The Du Nouys Ring Method

The basis of this method is to measure the force required to detach a ring or loop of wire from the L/L interface [9]. As a first approximation, the detachment force is taken to be equal to the interfacial tension y, multiplied by the perimeter of the 

Values of the correction factor / were tabulated by Harkins and Jordan [see Eq. (5.7)], and a theoretical account of / was given by Freud and Freud [11]. 

When using the du Nouy method for determining y, it is essential that the ring is kept horizontal during the measurements. Moreover, the ring should be free from any contaminant this is usually achieved by using a platinum ring that is flamed before use. 

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STATIC INTERFACIAL TENSION DETERMINATION AT A LIQUID/GAS INTERFACE BY THE DU NOUY RING METHOD... 

The chemistries of commercial oil vehicles vary with respect to the ratio of aromatic, paraffinic and cycloparaffinic carbon (generally about 20, 40, and 40 percent, respectively), degree of oxidation, and amount and character of impurities. The surface energies of the high and low viscosity ink vehicles shown in Figure 8 were 31.3, and 26.0 mN/rn, respectively (as measured by the Du NoUy ring method). 

Figure 6.4 Liquid surface tension determination by the Du Nouy ring method A maximum upward pull...

The Du Nouy Ring Method A ring with perimeter L is immersed in to a liquid and then lifted up. The maximum pull exerted on the ring by the surface tension forces is measured (see Fig. 7a). At the maximum puU the force vector will be exactly parallel to the direction of motion and the contact angle will be zero. The surface tension is then calculated from y = F IL, where is the maximum force during the lift off. A correction for the weight of the volume of... 

Effects of anion on surface tension. Surface tension measurements by the du Nouy ring method kept the type of cation constant while the anions were varied to include [BMIM][Bp4], [BMlM][PFg], BMlM][Tf2 N], and [BMIMJpFjSO ] and four additional cation-anion pairs. The dry ionic liquids showed the expected decreasing trends in surface tension with increasing temperature for each ionic liquid cation-anion series [5]. Increasing anion size appeared to correlate with a decrease in surface tension of the ionic liquid - as noted in reference [34]. 

Equation (2.7) shows the relationship for the calculation of surface tension by the du Nouy ring method. In this equation, Pj is the total force on the ring which is measured, is the weight of the ring, R is the radius of the ring, and is the ideal surface tension. In practice, a meniscus correction factor is required because the size and the shape of the surface inside and outside the ring are not the same. Therefore, surface tension must be... 

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.,... 

Before equations such as Eqs. 6, 7 and 8 can be used, values for the surface energies have to be obtained. While surface energies of liquids may be measured relatively easily by methods such as the du Nouy ring and Wilhelmy plate, those of solids present more problems. Three approaches will be briefly described. Two involve probing the solid surface with a liquid or a gas, the third relies on very sensitive measurement of the force required to separate two surfaces of defined geometry. All involve applying judicious assumptions to the experimental results. 

The interfacial shear viscosities are measured by the deep channel viscous traction surface viscometer (5) at the Illinois Institute of Technology. The oil-water equilibrium tensions are measured by either the spinning drop or the du Nouy ring (6) method. 

Many methods for the measurement of surface and interfacial tensions, details of the experimental techniques, and their limitations are described in several good reviews (27-29). Some methods that are used most in emulsion work are the du Nouy ring, drop weight or volume, pendant drop, and the spinning drop. The spinning drop technique is applicable to the very low interfacial tensions encountered in the enhanced oil recovery and microemulsion fields (30). In all cases, when solutions rather than pure liquids are involved, appreciable changes can take place with time at the surfaces and interfaces. 

Wilhelmy Plate Method A method for determining surface or interfadal tension on the basis of measuring the force needed to pull an inert plate, held vertically, through an interface. Also termed the Wilhelmy slide method. See also du Nouy Ring Method. 

The du Nouy ring and Wilhelmy plate methods (Figure 2.4) are the two most frequently used techniques for measuring surface tension at the liquid-air interface or interfacial tension at a liquid-liquid interface. Only the du Nouy method can be applied to measure interfacial tension. Both these techniques are based on pulling an object with a well-defined... 

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. 

Lunkenheimer, K. and Wantke, K. D., On the applicability of the du Nouy (ring) tensiometer method for the determination of surface tensions of surfactant solutions, J. Colloid Interface ScL, 66, 579-581 (1978). 

The dynamic process of adsorption of emulsifiers and the equilibrium state of the interfacial film can be measured by the change in interfacial tension as a function of time. Dynamic interfacial tension techniques exist that measure without disturbing the interface. Various such techniques to measure interfacial tension have been reported in the literature (Addison and Hutchinson, 1949 Padday and Russel, 1960). The Wilhelmy plate technique is preferred over other techniques because the values obtained are more accurate than those obtained using other techniques such as the capillary rise or du Nouy ring methods (Padday and Russel, 1960). In the latter two methods, the long equilibration time (3-60 hours) and difficulties in accurately positioning the... 

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... 

The surface tension of the surfactant solutions was measured by the Wil-helmy plate method, whereas the surface tension of the oils was measured by the Du Nouy ring technique on a Krtiss KIOT digital tensiometer. The interfacial tension of the oil-solution interfaces was measured by the pendant drop method. 

A method that has been rather widely used involves the determination of the force to detach a ring or loop of wire from the surface of a liquid. Originally developed by du Nouy, this method is based on using a ring (platinum) and measuring the force when it is dipped in the liquid surface. 

Figure 3.8 Illustrations of the Wilhelmy plate (upper) and du Nouy ring (lower) surface tension methods. Both illustrations are for wetting contact angles.

Best surfactant behaviour was achieved in the case of the polyether trimethylsilane by introducing a hexyl spacer group in the molecule structure. For testing the wetting ability a droplet (50 pi) of an aqueous surfactant solution is applied by syringe to a clean sheet of polypropylene. Afterwards the increase of the diameter of the droplet is measured. The surface tension of the aqueous solution is usually determined by the well known ring method of du Nouy [5]. 

Table 1.2. Surface tensions of water in mN m , obtained by various investigations using different techniques. Temperatures in degrees Celsius. Abbreviations for methods CR = capillary rise, WP = Wilhelmy plate, DNR = Du Nouy ring, DM = other detachment method or object in the surface. HD = hanging (pendent) drop, SD = sessile drop, MBP = maximum bubble pressure DW = drop weight.

It is hence essential to be able to measure and modify the surface free energy of the solid if criteria other than trial and error are used to predict the wettability (and the colloidal stability, for that matter) of a solid drug, since the surface tension of the liquid is readily measurable by any well-establi.shed method (Wilhelmy plate, du Nouy ring, drop shape analysis, etc. see. e.g. Ref. 37). In the case of solids, only indirect methods are available to estimate Ys and Ysl here we shall give details on the simplest one. contact angle measurements, and the reader is referred to other chapters of this volume for details on other methodologies. The technique is based on Young s equation (37) ... 

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