Effects of Degassing and Ionic Strength on AFM Force Measurements in Octadecyltrimethylammonium Chloride Solutions

Abstract

Sakamoto et al. (Langmuir 2002, 18, 5713) conducted AFM force measurements between silica sphere and fused-silica plate in aqueous octadecyltrimethylammonium chloride (C18TACl) solutions and concluded that long-range attractive force is not observed in carefully degassed solutions. In the present work, AFM force measurements were conducted by following the procedures described by Sakamoto et al. The results showed the presence of an attractive force that was much stronger than the van der Waals force both in air-saturated and degassed solutions. The force was most attractive at 5 x 10(-6) M C18TACl, where contact angle was maximum. At this concentration, which is close to the charge compensation point (ccp) of the glass sphere, the long-range decay lengths (D) were 34 and 38 nm in air-saturated and degassed solutions, respectively. At 10(-5) M, the decay length decreased from 30 to 4 nm upon degassing. This decrease in decay length can be explained by a pH increase (from 5.7 to 6.6), which in turn causes additional surfactant molecules to adsorb on the surface with inverse orientation. The attractive force was screened by an added electrolyte (NaCl), indicating that the attractive force may be of electrostatic origin. Therefore, the very long decay lengths observed in the absence of electrolyte may be ascribed to the fact that the ccp occurs at a very low surfactant concentration.

Establishment of the Center for Advanced Separation Technologies

“…According to Fuerstenau and Pradip (2005), hemi-micelles begin to form at concentrations 2-3 orders of magnitude below the c.m.c. The AFM (Zhang et al, 2005) and SEM (Delamarche, et al, 1994) images of the surfactant-coated surfaces indeed showed patches, as is also shown in Figure 15. Further, the aggregation numbers of the hemi-micelles (or 'soloids') determined using the electron spin resonance (ESR) and fluorescence spectroscopic techniques showed the patch-wise adsorption mechanism (Fan et al, 1997;Ström et al, 2000).…”

Electrostatic attraction between overall neutral surfaces

“…This is a mean field (MF) theory, within which ions are treated as point-like particles obeying a Boltzmann distribution, the aqueous solution is taken as a continuous and homogeneous dielectric medium, and in most treated cases , the bounding surfaces are assumed to be homogeneously charged [1][2][3][4][5][6]. However, as many charged surfaces in soft and biological matter are heterogeneous over mesoscopic length scales, several experimental [7][8][9][10][11][12][13][14][15][16] and theoretical studies have investigated the effects of surfacecharge heterogeneity on the inter-surface electrostatic interactions.…”

“…Several experimental studies have examined the interaction between two such overall neutral surfaces, made of positive and negative domains ("patches") that are much larger than the molecular size [7][8][9][10][11][12][13][14]. For example, negative mica surfaces can be coated with neutralizing cationic surfactants that later dissociate and form positively charged bilayer patches.…”

“…For example, negative mica surfaces can be coated with neutralizing cationic surfactants that later dissociate and form positively charged bilayer patches. An unexpected attraction was measured between two such surfaces [7][8][9][10][11][12][13][14]. At large inter-surface * andelman@post.tau.ac.il separations (beyond tens of nanometers), hydrophobic and dispersion interactions were ruled out as a possible origin for this attraction.…”

Surface Forces Measured Between Xanthate-Coated Gold Surfaces

“…It was shown also that the long-range attractions disappeared when the force measurements were conducted in degassed solutions (10). Follow-up experiments showed, however, that strong hydrophobic force can still be measured between silica surfaces immersed in degassed cetyltrimethylammonium bromide (CTAB) solutions (13). The force curves obtained between hexadecanethiol (C 16 SH)-coated gold surfaces also showed steps and showed no significant hydrophobic force (14).…”