Electrostatic Patterns on Surfactant Coatings Change with Ambient Humidity

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.