Determining the complex refractive index of cellulose nanocrystals by combination of Beer-Lambert and immersion matching methods

• Published in: Journal of quantitative spectroscopy & radiative transfer 2019-09, Vol.235, p.1-6
• Main Authors: Niskanen, Ilpo, Suopajärvi, Terhi, Liimatainen, Henrikki, Fabritius, Tapio, Heikkilä, Rauno, Thungström, Göran
• Format: Article
• Language: English
• Subjects: Accurate measurement; Beer-Lambert; Cellulose; Cellulose derivatives; Cellulose nano-crystals; Cellulose nanocrystal (CNCs); Cellulose nanocrystals; Complex refractive index; Crystallinity; Immersion liquid methods; Immersion matching method; Matching methods; Nanocellulose; Nanocrystals; Nanoparticles; Particle shape and size; Particle size analysis; Refractive index; Spectroscopy
• ISSN: 0022-4073; 1879-1352; 1879-1352
• DOI: 10.1016/j.jqsrt.2019.06.023

•Determination of complex refractive index of CNCs as a function of wavelength.•The data analysis is based on Beer-Lambert and immersion liquid method equations.•The refractive indexes of CNCs are considerably lower than original cellulose.•The method is independent of the shape or size of a particle.•The accuracy of the refractive index was better than ±0.005 refractive index units. Nanocelluloses have received significant interest due to their unique structural, mechanical, and optical properties. Nanocellulose refractive indices can be used to indicate many crucial characteristics, such as crystallinity, transparency, and purity. Thus, accurate measurement is important. This study describes a new method to determine the wavelength dependent complex refractive index of cellulose nanocrystals (CNCs) by the measurement of light transmittance with a spectrophotometer. The data analysis is based on a combination of the Beer-Lambert and immersion liquid matching equations. The immersion liquid method's main advantage is that it is independent of particle shape and size. Moreover, the measurement is easy and relatively quick to perform. The present procedure is not restricted to the nanocellulose and could potentially be applied to other nanomaterials, such as hyphenate nanoparticle-based, lignin nanoparticles, nanopigments, biological entities, structural elements of dielectric metamaterials, and nanoparticle-based composites. [Display omitted]