Search

The structure of adsorbed layers of several polyoxyethylene alkyl ether (CnEm) nonionic surfactants on silica and graphite surfaces has been imaged using atomic force microscopy as a function of temperature up to their cloud points. For all surfactants with a cloud point within the experimentally accessible range, the adsorbed layer morphology on silica evolved from globules at low temperatures first into rods and then a mesh with increasing temperature. This mesh structure was retained even when the solutions were heated above their cloud points into the two-phase coexistence region. Only C12E3 was observed to form a laterally unstructured bilayer. On graphite, all surfactants formed straight, parallel hemicylinders at all temperatures examined.

The adsorption of the amphiphilic diblock copolymer poly(2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate) (DMA−MMA) at the mica/solution interface has been studied, over a range of solution pH values, using in situ atomic force microscopy. In contrast to the adsorption of the homopolymer poly(2-(dimethylamino)ethyl methacrylate (DMA), which exhibited a featureless adsorbed layer, adsorbed layers of the DMA−MMA copolymer exhibited lateral structure at all pH values studied. At low solution pH, images of the adsorbed layer reveal discrete domelike structures, separated by regions with no lateral features. Force−distance data recorded between these domes indicate adsorption in the form of positively charged material. It is proposed that the adsorbed layer forms via the adsorption of DMA−MMA micelles directly to the mica surface, followed by relaxation of the DMA chains to the surface. This relaxation is driven by electrostatics and results in an adsorbed layer that is essentially a layer of DMA chains with the MMA cores protruding as domes at the sites of the original micelle adsorption. The domes are evident over the entire surface, although arranged in a disordered manner. The distance between the domes is seen to decrease as the pH of the adsorbing copolymer solution is increased from 4 to 7, due to the increased number of negatively charged adsorption sites on the mica surface. Adsorption of charged latex particles shows that objects without molecular freedom adsorb in a close-packed manner. Close packing of adsorbed units is also observed when the DMA−MMA micelles are adsorbed from a solution at natural pH (pH 8.3), where the relaxation of the coronal DMA chains is electrostatically hindered.

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.

Polymer−surfactant mixtures have broad application in modern society; however, their interfacial behavior is not well understood. Soft-contact atomic force microscopy has been used to visualize the adsorbed layer at the graphite/solution interface in mixed aqueous solutions of sodium dodecyl sulfate (SDS) and poly(vinylpyrrolidone) (PVP). The nonequilibrium adsorbed layer is remarkable, consisting of micrometer-sized domains of surfactant-rich, ordered aggregates interspersed by disordered regions of ill-defined polymer-rich adsorbate. The ordered domains grow to cover the entire substrate after many hours. The nonuniform surface coverage and the long but finite equilibration times have important implications for colloid stability. These first observations of the heterogeneous nature of mixed polymer−surfactant adsorption highlight the importance of applying direct visualization to further our understanding of these ubiquitous formulations.

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.

The aggregated structure of sodium dodecyl sulfate (SDS) adsorbed to the graphite−solution interface has been determined in the presence of the divalent ions:  Mg²⁺, Mn²⁺, and Ca²⁺. Divalent ions are expected to alter the electrostatic interactions between charged headgroups and thus to cause changes from the structure formed with monovalent ions. However, atomic force microscopy reveals that the adsorbed structures are long (μm) and thin (~5 nm) and thus very similar in appearance to those observed in the presence of only monovalent counterions. Energetic considerations suggest that under most conditions the aggregates are hemicylinders, although not necessarily of constant curvature. When Mn²⁺ is added at constant low SDS concentration, the narrow dimension of the structure (the period) decreases linearly with solution Debye length down to a limiting period of about 5 nm. When Mg²⁺ is added at constant low SDS concentration, the period decreases slightly and then remains roughly constant. This suggests that the aggregate diameter increases. For both of these ions, force measurements suggests that there is a transition from a thin layer to a thicker layer as the divalent ion is added. At surfactant concentrations several times greater than the critical micelle concentration, the addition of divalent ions has no observable effect on the adsorption. We did not observe a transition to a flat layer at any of the conditions examined.

A surface aggregate phase transition is described. Atomic force microscopy has been used to image the equilibrium association of sodium dodecyl sulfate (SDS) and 1-dodecanol molecules at the interface between graphite and aqueous solutions. In pure SDS solutions, the molecules associate into long, parallel hemicylindrical surface aggregates over a concentration range from about one-third to at least 10 times the critical micelle concentration (cmc). Above the cmc, dodecanol has little influence on the surface aggregate structure, probably because dodecanol is partitioned into the bulk micelles. Below the cmc, dodecanol causes a transition from hemicylindrical aggregates to a two-phase mixture in which flat sheets coexist with swollen hemicylindrical aggregates. In this mixture, the hemicylindrical aggregates are preferentially located at and parallel to steps on the underlying graphite substrate. Under conditions where hemicylinders and flat sheets coexist, an increase in bulk dodecanol concentration results in an increase in surface coverage by flat sheets. No bulk solution changes were detected by NMR in the region where the surface phase transition was observed.

The structure of surfactant surface-aggregates can be controlled by mixing two surfactants which, individually, form different surface-aggregate structures. Using atomic force microscopy (AFM), we have found that mixtures of (dodecyldimethylammonio)propanesulfonate (DDAPS) and dodecyltrimethylammonium bromide (DTAB) at the interface between mica and aqueous solution form aggregates (“hemimicelles”) which have a length dictated by the proportion of each surfactant in the mixture. In pure DDAPS, the two axes of the aggregate parallel to the mica−solution interface are similar in length and similar to the diameter of a bulk micelle. As the mole fraction of DTAB in solution is increased, one axis of the surface aggregates increases while the other remains almost constant. In the limit of pure DTAB solutions, the aspect ratio is large (>10). The length of the mixed aggregates is not a linear function of the mole fraction of DTAB in bulk, but this can be partially rationalized on the basis of electrostatic interactions which concentrate DTA⁺ at the mica−solution interface at low bulk concentration. Both DTAB and DDAPS form approximately spherical aggregates in bulk solution, so the surface shape transformation cannot be accompanied by an analogous bulk transformation.