Characterization of Suspensions Using Particle Surface Area

By AZoNano.com Staff Writers

Topics Covered

Introduction
Particle Surface Area and Particle Shape
The Practical Consequence of Particle Surface Area
Particle Surface Area Measurement
The Wetted Particle Surface Area of Suspensions
The Nature of Suspensions
     The Acorn Area™
About XiGo Nanotools

Introduction

Particle size is important to manufacturing and processing, determining performance parameters such as the setting time of cement, the taste of food and the sintering shrinkage of metallurgical and ceramic materials.

Speaking practically, the particle surface area of a given suspension is a consequence of two important properties - the concentration and size of particles.

Particle Surface Area and Particle Shape

A factor of considerable importance in determining the overall properties of colloidal systems is particle asymmetry, especially physico-mechanical asymmetry.

The property of existing in different forms is known as polymorphism and can potentially be found in any crystalline material. Polymorphs have different stabilities and may spontaneously convert from a metastable form to the stable shape/structure at a particular temperature.

The Practical Consequence of Particle Surface Area

The greater the concentration of colloidal particles in a system, the greater the particle surface area of the system and, hence, the greater the ability of the material to react with its environment. As particle size is reduced, the internal surface is exposed, and with it a potential change in the number and/or type of surface chemical sites and groups.

In a cubic particle with a side of 1cm, only two or three molecules in 10 million are surface molecules. When divided into 10nm particles, more molecules/atoms become "surface moieties" and the ratio rises to nearly 1:4. At a particle size of 1nm, 80% of the atoms are at the surface.

Hence, at the interfaces between the dispersed phase and the dispersion medium, electrical surface charge effects that are normally negligible for solids become dominant in the description of colloidal behavior, and play a very important role in determining the physico-chemical properties of the system as a whole.

In the case of catalysts, it is not just the particle surface area which has an important effect, but also the associated increase in surface energy as the material is finely divided.

Taking again the example particle (1cm cube), the increase in surface energy, per gram of material, associated with subdivision down to a 10nm cube is around 10 million Joules, which is equivalent to about 3kW hours of energy.

When nanoparticulate suspension are used as the electrolyte in a battery cell, the energy storage capacity and charge/discharge rate are directly controlled by the particle surface area.

There is also increasing evidence that, specifically with nanoparticulate materials, it is the particle surface area and not particle size that is the defining metric that controls toxicological interaction; hence the drive to develop reformulations based on nanotechnology.

For all these reasons, the particle surface is a critical measure in the characterization of suspensions, especially colloidal dispersions and nanoparticulate systems.

Particle Surface Area Measurement

A common method of particle surface area determination is nitrogen (N2) gas adsorption. In this method, N2 is adsorbed on a sample kept at liquid N2 temperature at a series of different pressures.

The technique requires that the sample be degassed to drive off any adsorbed material (sample conditioning). It also critically needs a source of liquid N2 to maintain the proper sample temperature, and only works on dry powder samples.

Drying wet suspensions inevitably results in aggregates and agglomerates, which will usually result in a serious underestimate of the surface area when measuring it by gas adsorption. For accurate measurements on wet suspensions, the surface area must be directly measured.

The Wetted Particle Surface Area of Suspensions

The Nature of Suspensions

The large majority of manufactured industrial products, and increasingly healthcare products as well, involve, either in the final state or at some stage of their production, suspensions of particulate materials or dispersed emulsion droplets, often at high volume fractions.

For example, in cosmetics, the importance of suspension properties to application quality and color is significant - the dispersion of colored organic/inorganic pigments and dyes has a crucial effect on brightness and gloss.

In sunscreens, the quality of the dispersion not only affects the formulation aesthetics but also the UV-blocking performance.

The dispersion state of any solid material impacts suspension properties directly. For example, as particulate material is added to any liquid medium its flow becomes increasingly non-Newtonian and, with high particle concentrations, can become thixotropic.

Several dry powders start as massive solid phases and require size reduction even prior to dispersion of the powder in a liquid. Conversely, agglomerates are formed when fine particles are handled, shaken, rolled or stored in a single position; deagglomeration and stabilization are necessary to obtain optimal dispersion.

The Acorn Area™

One highly powerful analytical tool to study molecular structure and dynamics is high resolution nuclear magnetic (NMR) spectroscopy. Such instrumentation has traditionally required extremely large, powerful magnets to achieve good resolutions.

With the advent of small, powerful permanent magnets, however, a commercial instrument - the Acorn Area™ - using low resolution NMR is now available to measure the wetted particle surface area of suspensions.

This technique is based on the fact that liquid in contact with - or 'bound" - to the particle surface behaves differently from the bulk or "free" liquid. The NMR relaxation time of bound versus bulk liquid is markedly different, with the relaxation time of the latter being much longer.

It is also feasible to use NMR to measure other characteristics of dispersions, such as diffusion, from which size can be estimated. Also, NMR can directly measure volume fraction and, potentially, the molecular weight of polymers.

Another application of NMR with important practical uses is the ability to determine competitive adsorption and/or displacement of polyelectrolytes, macromolecules and surfactants at interfaces.

About XiGo Nanotools

XiGo Nanotools was founded by Sean Race and Dr. David Fairhurst in 2005 with the mission to provide new innovate “tools” for the emerging nanomaterials industry.

The Acorn Area is designed to measure the wetted surface area of concentrated dispersions with little or no sample preparation, providing a viable complementary technique to BET surface area, analyzing nanoparticles as they are made or used, dispersed in liquids.

Xigo Nanotools' goal is to provide scientists, researchers, and corporations with tools that are easy to use and serve as wide and diverse a customer base as possible. They have incorporated the latest technology available into an integrated, high quality package that provides precise measurements in a very small footprint.

This information has been sourced, reviewed and adapted from materials provided by XiGo Nanotools.

For more information on this source, please visit XiGo Nanotools.

Date Added: Nov 14, 2013 | Updated: Nov 15, 2013
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