Thesis subject
BSc - Effect of water and surfactant in the interaction between hydrophobic surfaces in apolar media
Dry-cleaning is a process for removing soils and stains from fabrics and garments using a non-aqueous solvent with added detergent. Traditional dry-cleaning methods employ organic solvents such as PERC (perchloroethylene), which is toxic and environmentally harmful. PERC has many adverse effects such as damages kidneys and liver, and causes gastrointestinal irritation. It can also cause eye, nose and throat irritation. Repeated dermal exposure may result in dry, scale, and fissured dermatitis.
Carbon dioxide is non-toxic, nonflammable, ecologically sound, cheap, and available on a large scale. An important difference between dry cleaning with PERC or other currently used solvents, and dry-cleaning with CO2 is that carbon dioxide dry cleaning needs a substantially higher pressure (45-60 bar) than atmospheric pressure. Therefore, a new process for dry-cleaning has to be developed. Literature on dry-cleaning using liquid CO2 revealed that removal of oily soil was comparable or in some cases better than PERC, however, detergency of small particulate soil (size less than 20 μm) was inadequate compared to the cleaning performance of PERC. Higher input of mechanical action could not improve the cleaning performance of particulate soil in liquid CO2 and as a result the perspectives of liquid CO2 as a dry cleaning solvent remained limited. Cleaning efficiency can be improved by the use of surfactants in the system. But finding a surfactant for liquid CO2 is another challenge. Liquid CO2 near the critical pressure and temperature is a very poor solvent and hence solubility of surfactants is very low. Our research is focusing on the following three things:
- Finding a suitable surfactant or a surfactant formulation
- Understanding of the fundamental science behind the dry-cleaning which involves knowledge of colloid and interfacial science in apolar media and
- Improvement of the hydrodynamics involved in the dry-cleaning process using liquid CO2
The apolar solvents always contain a trace amount of water dissolved in them . The maximum amount of water that can be dissolved in the apolar solvents depends on the dielectric constant of the medium. In this study we will focus on the solvents with very low dielectric constants e.g. n-hexane and cyclohexane in which the maximum solubility of water is ~ 0.01%. The presence of this small amount of water plays a significant role in many industrial processes such as dry-cleaning using liquid CO2, tertiary oil recovery and nano-particle synthesis.
Our previous research has shown that the molecular force of interaction between a hydrophilic particle and a hydrophilic surface through n-hexane with saturated amount of water is ~ 1 order of magnitude higher than that through dry n-hexane. We hypothesized that this could be due to the formation of water capillary bridge between the two interacting bodies when these come within a very short distance (nm-μm range depending on the system involved). We have measured these forces using a colloidal probe atomic force microscopy.
In this present study we would investigate the effect of water on the hydrophobicity of the surfaces. The force of adhesion between a hydrophobized colloidal particle and a hydrophobic surface interacting through a water saturated apolar solvents will be measured in the same way as mentioned above.
Further to this, the wettability of the modified hydrophobic surfaces with water through the apolar solvents can also be measured using a goniometer. In addition, the effect of surfactants on adhesion force will also be studied by atomic force microscopy.
Experimental techniques:
- Preparation of the model hydrophobic surfaces
- Measurement of adhesion force: Dynamic force spectroscopy
- Confocal microscopy to visualize presence /absence of water capillary bridge
- Equilibrium and advancing/receding contact angle measurement using Goniometer