NAMS 2002 Workshop - ICOM 2008

Nanofiltration and Reverse Osmosis II - Imaging and Characterization – 4
Tuesday July 15, 10:30 AM-11:00 AM, Maui
Probing Polyamide RO membrane Surface Charge, Energy, and Potential
With Advanced Contact Angle Titrations
G. Hurwitz (Speaker), University of California, Los Angeles, Los Angeles, CA, USA
E. Hoek, University of California, Los Angeles, Los Angeles, CA, USA, hoek@seas.ucla.edu
Contact angle titrations are performed to evaluate surface charge, surface
tension, and surface potential of a polyamide reverse osmosis (RO) membrane.
Contact angle titration involves measuring equilibrium contact angles for both
buffered and unbuffered aqueous drops over a range of pH values. The buffered
titration gives the fractional ionization of surface functional groups and the
effective pKa. The unbuffered titration gives the maximum surface charge density
of the membrane. These measured parameters are then combined with the
Grahame equation to estimate the membrane surface (zeta) potential. Zeta
potentials calculated from the contact angle titrations compare well with those
calculated from streaming potential measurements across a range of ionic
strength and pH values.
In addition to direct surface titrations, contact angles of a non-aqueous polar
liquid and an apolar liquid are measured to enable calculation of Lifshitz-van der
Waal, electron-donor, and electron-acceptor surface tensions. These contact
angle measurements are augmented by measured contact angles of various
aqueous electrolytes to provide further insight into how specific ion interactions
influence electron-donor/acceptor components of surface tensions for polyamide
RO membranes. The polyamide membrane becomes more hydrophilic as NaCl
concentration increases from 0 to 1 M. The higher hydrophilicity results from a
larger ratio of electron-donor to electron-acceptor functionality being expressed
as the contact angle droplet ionic strength increases. Hydrophilicity also
increases with increasing solution pH and in the presence of a few millimoles of
divalent cations. However, there are no discernable differences among calcium,
barium, magnesium, and strontium at a fixed concentration.
In summary, these results suggest that contact angle analyses can be used to
probe membrane surface chemistry to a greater degree than is traditionally
pursued. Contact angle titrations may be combined with multiple probe liquid
contact angle analyses to elucidate membrane surface charge, tension, and
potential. In a more practical sense, the polyamide membrane evaluated
becomes more hydrophilic as pH, ionic strength, and minerals content increase.
Increased membrane hydrophilicity will no doubt have significant impacts on
membrane transport and surface fouling phenomena. Additional research is
needed to determine if this behavior is reproducible for other RO membranes