Earlier work describing the use of p‐nitrophenol (PNP) in measuring specific surface of finely divided solids is supplemented here by a description of recent investigations which enable the usefulness of the method to be more clearly defined. Normally PNP is used in aqueous solution, but for some materials liable to dissolve in, swell in, or react chemically with, water, e.g. sugar, certain inorganic oxides and textile fibres, an organic solvent is used. The method is suitable for a wide variety of solids, both non‐porous and porous, provided they either form a hydrogen bond with PNP or have aromatic nuclei. Thus, with appropriate solvents, oxides, textile fibres with polar molecules and carbons can be used, but not, for example barium sulphate. With porous solids the results may or may not reveal a lower accessible surface than the area measured by nitrogen, depending on the pore size distribution. With porous solids the method enables a rapid estimate of the relative proportion of small and large pores to be made. Also, it can be used to measure the external specific surface only, as distinct from the total surface of some porous granular solids and fibres. Normally PNP appears to be adsorbed flatwise from water or organic solvents with an effective molecular area of 52.5 Å2. (An earlier value for this area given for special cases of adsorption from non‐aqueous solutions is now shown to be invalid). In some cases on polar inorganic solids it is adsorbed end‐on with an effective area of 25 Å2. The mode of adsorption is indicated by the type of isotherm.
The Langmuir film balance and the Davies surface‐film viscous traction apparatus have been used with films of cellulose triacetate alone and in presence of two different disperse dyes; these were C.I. Disperse Red 13, an azo dye having both proton‐donating and proton‐accepting groups in its molecule, and C.I. Disperse Yellow 13 (methoxybenzanthrone), which has only proton‐accepting groups. The results show that a 1:1 complex is formed between either dye molecule and a hexa‐acetylcellobiose unit of the cellulose triacetate molecule, and possibly also a complex with an aggregate of the yellow dye. This indicates a face‐to‐face association of the dye molecules (which are planar) with cellulose acetate. The association is probably a hydrogen‐bonding interaction.
A new method is described whereby the specific surface of colorants, i.e. pigments or insohrble or watersoluble dyes, in cellulose can be nleasltred. The principle of the method is the selective adsorption of p-nitrophenol (PNP) from water by the colorant particles but not by the cellulose, thus permitting determination of the surface of the particles and the egkct thereon of various treatments.Results with organic pipnents in spun-dyed viscose rayon agree with calculations of specific surface from particle-size analysis of the pigments before incorporation. This shows that the whole surface of the colorants in the cellulose is accessible to PNP and thus measurements can be made with a variety of dyes.Azoic dyes and vat dyes give results that correspond to average particle diameters of N 0 . 1 4 6 pm and the method detects the increase in their particle size on soaping. A direct cotton dye of low light fastness and a reactive dye give specific surface values much too low for a monolayer and they appear to exist as multilayers in cellulose. A direct cotton dye of high light fastness appears to be present as three-dimensional aggregates of diameter N 0.08 pm, in agreement with published electron micrograph values for similar dyes.
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