Liquid displacement techniques

2 Liquid/gas methods (bubble point, liquid expulsion permporometry) 

This very simple and established method has become a standard technique used by suppliers to measure the largest active pores (as well as cracks or pinholes) in a membrane. The principle is to measure the pressure needed to force air through a liquid-filled membrane. The bottom of the filter is in contact with air and, as the air pressure is gradually increased, air bubbles penetrate through the membrane at a certain pressure. The pressure and pore radius are related by the Laplace equation 

An air bubble will pass through the pore when its radius is equal to that of the pore (Fig. 4.17), assuming that the contact angle is zero. Penetration will first occur through the largest pores and since the pressure is known, the pore radius can be calculated from Laplace equation. 

With water as the wetting medium, the water/air surface tension is relatively high and it is necessary to apply a high pressure if small pores are present (145 bars for a pore radius of 0.01 pm) water can then be replaced by another liquid (e.g. alcohols or hydrocarbons). Nevertheless, as the method is dependent on the type of liquid used (different wetting effects), z-propanol is often used as a 

Standard liquid. The rate at which the pressure is increased and the pore length can also influence the measurements. 

---

A liquid-liquid displacement technique (biliquid permporometry) was also found to be well adapted to the characterization of meso- and macroporous membranes [44]. It is based on the combined principles of bubble pressure and solvent permeability. In this case the applied pressure P and the flux J through the membrane are measured simultaneously. The recorded P and J values, introduced in the Laplace equation directly, give the pore equivalent radius and... 

The porometer used by Hsieh et al. [6S]. employed a widely used liquid displacement technique adopted from an ASTM procedure [66], which uses an external air or nitrogen source at pressures up to 1 MPa, allowing pore sizes in the radii range 0.05 to 10 pm (see also BS 3321, BS 1752 (ISO 4793) and BS 5600 (ISO 4003)]. 

Once a geometric model for the pore and a model for the gas transport through the pores is chosen, this method (as the liquid displacement technique) allows determination of the absolute distribution (differential or integral) of the number of pores active to flux for the membrane. The model used should depend on the working con-... 

Various aspects of in vitro gas production test have been reviewed by Getachew et al. [33], and these authors reported that gas measurement were centered on investigations of rumen microbial activities using manometric measurements and concluded that these methods do not have wide acceptability in routine feed evaluation since there was no provision for the mechanical stirring of the sample during incubation. Another in vitro automated pressure transducer method for gas production measurement was developed by Wilkins [34], and the method was validated by Blummel and Orskov [35] and Makkar et al. [36]. There are several other gas-measuring techniques such as (i) Flohenheim gas method or Menke s method [37] (ii) liquid displacement system [38] (iii) manometric method [39] (iv) pressure transducer systems manual [40], computerized [41], and combination of pressure transducer and gas release system [42]. 

The three most common ways of obtaining true density measurements are gas pycnometry (gas displacement), liquid displacement, and flotation in a liquid. These three techniques have been compared based on accuracy, ease of use, and instrumentation [63], and the results are summarized in Table 4. Gas pycnometry will be discussed in this section because of its wide use and ease of operation. 

Nguyen et al. [205] designed a volume displacement technique that was used to measure the capillary pressures for both hydrophobic and hydrophilic materials. One requirement for this method is that the sample material must have enough pore volume to be able to measure the respective displaced volume. Basically, while the sample is filled wifh water and then drained, the volume of water displaced is recorded. In order for the water to be drained from fhe material, it is vital to keep the liquid pressure higher than the gas pressure (i.e., pressure difference is key). Once the sample is saturated, the liquid pressure can be reduced slightly in order for the water to drain. From these tests, plots of capillary pressure versus water saturation corresponding to both imbibitions and drainages can be determined. A similar method was presented by Koido, Furusawa, and Moriyama [206], except they studied only the liquid water imbibition with different diffusion layers. 

Calcium chloride solutions (pH =6.2) labeled with Ca or 36ci were displaced vertically downward through columns of homogeneous, repacked, water-saturated sandy soil by a chemically identical solution labeled with Cl or Ca, respectively. Constant water fluxes, and solution activities of 1 to 15 pCi/dm, were used. Calcium solutions were analyzed by titration with disodium dihydrogen ethylenediamine tetraacetate to a murexide end point (11). The activity of radioactively labeled solutions was obtained by liquid scintillation techniques. Concentrations of adsorbed calcium were calculated from isotope dilution. The concentration of calcium chloride in the influent solution was 0.08 N. Because exchange of calcium for itself was the only chemical process affecting transport, the calcium chloride concentration remained constant at 0.08 N throughout each experiment, both within the column and in the effluent. 

The chromatographic separation of enantiomers, often referred to as enantioseparation, has received a great deal of attention in recent years. Both liquid (LC) and gas (GC) chromatographic procedures are used. The former is extremely useful for enantioseparations because of the available variations in scale, mechanism, and technique. It has been used in enantioseparations from analytical to preparative in scale, taking advantage of various modes of diastereoisomeric interactions andusing elution and displacement techniques. All the chromatographic methods involve diastereoisomeric interactions between the enantiomers of interest and... 

As a result of these shortcomings, porosity characterizations generally involve both gas permeability measurements to provide estimates of the avasege pore size and liquid displacement to measure maximum pore size in the membrane surface. The techniques rely on a number of assumptions for their results to heve physical menning. Depending on the stiucture of die asymmetric membrane, some of the assumptions may be satisfied only merginally. In such a case, the characterizations are primarily useful as empirical indices for comparison of differenl samples, and the fundamental meaning of the numbers derived from such analyses is questionable. 

Another more recent technique, called permporometry (or liquid displacement porosimetry), is based on the controlled blocking of pores by condensation of a vapor, present as a component of a gas mixture, and the simultaneous measurement of the gas flux through the membrane [42,43]. By measuring the gas transport through the membrane upon decreasing relative vapor pressure, the size distribution of the active pores can be found in the limit of validity of the Kelvin equation. The calculation of the number of pores can be performed by assuming a Knudsen type of transport regime. 

Three methods are common, viz, the liquid displacement method, the sink-float method and the density gradient column method. Each of these is a common, standard technique and is fully described in ISO 10119, 1992 (for the determination of the density of carbon fiber), and also in ASTM D 276-87 (reapproved in 1993), which in fact also refers to ASTM D 1505, ASTM D 792, and AATCC, Method 20 (1990) (Fiber identification), each of which deals with the above techniques. ISO 10119 is a very good and concise description of the techniques. However the measurement liquids specified in ISO 10119 of ethanol, methanol, acetone, tricloroethane, and carbon tetrachloride, although suitable for carbon fibers, are not at all suitable for the general range of textile polymers, with the exception perhaps of ethanol and methanol. ASTM D 276 87 recommends the use of / -Heptane for universal application, except, of course for the olefins, such as polyethylene. A range of typical fiber densities is given in Table 4. 

Alternatively, the material can be determined by the water-displacement technique. The sample is weighed carefully in air with a sensitive analytical balance. The same sample is then suspended from a wire attached to the balance, then placed in water (or any liquid that will cause it to sink). The weight of the sample in the liquid, less the weight of the wire, is used to determine the mass of liquid displaced by the sample, which provides the volume of the sample. The density can then be calculate. 

An analysis of the techniques presented shows that in chromatography with a mobile gas phase the elution technique is used almost exclusively. The displacement technique suits chromatography with a mobile liquid phase in gas chromatography it may be an auxiliary method for the preliminary concentration of certain components. 

There are a number of other techniques besides cut-off measurements for characterising ultrafiltration membranes. However, typical methods for microfiltration membranes, such as mercury intrusion or scanning electron microscopy cannot be used for the characterisation of ultrafiltration membranes. For this reason, other techniques have been developed such as thermoporometiy, liquid displacement and permporoinetry as have been discussed in chapter IV. Other more general techniques which are applicable are gas adsorption-desorption, permeability measurements and modified cut-off measurements. v, ... 

The technique is similar to the liquid extrusion technique in that the nanofiber mat is saturated with a wetting liquid and gas pressure is applied to one smface of the mat. The surface free energy of the liquid with the fiber mat needs to be less than that of the mat with the gas. As with liquid extrusion, the liquid coltmm occupying through-channels will be displaced by the gas. In flow porometry, the gas displaces the liquid (and continues to flow through the emptied channel as well) and the flow rate of gas as a function of the differential pressure is recorded. 

---