Nouy ring

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. 

Figure D3.6.1 General principle of direct force-transducer-based tensiometers (Wilhelmy plate and/or du Nouy ring method). I/O is input/output.

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

Static interfacial tension analysis du Nouy ring method, 635-636,640-641 vs. dynamic tension, 632 (table)... 

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

Experimental Procedure -- The surface tensions of Triton X-100 solutions, with or without other additives, were measured using a du Nouy ring tensiometer (Fisher Scientific Company) at 25°C. 

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. 

The advantage of the ring over the plate is that and are independent of position but this is to a large extent offset by the disadvantage of the much more complicated geometry. Du Nouy rings are often employed in the detachment mode. Under idealized conditions this detachment may proceed as sketched in fig. 1.22. Maximum pull is obtained in situation b), beyond which the liquid necks spontaneously. When the ring is completely wetted the capillary force equals 27t(R- a)y + 2ti(R + a]y, similar to the flat plate situation. Usually R a so that... 

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.

Abbreviations CR = capillary rise DM = detachment method (including Wilhelmy plate and Du Nouy ring) DW = drop weight ( stalagmometer ) HD = hanging (pendant) drop MGP = maximum gais bubble pressure SD = sessile drop. 

Surface Tension Measurements. The surface tension of each HA or FA solution was measured using a du Nouy ring tensiometer as previously described (13). 

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). 

Inductive sensor Du Nouy ring Sample vessel... 

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

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. 

Figure 19. Suspension-air and suspension-toluene surface and interfacial tensions for aqueous suspensions of Na-montmorillonite, as measured with the du Nouy ring technique at 24.5 °C. The broken lines show the values measured for pure water. (Reproduced with permission from reference 44. Copyright 1994 National Research Council of Canada.)...

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) ... 

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... 

Surface Tension, Capillarity, and Contact Angle, Fig. 7 Surface tension measurement, (a) Du Nouy ring method, (b) Wilhelmy plate method... 

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]. 

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