Drop weight and volume methods

For foams, it is the surface tension of the foaming solution that is usually of most interest. For this, the most commonly used methods are the du Noiiy ring, Wilhelmy plate, drop weight or volume, pendant drop, and the maximum bubble pressure method. For suspensions it is again usually the surface tension of the continuous phase that is of most interest, with the same methods being used in most cases. Some work has also been done on the surface tension of the overall suspension itself using, for example, the du Noiiy ring and maximum bubble pressure methods (see Section 3.2.4). 

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. 

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]. 

Instead of measuring the weight directly we may calculate it from the volume and the density the drop volume method has been applied by Harkins chiefly to the measurement of the tension between two liquid phases, and it probably falls little short in accuracy from the previous method. More frequently it has been j modified, especially for biochemical purposes, as a drop number method that is, a known volume of liquid iFallbwed oo nov. of a tube, and the number of drops formed is compared with that formed by a standard fluid. This method is necessarily very rough. 

Note 1 As was mentioned in Note 1 under item B the US MIL-STD-650 (Ref 21) does not make any distinction betw Bulk Apparent Densities. It defines them in Method 201.1 as "weight per unit of outside volume, which may include voids in Method 201.2 as "weight per unit of apparent volume and in Method 201.3 as "weight per unit of packed volume , which means that the specimen is dropped and tapped to dispel voids Note 2 Accdg to Ref 13, p 877, "It is not always possible to say whether a given measurement refers to bulk specific gravity, to apparent specific gravity, or to some intermediate value, since the determination of solid volume depends on the nature of the substance used to permeate the voids and on the nature of the pores in the solid ... 

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 ... 

Figure 4.8 Correction factor for drop volume and drop weight methods...

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 ). 

The drop weight, or drop volume method (sec. 1.6) is intrinsically dynamic the time scale can be varied by applying a variable pressure on the capillary. The volume of the drop is measured as a function of time, emd theory is needed to dafve y(t). Practically speaking, this technique is convenient although the interpretation may offer problems temperature control Is simple, the accuracy is = 0.1 mN m and LG and LL Interfaces can both be studied. 

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. 

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 volumes of the primary and satellite drops that are formed. In this case one can write Tate s law for the weight of the drop... 

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