Interfacial Forces at Layered Surfaces: Substrate Electrical Double-Layer Forces Acting through Ultrathin Polymer...

Area of Science:

• Materials Science
• Surface Chemistry
• Colloid Science

Background:

• Surface coatings are used to tailor material interfacial interactions.
• It is assumed coatings solely determine interfacial forces, neglecting substrate contributions.
• Interfacial interactions occur over finite length scales, questioning coating effectiveness at screening substrate forces.

Purpose of the Study:

• To investigate if bulk substrate material properties influence interactions through thin coatings.
• To determine the critical coating thickness required to screen subsurface forces.
• To understand the interplay between coating and substrate properties in determining interfacial forces.

Main Methods:

• Plasma polymer layers of varying thicknesses were deposited on silicon wafers from ethanol vapor.
• Colloid-probe atomic force microscopy (AFM) was used to measure electrical double-layer forces in solutions of varying ionic strengths.
• Data were fitted using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory.

Main Results:

• Thicker ethanol plasma polymer coatings (>18 nm) exhibited surface potentials independent of thickness, dominated by surface carboxylate groups.
• For coatings thinner than 18 nm, surface potentials increased as film thickness decreased.
• Electrical double-layer forces for thin coatings showed contributions from both the coating and the underlying substrate (SiO2).
• Theoretical calculations supported the model of superimposed forces from coating and substrate.

Conclusions:

• Thin coatings (<18 nm) do not completely screen the surface potential of the underlying substrate.
• The superposition of coating and substrate surface potentials influences interfacial interactions for thin films.
• Coating design for controlling material interactions must consider the influence of substrate properties when using thin films.