Preface

Colloid and surface chemistry is a subject of immense importance and has implications both to our everyday life and to numerous industrial sectors from paints and materials to medicine and biotechnology.

When we observe nature, we are impressed by mosquitos and other small insects that can walk on water but are drawn into the water when detergents (soaps) are added in their neighbourhood. We are fascinated by the spherical shape of water and even more by the mercury droplets that can roll around without wetting anything. We know that for the same reasons we should use plastic raincoats when it is raining. We are also impressed by some of natural wonders like the "delta" created by rivers when they meet the sea and the non-sticky wings and leaves of butterflies, lotus and some other insects and plants. We are also fascinated by the blue colour of the sky and the red colour of the sunset.

When we are at home we are constantly surrounded by questions related to colloids or interfaces. We would like to know how detergents really clean. Why can we not just use water? Can we use the detergents at room temperature? Why do we often clean at high temperatures? Why do so many products have an expiration date (shelf-life) of a few days or weeks? Why can't milk last for ever? And why this "milky" colour that milk has? Is it the same thing with the well-known drink Ouzo? Why does Ouzo's colour change from transparent to cloudy when we add water? And what about salt? Why does it have such large effect on foods and on our blood pressure? Why do we so often use eggs for making sauces?

Those who have visited the famous VASA museum in Stockholm are impressed by the enormous efforts made in preserving this ship, which sank 400 years ago, after it was taken out of the sea. Why was a solution of poly(ethylene glycol) spayed on the ship?

Of course, similar problems occur in industries that focus on the development and manufacturing of a wide range of products ranging from paints, high-tech materials, detergents to pharmaceuticals and foods. In addition, here it is not just curiosity that drives the questions! Paint industries wish to manufacture improved coatings that can be applied to many different surfaces but they should also be environmentally friendly, e.g. should be less based on organic solvents and if possible exclusively on water. Food companies are interested in developing healthy, tasty but also long-lasting food products that appeal to both the environmental authorities and the consumer. Detergent and enzyme companies have worked in recent years, sometimes together, to develop improved cleaning formulations containing both surfactants and enzymes that can clean much better than before, working on more persistent stains, at lower temperatures and amounts, to the benefit of both the environment and our pocket! Cosmetics is also big business. Many of creams, lotions and other personal care products are complex emulsions and the companies involved are interested in optimizing their performance in all respects and even connecting consumer's reactions to products characteristics.

Some companies, often inspired by nature's incredible powers as seen in some plants and insects, are interested in designing surface treatment methods that can result in self-cleaning surfaces or self-ironing clothes; surfaces that do not need detergents, clothes that do not need ironing!

There are many more other questions and applications! Why can we get more oil from underground by injecting surfactants? Can we deliver drugs for special cases in better and controlled ways?

All of the above and actually much more have their explanation and understanding in the principles and methods of colloid and surface chemistry. Such a course is truly valuable to chemists, chemical engineers, biologists, material and food scientists and many more. It is both a multi- and interdisciplinary topic, and as a course it must serve diverse needs and requirements, depending on the profile of the students. This makes it an exciting topic to teach but also a very difficult one. Unfortunately, as Woods and Wasan (1996) showed in their survey among American universities, a relatively small number of universities teach the course at all! This is a problem, as several universities try to "press" concepts of colloid and surface chemistry (especially the surface tension, capillarity, contact angle and a few more) into general physical chemistry courses. This is no good! This is no way to teach colloids and interfaces. This exciting topic is a science by itself and deserves at least one full undergraduate course and of course suitable books that can be used as textbooks.

This brings us to the second major challenge, which is to have a suitable book for teaching a course to undergraduate students of a (technical) university. More than ten years ago, we were asked to teach a 5 ECTS theory course on colloids and interfaces at the Technical University of Denmark (DTU). The time allocated for a typical 5 ECTS point (ECTS = European Credit System) course at DTU is one four-hour block a week during a 13 week semester, followed by an examination. This course is part of the international program of our university, typically at the start of M.Sc. studies (7th-8th semester) and can be followed by students of different M.Sc. programs (Advanced and Applied Chemistry, Chemical and Biochemical Engineering, Petroleum Engineering). B.Sc. students towards the end of their studies can also follow the course. In Denmark, students submit a written anonymous evaluation of the course at the end of semester, providing feedback on the teaching methods and course content, including course material (books used).

Our experience from 12 years of teaching the course is that we found it particularly difficult to choose a suitable textbook that could fulfil the course requirements and be appealing to different audiences and the increasing number of students. This may appear to be a "harsh comment". First of all, there are many specialized books in different areas of colloid and surface chemistry, e.g. Jonsson et al. (2001) on surfactants and Israelachvili (2001) on surface forces. These and other excellent books are of interest to researchers and also to students as supplementary material but are not suitable -and we do not think they were meant by these authors to be - as a stand-alone textbook for a general colloid and surface chemistry course. Then, there are some books, for example Hunter (1993) and Barnes and Gentle (2005), that focus either only on colloid or on surface science. There are, nevertheless, several books that, more or less, target to cover a large part of a standard colloid and surface chemistry curriculum. Examples are those written by Shaw (1992), Goodwin (2004), Hamley (2000), Myers (1991) and Pashley and Karaman (2004). Some of these could and are indeed used as textbooks for colloid and surface chemistry courses. Each of these books has naturally their own strengths and weaknesses. We have reported our impressions and those of our students on the textbooks which we have used in the course over the years in a previous publication (Kontogeorgis and Vigild, 2009) where we also discuss other aspects of teaching colloids and interfaces.

What we can state, somewhat generally, is that unlike other disciplines of chemical engineering (which we know well, both of us being chemical engineers) we lack in colloid and surface chemistry what we could call "classical style" textbooks and with an applied flavour. In other fields of chemical engineering, e.g. unit operations, thermodynamics, reaction engineering and process control, there are textbooks with a clear structure and worked out examples along with theory and numerous exercises for class or homework practice. We could not generalize that "all is well done" in the textbooks for so many different disciplines, but we do see in many of the classical textbooks for other disciplines many common features that are useful to teachers and of course also to students. As the structure in several of these disciplines and their textbooks is also rather established, things appear to be presented in a more or less smooth and well-structured way.

We felt that many of these elements were clearly missing from existing textbooks in colloid and surface chemistry and this book makes an attempt to cover this gap. Whether we succeed we cannot say a priori but the positive comments and feedback from the students to whom parts of this material has been exposed in draft form over the years is a positive sign.

Thus, this book follows the course we have taught ourselves and we hope that the content and style may be appealing to others as well, both colleagues and students. We would like to present the main elements of this book in two respects (i) content and (ii) style.

First of all, the book is divided approximately equally into a surface and a colloid chemistry part although the division is approximate due to the extensive interconnection of the two areas. The first two chapters are introductory, illustrating some applications of colloids and interfaces and also the underlying - for both colloids and interfaces - role of intermolecular and interparticle/intersurface forces. The next two chapters present the concepts of surface and interfacial tension as well as the "fundamental" general laws of colloid and surface science; the Young equation for the contact angle, the Young-Laplace and Kelvin equations for pressure difference and vapor pressure over curved surfaces, the Harkins spreading...