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Introduction

Abstract

This chapter describes the fact that colloidal and surface chemistry occurs naturally. Eight typical kinds of colloidal dispersion systems are reviewed briefly, particularly those affecting snow, coffee, green tea, black tea, and stained glass. Several examples of colloidal organization and the importance of colloidal and interface science among many sciences are discussed. Finally, great scientists who contributed to the development of colloid and surface chemistry are considered briefly.

Keywords

Colloidal crystal; Colloidal dispersion; Colloidal organization; Electrical double layer; Green tea; Stained glass; Snow; Water droplet

1.1. Common Colloidal Dispersions

Colloidal and surface chemistry have always existed. Rain, fog, and mist are typical examples of liquid-in-gas-type colloidal dispersion. These types of colloidal dispersions are further distinguished depending on their sizes. In Japan, for example, they are named moya ("dry fog," smaller than 10 µm in diameter), kiri (~10-100 µm), kirisame (~100-300 µm), sitosito-ame (~0.3-1 mm), ame ("rain"; ~1 mm), and suko-ru ("squall"; larger than 1 mm). Ice-fog is the dispersion of ice particles in the air (solid-in-gas type). Cloud refers to the concentration of water or ice particles (~2-40 µm) in the air. Gravity causes clouds to descend at the velocity of approximately 20 mm/min without wind; however, even a faint ascending air current can keep a cloud aloft for a long time. Snow is crystallized ice from saturated water vapor in a cloud and is a typical example of gas-phase crystallization. The maximum crystal growth rate of snow is approximately 30 µm/s and is surprisingly similar to that of colloidal crystallization. Observation of snow crystals has a long history. Johannes Kepler (1571-1630) discussed why snow crystals are hexagonal (1611). Rene Descartes (1596-1650) observed and sketched many shapes of snow crystals (1637). Wilson Alwyn Bentley (1865-1931) and William Jackson Humphreys (1862-1949) published a famous book titled Snow Crystals (1931),1 which included many beautiful pictures of snow crystals. Ukichiro Nakaya (1900-1962), a famous scientist who studied snow, said, "Snow is a letter from the sky,"2,3 meaning that the state of the sky becomes clear by studying the morphologic characteristics of snow crystals. He made the Nakaya diagram showing the relationship between various crystal structures of snow as a function of the degree of supersaturation and temperature of water vapor in the sky (Fig. 1.1). For example, hexagonal polyhedric snow crystals grow in a low supersaturation state of vapor, whereas various crystal structures, including needlelike, folding fan-like, sheathlike, and dendritic crystals, form in a state of high supersaturation. Fig. 1.1 is newly redrawn by the author based on several types of Nakaya diagrams reported previously.
Figure 1.1 Schematic representation of a Nakaya diagram of snow. Smoke, the dispersion of the liquid-in-gas or solid-in-gas colloid type, is also familiar. Smog is a new word combing smoke and fog, and relates to the pollution problem. Mist and fume are also typical examples of colloidal dispersion systems in solid-in-gas and liquid-in-gas systems. Furthermore, aerosol is a general name for colloidal dispersions of solid-in-gas and liquid-in-gas systems. A large, red sunset may be beautiful, but this also means air pollution is a serious problem in the presence of colloidal particles. Colloidal dispersion systems in the liquid phase are most familiar. For example, turbid water in a pond contains colloidal particles of soil. Mayonnaise and milk are typical examples of liquid-in-liquid-type colloidal systems. Most paints are solid-in-liquid-type dispersions. Coffee,4,5 black tea,6 green tea,7 and Miso soup8 are also solid-in-liquid-type colloidal dispersions. The monodispersed colloidal spheres are now available commercially thanks to the recent advancement in the field of synthetic colloidal chemistry. Fig. 1.2 shows typical colloidal crystals of polystyrene spheres (D1K88, 137 ± 16 nm in diameter) in the stock (A) and diluted (B) suspensions in a bottle and a test tube, respectively. Iridescent colors are beautiful. To remove most of the ionic impurities in the suspension as completely as possible, the stock suspension was deionized with the mixture of ion-exchange resins 10 years previously. The sample suspensions were prepared by diluting the stock suspension with deionized pure water and the ion-exchange resins were introduced again. The author found the viscosity of the sample suspension containing the resins increased with time and reached saturation after several years. Complete deionization of the sample spheres took such a long time because deionization is one of the equilibrium reactions proceeding between colloidal spheres and solid resins dispersed in the aqueous phase. Colorful single crystals are observed in Fig. 1.2, B.
Figure 1.2 Colloidal crystals of polystyrene spheres (D1K88, 137 ± 16 nm in diameter) in the stock (A, ? = 0.062) and diluted (B, 0.00053) suspensions. Fig. 1.3 shows one of the typical microscopic pictures of colloidal green tea leaves. In most Japanese hotels, a guest will find in her or his room a tea set to prepare green tea; the tea set is composed of green tea leaves in a paper filtering bag, a tea cup, and a water heater, for example. One day, after preparing tea and leaving it on the desk for several minutes, I was excited to find a broad ring forming in the cup. Fig. 1.4 shows typical examples of the broad ring patterns of green tea.7 The broad rings of the hills created by accumulated green tea particles stayed on the inclined wall of the cup, and did not drop into the deepest central area. This was my first finding of sedimentation patterns. The sedimentary patterns were always observed irrespective of the kind of particles, cup, and observed places. The smaller the colloidal particles, the larger the broad rings that form up the inclined wall. Therefore approximate information on the size distribution of the particles is available from the sedimentary broad ring patterns. The details are described in Chapter 5.
Figure 1.3 Microscopic observation of green tea leaves. Objective lens ×40, eye lens ×10; full scale is 25 µm. (From Okubo T: Sedimentation and drying dissipative structures of green tea. Colloid Polym Sci 285:331-337, 2006.)
Figure 1.4 Sedimentary patterns of green tea in the several cities in Japan. A, Sapporo, B, Yonezawa, C, Kumamoto, and D, Ishigaki in Japan. (C and D from Okubo T: Sedimentation and drying dissipative structures of green tea. Colloid Polym Sci 285:331-337, 2006.) Biologic polymeric substances are grouped into three categories: proteins (polypeptides), polysaccharides, and polynucleotides. Many are assemblies of anionic polyelectrolytes. Spherical proteins of hemoglobin and fibrous collagen or keratin exist in the state of solid-in-liquid-type colloidal systems. Sponge, hydrated silica gel, and stained glass are typical examples of gas-in-solid, liquid-in-solid, and solid-in-solid systems, respectively. Stained glass is the beautiful colloidal system composed of metal particles 100 to 300 nm in diameter dispersed in a glass solid.9 An example of stained glass is shown in Fig. 1.5. Stained glass can be seen almost anywhere. Only light in a certain range of wavelengths is transmitted through the stained glass by the quantum effect, and the transmitted light is colorful (e.g., reddish, greenish, or yellowish, depending on the kind of the metal particles). Usually, a stained glass window in a church is beautiful when we see it in the daytime. Many years ago, I was impressed to find beautiful stained glass when I looked at a lighted church in the evening. Table 1.1 describes examples of colloidal systems grouped by the combination of the interfaces formed between the solute and the medium. Gas-in-gas systems do not exist in equilibrium, because gas and gas are mixed homogeneously and no interfaces between gas and gas exist.
Figure 1.5 A typical example of stained glass in Notre-Dame Cathedral, Strasbourg, France.

Table 1.1

Types of Colloidal Dispersions

Dispersion medium Gas --- Aerosol
Fog, mist, cloud Aerosol
Smoke, smog Liquid Foam
Foam Emulsion
Mayonnaise, milk Solid dispersion
Paint, coffee Solid Solid foam
Sponge Solid...