Introduction
This chapter summarizes recently published work on the hydrophobic interaction ("Long-Range Attraction between Silanated Silica Materials Studied by an Electrolyte Titration with Atomic Force Microscopy." D.E. Aston, J.C. Berg, Colloids and Surfaces A 163: 247-63 (2000). The study takes one simplifying step backward from our study of deformable oil-water interfaces to rigid hydrophobic surfaces in order to study long-range attractions often attributed to the hydrophobic effect. A parametric study of long-range interactions, electrostatic and hydrophobic, with changing electrolyte concentration is conducted to distinguish the contribution of the hydrophobic effect to the net interaction from forces of electrostatic origin by quantitative assessment of dynamic cantilever snap-in. The relatively simple and efficient experimental procedure for determining the strength and range of the hydrophobic force is accomplished in the sphere-plate geometry between silanated glass and quartz by a systematic electrolyte titration of electrostatic forces. Snap-in distance from atomic force microscopy (AFM) is plotted against electrolyte concentration and reveals both a minimum at some characteristic salinity and a snap-in plateau for higher concentrations. A single titration curve produced from a large enough database allows successful deconvolution of the hydrophobic force from other interactions.
Background
Long-range attractions observed between non-polar
constituents in aqueous media that are not explained by the well-characterized
DLVO forces have long been grouped under the single heading of hydrophobic
effect. There is no mutually agreeable theory to explain the collective
results presented in the literature to date. It appears that a single
explanation—though desirable for its simplicity and elegance—is not sufficient
for the different mechanisms of long-range “hydrophobic forces” that are
observed and substantiated.
Since conventional AFM is limited to regimes of
repulsion or weak attraction due to cantilever instabilities, it is useful
to measure the stronger attractions from force-gradients where they exceed
the spring constant. The sphere-plate separation is no longer controllable
within this attractive regime, and equilibrium forces are not accessible
because the snap-in event is a dynamic transition from a non-contacting
equilibrium state to a contacting one. Even though no static force
information is available directly during snap-in, the distance traveled
is enough to fit the parameters of the hydrophobic forces when the system
is correctly characterized. It may also be possible to deconvolute
equilibrium forces from the transient cantilever motion recorded during
snap-in. This will be discussed in another section..
Experimental Summary and Conclusions
Instead of relying on an external force to counterbalance
long-range attractions between two interfaces—either by a very stiff and
insensitive cantilever or a custom design for variable cantilever stiffness,
a repulsive interparticle force that can be manipulated independently from
the interaction(s) of interest may be employed as an internal control on
cantilever stability. A repulsive electrostatic double-layer interaction
seems to be the best candidate for a controllable colloidal force, especially
since both surfaces of interest are usually negatively charged under a
wide range of environmental conditions. The double-layer interaction
is often comparable in range and opposite in magnitude to the hydrophobic
attraction and is readily adjusted in range by screening charges with electrolytes.
A theoretical fit of force profiles or snap-in distances vs. concentration
leads to quantified hydrophobic parameters when the surface potentials
or charge densities are known.
Electrostatic force titration in the AFM liquid
cell by incremental addition of electrolyte shows the strength and range
of the hydrophobic force between stable, robust surfaces from snap-in distances.
Silica substrates hydrophobized by complete chemical reaction with silane-coupling
agents yield very stable films for an AFM electrolyte titration.
Adsorbed macromolecules, self-assembled monolayers, and other adsorbates
are not practical surfaces for unambiguous investigations because they
are not strongly attached to present unaltered surfaces throughout the
experiment. When van der Waals and electrostatically screened interactions
are appropriately accounted for, the hydrophobic or other non-DLVO force
may be quantitatively characterized, even in the case where the hydrophobic
effect is directly affected by the presence of ions. A computational
study of electrostatic and hydrophobic forces parameters—holding the van
der Waals interaction constant—was conducted to identify trends with changing
electrolyte concentration from which the hydrophobic interaction might
be assessed. The AFM measures local surface forces and adhesion via
snap-in distances and force profiles in aqueous solutions between glass
or fused quartz plates and glass spheres made hydrophobic by exposure to
silane vapor.
The computational parametric study was used to fit
the experimental results and has illustrated the competition of double-layer
and hydrophobic forces that show a characteristic, non-monotonic trend
in long-range attraction with increasing electrolyte. The strength
and range of the hydrophobic force may be readily quantified from a titration
curve, provided the substrates are adequately charged and sufficient snap-in
vs. electrolyte concentration data are available. The interesting
and reproducible observation of a minimum snap-in distance for some intermediate
salinity is qualitatively modeled with only a single exponential function
for the hydrophobic effect. The electrolyte titration distinguishes
this from a longer-range electrostatic correlation previously observed
and explains our results quite reasonably. This first thorough electrolyte
investigation of stable, chemically modified surfaces supports the premise
that the presence of electrolyte does not affect the hydrophobic interaction.