AFM-ANALYSIS OF THE SURFACE OF THE PROFILED SULFOCATION-EXCHANGE MEMBRANE AFTER ITS CONTACT WITH PHENYLALANINE

Abstract

The AFM analysis of surface roughness of ion-exchange membranes is of great practical importance, since surface properties affect operational, physico-chemical, transport and equilibrium characteristics. The purpose of this work is to find out the influence of alkylaromatic amino acid phenylalanine on the surface properties (microrelief and roughness) of the heterogeneous profiled sulfocation-exchange membrane MK-40.

Heterogeneous sulfocation-exchange membrane MK-40pr with geometrically inhomogeneous (profiled) surface was employed as an object of investigations. Investigations of the surface micro-relief of the membrane were performed by atomic force microscopy (AFM) with the use of scanning probe microscope produced by NT-MDT Corporation (model Solver P47 Pro) (Russia, Zelenograd) performed in semi-contact mode applied to the air-dry samples.

The geometrical inhomogeneity of the surface of the profiled sulfocathion-exchange membrane MK-40_pr after chemical conditioning and its contact with aqueous solutions of phenylalanine concentration 0.15 mol/L for 80 hours was studied by atomic-force microscopy. For membranes after contact with phenylalanine, an increase in the homogenous of the surface is found in comparison with the conditioned membrane sample. This is due to a decrease in the number and size of macropores and large defects on the surface of the membrane due to the sorption of the amino acid.

The analysis of the microprofile surfaces of the membrane MK-40_pr confirms the fact of a decrease in the roughness of its surface in the amino acid form due to the decrease in the water content of the membrane. Absorption of the amino acid promotes the hydrophobization of the membrane surface due to the intensive structure formation in the ion-exchanger - polymer carrier system. Evidence of the hydrophobization of the surface and the decrease in the water content of the membrane in the form of an amino acid is information obtained in the phase contrast regime. Upon transition to the amino acid form, the value of the phase shift of the probe oscillations and the number of light regions characterizing the hydrophobicity of the membrane surface sharply increase.

The effect of phenylalanine on the properties of the surface is in the decrease of all the amplitude roughness parameters of the surface of the conditioned membrane MK-40_pr.  Image of the surface for the conditioned sample of sulfocation-exchange membrane MK-40pr appears as a developed chaotic structure with micrometer-scaled roughness: arithmetical mean roughness R_а fits with 186.4 nm at R_z=953.1 nm. Surface of the MK-40_pr membrane after its contact with phenylalanine has a more smoothed relief: surface roughness R_z fitted with 745.0 nm, while ariythmetical mean scale of roughness R_a – 172.8 nm.

The correlation of the histograms the heights density distributions on the surface revealed that the narrower distribution of heights is typical for the conditioned membrane sample. As for the conditioned sample of MK-40pr membrane the maximum density corresponds to the mean value of the surface roughness equal to 1 mm, while for the sample of membrane after its contact with phenylalanine histogram is characterized by a spread maximum and a decrease of the mean roughness value up to 0.6 – 0.8 mm, as well.

ACKNOWLEDGMENTS

This work was financially supported by RFBR grant (project No. 18-08-01260).

AFM images of the membrane surface were obtained on the equipment of the Collective Use Center of Voronezh State University. URL: http://ckp.vsu.ru.