Surface tension sedimentation volume

An extreme in sediment volume was usedt as a criterion for the effective cancellation of interparticle attraction by the continuous phase. Nylon-6,6 dispersions consisting of 1.0 g of solid in 10 ml of /2-propanol-thiodiethanol mixtures of various compositions were allowed to settle to sedimentation equilibrium. Listed here are the equilibrium sediment volumes, the volume/volume compositions, and the surface tensions of the media ... 

Plot the sediment volume versus the surface tension of the continuous phase for these dispersions. What is the apparent surface tension of the nylon-6,6 Briefly describe some precautions that must be observed in interpreting results such as these in terms of 7 values for the dispersed solid. 

The measurement of contact angles on solids in powder or particle form is a challenging task, and numerous methods have been proposed 7. In this section, three successful indirect approaches are discussed capillary penetration into columns of powders, sedimentation volume of particles, and solidification fronts of particles. These methods are indirect because they provide the solid surface tension rather than the contact angle. For the sake of completeness, various direct approaches are also summarized briefly. 

In view of the possibility that the sedimentation volume of particles may show extrema when A = 0, such an extremum in the sedimentation volume may provide a means to determine the solid-vapor surface tension of the particles. The solid-vapor surface tension ysV of particles would be equal to yn. the surface tension of the suspending liquid at which the sedimentation volume extremum occurs. 

Therefore to determine the particle surface tension by using the sedimentation volume technique, the required basic procedures in the experiments can be summarized as follows ... 

Determine the liquid surface tension y % at which an extremum in the sedimentation volume occurs. 

In practice, sedimentation volume experiments are performed with binary liquid mixtures as the suspending liquids, in order to have a sufficiently large range of surface tension and to be able to adjust the liquid surface tension to any specific value. 

Figure 9 Sedimentation volume of Nylon-6,6 particles given in Figure 8 plotted as fiinction of the surface tension, YlV of the suspending liquid in each tube. (Reproduced with permission from Ref. 40. Copyright 1986, Academic Press Inc.)...

Figure 10 Sedimentation volume, Vge j, as a function of the surface tension, ylV suspending liquid...

Overall, wettability measurement of small particles is a difficult problem that is further aggravated in the case of heterogeneous surfaces. Some of these problems can result from the presence of patches of different composition in the same particle. It is considered that if these patches are below a critical size of 0.1 mm, the surface is homogeneous regarding its wettability. Several indirect techniques have been developed to measure the surface tension, and thus the wettability of small particles. In these techniques, the surface tensions of the particles are derived from thermodynamic models and include the advancing solidification front or freezing front, sedimentation volume, and particle adhesion techniques [44, 45]. 

Experimental procedure In order to perform an appropriate experiment, one needs to prepare a series of liquids or binary liquid mixtures with a range of surface tensions that covers the surface tensions of the particles of interest. For this purpose, it is preferable to have relatively tall cylinders with a small diameter and increments (e.g. 0.01 ml), in order to maximize the accuracy of the final sedimentation volume, Vsed, readings. It should be noted that the liquids used must satisfy the following criteria, i.e. (i) liquids should be chemically inert with respect to the sample particles, (ii) liquids... 

Table 14.4. Liquid surface tension, /iv, at which maximum (Max) or minimum (Min) in sedimentation volume occurs, for several polymers PTFE, polytetrafluoroethylene PVDF, poly(vinylidene fluoride) PVF, poly(vinyl fluoride) HOPE high-density polyethylene PA 66, poly(hexamethylene adipamide) (nylon 6, 6) PSF, polysulfonate. Experiments were performed at room temperature, and the extrema are to be understood as being the surface tensions of the polymers (from refs (65, 66)) ... 

Figure 14.23. Sedimentation volume, Vsed, as a function of the surface tension, yiv, of the suspending liquid for a PTFE powder the extrema indicate the particle surface tension (from ref (5))...

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