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Study Guide - Practical 3
Colloidal Systems and Surface Tension
Introduction
Colloidal Systems
The term "colloidal systems" is used in systems that contain either large molecules or very small particles. With respect to particle size, they range between 1 and 1000 [nm].
Simple colloidal dispersions are two-phase systems consisting of a disperse phase (for example, a powder) finely distributed in a dispersion medium. The colloidal dispersions that are most relevant for the aim of this course are solid/liquid dispersions and emulsions. The complete classification of dispersions was created by W. Ostwald over 100 years ago, but it is still valid today and is reported in Figure 1.
Figure 1: Classification of Colloidal Systems
All three dimensions of a colloid don’t need to be smaller than 1 [μm]. Colloidal behavior can also be observed for fibers, where only two out of three dimensions fall within the colloidal range, and in films, where only one dimension does.
It is important to introduce some key concepts in colloidal chemistry. Some of these terms are reported below.
- Monodisperse systems: Systems in which all particles are approximately the same size.
- Polydisperse systems: Systems containing particles of varying sizes.
- Lyophobic or hydrophobic colloids: The particles are incompatible with the dispersion medium, which is organic in the case of lyophobic colloids, aqueous in the case of hy- drophobic. The thermodynamic instability of these systems is visible in their tendency to clump, aggregate, agglomerate, and flocculate.
- Lyophilic or hydrophilic colloids: The particles are compatible with the medium as they interact with it. Unlike lyo-/hydrophobic colloids, they form spontaneously and are ther- modynamically stable.
- Amphiphilic colloids: The molecules have affinity for both polar and nonpolar solvents. These form the large class of surface-active and related substances that include micelles. They are thermodynamically stable.
Kinetic Stability
Dispersions are heterogeneous systems in which fine particles (solid, liquid, or gas) are dis- tributed within a continuous phase. These systems are inherently thermodynamically unstable because the dispersed phase tends to minimize its surface area to reduce interfacial energy, leading to aggregation or phase separation over time. However, many dispersions remain ap- parently stable for prolonged periods due to kinetic barriers that prevent or significantly slow down these processes. This phenomenon is referred to as kinetic stability.
A dispersion is said to be kinetically stable when the particles do not aggregate or sediment rapidly, even though the system is not at its lowest free energy state. The stability arises from repulsive interactions (such as electrostatic or steric repulsion) that introduce an energy barrier between particles. When this barrier is sufficiently high compared to the average thermal energy of the system (typically on the order of kT , where k is the Boltzmann constant and T the temperature), particle aggregation becomes an unlikely event. Thus, the system can remain dispersed for long periods despite the thermodynamic drive toward separation.
Brownian motion plays a central role in the kinetic behavior of dispersions. It refers to the random, thermally driven motion of colloidal particles in a fluid, which results from collisions with solvent molecules. This motion keeps the particles in continuous movement, increasing the probability of inter-particle encounters. The collision frequency between particles depends on their concentration, size, and diffusivity, the latter being inversely related to particle size and fluid viscosity.
However, not all collisions lead to aggregation. Whether two particles stick together after a collision depends on whether they can overcome the energy barrier created by repulsive forces. In electrostatically stabilized systems, this barrier originates from the overlap of electrical double layers surrounding charged particles. If the barrier is high enough, most collisions are elastic (particles bounce off each other), preserving the dispersed state. Only particles with sufficient kinetic energy can overcome the barrier and aggregate, which is statistically rare at moderate temperatures.
In summary, while dispersions are not thermodynamically stable, they can exhibit remarkable kinetic stability due to the presence of inter-particle repulsive forces that create energy barriers against aggregation. The interplay between Brownian motion, collision frequency, and the
Figure 2: Experimental setup for the determination of the surface tension of a soap film
The surface tension is then expressed as the derivative of the work performed to disrupt the film with respect to the area of the soap film:
γ =
dW dA
K ds 2 L ds
K
2 L
The factor of 2 in the denominator arises because the soap film possesses two liquid–air inter- faces, and surface tension acts on both.
This expression establishes the relationship between surface tension and the work required to increase the surface area of the film:
dW = K · ds = γ · 2 L · ds = γ · dA −→ W = γ · ∆A
