The Acorn Area™ is designed to measure the wetted surface area of concentrated particles in liquids. This patented (NMR) based method offers many advantages in comparison with conventional surface area techniques. Measurements take about 5 minutes from start to finish. No sample preparation— such as drying or degassing— is required. High concentration samples are measured directly, without dilution. Acorn Area measurements take about 5 minutes, much faster than any other surface area measuring technique.
Why Wetted Surface Area?
The wetted surface area of particles influences many aspects of product performance. For example, the hiding power of pigments, the activity of catalysts, the potency and bioavailability of drugs. In the past, surface area measurements were made using methods such as gas adsorption. Unfortunately these methods are only useful for analyzing dry powders. Wetted surface area measurements tell you how well dispersed the particles are in the liquid. If the wetted surface area is low, the particles are not well dispersed. If the surface area is high, then the particles are well dispersed. However, the majority of products involve concentrated suspensions of particles in a liquid, either in the final state or at some stage of production. While dry powder methods are suitable to test incoming dry powder raw materials, they cannot provide information about the particles in liquids. It isn’t a simple matter to separate out particles that are in suspension for dry powder gas adsorption measurements because most commercial formulations include components such as surfactants and or polymers to keep the particles from aggregating. As these formulations are dried for measurement in gas adsorption methods, surfactants and polymers tend to form clumps of particles that would reduce the true surface area of the particles when dispersed in the liquid. Formulators tend to use particle size measurements to estimate surface area because of the speed of measurement. While size measurements are rapid, they are also less sensitive to the presence of small particles in the suspension because the light scattered from a particle size distribution is dominated by the largest particles. The smallest particles are often the “bad actors” in a formulation. They have the highest surface area and diffuse rapidly in a formulation. Particle size measurements are also limited to very dilute conditions far removed from the end use formulation. The process of dilution and sonication which often occurs in the sample preparation process for particle size measurements often obscures stability and performance issues with formulations at concentration in which they are actually used. When a formulation is concentrated, it increases the tendency of small particles to interact. To date this has not been possible other than to estimate the surface area from particle size measurements or to perform time- consuming adsorption isotherm or titration measurements.
What does it mean when the wetted surface area is low?
Mixing particles in liquids requires some form of mixing, and often additives to keep the particles well dispersed. If the particle mixing is not optimized, the wetted surface area will be low. The particles may not do the job they were intended to do, such as hiding the color of the substrate in a paint or producing the proper color in an ink. Or the aggregates may create production problems such as clogging nozzles in 3D printers.
What does it mean when the wetted surface area is high?
If the wetted surface area is high, it means that the particle size is small. It also means that more surfactant or polymer may be required to produce a stable formulation. The must be a balance between the wetted area and the concentration of additives to produce a stable formulation. While you may have optimized the particle size for good product performance, to achieve good storage stability, you must balance the particle size with the correct choice of stabilizer. The challenge with particle size analyzers in this regard is that dilution disrupts the balance between the concentration of surfactant and or polymer in a formulation. It is also a destructive measurement. Wetted surface area measurements require no dilution and are non-invasive, you can make measurements on exactly the same sample over a period of hours, days, or weeks.
How it Works
To make measurements, we compare the behavior of the liquid without particles to the particulate suspension. When we stimulate the suspension or liquid with NMR, we observe an exponentially decaying signal that can be described by a characteristic intensity and a characteristic time (aka relaxation time). The presence of particles in the suspension reduces the relaxation time of the suspension in comparison to the liquid without particles. By understanding the change in relaxation of the suspension relative to the liquid without particles and knowing the particle concentration, we can relate the relaxation time to wetted surface area. We can consider the liquid in the suspension to be separated into two distinct domains: 1) surface liquid in close contact with the particle surface; 2) bulk liquid far away from the particle surface. The NMR relaxation time of liquid on the particle surface is much shorter than that of the bulk liquid. Each domain has a characteristic relaxation time. In many cases, liquid molecules move freely between these two domains. In this case, we observe a weighted average of the amount of liquid in each domain, weighted by the relaxation time of each domain.
This difference in relaxation time between the surface and bulk liquid can be several orders of magnitude. As particles get smaller, the proportion of surface liquid in the suspension increases dramatically, in proportion to the radius R squared. We are interested in the difference between the relaxation time of the liquid without particles and the suspension. Tha is the driving force for measurement. Consequently these measurements work best when the particle concentration in the suspension is high.
Making a Measurement
To make a measurement, place a 0.5 ml sample in a sample tube and insert it into the Acorn Area. The sample tube is located within a coil between two permanent magnets. A static, uniform magnetic field, Bo, causes the liquid molecule to align with Bo; this process typically takes a few seconds. At the start of the measurement, (RF) pulses in the coil produce large temporary magnetic fields, creating a change in orientation of the liquid molecules. When the temporary magnetic field stops, the liquid molecules then realign with the static magnetic field, Bo. This realignment induces a decaying voltage in the coil. By using specific RF pulse sequences, the sample T1 (longitudinal) and T2 (transverse) relaxation times are measured. Although T1 and T2 are different, their shift between liquid in the bulk and liquid on the surface is similar, and both can be used to determine the wetted surface area.
Exclusive Software
AreaQuant™, the Acorn Area software, measures T1 using an inversion recovery pulse sequence, and measures T2 using the CPMG pulse sequencing method.
The resulting data are analyzed to determine the relative proportion of surface and bulk liquid and the corresponding wetted particle surface area. There are no assumptions about the sample particle size (distribution) or shape used to determine surface area; the wetted area is measured directly. The user inputs the particle dispersion weight concentration, the liquid and particle density. The particle-to-liquid volume ratio (used in the calculation of surface area) is automatically determined.
There are two Operational Modes in the software setup: Standard (or QC) Mode and Advanced (or R&D) Mode. The former is the mode used to measure surface area. In R&D mode, the instrument behaves like a traditional NMR spectrometer capable of a wide range of low-resolution NMR measurements.
Measurements with the Acorn Area are simple and easy. Dispersions can be measured non-invasively, without dilution. The upper limit in concentration is essentially unlimited; the lower limit is about 1-2%. Using this automatic mode, AreaQuant™ then determines all the measuring parameters needed such as 
the amplitude and duration of the pulses, the number of repetitions, etc. without operator intervention. This ensures measurements are made with the highest resolution and repeatability without a great deal of operator training. To simplify method development, the software also features an automatic test parameter file configuration based on an estimated T1 or T2 time input by the user. Once a set of parameters are defined, operation of the Acorn Area is simple: just enter the operator name and sample information – the software does the rest!
AreaQuant™ operates on Windows® 10&11. Data files are saved in XML format, easily opened with Microsoft Excel. Reports are saved in PDF format. Graphs may be copied to the clipboard or saved in JPG or PNG formats while tabular data can be copied to the clipboard.
