The Importance of Liquids - Introduction - Liquid-State Physical Chemistry: Fundamentals, Modeling, and Applications (2013)

Liquid-State Physical Chemistry: Fundamentals, Modeling, and Applications (2013)

1. Introduction

Whilst liquids and solutions are known to play an essential role in many processes in science and technology, the treatment of these topics in the literature have in the past appeared to be limited to either rather basic or rather advanced levels, with intermediate-level texts being scarce, despite the practical importance of liquids and solutions. A brief outline of the differences and similarities between solids, gases, and liquids would help to clarify the reasons for this, and this book represents an attempt to remedy the situation. In this first chapter, an outline of what will be dealt with, and the reasons for these choices, will be given.

1.1. The Importance of Liquids

A brief moment of reflection will make it clear that liquids play an important role in daily life, and in the life sciences and natural sciences, as well as in technology. Hence, these areas of interest will be considered briefly in turn.

Undoubtedly, the most important liquid is water. Water is essential for life itself, and its interaction with other liquids, ions and polymers is vital to many life processes. The basic component of blood, the solvent, is water, and blood itself is an example of a highly complex dispersion of red and white blood cells within a complex mixture of water, ions, and polymeric molecules. As is well known, blood not only transports oxygen through the body but also distributes required molecules to a variety of locations in the body, as well as removes waste material. The miscibility of water with other liquids (such as alcohol) is well known and exploited in alcoholic drinks.

Two other arbitrary examples of liquids that are highly relevant to daily life are petrol – a complex mixture of several types of aliphatic molecules and other species – for cars, and milk – a dispersion of fat globules stabilized in water by a complex of large and small molecules. Without petrol, modern society would be unthinkable, while milk provides a valuable (some say indispensable) part of the nutrition of humankind.

In technology many processes use solvents. With the current drive in industry to abolish the use of volatile organic compounds, the importance of water as a solvent – though already considerable – will be increased. Nonetheless, organic solvents are still used to a considerable extent, and properties such as rates of evaporation, miscibility, viscosity, thermal expansion, compressibility and thermal conductivity feature extensively in many technological processes. The re-use of solvents is also an important issue.

From a scientific point of view, liquids have a less extensive history of understanding than do gases and solids. The reasons for this will become clear in Section 1.2: a natural reference configuration with respect to structure and energy is missing. This implies that, for almost all aspects of liquids, it is not one largely dominant factor that determines the behavior, but rather several smaller factors must be taken into account, and this renders the overall description complex. Hence, the scientific problems posed by liquids is highly challenging, and the modeling of systems may range from simple semi-empirical models via sophis­ticated “physical” models to ab initio models and computer simulations. Yet, each of these models has its own value in the understanding of liquids with regards to their complexity from a structural, dynamic, and energetic point of view.