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Often, colloidal particles are suspended in water. In this case, they accumulate a surface charge and an electrical double layer forms around the particles. The overlap between the double layers of two approaching particles results in a repulsive potential, which leads to particle stabilization. When salt is being added to the suspension, the electrical double layer repulsion is screened and van der Waals attraction induces fast aggregation. The figure on the right shows the typical dependence of the stability ratio W versus the electrolyte concentration, whereby the regimes of slow and fast aggregation are indicated. The table on the left summarizes concentration ranges of CCC for difference net charge of the counter ion expressed in units of elementary charge. This dependence reflects the Schulze-Hardy rule, which states that the CCC varies as the inverse sixth power of the counter ion charge. The CCC also depends somewhat on the type of ion even if they carry the same charge. This dependence may reflect different particle properties or different ion affinities to the particle surface. Since particles are frequently negatively charged, multivalent metal cations thus represent highly effective flocculants.

Adsorption of an oppositely charged species (e.g., protons, specifically adsorbing ions, surfactants, or polyelectrolytes) may destabilize a particle suspension by charge neutralization or stabilize it by buildup of charge, leading to a fast aggregation near the charge neutralization point, and slow aggregation away from it.

Quantitative interpretation of colloidal stability was first formulated within the DLVO theory. This theory confirms the existence slow and fast aggregation regimes, even though in the slow regime the dependence on the salt concentration is predicted to be much stronger than observed experimentally. The Schulze-Hardy rule can be derived from DLVO theory as well.

Experimental Techniques
Numerous experimental techniques have been developed to study particle aggregation. The most frequently used are time-resolved optical techniques, based on transmittance or scattering of light.

Light Transmission. The variation of transmitted light of an aggregating suspension can be studied with a regular spectrophotometer] in the visible region. As aggregation proceeds, the medium becomes more turbid, and its [[Absorbance|absorbance increases. The increase of the absorbance can be related to the aggregation rate constant k and the stability ratio can be estimated from such measurements. The advantage of this techniques is its simplicity, but its disadvantage is that it can be only reliably used for larger particles or that detailed corrections from larger clusters must be considered. Smaller particles aggregate rapidly, and in such systems it is difficult to extract the stability ratio from the transmittance quantitatively.

Light scattering. These techniques are based on proning the properties of the scattered light from an aggregating suspension. Static light scattering yields the change in the scattering intensity, while dynamic light scattering the variation in the apparent hydrodynamic radius. At early-stages of aggregation, the variation of each of these quantities is directly proportional to the aggregation rate constant k. At later-stages, one can obtain information on the clusters formed (e.g., fractal dimension). Light scattering works well, provided effects of multiple scattering can be neglected, which means that the suspension should not be too turbid. Since scattering becomes increasingly important for larger particles or larger aggregates, multiple scattering effects have to be considered in such systems. For this reason, backscattering techniques or diffusing-wave spectroscopy has been used to study aggregation processes in highly turbid suspensions.

Single particle counting. This technique offers excellent resolution, whereby clusters made out of tenths of particles can be resolved individually. The aggregating suspension is forced through a narrow capillary particle counter and the size of each aggregate is being analyzed by light scattering. From the scattering intensity, one can deduce the size of each aggregate, and construct a detailed aggregate size distribution. If the suspensions contain high amounts of salt, one could equally use a Coulter counter. As time evolves, the size distribution changes, and from this variation the aggregation and breakup rates involving the different clusters can be deduced. The disadvantage of the technique that the aggregates are forced through a narrow capillary under high shear, and the aggregates may disrupt under these conditions.

Indirect Techniques. As many properties of colloidal suspensions depend on the state of aggregation of the suspended particles, various indirect techniques have been used to monitor particle aggregation too. While it can be very difficult to obtain quantitative information on aggregation rates or cluster properties from such results, for practical applications they can be most valuable. Among these techniques settling tests are most relevant. When one inspects a series of test tubes with suspensions prepared at different concentration of the flocculant, stable suspensions often remain dispersed, while the unstable ones settle. Automated instruments based on light scattering to monitor suspension settling have been developed, and they can be used to probe particle aggregation. The scheme of such instrument is shown in the animated figure on the right. One must realize, however, that these technique may not always reflect the colloid stability correctly. For example, larger primary particles may settle even in the absence of aggregation, or clusters may remains in suspension when they gel. Other indirect techniques capable to monitor the state of aggregation include, for example, filterability, rheology, absorption of ultrasonic waves, or dielectric properties.