Paper chemistry colouration

In order to better discuss the role of surface chemistry in photography, it is useful to consider some of the basic features of photographic elements (paper, film, etc.) morphology, exposure and processing. As a start, consider a multilayer photographic element such as that pictured in Figure 5.1. The layer structure illustrated is typical of a simple colour print material, such as colour paper or colour movie print films. Such materials comprise many different types of layers (1), and each layer is prepared as a thin film coating. 

The potential value of the application of molecular orbital methods in colour chemistry is immense. In essence, the reason for this is that the methods enable, in principle, many of the light absorption properties of dyes, from a knowledge of their chemical structure, to be calculated with the aid of a computer. Thus, the colour properties of any dye whose structure may be drawn on paper may be predicted, with some expectation of accuracy, without the need to resort to devising a method for the... 

The chemistry and applications of the colour change grouping, containing all the well-known isms of chromic phenomena, namely photochromism, thermo-chromism, ionochromism, electrochromism and solvatochromism, as well as the lesser-known ones such as tribochromism and vapochromism, are covered in Chapter 1. These chromic phenomena impinge on our everyday life, e.g. in photo-chromic spectacle lens, thermochromic temperature indicators, fax paper, smart windows and mirrors and in visual displays. 

Figure 1.1 Paper chromatography. Even today lower school science lessons often make use of paper chromatography to separate the colours of Smarties, for example. Reprinted with permission (J. Nicholson, Chemistry in Britain 30 (1994) 658).

Prussian Blue (Berlin Blue) is the fine mineral colour containing both ferrous and ferric iron united with the cyanogen radicles. It has been on the market since 1700. Together with ultramarine and the coal-tar blues it is still much used for the manufacture of paint, for paper and cloth printing, etc. For the latter purpose it is often produced on the fibre itself by imprinting with potassium ferrocyanide, followed by steaming (see Vol. I., Martin s Industrial Chemistry ). 

One of the true arts, still practised in papermaking, is the colouration of paper. Chapter 10 covers this subject with great delicacy. Explanations are given of how colour is perceived by the human eye and how differences in shade are distinguished. The chemistry of various chromophores is discussed as well as the conditions under which these chemicals are best used e.g. acid dye use in crepe paper, basic dyes in newsprint and direct dyes in printing and writing papers. 

Bozhenov, P.I., and Kholopova, L.I., "Coloured Clinker Cements," Supplementary paper, Sixth International Congress on the Chemistry of Cement, Section III-8, Moscow, 1974. 

In 1850, F. Runge published his book "Zur Farbenchemie. Musterbilder fur Freunde des Schonen und zum Gebrauch fiir Zeichner, Maler, Verzierer und Zeugdrucker" (On colour chemistry. Examples for lovers of beauty and for use by artists, painters, ornamenters and cloth printers), in which he described for the first time the migration of various dyes on paper using water as an eluent. 

The most widely-used industrial thermochromic system is the referred to as the leuco dye type. The term leuco describes a dye that can acquire two forms, one of which is colourless. This micro-encapsulated composite system relies on colour formation from the interaction of three materials a colour former (the leuco dye), an acidic developer and a low-melting, non-volatile hydrophobic solvent, a set of ingredients similar to those used in thermally-sensitive paper as described above. However, in the leuco dye thermochromic systems, it is curious that, even though similar chemistry is involved, the composite material is coloured at low temperatures and decolourises as the temperature is raised. To explain this effect, a mechanism illustrated in simplified form in Scheme 11.4 has been proposed. At temperatures below the melting point of the solvent, the system is heterogeneous with the colour former existing as the coloured... 

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