Measurements of Wetted Surface Area of Nanoparticles

By AZoNano.com Staff Writers

Topics Covered

Introduction
Surface Area and Particle Size
Direct Wetted Surface Area Measurement
Estimation from Dry Powder Surface Area Measurement
Estimates from Particle Size Measurements
About XiGo Nanotools

Introduction

The majority of products in healthcare, and in many other industries, involve a concentrated dispersion of particles in a liquid at some stage of production. Proper dispersion of the particles is required to make sure that product performance is maintained.

In cosmetics, adequate dispersion is essential to ensure the right color and application properties. Dispersion uniformity of dyes and pigments also affects hiding power, gloss and brightness. Dispersion quality does not just affect the formulation aesthetics but also the performance (e.g. SPF factor).

In pharmaceuticals, poorly soluble pharmaceutical active ingredients (APIs) need optimal dispersion for maximization of bioavailability and uniformity of dose. The importance of the dispersion process and its impact on the quality and economics and quality of the subsequent product has long been recognized.

Several dry products start as solid aggregates and need size reduction before dispersion of the powder in a liquid. Conversely, agglomerates are formed when fine particles are handled, shaken, rolled or stored in a single position. De-agglomeration and stabilization are therefore necessary to obtain optimal dispersion.

Furthermore, all dispersions are thermodynamically unstable. They will, through random motion of the particles over time, aggregate, because of the natural and dominant tendency to decrease the large specific surface area and excess surface energy.

This tendency becomes more important as particle size decreases, and is highly significant for colloidal-sized particles (less than 1μm) and especially so for nanoparticulate systems (<100nm).

Surface Area and Particle Size

It is obvious that controlling particle size is essential for processing and manufacturing. However, it must be noted that as the particle size is reduced, an exceptionally large particle surface area-to-volume ratio is created.

Reducing particle diameter from 10 microns to 1 micron increases the surface area/gram by a factor of 100. As particles approach the nanoscale, the specific surface area becomes orders of magnitude larger than for particles of even only a few micrometers in size, and it matters little what the particle shape is.

Direct Wetted Surface Area Measurement

A new commercial instrument, The Acorn Area™ (shown in Figure 1), is now available to measure the wetted surface area of suspensions using low resolution NMR.

This technique depends on the fact that surface liquid in intimate contact with the particle surface, behaves differently from the bulk or “free” liquid. The NMR relaxation time of surface liquid is typically orders of magnitude shorter than bulk liquid. This observation is used to measure the wetted particle surface area.

There are no assumptions made about the particle size (distribution) or shape in the determination of the surface area. Measuring virtually any particle in any fluid is accessible and the technique does not require dilution of the sample.

Figure 1. Acorn Area Particle Analyzer from Xigo Nanotools

Figure 2 shows the impact of milling time on relaxation data from the Acorn Area for a pharmaceutical dispersion. Samples were measured as milled without dilution taken at 30, 60, 90, and 120 minutes milling.

This experiment makes use of a CPMG pulse sequence to measure T2, which is calculated from a single exponential fit of the relaxation curve. At fixed particle volume, the change in relaxation time is inversely related to the change in relaxation time.

Figure 2. Pharmaceutical API NMR Relaxation as a Function of Milling Time

The formula for calculating the surface area from the measured NMR relaxation time is:

Rav = ψpS L ρp (Rs-Rb) + Rb      (1)

Where:

Rav =1/T2 av, is the average relaxation rate constant
ψp is the particle volume to liquid volume ratio
S is the total surface area per unit weight
L is the surface layer thickness of liquid
ρp is the particle density
Rs is the relaxation rate constant for the surface liquid
Rb is the relaxation rate constant for the free or liquid

Using a standard reference material we can define a constant, Ka = L ρb (Rs – Rb) so that the equation (1) reduces to:

Rav = Ka S ψ p + Rb        (2)

The surface area can then be calculated from:

         S = Rsp Rb/ Ka ψp      (3)

Where:

Rsp = Rav/Rb - 1

A key practical application is the ability to determine competitive adsorption and/or displacement of polyelectrolytes, macromolecules and surfactants at interfaces. Figure 3 illustrates the change in the relaxation rate as a function of surfactant concentration.

Figure 3. Surfactant Adsorption Isotherm

Estimation from Dry Powder Surface Area Measurement

The most common method of surface area determination is gas adsorption. N2 is adsorbed on a sample kept at liquid N2 temperature at a series of different pressures.

First the sample must be degassed to drive off any adsorbed material (sample conditioning); this requires a source of liquid N2 to maintain the proper sample temperature. This technique requires the material must first be a dry powder. Drying particles dispersed in a liquid result in aggregates and agglomerates. For particle dispersions it is essential that the surface area is measured directly.

Estimates from Particle Size Measurements

Wetted surface area estimates from particle size measurements assume spherical particles and monodisperse size distribution, a condition that is not met clearly by most real-world dispersions.

Particle size measurements are typically performed at very low concentrations, and are not very sensitive to the presence of a “tail” of small particles contained in broad distribution particle dispersion.

Any surface area calculated for such materials is only a crude approximation and it is well-recognized that particle shape, surface irregularities and porosity will inevitably lead to estimated values significantly less than the true value.

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.

Our 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. We 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 13, 2013 | Updated: Nov 20, 2013
Ask A Question

Do you have a question you'd like to ask regarding this article?

Leave your feedback
Submit