Effective refractive index of calcium carbonate pigment slurries by a surface-plasmon-resonance sensor
Introduction
Various pigment slurries are widely used in industry, especially in the paper industry which consumes a significant amount of pigments. Pigments are used either as fillers or as a coating layer on body stock (non-coated paper). When used as a coating layer, the thickness of the layer is typically about 10 μm. Pigments give the paper its white colour due to their optical properties such as refractive index, extinction coefficient and light scattering. Other properties provided by pigments are opacity and gloss. These properties are closely connected to the refractive index of the pigment. Pigments are applied also in paints to give the appropriate colour. Generally, pigments increase light scattering from the surface of the material.
If the size of pigment particles is comparable to the wavelength of the light, scattering is the dominant effect when light and particles interact. Thus, pigment slurries, having pigment particles whose size centres around 1 μm, are optically opaque and turbid. In such cases, transmission of light is weak and an optical measurement method based on light transmission is not practical, whereas reflection spectroscopy is more effective because there is no need to dilute the slurry. Calcium carbonate pigments, for example, scatter most of the incoming light and, thus, their colour appears to be white. When looking for the optimal light scattering conditions: particle size, shape, refractive index, absorption coefficient, particle orientation and packing of the pigments have to be optimized if excellent opacity is to be a feature of paper products.
In this study we investigated irregular cylinder shaped, birefringent calcium carbonate pigments in a water matrix. We prepared different loadings of slurries by weighing solid particles and mixing them in water. The object of the study was to find out a means of detecting the pigment loading, to determine the effective refractive index of the slurries and to observe the absorption as a function of calcium carbonate loading. In the pigment industry there is a desire to measure the quality of calcium carbonate slurries. Therefore, a simple on-line detection system would have significance for such an industry. Because of the high calcium carbonate loading of the slurry the optical measurement of the quality has been rather problematic, so far, due to the strong turbidity of the slurry. Therefore, the applicability of a surface-plasmon-resonance sensor (SPRS) for the detection of optical properties of high calcium carbonate loading slurries was investigated. The idea of sensing, which is based on the use of light-induced, surface-plasma wave oscillations, was introduced by Kretschmann [1] and further developed by others [2], [3]. Lately, SPRS has found various applications in detection of chemical changes and physical changes [4] and also biological [5] changes of liquids and other phases. Recently, experiments have been applied to the detection of the effective refractive index of commercial calcium carbonate slurries using an ATR-reflectometer and SPRS [6]. Here the number of different calcium carbonate slurries was larger than quoted in Ref. [6]. In addition, the connection between the reflectance signal and the effective absorption of the slurries using the SPRS was investigated.
Section snippets
Theory
In the case of SPRS (see Fig. 1), the reflectance is determined in the ATR-mode, i.e. the angle of incidence exceeds the critical angle of reflection. Thus, only the evanescent component of incoming p-polarized light wave penetrates into the rarer medium, decaying exponentially along the normal of the metal film-slurry interface of the prism. The intensity reflectance can be expressed as follows [2], [3]:
Here, θ is the angle of incidence
Materials
The investigated pigment was PCC (precipitated calcium carbonate). The PCC particles used in this study are optically anisotropic and their shape resembles a cylinder. Furthermore, calcium carbonate is a negatively birefringent material, transparent in the spectral range 0.2–2.0 μm. Therefore, its intrinsic absorption is low in the UV–VIS spectral range. The length of particles is around 1 μm and width around 0.4 μm. Thus, their dimensions indicate light scattering in UV–VIS range. The
Measurement method
A schematic diagram of the SPRS is shown in Fig. 1. A thin (50 nm) silver film was evaporated onto one face of a prism (Schott FK11 glass). Optical constants of silver and other metals, as a function of wavelength are available [7]. The light source was a helium neon laser (633 nm) that produces linearly polarized light. The stability of the laser was monitored using a beam splitter, to provide a reference signal. The intensity of the probe and reference signals were measured using commercial
Experimental and discussion
The SPR-curves for six calcium carbonate loadings ranging from 0 to 50 g, mixed in 100 cm3 of water were measured. The reflectance curves obtained from such slurries are shown in Fig. 3. Fig. 3 shows the minimum position of the reflectance curve, which gives the angle of surface-plasmon-resonance. Fig. 3 also shows that the SPR-angle shifts towards a higher angle of incidence while the loading of the pigment into the slurry increases. The reflectance minimum (Rmin) depends also on the loading
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