Determination of Contact Angles of Nanosized Silica Particles by Multi-Angle Single-Wavelength Ellipsometry

Abstract

The determination of nanoparticle wettability is an area of great practical importance to materials science and engineering fields. There has been some recent interest in using spectroscopic analysis to indirectly categorize behaviour of monolayers, using the reflective and refractive properties of immersed particles. A method is developed here to calculate specific contact information for nanoparticles at an air–water interface, using single wavelength ellipsometry. A two-layer model is used that considers the refractive index of the immersed and non-immersed particle portions. Pressure–area isotherms and Brewster angle images were used to confirm and categorize the nature of a packed monolayer, and ψ and Δ measurements taken of the interface. Immersion thickness was calculated and related to an equilibrium contact angle, and combined thickness data for the two layers was successfully related to the diameter of the particles.

Particle detachment from fluid interfaces: theory vs. experiments

“…Different techniques have been developed to obtain θ for colloidal particles, 18 based on direct visualization [19][20][21][22][23] or indirect measurements [24][25][26][27][28][29][30] . The gel-trapping technique (GTT) 19,20 and freeze-fracture shadow-casting (FreSCa) cryo-SEM 21,22 are applicable for contact-angle measurements of nano-and microparticles.…”

“…For instance, using a Langmuir trough, 26 θ can be estimated from the pressure-area-isotherms differences between a surfactant and a surfactantparticle system, assuming there are no surfactant-particle interactions. Alternatively, using an ellipsometer 27,28 and applying a two-layer model to describe a particle-laden interface, one can deduce the average particle position relative to the interface and then calculate θ.…”

Silica nanoparticle sols. Part 3: Monitoring the state of agglomeration at the air/water interface using the Langmuir–Blodgett technique

“…Frequently, chloroform has been used as a spreading solvent in LB experiments [9,15,16] since it is almost insoluble in water and evaporates quickly. In the present study, we were following the work of Hunter and coworkers [17] and used chloroform/ethanol mixture (50/50 vol/vol%). However, since carrying out the present experimental investigation, we noted that Hunter [14] extended his studies and found that chloroform coagulated hydrophobic silica nanoparticles (to some extent) and after a series of dispersion tests used chloroform/ethanol to disperse the unmodified silica nanoparticles and ethanol/acetone to more effectively disperse hydrophobic modified silica nanoparticles in LB experiments.…”

Particle and Particle-Surfactant Mixtures at Fluid Interfaces: Assembly, Morphology, and Rheological Description

“…In the last twenty years, several experimental techniques have been developed for the determination of the contact angle of particles at fluid interfaces: drop shape techniques [56,65,66], surface pressure-area isotherms [52,[67][68][69][70], Washburn capillary rise method [71][72][73], Atomic Force Microscopy (AFM) coupled to colloidal probe [74,75], Gel Trapping Technique (GTT) [56,67,76,77], Freeze Fracture Shadow Casting (FreSCa) [55,[78][79][80], excluded area method [81,82], Film-Calliper Method (FCM) [83], ellipsometry [53,[84][85][86][87][88], or neutron reflectivity [89]. It has been found that different methods to evaluate the contact angle of particles provide different values of for similar particles [2].…”