2: Colloidal Particles and Suspensions

an entity large in comparison with atomic dimensions so as to have a supramolecular structure and properties, but small enough to remain in suspension for a finite amount of time (at least of the order of hours) when dispersed in a liquid medium such as water

approximately 1 nm to 10 μm

clay fraction and fine silt fraction

fraction of soil with particle size less than 2 μm

fraction of soil with particle size 2–10 μm

clay minerals, oxides, organic matter (humic substances)

the surface of soil colloids

a dispersion of colloidal particles in the bulk of another phase, usually in a different aggregation state, most commonly solid particles in a liquid medium

macromolecules may be soluble in the liquid medium (e.g., proteins in water); oxide particles will not be soluble

solution has no phase boundary; a true suspension has phase boundary

a solution can be thermodynamically stable; a suspension can be kinetically stable

aggregate, condense, aggregate, condense

the process in which individual particles bind into clusters by intermolecular forces

the process in which aggregated clusters sediment into larger particles, losing their former identity

stability due to relatively slow motion of particles, in many cases due to energetic barriers (usually electrostatic); may last for many years

kinetic stability may be broken by addition of an indifferent electrolyte; thermodynamic stability may be broken by modifying thermodynamic variables, such as temperature

lyophobic, lyophilic

a solid-liquid system in which particles have low affinity for the dispersing medium

a solid-liquid system in which colloidal particles have great solvent affinity for the dispersing medium

solid-liquid interaction classifications in which the dispersing medium is water

a hydrophobic suspension

AgI, Au, clays, crystalline oxides (e.g., hematite, gibbsite)

nanoparticles can be invisible to the naked eye but revealed by Tyndall effect (dispersion of light); larger particles, e.g., those found in soil, usually give a milky appearance

hydrous (oxo) hydroxides, e.g., Fe(OH)3 and Al(OH)3

hydrophobic colloids aggregate but in aggregation, the individual particles are preserved and can be redispersed (peptized) by replacing the electrolytic solution by water or a more diluted solution; hydrophilic colloids in many cases do not show coagulation and may be at borderline between suspensions and true solutions

individual particles can be observed subject to thermal (Brownian) motion, but upon coagulation they cluster to form larger aggregates that precipitate relatively quickly

area per unit mass (m^2/kg)

particle size, surface roughness

actual particles, especially those found in natural environments, do not have perfectly flat surfaces but display defects and irregularities that increase the total area exposed to the surrounding phase

the reversible work per unit area (J/m^2) to increase the surface (e.g., change the shape of the phase)

when a molecule, immersed in the bulk of a condensed phase, is brought to the surface in the presence of a dilute gas or vacuum, attractive forces in the direction normal to the surface are missing, so that a net work should be done (the surface Gibbs free energy)

(N/m) the interpretation of surface energy as the force that should be applied to "stretch" the surface linearly a unit length in any direction

the Gibbs free energy needed to expand the surface per unit area when the condensed phase is in contact with another solid or liquid phase

$δW = γ_{AB} dA$

a molecule composed by a nonpolar part (usually a long aliphatic chain) and a polar part (either an ionizable group, such as sulfate or phosphate, or a highly hydrophilic group, such as a polyethylene oxide chain)

concentrations, chemical potentials of the different species, electrostatic potential, entropy

they should not change abruptly at the interface because they, and in most cases their derivatives, should be defined everywhere

the region about the interface across which physicochemical properties change

a plane placed at an arbitrary location in the interfacial region at which accumulation or depletion of chemical species (as well as extensive properties such as entropy) can be measured relative to the bulk of each phase

$Γ_i=\frac{n_{i+}-n_{i-}}{A}$, (mol/area) "the amount of solute adsorbed at the surface"

surface excess defined in with respect to a datum: when one of the phases is a solution, especially an aqueous solution, usually the solvent is set to have zero surface excess by special placement of the Gibbs plane

$dγ = -\sum_i Γ_i dμ_i$

since surface tension cannot be measured when dealing with solids, the Gibbs equation is more useful when dealing with interfaces not involving solid phases

$\mu_i=\mu^0_i+RT \ln{a_i}$
where $\mu^0_i$ is the standard chemical potential, $a_i$ is the solute activity

they show tendency to accumulate at the surface due to polarity reasons, giving a positive surface excess

due to polarity effects, they dislike the surface and thus show negative surface excess (surface depletion or negative adsorption)

the surface is saturated with this solute even at very low concentrations; after a particular temperature-dependent concentration value, there is almost no surface excess of this solute

• a structure, or cluster, in which amphipathic molecules group together with the nonpolar tails directed toward the interior and the polar heads in the exterior part in contact with water
• usually of colloidal size and display most colloidal properties

the concentration threshold at which the surface excess vanishes, indicating micelle formation

the particular substance, temperature, ion strength

micelles can dissolve nonpolar compounds in the inner part and also can interact with ions and polar molecules at the surface

amphipathic compounds and, eventually, micelles are usually not found in natural soils, but can be present as a consequence of human activity, such as detergent spillage