Measurement of polyphenol–membrane interaction forces during the ultrafiltration of black tea liquor

Abstract

This paper reports the use of atomic force microscopy (AFM) surface interaction measurements for a model polyphenol present in tea (theaflavin-3-gallate) with a regenerated cellulose ultrafiltration membrane. Differently treated membranes from throughout the fouling cleaning cycle were investigated. The adhesion of the foulant to the virgin and the fouled once/cleaned once (F1C1) membrane surface was stronger than that recorded for foulant–foulant interactions (1.87 and 1.72 N m −2, respectively, matching curves ranging between 0 and 5.5 N m −2 with a single shallow peak). Interestingly, NaOH cleaning of the virgin conditioned membrane reduced foulant–membrane adhesion compared to the virgin or F1C1 surface (0.96 N m −2) with a narrow spread of the adhesion (80% of measurements being within the range 0.5–0.99 N m −2). The uniform narrow distribution of attraction (approach) forces to the fouled and cleaned surfaces helps explain this due to uniformly charged foulant/hydroxyl ions adhering to these membrane surfaces, whereas the virgin membrane distribution was very wide demonstrating no charge modification of this surface. The present study has demonstrated how the AFM coated colloid probe technique can be used with multiple (200×) measurements across a surface to further aid understanding of the nature of fouling and cleaning mechanisms, particularly in protein/polyphenol systems.