W. Richard Bowen and Nidal Hilal 4

4.4 IMAgINg IN LIqUId ANd THE dETERMINATION OF sURFACE ELECTRICAL 117
Force (nN)
50
40
30
20
10
NaCl 10 –1 M
NaCl 10 –2 M
NaCl 10 –3 M
NaCl 10 –4 M
0
0 5 10 15 20 25 30 35
Distance (nm)
FIgURE 4.10 Force versus distance curves for the approach of an AFM tip to a
Cyclopore microfiltration membrane in NaCl solutions of various ionic strengths at pH 6.5.
Roughly understood, the imaging tip will follow an isopotential line
during the imaging process. At the high ionic strength, all of the calculated
lines lie close to the membrane surface (shaded, spinning the image
360° out of the plane of the paper would generate a pore). At the lower
ionic strength, some of the isopotential lines lie far from the surface, so
a clear, veracious pore image would not be obtained when such a line is
followed, as would be the case at low imaging forces. In conclusion, the
best imaging conditions in ionic solutions are at high imaging force and,
if possible and appropriate, at high ionic strength.
Though most membrane technologists now recognise the important
contribution of membrane surface electrical properties in defining the
separation characteristics, there remains confusion as to how best to
describe and quantify such interactions. There is an unfortunate tendency
in the applied membrane literature to use the terms ‘charge’ and ‘potential’
loosely, almost interchangeably. There is also a regrettable lack of
precision in the interpretation of membrane streaming potentials, which
are the basic data most commonly used to quantify membrane surface
electrical properties. The interpretation of streaming potential data can
be complex even for perfectly smooth and chemically uniform planar
surfaces. Most membrane surfaces have roughness comparable to electrical
double layer dimensions, so along-the-surface-membrane streaming
potential data give only some average property. Through-the-membrane
streaming potential data may only be usefully interpreted in comparison