Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX
Ordering in Surfactant Foam Films Transferred onto Hydrophilic and Hydrophobic Substrates Thaar M. D. Alharbi†,‡ and Gunther G. Andersson*,† †
Flinders Centre for NanoScale Science and Technology, Flinders University, Adelaide, SA 5001, Australia Physics Department, Faculty of Science, Taibah University, Almadinah Almunawarrah, Saudi Arabia
‡
S Supporting Information *
ABSTRACT: Foam films have been formed by aqueous dodecyldimethylphosphine oxide (C12DMPO) and aqueous hexadecyltrimethylammonium bromide (C16TAB) solutions and drained. The surfactant films were transferred onto hydrophilic and hydrophobic substrates at five different drainage times. Angleresolved X-ray photoelectron spectroscopy and metastable induced electron spectroscopy were used for investigation of the thickness and orientation of the surfactant molecules in the transferred foam film. Of the four combinations only the drained film formed by C16TAB transferred onto hydrophobic surfaces shows a strong degree of orientation of the surfactant molecules in the transferred film. The other three combinations do not show a preferred orientation of the surfactant molecules in the foam film. The phenomenon is discussed based on the interaction of the molecules within the films and the interaction of the formed films with the substrates.
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INTRODUCTION Foams are colloidal systems made of air bubbles dispersed in liquid solutions.1−3 Foams consist of two phases, which are the continuous liquid phase (e.g., aqueous surfactant solution) forming thin liquid films (foam films) and the dispersed gas phase (e.g., air). In a foam film the two surfaces forming the film come in close proximity of less than a few 10 Å.2 Foam films are metastable and are stabilized by a combination of attractive van der Waals forces across the film and repulsive forces due to the internal osmotic pressure as a result of the overlap of the electric double layers at the two surfaces of the foam film.1,2,4−6 Foams formed by surfactants usually show an enrichment of the surfactant at the surface of the foam film.7 The enrichment of the surfactant molecules at the foam film surface leads to the formation of an ordered structure of the surfactant molecules8 which can be transferred to solid substrates.9−12 It has been shown that nanoparticles such as single-walled carbon nanotubes,13 gold nanoparticles,14 mixed oxide nanowires,15 graphene oxide flakes,16 and silica nanoparticles17 can be inserted into the foam films and transferred with the foam film onto solid substrates. In a previous work the transfer of phospholipid films onto a solid substrate was investigated by means of electron spectroscopy.11 It was found that the larger fraction of the transferred fully drained phospholipid film showed ordering such that the phospholipid molecules have a preferred orientation in the transferred film. The smaller fraction showed a random orientation of the transferred phospholipid film. The aim of the present work is to investigate the conditions for retaining the ordered structure of foam films when they are transferred onto solid substrates. From the previous work the © XXXX American Chemical Society
question arises which factors contribute to the degree of order in transferred foam films. In general, the interactions between solute molecules and between solute and solvent molecules as well as between solute molecules and substrates surfaces are considered as being the dominant factors in the formation of ordered structures of films formed by molecules adsorbed to substrates.18 In the process of transferring foam films onto surfaces the formation of order within the surface of the surfactant solution and the formation of the interface between the surfactant layer and the substrate are separated. The separation allows studying the above-mentioned factors for formation of order in layers adsorbed to surfaces independently. In the first step the liquid surface is formed as interface with air. The structure of the liquid surface is mainly dominated by the presence of surfactant molecules at the liquid surface. Subsequently, the interface between the surfactant-rich surface and the substrate is formed. During the film formation the concentrations of the surfactant molecules in the outermost layer of the foam film increases as well as the surfactant molecules show a larger degree of orientation with the alkyl chain pointing toward the surface.8,19 In summary, the formation of foam films leads to orientation of surfactant molecules in the foam film and subsequently to the formation of the foam film, and subsequently the films are brought into contact with a surface. Thus, the interaction between the surface and the foam film occurs after the structure within the foam film has been formed. Understanding the conditions for the transfer of foam films onto substrates will help in Received: October 20, 2017 Published: October 26, 2017 A
DOI: 10.1021/acs.jpcc.7b10406 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Table 1. Surfactants Used in the Present Work
Figure 1. Formation and drainage of a C12DMPO-stabilized bubble created by injecting air through a glass pipet in a porous plate. The diameter of the bubbles was approximately 1.5 cm. The color of the foam films was faint.
hydrophobic and hydrophilic substrates at five different drainage times. For characterization, two surface sensitive techniques are used: X-ray photoelectron spectroscopy (XPS) as well as the angle-resolved version of XPS (ARXPS) and metastable induced electron spectroscopy (MIES). XPS was used for determining the elemental composition of the samples. ARXPS allows determining the thickness of the transferred films. MIES measures the valence electron structure of the outermost layer of the transferred films and thus reveals information about the molecular orientation of the surfactant molecules. Materials. C12DMPO was purchased from ABCR Chemicals (Germany) and was recrystallized twice. C16TAB (purity >96%) (Table 1) was purchased from Fluka and used without further purification. It is known from former publications that it can be assumed that the degree of purity of C16TAB purchased from such resources is sufficiently high.22 C12DMPO had been purified by recrystallization. All solutions were prepared with ultrapure water (18.2 MΩ, Milli-Q system). The concentration was kept constant at 15 × 10−4 mM for C12DMPO and 4.6 × 10−4 mM for C16TAB, which is about 5 times the cmc of each of the aqueous surfactant solutions. Using concentrations of about 5 times the cmc was found earlier to result in stable foam films for transfer to solid substrates.11 Substrate Treatments. The substrates used were n-type Si (100) wafers from MTI Corp. with a thickness of 0.525 mm, size of 1 × 1 cm2, and a low resistivity
