Directly probing surfactant adsorption on nanoscopic trenches and pillars (original) (raw)
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Langmuir, 2018
Force curves collected using an atomic force microscope (AFM) in the presence of adsorbed surfactants are often used to draw conclusions about adsorbed film packing, rigidity and thickness. However, some noteworthy features of such force curve characteristics have yet to be thoroughly investigated and explained. In this work, we collected force curves from tetradecyltrimethylammonium bromide (TTAB) films adsorbed on highly ordered pyrolytic graphite (HOPG), silica, and silica that had been hydrophobized by functionalization with dichlorodimethyl silane. Breakthrough events in the force curves from several different trials were compared to show that the breakthrough distance, often reported as the adsorbed film thickness, increased with concentration below the critical micelle concentration (CMC) but was approximately 3.5 nm on all surfaces between 2× and 10× CMC; an unexpected result because of the different surface chemistries for the three surfaces. We employed an AFM probe with a different force constant (k) value as well as a colloidal probe and the breakthrough distance remained approximately 3.5 nm in all cases. Gradient mapping, a variant of force mapping, was also implemented on the three surfaces and resulted in a new technique for visualizing adsorbed surfactant in situ. The resulting maps showed patches of adsorbed surfactant below the CMC and revealed that with increasing concentration, the size of the patches increased resulting in full coverage near and above the CMC. These results are, to our knowledge, the first time force mapping has been used to spatially track patches of adsorbed surfactant. Finally, layers of surfactants on an AFM tip were investigated by collecting a force map on a single AFM tip using the tip of a separate AFM probe. A breakthrough event was observed between the tips, indicating a layer of surfactant was present on at least one, if not both tips.
Counterion Effects on Hexadecyltrimethylammonium Surfactant Adsorption and Self-Assembly on Silica
Langmuir, 2000
Combining optical reflectometry and atomic force microscopy (AFM), we have studied the effects of the surfactant counterion on the adsorption isotherms, kinetics, and layer structure for cationic hexadecyltrimethylammonium (C16TA + ) surfactants on negatively charged silica surfaces. The adsorption kinetics suggest that the adsorption mechanism changes at the critical micelle concentration (cmc). A change in mechanism is also suggested by differences observed in the state of interfacial self-assembly on either side of the cmc. Above the cmc, increasing the binding affinity of the counterion (from chloride to bromide) increased the surface excess concentration by approximately 60% and changed the structure of the adsorbed surfactant layer from aggregates with circular projections to wormlike micelles. The addition of 10 mM KCl or KBr increased the surfactant surface excess concentration for both counterions. Below the cmc, the counterion has only a small effect on the structure of the adsorbed layer, and the isotherms are similar, provided the surfactant concentration is scaled by the appropriate cmc. By quantitatively analyzing the AFM images and comparing this to the surface excess concentration measured by reflectometry, we determined that surfactants pack differently in adsorbed aggregates than they do in aggregates formed by self-assembly in solution. Finally, we show that an impurity present in poly(vinyl chloride) tubing explains anomalous adsorption behavior previously reported for C16TAB on silica.
Langmuir
Surfactant adsorption at solid−liquid interfaces is critical for a number of applications of vast industrial interest and can also be used to seed surface-modification processes. Many of the surfaces of interest are nanostructured, as they might present surface roughness at the molecular scale, chemical heterogeneity, as well as a combination of both surface roughness and chemical heterogeneity. These effects provide lateral confinement on the surfactant aggregates. It is of interest to quantify how much surfactant adsorbs on such nanostructured surfaces and how the surfactant aggregates vary as the degree of lateral confinement changes. This review focuses on experimental evidence on selected substrates, including gold-and carbon-based substrates, suggesting that lateral confinement can have pronounced effects both on the amount adsorbed and on the morphology of the aggregates as well as on a systematic study, via diverse simulation approaches, on the effect of lateral confinement on the structure of the surfactant aggregates. Atomistic and coarse-grained simulations conducted for surfactants on graphene sheets and carbon nanotubes are reviewed, as well as coarse-grained simulations for surfactant adsorption on nanostructured surfaces. Finally, we suggest a few possible extensions of these studies that could positively impact a few practical applications. In particular, the simultaneous effect of lateral confinement and of the coadsorption of molecular compounds within the surface aggregates is expected to yield interesting fundamental results with long-lasting consequences in applications ranging from drug delivery to the design of advanced materials.
Langmuir : the ACS journal of surfaces and colloids, 2005
Forces have been measured between silica surfaces with adsorbed surfactants by means of a bimorph surface force apparatus. The surfactants used are the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) and the nonionic surfactant hexakis(ethylene glycol) mono-n-tetradecyl ether (C(14)E(6)) as well as mixtures of these two surfactants. The measurements were made at elevated pH, and the effect of salt was studied. At high pH the glass surface is highly charged, which increases the adsorption of TTAB. Despite the low adsorption generally seen for nonionic surfactants on silica at high pH, addition of C(14)E(6) has a considerable effect on the surface forces between two glass surfaces in a TTAB solution. The barrier force is hardly affected, but the adhesion is reduced remarkably. Also, addition of salt decreases the adhesion, but increases the barrier force. In the presence of salt, addition of C(14)E(6) also increases the thickness of the adsorbed layer. The force barrier...
Soft-contact Atomic Force Microscopy Imaging of Adsorbed Surfactant and Polymer Layers
Microscopy and Microanalysis, 2000
The technique of atomic force microscopy (AFM) soft-imaging is outlined with respect to characterizing the adsorption of surfactants and polymers at the solid/liquid interface. This method utilizes the electrostatic and steric repulsion forces between the scanning probe and the sample to allow sensitive placement of the imaging probe near to the delicate surface layer. Specifically, the mixed adsorption of sodium dodecylsulfate (SDS) and poly(vinyl pyrrolidone) (PVP) on graphite is examined. Unlike the adsorbed layer in a solution of either component, the adsorbed layer in the mixture does not cover the substrate uniformly until equilibrium is reached (often hours later). The interesting kinetic and coverage effects observed are significant to the many applications reliant on adsorption from polymer-surfactant mixtures, especially to the flocculation of dispersions.
Langmuir, 2009
Single-molecule total internal reflection fluorescence microscopy was used to obtain real-time images of fluorescently labeled hexadecanoic (palmitic) acid molecules as they adsorbed at the interface between fused silica and three different solvents: hexadecane (HD), tetrahydrofuran (THF), and water. These solvents were chosen to explore the effect of solvent polarity on the activation energy associated with the attachment rate, i.e., the rate at which molecules were transferred to the surface from the near-surface layer. Direct counting of single-molecule events, made under steadystate conditions at extremely low coverage, provided direct, model-independent measurements of this attachment rate, in contrast with conventional ensemble-averaged methods, which are influenced by bulk transport and competing detachment processes. We found that the attachment rate increased with increasing temperature for all solvents. Arrhenius analyses gave activation energies of 5 (2 kJ/mol for adsorption from HD, 10 (2 kJ/mol for adsorption from THF, and 19 (2 kJ/mol for adsorption from water. These energies increased systematically with the solvent polarity and, therefore, with the expected strength of the solvent-substrate interaction. We hypothesize that the adsorption of amphiphilic solute molecules from solution can be regarded as a competitive exchange between solute molecules and surface-bound solvent. In this scenario, adsorption is an activated process, and the activation energy for attachment is associated with the solvent-substrate interaction energy.
ACS Symposium Series, 2003
The aggregate structure of a nonionic trisiloxane surfactant at the liquid/solid interface was studied by Atomic Force Microscopy (AFM) as a function of substrate surface energy. The hydrophobicity of oxidized silicon wafer was gradually increased by increasing the amount of noctadecyltrichlorosilane (OTS) monolayer coverage. AFM soft-contact imaging and force measurements captured the variation in surfactant aggregate structures from spherical micelles to elongated micelles, defected monolayer, and continuous monolayer with increasing surface hydrophobicity. The aggregate structural evolution at the solid/liquid interface resembles the microstructural sequence of the surfactant in bulk solution at room temperature. It is speculated that the hydrophobic attraction between the surfactant and the solid surface induces two-dimensional analogues of the surfactant bulk microstructures. The hydrophobic attraction affects the geometric packing parameter in a similar fashion as the increase in surfactant concentration.