No abstract

No abstract

The aggregated structure of sodium dodecyl sulfate (SDS) adsorbed to the graphite-solution interface has been determined. Atomic force microscopy reveals that SDS adsorbs in periodic structures when the solution concentration is in the range 2.8-81 mM. Using previously obtained adsorption isotherms, we deduce that these structures are hemicylindrical, but we are not able to determine their length. The long axes of the hemicylinders lie parallel in grains which typically extend over (500 nm) 2 to (1000 nm) 2 , but the grain size can be reduced by adsorption of other species from solution. Two basic types of grain boundaries have been identified: broad boundaries, where the periodicity of both grains continue into the boundary for several periods, and narrow boundaries, where one or both of the hemicylindrical arrays terminate within a short distance. The period within each grain decreases when the concentration of SDS or the concentration of added NaCl is increased and approaches the diameter of bulk micelles at high concentration. In NaCl solutions, the period is proportional to the solution Debye length. We propose that this is a result of a decrease in interaggregate spacing rather than a decrease in aggregate size. Using a simple geometric argument, we suggest that the curvature of surfactant aggregates on hydrophobic surfaces will usually be lower than that of aggregates in bulk solution with which they are in equilibrium.

No abstract

A very thin layer (5-80 nm) of gas phase, consisting of discrete bubbles with only about 40 000 molecules, is quite stable at the interface between a hydrophobic solid and water. We prepare this gas phase from either ambient air or from CO(2)(g) through a solvent exchange method reported previously. In this work, we examine the interface using attenuated total internal reflection infrared spectroscopy. The presence of rotational fine structure in the spectrum of CO(2) and D(2)O proves that molecules are present in the gas phase at the interface. The air bubbles are stable for more than 4 days, whereas the CO(2) bubbles are only stable for 1-2 h. We determine the average gas pressure inside the CO(2) bubbles from the IR spectrum in two ways: from the width of the rotational fine structure (P(gas) < 2 atm) and from the intensity in the IR spectrum (P(gas) = 1.1 +/- 0.4 atm). The small difference in gas pressure between the bubbles and the ambient (1 atm) is consistent with the long lifetime. The dimensions and curvature of a set of individual bubbles was determined by atomic force microscopy. The pressures of individual bubbles calculated from the measured curvature using the Laplace equation fall into the range P(gas) = 1.0-1.7 atm, which is concordant with the average pressure measured from the IR spectrum. We believe that the difference in stability of the CO(2) bubbles and the air bubbles is due to a combination of the much lower pressure of CO(2) in the atmosphere and the greater solubility of CO(2) in water, compared to N(2) and O(2). As expected, smaller bubbles have a shorter average lifetime than larger bubbles, and the average pressure and the curvature of individual bubbles decreases with time. Surface plasmon resonance measurements provide supporting evidence that the film is in the gas state: the thin film has a lower refractive index than water, and there are few common contaminants that satisfy this condition. Interfacial gas bubbles are not ubiquitous on hydrophobic solids: bubble-free and bubble-decorated hydrophobic interfaces can be routinely prepared.

Small bubbles of gas are known to exist at the interface between hydrophobic solids and water. Two features of these bubbles are unexplained: the very low contact angle and the stability. A self-consistent explanation of both of these effects is that there is a film of contaminant at the air-water interface that decreases the surface tension and thus the contact angle, and also hinders diffusion of gases from the bubble, thereby increasing the lifetime. If, during the lifetime of the bubble, the surface tension increases faster than the area of the air-water decreases, the interfacial energy can lead to a stabilization of the bubbles.

To better understand the role of the interactions between surfactant, solvent, and a solid substrate on surface aggregation, we have studied the adsorption of a series of alkylpoly(ethylene oxide) C n E m surfactants on three different substrates: graphite, hydrophilic silica, and hydrophobic silica. Using atomic force microscopy (AFM), we find that adsorption to hydrophilic silica, with two exceptions, results in the formation of globular structures that are similar to bulk micelles. On silica that has been made hydrophobic by reaction with organosilane, adsorption results in a surface layer that is laterally homogeneous and is probably a monolayer with ethylene oxide groups in contact with the solution. A number of surfactants with ionic and zwitterionic headgroups were also observed to form monolayers on hydrophobic silica. This large perturbation from the solution-aggregate structure on the hydrophobic surface is driven by a minimization of the area of contact between water and the hydrophobic silica substrate. On graphite, the surface layer is either long, thin aggregates (consistent with a hemicylindrical structure) or a laterally homogeneous layer (consistent with a monolayer with the headgroups facing the solution). The nonionic C 12 and C 14 surfactants form hemicylinders, and the C 10 surfactants with the same headgroups form a laterally homogeneous layer. This suggests that above a critical alkyl chain length the interaction between the graphite and the surfactant tail is sufficient to orient a layer of alkyl tail groups parallel to the graphite surface, which then templates further adsorption. Below the critical alkyl length, the arrangement on graphite is similar to that on hydrophobic silica and is probably driven by a minimization of the water-graphite interfacial area. The critical alkyl length for (zwitterionic) sulfobeteine surfactants is not the same as for the nonionic poly(ethylene oxide) surfactants. This shows that the headgroup also plays an important role in determining the adsorbed structure. All measurements were performed in equilibrium with bulk micelles at or above the critical micelle concentration and at approximately 25 °C.

We have changed the structure of an adsorbed surfactant layer by modifying the nature of the interface in situ. Muscovite mica contains surface anions that can bind to a variety of cations in aqueous solution. Using an atomic force microscope (AFM), we have investigated the influence of the adsorption of the salts HBr, KBr, and N(CH2CH3)4Br on the adsorption of hexadecyltrimethylammonium bromide (CTAB) to mica. In the absence of salt, at twice the bulk critical micelle concentration, CTAB initially forms cylindrical surface micelles on mica. The cylinders transform to a flat bilayer structure within 24 h. The introduction of 10 mM K+ produces cylindrical aggregates that are stable, and a further increase in the concentration of K+ produces defects in the cylinders. These defects consist of aggregate termini and changes in the direction of the long axis of single aggregates. More defects are introduced by H+ than by K+ (at the same concentration). This is consistent with the known higher binding constant of H+ to mica. Using the introduction of defects as an indicator of the adsorption of cations in the presence of CTA+, we find that CTAB greatly slows adsorption of H+ but that the speed of K+ adsorption is not noticeably affected. The adsorption of K+ produces structures that are sensitive to the force that is applied by the AFM tip. At a critical repulsive force, the image changes discontinuously from a defective cylinder structure to a spherical or flattened disklike structure.