Introduction to Colloid Science

Contents

“Who do not honour the small things is not worth the big ones” [Dutch saying]

F oreword T his book is meant as an introduction to the field of colloid science, i.e. the study of the behaviour of micrometric particles in a fluid (or a gas). The book was written with a special emphasis on sediment particles. Sediment particles are complex colloidal particles due to their composition, shape and interaction with their environment. Characterization of the colloidal fraction of sediment is done by recording , among others, the particles’ size, shape and electric surface charge and evaluating their interactions. These properties are important for civil engineering applications, even though the size range of these particles and their interactions is microscopic. Large - scale sediment transport models for example require as input the settling velocity of individual particles. In concentrated areas, this velocity becomes a function of the particles’ concentration and particle-particle interactions lead to the creation of larger particles, called flocs . These flocs can settle and, when reaching the bed, consolidate in time. All these aspects, and related models, are treated in the present book.

The book was written for students having no special beforehand knowledge in colloid science, and a limited kn owledge in physics and chemistry, the two main disciplines relevant to this branch of science . The mathematics are also kept minimal. They are given when the derivations present no particular problem . The more lengthy derivations, often very elegant (but sometimes rather tedious) can be found in the references given as footnotes in the different chapters. Key words are highlighted in the text, which will enable the reader to find further information by searching for them in a web browser.

Chapter 1 Introduction, general definitions and some properties of clayey material

Introduction Mud and clayey systems have from origin been studied in the field of soil science 1, i.e. the branch of science concerned with the formation, classification and fertility properties of soils. In soil science, the classification of soils is of prime importance, as the properties of a soil are used by farmers to decide which types of crops , livestock and soil management are best for their piece of land. The information obtained by a soil survey can also be used by an architect or a builder to determine whether a given soil is suitable for a specific type of construction. In agriculture, soils are primary viewed for their chemical properties, i.e. their ability to bring nutrients to plants. At the beginning of the 19 th century chemical studies were extensively conducted which confirmed for instance the role of phosphate, nitrogen and potassium in plant growth, and commercial fertilizers were developed. The composition of a soil is also determinant for the type of plant s that are growing on it. The first report that clearly related the soil properties to the different climates, vegetation and parent rocks from which the soils were formed was written by Dokuchaiev in 1883 2. Many of the chemical changes occurring in a soil are related to the action of bacteria. At the end of 19th century it was for instance discovered that the bacteria which live in nodules on the roots of legume plants absorb nitrogen from the air and convert it to a form which plants can utilize. The diff erences in soil characteristics are connected not only to the (bio)chemistry but also to the mineralogical and physical properties of soils. These properties are important for construction, but also to the farmer as the porosity of a soil governs gas exchange with the atmosphere, water penetration and root growth. The porosity of a soil is strongly connected to the particle size distribution of the grains forming the soil and the soil’s state of consolidation. The mineralogical composition of a clay, which can be determined from X -ray crystallography and electron microscopy tests, will tell if a clay soil is prone to weathering, swelling and retain or exchange cations 3. This last property is pH -dependent. This shows that both chemical and physical properties 1 We will focus on natural soils. Another important research concerns the use of clays in ceramics, cosmetics and drugs which will not be discussed here. 2 Dokuchaiev, V.V. (1883) Russian chernozem, Monograph, Sankt -Peterburg. See also Greenland, Dennis James, and Michael Hilary Bermingham Hayes. The chemistry of soil constituents. Wiley and sons (1997). 3 a cation is a positively charged ion; an anion is a negatively charged ion. 1350 BC, Egypt : thanks to the fertile soil of the river Nile, Egypt became a powerful and long-lasting civilisation

Chapter 1 Introduction, general definitions are of importance to understand the diffusion and retentions of ions within the soil fabric. Similarly, the consolidation, permeability, strength and aging of soils are related to chemo -physical processes: by oxidation (chemical process) the permeability of a soil (physical property) can for instance been altered. In the context of sediment dynamics, i.e. the study of sediment transport, deposition and erosion, the first studies have been initiated starting from the work do ne on sand. We emphasize that we are here dealing with water-saturated bodies and not Aeolian transport. The studies we refer to are therefore relevant for fresh and salt water systems such as rivers, lakes, seas and estuaries. Aquatic sand transport, deposition and erosion has been monitored and modelled successfully in the past decades. The models have been tried on clays and clayey systems, but it was soon recognized that these systems were much more complex and the models and theories should be adapted4.

In this first chapter ( Chapter 1), clayey soils and suspensions are introduced from their classical definitions, derived primarily from soil science, in terms of particle size, clay mineralogy and physical properties (fluid, solid). We will start by defining colloidal sized particles. Colloids (= colloidal suspensions) will be the main topic of interest in the following chapters.

There are several differences between clay and sand particles. One of them is their size, which affects their diffusion behaviour in the water body, as detailed in Chapter 2. A major difference between sand and (colloidal) clay particles is that clay particles have the ability to aggregate. the reason behind this property is discussed in Chapter 3. The clay particles can aggregate between themselves, adsorb (poly)ions, stick to or interact with other particles (some of them are reviewed in Chapter 4). In Chapter 5, the different types of aggregation (also called flocculation) and the different types of aggregates (flocs) produced are reviewed. The rate of flocc ulation is modelled in Chapter 6.

From the concepts introduced in Chapters 2 – 6, it will become clear that the main reason sand models are not adapted to clayey systems is that because of their size, clay particles are extremely dependent on surface force s (interaction with other particles in suspension), whereas the dynamics of sand particles are mainly controlled by a volume force (gravity force). Sand particles can be assumed to have a constant Stokes’ settling velocity 5, which can be calculated from their density and volume whereas clayey particles have a time- dependent density and volume. Sediment transport models have as input the concentration of particles of a certain size and their Stokes’ settling velocity, it is therefore clear that some adjustment is 4 Leussen, W. van, Estuarine macroflocs and their role in fine-grained sediment transport, PhD thesis, RIKZ (1994) 5 Stokes’ settling velocity is defined in Chapter 2 required to the model s as aggregation implies that particles change both their size and their settling velocity.

The density and composition of the clay suspensions influence their rheology (Chapter 7) and the settling of their constitutive particles (Chapter 8). Once settled, mud suspensions will form a soil with a time -dependent permeability ( Chapter 9). Given a specific relation between permeability and density, the early stage of consolidation can be studied ( Chapter 10 ). This so -called early sta ge of consolidation, when mud is freshly deposited at the bottom of a water body, is important to study in connection to sediment dynamics: the strength of a freshly consolidated mud layer is a key parameter for sediment transport models as it will determine the amount of mud eroded from the bottom at a given water velocity. The link between Chapters 8 and 10 is discussed. In the last part of Chapter 10, we will briefly review the later stage of consolidation, when forces at the contact points between particles become predominant. This stage is extensively studied in soil science and the reader is referred to books in that field of science for further reading. Solutions, colloids and suspensions For the remainder of the book, we will be concerned with what is technically defined as a two- phase system: water and particles. Already we have to be more precise: water itself does contain particles. First of all water molecules of course, but also dissolved ions, which (see Chapter 3), can play a key role in the aggregation of clay particles. Even fresh water contains ions, for example HCO3− and Ca2+ ions which are the predominant ions found in river waters, and originate from the weathering of limestone or feldspar (carbonate rocks). These type of rocks are typically found in Europe, North America and Asia. The weathering of silicate rocks will produce different types of ions. In chemistry, one does therefore speak of a solution (water + ions) and not of water (as is done in civil engineering). In a large branch of colloid science, solutions are seen as a continuum. This means that the solution is a fluid with given properties and that no distinction is made for the individual ionic particles or water molecules in the water. This fluid has bulk properties such as density and ionic concentration. In this book, water will be seen as a continuum. The particles we refer to are predominantly clay particles, but, as we will see in Chapters 4 and 5 mud is composed of numerous other particles. These particles can either be seen as individual particles which interact with each other and the gravi ty field or as a continuum with a given (time -dependent) density. This continuum approach will in particular be used in Chapters 9 and 10 but it will become clear that all models presented in this book depend in a way or another on the properties of the microscopic constitutive particles.

Chapter 1 Introduction, general definitions Mainly depending on the size of the particles in suspension, three different types of fluids can be distinguished: solutions, colloids and suspensions. They are represented underneath: A solution is a homogeneous mixture that appears clear, such as salt water in a glass. In a colloid, such as milk, the particles are much larger but remain dispersed and do not settle. A suspension, such as mud, is a heterogeneous mixture of suspended particles that can either remain in suspension (for the colloidal fraction) or can settle (for the coarser fraction). A solution is a homogeneous mixture resulting from the dissolution of a solute (ex: salt) in a solvent (ex: water). In the case of salt in water, one speaks of electrolyte solutions (or electrolytes), as dissolved salts are charged and electrically conducting. It can happen that a solution is saturated in one of its solutes, in which case it cannot dissociate this solute anymore. This happens for example when too much salt is added in a glass of water: some salt crystals will remain at the bottom of the glass. Note that due to gravity, when the height of the water column is important, like in the sea, some stratification can be observed in the solution, and its density may vary as function of height.

A colloid (or colloidal suspension) is composed of particles or droplets (in which case one speaks of an emulsion – milk is for instance an emulsion) that are much larger than ions or molecules, but that are small enough not to settle out. These colloidal particles are dispersed in a solvent . We will only consider water or an electrolyte solution as solvent in the remaining of the book, as these are the most common solvents for sediment particles. If the colloidal particles or droplets are dispersed in a gas one refers to an aerosol (smoke and mist are aerosols).

To be classified as colloidal, a material must have one or more of its dimensions (length, width, or thickness) in the approximate range of 1-1000 nm. The particles in a colloid are large enough to scatter light, a phenomenon called the Tyndall effect. This can make colloidal suspensions appear cloudy or opaque. The Tyndall effect is an easy way of determining whether a fluid contains a significant amount of colloidal particles. When light is shined through a true solution, the light passes cleanly through the solution (ex: an electrolyte solution), however when light is passed through a colloidal solution, the substance in the dispersed phases scatters the light in all directions, making it readily seen (ex: milk).

John Tyndall (1820 – 1893) was a prominent 19th - century physicist. His initial scientific fame arose in the 1850s from his study of diamagnetism. Later he made discoveries in the realms of infrared radiation and the physical properties of air. Tyndall also published m ore than a dozen science books which brought state -of-the-art 19th century experimental physics to a wide audience. From 1853 to 1887 he was professor of physics at the Royal Institution of Great Britain in London. extract from the book The Glaciers of the Alps, by John Tyndall6 Expedition of 1857: the Lake of Geneva. Blueness of the water “On Thursday, the 9th of July, 1857, I found myself upon the Lake of Geneva, proceeding towards Vevey. I had long wished to see the waters of this renowned inland sea, the colour of which is perhaps more interesting to the man of science than to the poets who have sung about it. Long ago its depth of blue excited attention, but no systematic examination of the subject has, so far as I know, been attempted. It may be that the lake simply exhibits the colour of pure water. Ice is blue, and it is reasonable to suppose that the liquid obtained from the fusion of ice is of the same colour; but still the question presses —"Is the blue of the Lake of Geneva to be entirely accounted for in this way?" The attempts which have been made to explain it otherwise show that at least a doubt exists as to the sufficiency of the above explanation.

It is only in its deeper portions that the colour of the lake is properly seen. Where the bottom comes into view the pure effect of the water is disturbed; but where the water is deep the colour is deep: between Rolle and Nyon for example, the blue is superb. Where the blue was deepest, however, it gave me the impression of turbidity rather than of deep transparency. At the upper portion of the lake the 6 http://www.gutenberg.org/files/34192/34192-h/34192-h.htm#Page_33

Chapter 1 Introduction, general definitions water through which the steamer passed was of a blue green. Wishing to see the place where the Rhone enters the lake, I walked on the morning of the 10th from Villeneuve to Novelle, and thence throug h the woods to the river side. Proceeding along an embankment, raised to defend the adjacent land from the incursions of the river, an hour brought me to the place where it empties itself into the lake. The contrast between the two waters was very great: the river was almost white with the finely divided matter which it held in suspension; while the lake at some distance was of a deep ultramarine.

The lake in fact forms a reservoir where the particles held in suspension by the river have time to subside, and its waters to become pure. The subsidence of course takes place most copiously at the head of the lake; and here the deposit continues to form new land, adding year by year to the thousands of acres which it has already left behind it, and invading more and more the space occupied by the water. Innumerable plates of mica spangled the fine sand which the river brought down, and these, mixing with the water, and flashing like minute mirrors as the sun's rays fell upon them, gave the otherwise muddy stream a silvery appearance.” The Tyndall effect can be linked to theories developed by Rayleigh and Mie, in the frame of optics: when the particle is smaller than the wavelength, Rayleigh theory is used, which predicts that for this type of particles, short -wavelength light is scattered more than longer wavelengths. The blue colour (short wavelength) is therefore more scattered than the red (long wavelength): this is why the sky appears blue to us, as the gas molecules of the atmosphere are much more effective in scattering the blue wavelength. Some suspensions, containing nanoparticles, similarly have a blueish tinge. Clouds, on the other hand, are formed by water droplets that are much larger than gas molecules and scatter in the same way all the parts of the light spectrum: this is why clouds appear white (like milk does). A solution is clearly scattering much less light than a colloidal suspension, and this is why the way a fluid is scattering light can be used to determine if this fluid contains a substantial amount of colloidal-sized particles.

Note that the fact that a true solution might be coloured is not due to the scattering of light, but on the contrary to the adsorption of light by the atoms and molecules that compose this solution. What we see is not the colour absorbed, but its complementary colour, originating from the removal of the absorbed wavelengths. For example, beta-carotene, an organic pigment abundant in plants and fruits, has a maximum absorption at 454 nm (blue light) and consequently wha t we see appears orange (the complementary colour for that type of blue). (Beta- carotene takes its name from the carrot (daucus carota) from which it can be extracted.) A suspension (not necessarily colloidal) is a fluid composed of a solvent and particles in suspension. At low or at no shear rate, these fluids are usually containing colloidal particles to be classified as “suspensions”. If the particles constituting the suspension would be all much larger than a few microns the time the particles would remain in suspension would be very limited at low shear: sand particles for example, settle out very rapidly and large gas bubbles usually move rapidly towards the water/air interface. In natural systems, however, it is possible to keep fine particles in suspension by turbulent mixing. This mixing insures that sand can be transported over some considerable distance during storms for example. Typical orders of magnitude for ions, colloidal and sand particles: Ions colloidal particles sand typical particle size 10-10 m 10-6 m 10-4 m number of particles in a spoon (15 mL) 1025 1013 107 equivalent in number to number of sand particles in the Sahara 10000 times the population of Earth number of inhabitants of Paris T he unit to measure the number of ions in a litre is mol/L.

A mole (symbol: mol) corresponds to 6.02x1023 particles, which is the number of 12C particles contained in 12 g. One speaks of Avogadro’s number (symbol: N A) to express the fact that there are 6.02x10 23 particles in one mole (N A = 6.02x10 23 mol-1). A solution of 1 mol/L therefore contains 6.02x1023 particles per litre. The molar mass of a product enables to determine how many grams of product is contained in one m ol. The molar mass of NaCl salt (kitchen salt) is 58 g/mol. There are therefore 6.02x10 23 salt particles in 58 g of NaCl, which will give, when dissociated in water, 6.02x10 23 Na+ particles and 6.02x1023 Cl- particles. Comparatively, 58 g of clay particles which have a density of 2.6 kg/L (2600 kg/m 3) and a size of 1 micron will give approximatively 10 13 clay particles, i.e. 1010 times less particles, or, in other words, if 58 g of clay is mixed with 58 g of salt in one litre of water, for each clay particl e, there will be 10 10 ionic particles. The typical concentration of salt ions in natural systems ranges from less than mM (millimol/L or 10 -3 mol/L) for fresh water system to hundreds of mM for sea water. Clay concentrations are usual of the order of a few mg/L at sea to hundreds of mg/L and beyond in estuaries.

The difference between clay, silt and sand particle sizes Sand is a naturally occurring granular material composed of finely divided rock and mineral particles. It is defined by size, being finer th an gravel and coarser than silt. Here we differentiate sand, silt and clay based on size. Depending on the context, silt can refer to a type of mud or particles with a specific grain size. A similar confusing double definition exists for clays, see the table below. Particles that have a “clay” size may not necessarily be clay minerals (organic matter for instance can have a clay size but is not composed of clay minerals).

Chapter 1 Introduction, general definitions Sand feels gritty when rubbed between the fingers. Silt, by comparison, feels like flour. Even though the exact definitions, in terms of size, of sand, silt and clay are depending on countries, one assume in general that they can be defined as follows: T raditionally, sieves could not be made for particle sizes smaller than 63 µm. This is why the lower limit of 63 µm was adopted for sand. The size of silt particles is measured traditionally by hydrometer test. Nowadays, Static Light Scattering (laser diffraction) devices enable to measure the whole range of particle sizes (from 1 nm to 1 mm) with a single device, see Chapter 2.

The most common constituent of sand, in inland continental settings and nontropical coastal settings, is silica (silicon dioxide, or SiO 2), usually in the form of quartz, which, because of its chemical inertness a nd considerable hardness, is the most common mineral resistant to weathering. In contact with water or electrolyte, the presence of hydroxyl (silanol) groups have however been observed on the surface of silicas and silicates, leading to the creation of a surface charge. A n easy way to find out roughly the composition of a sediment is to use a jar test: Fill the jar halfway with sediment. Add water to fill up the jar, close the lid, shake, and set on the table. The sand will settle out quickly, in a matter of hours (like “Sand” in the example underneath). The middle jar (“Loam”) represents the mixture of clay and silt, which takes up to 24 hours to settle out. The third “Clay” jar represents how long it takes clay content to settle out, in this example up to three days. Loam is soil composed mostly of sand (particle size > 63 µm), silt (particle size > 2 µm), and a smaller amount of clay (particle size < 2 µm). Loam soils generally contain more nutrients, moisture, and humus than sandy soils, have better drainage and infiltration of water and air than silty soils, and are easier to till than clay soils. Loam, (combined or not with straw), has been used in construction since ancient times. The definition of mud, silt and clay Mud is composed, in part, of clay mineral and water. For the other parts, most types of mud are include ions and organic material. Particles of different sizes, such as clay, silt and sand particles can also be incorporated in mud. The most common definitions of mud, silt and clay in English, French and Dutch are given in the table below7.

Clay particles are colloidal particles when defined by their size (< 2µm). The lower size range of silt particles can also be seen as colloidal, but in most cases silt particles (when composed of mineral clay) are non -cohesive, which implies that their dynamics in the water column are dominated by the gravity force and hydrodynamics similarly to sand particles.

Clay particles can be found in all water bodies (sea, lake, river), but their relative ratio compared to silt and sand varies of course from site to site. A coastal wetland where clays and silts can be found are mudflats. Mudflats are formed when mud is deposited by tides (tidal flat) or rivers. Mudflats (French: vasière; Dutch: wad) can be seen as exposed layers of (wet) mud. A tidal flat is submerged approximately twice daily enabling estuarine silts, clays and organic matter to be deposited. On the Baltic Sea coast of Germany in places, mudflats are exposed not by tidal action, but by wind- action driving water away from the shallows into the sea. These wind - affected mudflats are called windwatts in German. Mudflat hiking (Dutch: Wadlopen; German : Wattwandern ; Danish: Vadehavsvandring ) is a recreation enjoyed by Dutch, Germans and Danes in the Netherlands, northwest Germany and in Denmark. Mudflat hikers are people who, with the aid of a tide table, use a period of low water to walk and wade on th e watershed of the mudflats. The study of the 7 See in particular for the French definitions: “ La vase” by J. Bourcart and C. Francis -Boeuf (1942)

Chapter 1 Introduction, general definitions morphology of mudflats is an on -going topic of research (see middle figure below where students fix instruments to a rig).

English French Dutch mud liquid or semi-liquid mixture of principally water and clay minerals boue from old Celtic word “baw” (cf gall. baw, “dirt” ): creamy substance that is found, after rainfalls, on roads . modder Mud and modder are etymologically related to Middle Low German and Dutch mudde and modde denoting something wet or dirty vase from old Saxon word “wase”: wet mud that is to be found under water (in lakes, rivers or seas) or deposited on mudflats slib probably derived from old Dutch slik/slijk: deposited wet mud (on mudflats) silt a - mud carried by running water and deposited as a sediment, especially in a channel or harbour. b - particles with a size > 2 µm and < 63 µm limon a - from latin “limo - limus” (“mud”): mud that accumulates due to water flows on river sides. b - particles with a size > 2 µm and < 63 µm silt a - from Middle Dutch silte, sulte “salt marsh brine”.

Contrary to slib, silt is found in open air and can therefore oxidize. b - particles with a size > 2 µm and < 63 µm clay8 a - mud that can be moulded when wet, used in bricks, pottery, and ceramics b - particles with a size < 2 µm c - clay (minerals) are hydrous aluminium phyllosilicates argile a - from latin “argila”: mud having plastic properties, used in bricks, pottery, and ceramics b - particles with a size < 2µm c – argiles (minéraux argileux) are hydrous aluminium phyllosilicates klei a - potklei: clay used in pottery b – klei is composed in majority of lutum (from latin “lut(um)”: clay) i.e. clay particles with a size < 2µm c – klei(mineralen) are hydrous aluminium phyllosilicates 8 Geologists and soil scientists call “clays” particles less than 2 μm. The Dutch klei (clay) is composed in majority of lutum particles, which are by definition particles smaller than 2 μm. Sedimentologists often use the limit 4–5 μm, and colloid chemists use 1 μm.

T idal flat W adden Sea in the Netherlands9 9 http://www.werelderfgoed.nl/werelderfgoed/waddenzee

Chapter 1 Introduction, general definitions Clay minerals Clay minerals are common weathering products and low-temperature hydrothermal alteration products. Clay minerals are very common in soils, in fine- grained sedimentary rocks such as shale, mudstone, and siltstone and in fine -grained metamorphic slate and phyllite. Depending on the soil's content in which it is found, clay can appear in various colours from white to dull grey or brown to deep orange - red.

Clay minerals are hydrous aluminium phyllosilicates. Phyllosilicates are minerals formed by parallel sheets of silicate tetrahedra, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Depending on the academic source, there are three or four main groups of clays: kaolinite, montmorillonite-smectite, illite, and chlorite. Chlorites are not always considered a clay, sometimes being classified as a separate group within the phyllosilicates. There are appro ximately 30 different types of "pure" clays in these categories, but most "natural" clay deposits are mixtures of these different types, along with other weathered minerals. Single mineral clay particles are in the clay (colloidal) size range. The identification of the type and structure of clay minerals requires special analytical techniques as x-ray and electron diffraction methods (XRD, XRF and SEM in short). Clay mineralogy The crystalline structure of clay minerals is built up from different types of sheets or layers. The fundamental building blocks of these sheets are the tetrahedron and the octahedron units. The tetrahedron is composed of either a central silicon or aluminium surrounded by four oxygen ions in a tetrahedral coordination . The octahedron is com posed usually of a central poly valent cation surrounded by six oxygen (O) or hydroxyl (OH) ions in an octahedral coordination. Whether a cation forms tetrahedral or octahedral coordination with oxygen depends on the relative size of the cations and anions involved.

Tetrahedral layer: All the tetrahedrons in a tetrahedral layer are in the same plane and therefore their tips point in the same direction . The unit cell fo rmula of the Si tetrahedron is ( Si4O10)4-. Electrical neutrality is obtained by replacement of four oxygens by hydroxyls (-OH) or by union with a sheet of different composition that is positively charged.

Octahedral layer: This sheet structure is composed of magnesium or aluminium in octahedral coordination with oxygens or hydroxyls. If the cation is trivalent, then normally only two-thirds of the possible cationic spaces are filled and the structure is termed dioctahedral. In the case of aluminium the comp osition is Al(OH) 3. This composition and structure form the mineral gibbsite. If the octaedrally coordinated cation is divalent then normally all possible cation sites are occupied and the structure is trioctahedral. In the case of magnesium the composition is Mg(OH) 2, which forms the mineral brucite.