Physical Methods Of Liquid Dispersion - Beckman Coulter LS 13 320 User Manual

Laser diffraction particle size analyzer

Table of Contents

Liquid Sample Dispersion

Liquid sample dispersion is process that attempts to suspend a dry powder in a liquid medium

before analysis. This medium can be aqueous or non-aqueous. Characteristics of the dry powder

dictate what medium is to be utilized as a diluent. Factors such as solubility, reactivity,

suspendibility and end use govern the choice of the media. For example, a non-aqueous medium

such as isopropanol is chosen over water as a diluent when determining particle size distribution

for a sample of fertilizer, which is soluble in water.

Many pharmaceutical powders, such as niacin, experience this solubility problem and require the

use of non-aqueous media. Powders can also be highly reactive. For instance, lithium hydride

explodes when in contact with water and cement cures in water. Next, suspendibility of large or

heavy powders in a medium may prompt you to use a highly viscous solvent such as glycerol. Finally,

pigments are often dispersed in toluene because toluene is the major component of the end

product. In the preceding examples, Beckman Coulter illustrates why sometimes it is beneficial to

select a non-aqueous medium over an aqueous one. Nevertheless, aqueous systems have fewer

precautions in the handling and disposal of waste, and water is far less costly than any organic

solvent.

CAUTION

The safe handling and disposal of hazardous organic materials have to be

practiced according to government regulations such as Occupational Safety and

Health Administration (OSHA) and Environment Protection Agency (EPA)

regulations.

The techniques that are used in the dispersion of powders in both aqueous and non-aqueous media

are quite similar. Dispersants aid wetting and stabilization of a suspension and can be ionic (anionic

or cationic) or non-ionic. However, concentrations of dispersants must be low. Otherwise, their

properties are reduced or even inverted (i.e., cause aggregation or flocculation). In dispersing a

powder in a nonaqueous medium, it is critical to select an organic solvent that is a good diluent as

well as dispersant, although there are many organic dispersant aids. Aqueous surfactants are

primarily responsible for reducing surface tension, hence aiding in the dispersion of powders in

aqueous systems. The first task in dispersing a dry powder is the wetting of the sample with diluent

or dispersant, if needed. Gentle spatulation of the resulting paste prior to addition of diluent further

aids the dispersion process. Once the diluent is added, a sample of the slurry can be placed on a slide

for a microscopic evaluation of the effectiveness of dispersion. Providing that the sample is well

dispersed, the use of a magnetic stir bar is suggested to keep the sample fully dispersed, so a

representative sample can be drawn for analysis while stirring is in progress. For example, in some

cases it is necessary to use sonication. This is the case with strongly aggregated pigments of earth

elements. Ultrasonics must be used with caution since it can cause aggregation, shattering and

heating if the duration is too long or the ultrasonic power is too strong.