Drop Volume Weight Method

Here one determines the volume V (or weight W) of a drop of liquid (immersed in the second, less dense liquid) which becomes detached from a vertically mounted capillary tip having a circular cross section of radius r. The ideal drop weight Wideai is given by the expression. 

In practice, a weight W is obtained that is less than Wideai because a portion of the drop remains attached to the tube tip. Thus, Eq. (4.30) should include a correction factor j), which is a function of the tube radius r and some linear dimension of the drop, i.e., (Eq. 4.31). 

Values of have been tabulated by Harkins and Brown [9]. Lando and 

Oakley [10] used a quadratic equation to fit the correction function to (r/V ). A better fit has been provided by Wilkinson and Kidwell [11]. 

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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]. 

If one considers a system consisting of water (with or without added electrolyte) + oil + surfactant (with or without a cosurfactant) at equilibrium, there will most likely be present more than two phases (due to the formation of emulsion or microemulsion). The determination of the interfacial tension, Yij> between the two liquid phases is, therefore, of much importance, in order to understand the forces which stabilize these emulsions or microemulsions. The interfacial tension can be measured by using a variety of methods, as described in detail in surface chemistry text-books (1-3). If the magnitude of yij is of the order of few mN/m (=dyne/ cm), then the methods generally used are Wilhelmy plate method or the drop volume (or weight) method (1-4). However, in certain systems ultra-low (or low) interfacial tensions have been reported. Since these low values are reported to be essential in order to mo-... 

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. 

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 ... 

Drop-weight method. With a mass m = 2.2 x 10 3kg/100 volume of one drop is... 

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. 

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