Double-Layer Interactions between Self-Assembled Monolayers of ω-Mercaptoundecanoic Acid on Gold Surfaces

Abstract

The atomic force microscope was used to investigate the interaction between 11-mercaptoundecanoic acid surfaces and 2-mercaptoethanesulfonic acid surfaces as a function of electrolyte and pH. The surface potential was found to peak at −80 mV at pH 6-8 in 0.1 mM 1:1 electrolyte. The Hamaker constant for the gold−gold interaction was found to be decreased from the value of 1 × 10-19 J in water to 4 × 10-20 J in ethanol. The dispersion interaction between gold surfaces following derivatization with 11-mercaptoundecanoic acid was drastically reduced. The terminal carboxyl groups did not ionize in ethanol. Even in water, extremely low degrees of ionization were observed, with just 1−2% of surface sites being ionized at pH 10. The force curves showed no jump-in nor adhesion above pH 7. We could not reconcile the ψafm−pH results with a simple diffuse layer model for the interface; however the ionization behavior could be explained with sodium ion binding to surface carboxyl groups with a binding constant pK Na = 6.3. The addition of micromolar amounts of Pb2+ caused almost complete neutralization of charged surface groups. These data could be modeled with K Pb = (1−4) × 10-10. An alternative modeling procedure based on incorporation of a zero-order Stern layer could only explain the experimental values if an extremely low value of the inner layer capacitance was assumed of 1−4 μF cm-2. However, this model may be able to explain the lack of jump-in in alkaline solution and the absence of adhesion at high pH.

Silicon addition to hydroxyapatite increases nanoscale electrostatic, van der Waals, and adhesive interactions

“…The magnitudes of the van der Waals values here are intermediate compared with that of typical ceramics in aqueous solution, which range from 1.6 to 94 zJ 39 and ϳ2 times less than that for gold surfaces (A ϳ 100 zJ). 40 Initial contact angle measurements are consistent with SiHA having a higher surface energy than HA, which is related to the Hamaker constant. A contribution to this could come from the fact that Si is more easily polarized than P because of lower atomic number with the same number of electron shells.…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…glass, silica, and silicon nitride [15,16,187,[427][428][429][430][431][432][433][434]; gold [327,429,435,436]; copper and nickel [437]; zinc and lead sulphide [171,172,438]; titanium oxide [169,339,340]; zirconia [165,166,410]; iron oxide [439]; tungsten [175]; cobalt [175]; and alumina [16,[439][440][441][442]. Different polymers [177,189,[443][444][445][446][447], Langmuir-Blodgett and other thin organic layers [126,329,411,448,449] have been analyzed. In recent years special interest in biologically relevant surfaces materials evolved.…”

Direct Measurement of Glycosaminoglycan Intermolecular Interactions via High-Resolution Force Spectroscopy

“…The measurement of repulsive DLVO forces between nonpolymeric charged surfaces is well documented in the literature using both the surface force apparatus (and similar instruments) [46][47][48][49][50][51] and the atomic force microscope. [20][21][22][23]30,[52][53][54][55][56][57][58][59][60][61][62] All of the theoretical models presented in this paper employ constant surface charge boundary conditions, rather than constant potential, for two reasons. First, both the substrate and the tip were electrically isolated from each other and from the instrument ground and, therefore, not electrically connected to any source that would maintain them at a constant potential.…”