Basic Theory of Interfacial Phenomena and Colloid Stability

Tharwat F. Tadros

• Publisher: De Gruyter
• Year: 2018
• ISBN:
  9783110587609
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

Volume 1 of Formulation Science and Technology is a survey of the theory of formulations in a variety of fields, as well as their rheological characterization. It offers in-depth explanations for research scientists, universities, and industry practitioners looking for a complete understanding of how different formulations behave and how to influence their performance.

1General introduction

1.1General classes of formulations

Several classes of formulations are encountered in the chemical industry of which disperse systems (liquid type formulations) are perhaps the most common, e.g. suspensions (both within the colloid range, i.e. 1 nm–1 μm and outside the colloid range, i.e. > 1 μm), emulsions (also within the colloid range and outside the colloid range), suspoemulsions (mixtures of suspensions and emulsions), multiple emulsions, microemulsions, latexes, pigment formulations, ceramics, etc. In additions, many chemicals are formulated as “solids”, e.g. grains, granules, tablets, etc. A third class of formulations are described as “semi-solids”, e.g. gels, ointments, etc.

For formulation of any chemical, it is necessary to understand the interfacial phenomena at the interface [1, 2]. This includes analysis of the process of charge separation at the interface and formation of electrical double layers. The latter determine the electrostatic repulsion between the particles or droplets in the formulation. This repulsion must overcome the permanent van der Waals attraction that is universal in all disperse systems. Another important fundamental investigation is to consider the adsorption of surfactants at the liquid/liquid and solid/liquid interface. Surfactants, both of the ionic and nonionic types, are frequently used in the formulation of most chemicals. The adsorption and conformation of surfactant molecules at the interface determine their ability to stabilize the dispersion. Another important fundamental investigation is to consider the adsorption and conformation of polymeric surfactants that are frequently used in many disperse systems. This determines the repulsive energy between particles or droplets that counteracts the van der Waals attraction. A very important subject that must be considered is the colloid stability of the final formulation. In most cases one starts with a colloidally stable dispersion which is then modified to reduce various phenomena such as creaming or sedimentation. One should also control the flocculation of the whole dispersion. In general, the formulation is modified to produce weak and reversible flocculation that is essential in many applications. Another instability problem that must be controlled is the process of Ostwald ripening that occurs as a result of the difference in solubility between small and large particles or droplets. On storage of the formulation, the smaller particles or droplets dissolve and become deposited on the larger ones. This results in a shift of the particle or droplet size distribution to larger values. This shift can cause enhanced creaming or sedimentation and/or enhance flocculation. A special instability problem that occurs with emulsions is coalescence that results from the thinning and disruption of the liquid film between the droplets.

Apart from the above colloid stability phenomena, a very important subject that must be considered is the bulk property of the dispersion. This determines the final state of the dispersion, such as its separation and production of strong gels. The bulk properties of any formulation can be investigated by measuring its flow characteristics or rheology. Such measurements can be carried out both at high and low deformation conditions. Such information can be used to analyse the stability/instability of the formulation as well as to predict of its long-term stability.

It is clear from the above introduction that for a comprehensive understanding of the various phenomena, one must consider the basic theory of interfacial phenomena and colloid stability as well as the basic principles of formulation types. These subjects are considered in Volumes 1 and 2. The industrial applications of the various formulations are described in Volumes 3 and 4.

In this introductory chapter, I will first consider the general classification of disperse systems and this is followed by a general description of the interfacial region that determines the physical stability/instability of the formulation. Two sections will be devoted to the formulation of solid dosage forms and semi-solid formulations.

1.2Classification of disperse systems

A general classification of disperse systems is given in Tab. 1.1.

It should be mentioned that colloidal particles possess characteristic properties between those of true solutions (with molecules that can diffuse through membranes) and suspensions that can sometimes be easily observed by the naked eye [1]. Clearly colloidal particles are unable to diffuse through membranes and when dispersed in a liquid medium they form a heterogeneous (two-phase) system that can scatter light [1].

Tab. 1.1: Classes of colloidal dispersions.

Solid	Liquid	Suspension
Liquid	Liquid	Emulsion
Gas	Liquid	Foam
Liquid	Gas	Aerosol
Liquid	Solid	Gel
Solid	Gas	Smoke
Solid	Solid	Composite

The best definition of colloids is perhaps “systems in which a significant proportion of the molecules lie in or are associated with interfacial regions”. Simple considerations suggest that the lowest limit for colloids (whereby one can distinguish between molecules in the interfacial region and the bulk) is 1 nm. The upper limit for colloidal dispersions lies in the region of 1,000 nm (1 μm), whereby a significant proportion of the total molecules lie at the interface. Unfortunately, the exact range of colloid size is difficult to ascertain in a precise manner. For that reason Ostwald [1] described colloids as “the world of neglected dimensions”.

To visualize colloidal particles, both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are effectively used with a resolution down to 5 nm for TEM. For metallic colloids (such as gold sols), a drop of the dispersion is deposited on a TEM grid, dried and observed directly in the microscope. The images, which are the two-dimensional representation, are captured on film or digitally and the size distribution is determined from these using an image analyser. For colloids such as organic polymers, proteins and bio-colloids that can be damaged by the electron beam, a carbon or gold replica is prepared that is floated off the sample and observed in the microscope instead of the colloid. Many other techniques can be used for measuring the size of colloidal particles, of which light scattering is perhaps the most convenient to use. Both static (elastic) and dynamic (quasi-elastic) light scattering methods can be applied and these will be described in detail in Chapter 9 of Vol. 2.

It should be mentioned that colloidal particles are not always spherical and many other shapes are encountered in practice, e.g. ellipsoids, rods, discs, etc. Measurement of particle size and shape distribution is essential since these determine many of the properties of the colloidal dispersion, such as its flow characteristics (rheology), solubility rate, stability, appearance of the dispersion, processing, etc. The particles are described as monodisperse if they are of the same size, otherwise they are polydisperse. It is therefore important to determine the particle size distribution and polydispersity index.

Several examples of dispersion classes can be quoted. One of the earliest colloidal dispersions of the solid/liquid type is colloidal gold which was used in the fourth and fifth century BC by ancient Egyptians and Chinese to make ruby glass and for colouring ceramics [1].Michel Faraday [1] prepared the first pure colloidal gold dispersion by reducing a solution of gold chloride with phosphorous. He recognized the colour of the dispersion was due to the small size of the gold particles. Nowadays such small size particles covering the size range 1–100nm are referred to as nanoparticles. The early work of Brown on the random drifting of dispersed particles induced by thermal energy (referred to as Brownian motion)was given a theoretical treatment by Einstein [1]. Brownian motion of colloidal particles and the resultant dynamics are unique and important characteristics of colloidal systems [1].

A good example of a naturally occurring colloid of the liquid/liquid type is milk, which consists of fat droplets dispersed in an aqueous medium also containing casein micelles that are also colloidal in nature. When milk is first obtained from cows, the fat droplets may exceed 1 μm and this results in creaming of these droplets. However, when milk is homogenized (using high pressure homogenizers) the fat droplets are subdivided into submicron droplets (nanodroplets) and this prevents the process of creaming.

An example of a gas/liquid system (foam) is the beer head. When beer is poured into a glass one observes a head of foam and the air bubbles are stabilized by protein present in the beer. Several examples of liquid/gas dispersions (aerosols) can be quoted, such as fog or mist. One of the main applications of aerosols is in pharmacy for oral and topical use. Pharmaceutical aerosols are dosage forms containing therapeutically active ingredients intended for topical administration, introduction into body cavities, or by inhalation via the respiratory tract. The aerosol product consists of two components, namely concentrate containing the active ingredient and propellant(s). The latter provides the internal pressure that forces the product out of the container when the valve is opened and delivers the product in the desired form.

Liquid/solid dispersions or gels are semi-solids consisting of a “three-dimensional” network in which the liquid is entrapped. The network can be either suspensions of small particles or large organic molecules (polymers) interpenetrated with liquid. In the first case, the inorganic particles, such as bentonite, form a three-dimensional “house of cards” structure through the gel. This is a true two-phase system. With polymers, either natural or synthetic, the molecules tend to entangle with each other due to their random motion. These systems are actually single-phase in the macro-sense; the organic molecules are dissolved in the continuous phase. However, the unique behaviour of polymers, leading to high viscosities and gel formation,makes it possible to consider the gel as a two-phase system on the microlevel; the colloidal polymer molecule and the solvent. Gels find use as delivery systems for oral administration, for topical drugs applied directly to the skin or eye as well as for long acting forms of drugs.

Examples of solid/gas dispersions are smoke and dust as well as particles produced in coal fires and diesel engines. Several examples of solid/solid dispersions can be quoted such as painted glass, pigmented plastics as well as dispersions of silica in plastic to enhance the mechanical properties of the system.

1.3The interfacial region

In all disperse systems such as suspensions, emulsions, foams, etc., the structure of the interfacial region determines their colloidal properties [1, 2]. The larger the interfacial area, i.e. the larger the surface-to-volume ratio of the particle or droplet, the more important the role of the structure of the interfacial region. For convenience, I will list the topics of colloid and interface science under two main headings, namely disperse systems and interfacial phenomena. This subdivision does not imply any separation for the following reasons. All disperse systems involve an interface. Many interfacial phenomena are precursors for formation of disperse systems, e.g. nucleation and growth, emulsification, etc. The main objectives of the present book are to cover the following topics: The basic principles that are involved in formulation of chemicals and their stabilization.

Several interfacial phenomena may be considered when dealing with dispersions as summarized in Section 1.1 [1, 2]:

(i)Charge separation and formation of electrical double layers.

(ii)Wetting of powders and the role of surfactants.

(iii)Adsorption of surfactants and polymers at the solid/liquid and liquid/liquid interfaces and the role of the structure of the interfacial region.

The physical stability/instability of any disperse system is determined by the properties of the interfacial region. In actual fact, colloid and interface science are one individual subject. This is particularly the case with charged interfaces that form electrical double layers and those interfaces that contain adsorbed surfactants and/or polymers. With systems containing electrical double layers, repulsion between the particles or droplets takes place as a result of the overlap of double layers [1, 2]. This is particularly the case at low electrolyte concentrations and low valency...