ADSORPTION OF ETHOXYLATED NONIONIC SURFACTANTS FROM SILOXANE-BASED SOLVENT AND AQUEOUS SYSTEMS: USE OF QCM AND MODEL POLYMERIC SURFACES

Abstract

Adsorption behavior was quantified with pure ethoxylated nonionic surfactants onto different polymeric surfaces (hydrophilic cotton and hydrophobic polyester) and model hydrophilic gold surface. The polymer materials used for the study were characterized using SEM. The role of ethylene oxide group variation in surfactant-polymer interaction was established using pure surfactant with the same alkyl chain length but varying ethoxylate chain lengths. It was observed that surfactant with more ethylene oxide groups per molecule, being more hydrophilic, interacts favorably with cotton in the hydrophobic siloxane solvent environment. The adsorption of the pure surfactants on model gold surface from hydrophobic solvent and water was also established using the quartz crystal microbalance with dissipation monitoring (QCM-D) system. Effect of ethylene oxide chain length and surfactant concentration on the extent of adsorption was quantified. At the gold-water interface, the plateau adsorption for C₁₂ E₃ (15.9 × 10⁻⁶ mole/m²) is about four times higher than for C₁₂E₈. An opposite trend was observed for adsorption of the surfactants on gold in the hydrophobic D5 environment. Information about thickness, adsorption and desorption kinetics, and structure of adsorbed layer was obtained from the QCM-D frequency-dissipation data. The study is an important contribution towards fundamental understanding of applications involving the use of ethoxylated nonionic surfactants.

Keywords:

• Adsorption
• D5
• Ethoxylated nonionic surfactants
• QCM
• Siloxane solvent

Acknowledgments

The QCM work was supported by the NSF-EPSCoR program and Center for Nanoscale Science and Engineering (CeNSE). RCTF provided financial support for A. Ladhe. The authors thank John May from ERTL (Environmental Research Training Lab) for the technical support in GC analysis.

Notes

The CMCs of the surfactants in D5 are not known. The CMC of C₁₂E₃ in benzene is 4 mM (Caruso et al., 1996). The CMCs of these surfactants in D5 may be of the same order of magnitude.

The listed Q_max values are for 0.79, 0.62, and 0.46 mM concentrations of C₁₂E₃, C₁₂E₅, and C₁₂E₈, respectively (250 mg/L).

^a Data calculated using Figure 2.

^b Data calculated using Figure 3.

C_eq = 250 mg/L, i.e., 0.79 mM C₁₂E₃, 0.62 mM C₁₂E₅, and 0.46 mM C₁₂E₈. Flow rate was maintained at 0.1 mL/min during adsorption and desorption experiments.

^a The regression coefficients R² are calculated for each set containing at least 500 data points.