Electrostatic Patterns on Surfactant Coatings Change with Ambient Humidity Juliana Silva Bernardes, Camila Alves Rezende, and Fernando Galembeck* Institute of Chemistry, UniVersity of CampinassUNICAMP, P.O. Box 6154, 13084-971, Campinas-SP, Brazil ReceiVed: August 3, 2010; ReVised Manuscript ReceiVed: October 5, 2010 Sodium dodecyl sulfate (SDS) deposited by dewetting on mica under well-defined experimental conditions presents patterns with characteristic morphology and electric potential distribution. These are formed by surfactant steps and terraces, with more negative potential on the steps than on the terrace surfaces. Kelvin force microscopy results show that the electric potential on the sample surfaces becomes more negative just by increasing the atmospheric relative humidity from 30 to 70%, within a grounded and shielded environment. The kinetic effect of the potential variation was also evaluated by keeping the sample at the same humidity for short (2 h) and longer times (24 h). In the last case, the electric potentials changed to more negative values, nonreversibly. Changes on the surface electric patterns are not paralleled by morphological changes in the lamellar structure, since no significant variations in the topography are observed. It is also interesting to note that, despite the persistent negative potential values acquired under high humidity conditions, the potential difference between each terrace surface and its neighboring step is constant and within a narrow range (60-100 mV), indicating that the negative charge is uniformly distributed throughout the crystal. 1. Introduction Dewetting of liquid films form interesting patterns on the substrate, whose morphology depends on the liquid motions during dewetting and thus on the interactions among the various components of the system (substrate, solvent, and solutes). 1-3 Micro- and nanosized structures produced by this process are of great interest for the fabrication of microelectronic and optical devices and also for the understanding of the process evolution resulting in the final patterns. 4-6 Polymers receive special attention in this field due to their importance in many applica- tions, and more recently, other long chain molecules, such as ionomers and surfactants have also be considered. 7-11 We have shown in a recent work that a drop of SDS aqueous solution deposited on mica form characteristic patterns by drying and dewetting. 7 These are composed by branched structures topped by a flat surface and formed by steps and terraces with surfactant molecules organized in bilayers. Besides, images obtained by Kelvin force microscopy (KFM) revealed well- defined potential distributions on this sample, with marked differences between the steps and the terraces. Modern techniques of microscopy, mainly scanning probe microscopy (SPM), are revealing interesting properties of solid surfaces, regarding the topography, and also electric, magnetic, hydrophilic, and other surface-related properties. 12-15 In the case of surfactants and other amphiphilic molecules, these studies are especially important because the self-organized structures formed by these molecules in liquid media are well-character- ized, but little is known about their morphology after drying. Moeller et al. 16 used KFM to analyze assemblies of perfluoro- alkyl alkanes spin-cast on mica, silicon, or graphite substrates. These molecules self-assemble into spirals, ribbons, and other structures, always presenting more negative electric potential than the substrates, under relative humidity (RH) within a 20-25% range. As the humidity is increased to 95% RH, the structures undergo significant changes in topography, with spiral assemblies changing into toroids, followed by changes in the electric potential values. Microscopy techniques suitable for maping electric force (EFM) and electric potential (SEPM or KFM) adjacent to a surface have been used in this laboratory in the past years showing electric patterns more or less complicated on the surface of insulating solids. 17-24 Domains with both positive and negative charges coexist in neighboring areas, and the resulting patterns are very stable in some cases, while in others they respond to external stimuli, such as atmospheric relative humidity. Particles of Sto ¨ber silica and aluminum phosphate were analyzed, as well as polymers, like polyethylene and different kinds of latex films. In the case of poly(styrene-co-hydroxyl methacrylate) (PS-HEMA) latex, films are shaped from spherical particles with core-shell ionic distribution. A comprehensive analysis by KFM and elemental imaging in transmission electrical microscopy (ESI-TEM) showed a close relationship between electric potentials measured at the surface and the distribution of potassium and sulfate ions, remaining from synthesis. 17,25 In other cases, when the sample composition is not well- defined, charge carrier identification is more difficult. Formation and dissipation of electrostatic potentials on silica surface, 23,24 cellulose, 26 polyethylene, 21 metals, 27 and particles of Sto ¨ber silica 22 and aluminum phosphate, 22 under variable relative humidity conditions are consistent with a model based on the preferential adsorption of positive or negative water clusters on the surface. Atmospheric water is adsorbed and dissociates, forming [H(H 2 O) n ] + or [OH(H 2 O) n ] - ions that adsorb on the surface depending on the local prevailing potential and also on specific interactions. The formation and dissipation of electric potentials on these surfaces are thus dependent on the atmo- spheric humidity. In the present work, characteristic electric potential patterns formed on SDS steplike samples are analyzed under variable relative humidity, under short (2 h) and long (24 h) times. Changes on the electric potential patterns measured using Kelvin * Corresponding author, fernagal@iqm.unicamp.br. J. Phys. Chem. C 2010, 114, 19016–19023 19016 10.1021/jp107291j  2010 American Chemical Society Published on Web 10/19/2010