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.
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.