Journal of Molecular Catalysis A: Chemical 167 (2001) 225–233 Ionic surfactant films imaged by atomic force microscopy G. Ceotto a , E.F. de Souza b , O. Teschke c,∗ a Departamento de Fisica, Universidade Federal de Vicosa, 36571-000 Vicosa, MG, Brazil b Instituto de Ciencias Biologicas e Quimica, Pontificia Universidade Catolica de Campinas, 13020-904 Campinas, SP, Brazil c Nano-Structure Laboratory, IFGW/UNICAMP, 13081-970 Campinas,SP, Brazil Received 6 April 2000 Abstract Forces acting on atomic force microscope (AFM) tips responsible for image formation are measured during scanning of films of ionic surfactant molecules adsorbed from aqueous solutions onto hydrophilic substrates. Near the critical micelle concentration mica substrate images show aggregate regions at the interface. Force versus distance measured curves show that patches form a thicker structure than the formed at partially covered regions, in agreement with the fact that at patches the adsorbates are perpendicularly oriented to the substrate plane. However, AFM topographic images registered at low scanning speed (5 m/s) show that these patched regions appear as holes, forming inverted images. In AFM imaging of soft structures, as surfactants or biological material, inverted in contrast with images may be observed when there is a specific tip penetration through each scanned layer. This penetration is adjusted by changing the tip force set point, consequently different topographic profiles are obtained. The precise force set point to obtain the correct contrast in scanned images is obtained by the analysis of the force versus distance curves that show the normal to the scanned plane structure profile. Adsorption patterns as a function of time may be conveniently monitored and the adsorption rate may be determined. © 2001 Published by Elsevier Science B.V. Keywords: Ionic surfactant; Adsorbed films; AFM imaging; Adsorption dynamics 1. Introduction Understanding the adsorption mechanism of sur- factant molecules at the solid–liquid interface, is an important step toward modeling industrial processes which use surfactants on a large scale, such as deter- gency, water purification, oil recovery, and ore refine- ment by flotation [1]. An intermolecular interaction in bulk solution leads to a variety of self-assembled structures like micelles or liquid-crystalline struc- tures which have been well-studied [2]. At an in- terface, however, the normal self-assembly process ∗ Corresponding author. Fax: +55-19-788-5376. E-mail address: oteschke@ifi.unicamp.br (O. Teschke). is perturbed by competing surfactant–surface and solvent–surface interactions, which can, in principle, lead to novel structures termed ‘hemimicelles’ [3]. Over the last few decades, the adsorption characteris- tics of a wide variety of surfactant–solvent–substrate systems have been investigated, traditionally by ad- sorption isotherm [4] and more recently by fluores- cence decay [5] and neutron reflection [6]. Imaging hard samples with atomic resolution re- quires a probe with atomic dimensions. The atomic force microscope (AFM) obtains its topographical in- formation from the short-ranged repulsion resulting from the overlap of electronic shells between tip and sample [7]. However, the presence of long-ranged in- teractions such as the double layer electrostatic force 1381-1169/01/$ – see front matter © 2001 Published by Elsevier Science B.V. PII:S1381-1169(00)00510-0