/m and (c) by the sessile drop method, 7 = 40 mN/m. Explain how these discrepancies might arise. Which value should be the most reliable and why ... 

As we shall have occasion to note in dealing with solutions, the composition of the surface phase is very different from that of the bulk liquid. When a liquid interface is newly formed the system is unstable until the surface phase has acquired its correct excess or deficit of solute by diffusion from or into the bulk of the solution. This process of diffusion is by no means instantaneous and, as has been observed in discussing the drop weight method, several minutes may elapse before equilibrium is established. In the ripple method the surfece is not renewed instantaneously but may be regarded as undergoing a series of expansions and contractions, thus we should anticipate that the value of the surface tension of a solution determined by this method would lie between those determined by the static and an ideal dynamic method respectively. 

Goard has determined the surface tensions of a few salt solutions hy the drop weight method employing the method of Iredale for calculating the surface tension, the following values were obtained. 

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.

Drop-weight method. To determine the surface tension of a hexadecane (Ci6H34) you let it drop out of a capillary with 4 mm outer and 40 /. m inner diameter. Hexadecane wets the capillary. Its density is 773 kg/m3. 100 drops weigh 2.2 g. Calculate the surface tension of hexadecane using the simple Eq. (2.15) and the correction factor /. It was concluded that / should be a function of rc/V 1/3, with V being the volume of the drop. Values for the correction factor are listed in the following table (from Ref. [1], p. 19). Is it necessary to use the correction ... 

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. 

Surface tension determined by drop weight method of Adamson. 

Permprasert, J. and Devahastin, S. 2005. Evaluation of the effects of some additives and pH on surface tension of aqueous solutions using a drop-weight method. Journal of Food Engineering 70 219-226. 

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. 

A rather complex theory of the drop weight method, which makes it possible to tabulate the data required in order to determine the surface tension, has been worked out in some detail [27]. In the first (roughest) approximation, it can be assumed that, at the moment of detachment the gravity force acting on a drop, P, is balanced by the surface tension forces,... 

The drop-weight method depends, as do all of the detachment methods, on the assumption that the circumference times the surface tension is the force holding two parts of a liquid column together. When this force is balanced by the mass of the lower portion, a drop breaks off (Fig. 18.4a) and... 

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... 

It is a well-known fact that when a drop is formed at a nozzle under pseudostatic conditions, its size is determined solely by the balance of interfacial tension and gravity or buoyancy forces. This forms the basis of the drop-weight method of determination of surface and interfacial tension. 

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.,... 

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