W. Richard Bowen and Nidal Hilal 4

56 2. MEASUREMENT OF PARTICLE ANd SURFACE INTERACTIONS
the deposition of mercaptoundecanoic acid (MUA). Force measurements
at different electrolyte concentrations and pH values were fitted with a
modified DLVO theory to determine surface potentials and Hamaker
constants. The adsorption of the MUA itself was found to substantially
decrease the magnitude of the dispersive forces between the gold surfaces.
It was also found that the degree of ionisation of the surface carboxy
groups was very low. The authors applied two models to explain
these results. It was concluded that either a thicker-than-usually-expected
Stern layer was present or that strong competitive binding of cations was
the cause. Surface interactions between particles and surfaces with more
complex attached monolayers, such as proteins, have also been investigated.
Bowen et al. [79] investigated systems of silica microspheres and
surfaces co-incubated to give an adsorbed layer of bovine serum albumin
(BSA) at different values of pH and NaCl concentrations. Figure 2.5
shows a plot of representative force curves obtained as a BSA-coated
silica sphere approached a BSA-coated silica surface in aqueous NaCl
solutions of differing ionic strengths. As the ionic strength was increased,
both the range and magnitude of repulsive interactions increased. For
the three highest NaCl concentrations, experimental observations were
in good agreement with the values predicted from DLVO theory and zeta
potential values. The zeta potentials for the BSA were calculated using a
site-dissociation-site-binding approach from the amino acid sequence [58,
80]. At the lowest ionic strength examined (0.0002 M NaCl), the observed
values were higher than those predicted from theory.
Normalised force, F/R (mN/m)
1
0.1
0.01
0.0001
0 5 10 15 20 25 30
Distance [nm]
FIGuRE 2.5 Plot of normalised force versus separation distance plot for silica surfaces
coated with monolayers of BSA at four concentrations of NaCl at pH 8.0: (�) 0.1 M; (�) 0.01 M;
(�) 0.001 M; (�) 0.0002 M. The lines are theoretical predictions from DLVO theory.