A quartz crystal microbalance study of the removal of solid organic soils from a hard surface in aqueous surfactant solution

doi:10.1016/S0927-7757(98)00777-8

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volume 146, Issues 1–3, 15 January 1999, Pages 185-197

https://doi.org/10.1016/S0927-7757(98)00777-8 Get rights and content

Abstract

A quartz crystal microbalance (QCM) has been employed to monitor the removal of two model solid organic soils, dotriacontane and tripalmitin, from the hard surface of the QCM crystal in aqueous surfactant solutions of octa-ethyleneglycol mono n-dodecyl ether (C₁₂E₈). We have investigated the effect of varying the thickness of the soil coating on soil removal and the effect of soaking the soil in high-purity water for an extended period of time before adding surfactant. The QCM results support the view that net soil removal is preceded by a stage of water and surfactant penetration into the soil. The rate of penetration and rate of removal depends on the soil type. Water and surfactant take longer to penetrate dotriacontane compared to tripalmitin coatings. The removal process also occurs over a longer period of time in the case of dotriacontane coatings. The percentage of material removed is less for dotriacontane, compared to tripalmitin coatings. The initial coating thickness on the hard surface does not appear to govern the final percentage of soil removed, at least in the thickness range accessible to the QCM (approximately ≤800 nm). Immersing the soil coated surfaces in water for a relatively long time, hastens the onset of the removal stage after surfactant is added but does not significantly influence the rate and extent of removal from the hard surface.

Introduction

A major focus of the soaps and detergents industry is to develop lower cost and more efficient product formulations for household and industrial cleaning products. Cleaning products ideally need to be optimised to remove the maximum amount of soil from surfaces in the minimum amount of time with the minimum input of mechanical or thermal energy. Fundamental studies of the detergency process may contribute to the optimisation process.

The quartz crystal microbalance (QCM) can be a sensitive technique for monitoring the mass changes that may take place when surface deposited (or adsorbed) materials are subjected to chemical or biological events [1]. The amount of coating material on the surface of a QCM crystal (as well as the coating thickness if the density of the material is known) can be determined from the change in the resonance frequency of the QCM crystal. Sauerbrey was the first to derive the equation which quantitatively relates the change in frequency (ΔF) to the change in mass (Δm) [2]: $Δ F=− 2F_{o}^{2} A ρ_{q} μ_{q}^{1/2} Δ m$ where ΔF is the measured shift in the resonance frequency (Hz) of the QCM crystal, F_o is the fundamental resonance frequency of the QCM crystal, Δm is the mass change (g), A is the electrode area, ρ_q is the density of the quartz, and μ_q is the shear modulus.

Two groups have recently independently employed the QCM to investigate the removal of solid organic soils from a hard surface [3], [4]. Shimomura et al. [3] studied the removal of proteinaceous material, such as gelatin, ovalbumin and keratin, from a hard surface in aqueous solutions containing enzyme and surfactants. We have previously reported [4] a preliminary study of the removal of a mineral oil, dotriacontane, and a triglyceride, tripalmitin, from a hard surface in aqueous nonionic surfactant solutions. Here, we extend our work on dotriacontane and tripalmitin, and examine the effect of varying the thickness of the soil coating on the removal process. We also investigate the effect of soaking the soil coatings in water before adding surfactant. The surfactant used in this study is octa-ethyleneglycol mono n-dodecyl ether (C₁₂E₈).

Section snippets

Materials

Octa-ethyleneglycol mono n-dodecyl ether (C₁₂E₈) was purchased from Nikko Chemicals, Japan. Tripalmitin (glycerol tripalmitate; glycerol trihexadecanoate) and dotriacontane (dicetyl) were obtained from TCI Tokyo Kasei and Sigma, respectively. The surfactants and soils were not further purified. De-ionised tap water was passed through a Milli-Q Plus Ultra-Pure Water System (Millipore, Australia) to obtain high-purity water.

QCM crystal coating procedure

The gold covered QCM crystals were coated with the solid organic soils by

Results and discussion

The frequency response of the QCM crystals coated with the two model solid organic soils has been investigated in high-purity water. Some representative plots of the change in frequency (ΔF) versus time are shown in Fig. 1. Some other representative plots are provided in the earlier preliminary report [4]. In all the plots in Fig. 1, the data just prior to time zero corresponds to the resonance frequency of the coated QCM crystal in air, before immersion in water. The origin (Time=0 min,

Conclusions

The QCM results reported in this work are in accord with the view that for solid organic soils on hard surfaces, the detergency process involves two major events which have different time intervals over which they dominate. The first dominant event involves penetration of surfactant and water into the soil, i.e. liquefaction. In this work, penetration was observed to occur more rapidly in the case of tripalmitin than in the case of dotriacontane. The next dominant event involves the net removal

Acknowledgements

The research in this paper formed part of the requirements for AW to obtain the degree of Master of Applied Science at The Royal Melbourne Institute of Technology. AW thanks the CSIRO for providing him with the opportunity to complete the requirements for this degree. CJD has been extremely fortunate to be associated with Tom Healy for 15 years; first as one of Tom’s (and Franz Grieser’s) post-graduate students and then as a colleague. It is a pleasure to be able to contribute to this issue in

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¹: Present address: Max-Planck Institute of Colloids and Interfaces, Rudower Chaussee 5, D-12489, Berlin, Germany.

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A quartz crystal microbalance study of the removal of solid organic soils from a hard surface in aqueous surfactant solution

Abstract

Introduction

Section snippets

Materials

QCM crystal coating procedure

Results and discussion

Conclusions

Acknowledgements

Colloids Surf. A: Physicochem. Eng. Aspects

Chem. Rev.

Z. Phys.

Textile Res. J.

Langmuir

Langmuir

Langmuir

J. Phys. Chem.