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Membranes

Development of nanocomposite membranes is of major research interest for water and wastewater treatment. Hollow fiber membranes (HFM), e.g., contain a semi-permeable barrier in the form of a hollow fiber. The primary interest in HFMs relates to their pore diameter and pore distribution. In this section, we look at a research example discussing hollow fiber filter membranes for biomedical application characterized by capillary flow porometry, a technique offered by Anton Paar – along with other instruments for measuring nanomaterials.

Pore size of hollow fiber membranes for biomedical applications

Introduction

Ultrafiltration techniques like tangential flow filtration (TFF), also known as cross-flow filtration (CFF), are both fast and efficient methods for the separation of biomolecules and the purification of drinking water, and they are suitable for applications in immunology, protein chemistry and public health. The membranes used are tubular rather than flat sheets, so they can operate in a continuous mode with less potential for clogging. The hollow-fiber construction of these membranes represents a challenge to determining the sizes of pores in the fiber wall. The difficulty of analyzing hollow fibers has been overcome by a special preparation technique.

Experimental

Characterization techniques: Capillary flow porometry measurement of pore size distributions in 4 distinct hollow fiber membranes, denoted HF1-4, was performed using Anton Paar’s Porometer 3G. A length of each fiber (1 mm diameter, wall thickness 100 µm) was cut and glued into a specialized sample holder. After wetting with a fluorocarbon having zero contact angle with the samples (Porofil, Anton Paar), the holder was installed into the unit. Air pressure was applied to the interior lumen to push the wetting liquid through the pores to the outside; pressure and resulting air flow were measured automatically. Pore size was calculated using the Washburn equation, and the largest pore size computed from the bubble-point pressure.

Results and discussion

Pore size values are presented in Table 1. Sample HF2 had pores significantly larger than the other 3 samples. Interestingly, while the minimum and mean pore sizes of sample HF3 were smaller than those of sample HF4, the largest pore in sample HF3 was approximately 50 % larger than the largest pore in HF4.

Sample	Maximum Pore Size (nm)	Mean Flow Pore Size (nm)	Minimum Pore Size (nm)
HF1	460	238	146
HF2	1113	980	750
HF3	751	155	91
HF4	517	183	108

Number distributions were calculated based on the internal geometric surface of the sample fibers (from gross dimensions) and are shown in the figure below. HF3 and HF4 appear much more similar in terms of overall size distribution, the greatest difference between them being the number of pores.

Determining the details of the critical pore sizes of a membrane in this way allows proper selection for the target separation/purification process.

Additional information

Instruments:

Porometer 3Gzh

Application report:

Measuring Pore Size in the Walls of Hollow Fiber Membranes

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