Using Ultrasound Scanning to Characterize Colloidal Particles

Ultrasound scanning is a highly promising new technique for product characterization and stability determination of colloidal and aqueous-based formulations. The stability of colloidal and fluid/particle dispersions is of increasing importance to the cosmetic and personal care industry where differences in raw materials can affect the batch-to-batch quality of products. Additionally, product developers in the industry need tools to predict formulation stability to reduce development time. 

Ultrasound and Colloidal Systems 

Ultrasound is a sound whose frequency is higher than sound that is normally detectable by human hearing. In most individuals, the limit of detection is well below 20 kHz, but some mammals, such as bats and dolphins, can hear, communicate and locate using ultrasound frequencies as high as 80 kHz. Possibly the best-known use of ultrasound is in scanning the womb to image a fetus. Amazing “videos” of the moving fetus have been produced-a reminder of the possibilities for ultrasound in materials characterization. Ultrasound imaging techniques are beginning to finding their way into areas that are more prosaic and that offer considerable advantages for characterizing optically opaque materials.

In any aqueous or colloidal system, particles are in a flux of constant motion known as Brownian motion. Numerous papers^1–4 have been written about the motion of particles in colloidal systems. For example, Tanaka⁴ suggested that colloidal systems undergo a complex and dynamic rearrangement at the microscopic level; coarsening of the continuous liquid and oil phases causes oil droplets to form.

Changes in macroscopic structure over time may lead to changes in a product’s odor, appearance or feel. Thus, assessment of product stability over time is essential in a market where product image for the customer is of vital importance.¹ Over periods of minutes or even hours, changes in a formulation structure may be undetectable. However, as time increases, the random motion of particles causes macroscopic structures to form and these can lead to creaming, flocculation, phase separation and other instabilities in the system. The key question is: At what stage does this macroscopic structure impact the product?¹ 

Ultrasound will often penetrate materials that are opaque to light.¹ Examples include concentrated colloidal systems such as milk, creams and pastes. It is also complementary to light scattering because it responds to a different set of properties in the sample such as thermophysical and elastic properties of the particles and suspending medium.

In addition, ultrasound is able to detect very small changes in the motion of nano- and microparticles.¹ This motion is caused by thermal energy that gives rise to Brownian motion of particles and is sensitive to the formation of flocs and gels that change the structure of the formulation. This increased sensitivity is explained by the fact that sound travels more slowly than light, and is sensitive to phase and amplitude. Therefore, acoustic measurements allow measurement of phase and amplitude in both time and frequency domains whereas light sensors usually only detect intensity (see Phase and Amplitude in Sound). 

Ultrasound technology has applications in many fields of medicine and science. In formulation science, ultrasound is useful for monitoring a variety of nucleation events, such as crystallization, Oswald ripening, flocculation and phase separation. This article will focus on using ultrasound as an analytical tool for determining product shelf life in colloidal systems. 

An Ultrasound Scanner

At the University of Leeds, measurement of colloid stability using ultrasound scanning and mathematical modeling has been in development for a number of years.¹ Recently, an apparatus that determines stability of emulsions and dispersions has been developed. This ultrasound scanner^a for colloidal systems (USCS) is a noninvasive and nondestructive technique that allows rapid screening of a tube containing 10­–500 mL of colloidal material and provides macroscopic and microscopic information about the temporal and spatial disposition of particles.¹ 

A USCS model with a six-cell sample charger is shown in Figure 1a. The cells, or tubes, are 300 mm high and 35 mm by 35 mm across internally. The glass windows permit simultaneous optical examination of the samples. Figure 1b is a photograph of the ultrasound microscope.

The USCS works by measuring the frequency dependency of the speed of sound and its attenuation through the sample at different heights up the tube. A pair of ultrasound probes (for transmitting, Tx, and receiving, Rx, repeatedly scan from the bottom to the top of the tube (Figure 2). The resulting data, stored in a computer on a spreadsheet, yields a picture of the changing concentration of the dispersed phase against height and time.

Macroscopic changes in the sample on a scale of millimeters is obtained from the USCS measurement of the group speed of sound, obtained from the change in phase per wavelength,  from which the concentration of a separating phase, for example a serum phase or a cream phase, can be calculated. This concentration is plotted against time and position to provide a picture of changes in the sample that ultimately become visible to the naked eye, perhaps days or weeks after they have been detected ultrasonically. 

Microscopic information, on the other hand, is obtained from the frequency dependence of the speed of sound that is determined from ultrasound signal phase and from changes in the attenuation of the signal that is determined from ultrasound signal amplitude. This spectroscopic ultrasonic information can be analyzed in terms of the scattering of sound by individual colloidal particles too small to be seen by the naked eye or even an optical microscope. 

During flocculation, individual colloid particles move more closely together under the influence of osmotic forces that are a common cause of destabilization in modern colloidal preparations. The close approach and reduction in relative Brownian motion causes the overlap of thermal field scattering that arises due to heating and cooling of particles during the compression and rarefaction when ultrasound passes.¹ The ultrasound microscope shown in Figure 1b operates at frequencies up to 200 MHz and is capable of imaging with a resolution of micrometers in samples at depths of several millimeters. This instrument is being used to study the impact of cosmetic creams on skin.

Exemplary Data

Figure 3a and Figure 3b show some typical experimental data that is obtained using ultrasound measurement. These figures clearly demonstrate a strong correlation of ultrasound measurements obtained from an ultrasound attenuation spectrometer that only measured attenuation, compared with images obtained from a confocal laser scanning microscope^b. The units of ultrasound attenuation on the vertical scale are dimensionless units called Nepers taken as log to the base e and adjusted for both the intensity reduction (meter–1) and the measurement frequency (MHz). 

Figure 4 shows some graphical data obtained with the USCS showing the effects on the oil particles at surfactant^c concentrations of 0%, 0.6% and 4%. Figure 4 shows the effects of flocculation on a log color scale that has been used so that small changes associated with flocculation are emphasized. These results indicate how increasing levels of surfactant first destabilize the emulsion through depletion flocculation and then cause stabilization through the formation of a gel. In Figure 4a the lower row of pictures indicates that the cells contain increasing degrees of flocculation as time passes, but that this flocculation is uniform from the bottom to the top of the cell. The upper row shows a stable emulsion; the lower row shows the effects of 4% surfactant concentration on the flocculation of the oil particles. In Figure 4b, the lower level of surfactant has created flocs. The flocs then cream, destabilizing the emulsion.

As can be clearly seen by comparing Figure 4a and Figure 4b, the technique is capable of simultaneously mapping the macroscopic distribution of the dispersed oil phase and the microscopic disposition of the particles relative to each other.⁵ Macroscopic disposition means changes in particle concentration on a scale that could become perceptible to human senses. This is the scale that is finally important when the customer assesses the product. Microscopic disposition means the relationship between individual particles. It is important for detecting the processes that lead to product instability long before they become obvious on a macroscopic scale. 

Figure 5 demonstrates the use of USCS analysis of a parental emulsion. This figure shows that the onset of instability in the parental emulsion starts after approximately day 17. This instability was not apparent using conventional techniques. Additionally, results show cream and serum formation. The data is plotted in the form of percentage change in the speed of sound from its value at time zero. 

Conclusion 

Ultrasound measurement is a new technology that can be used for product characterization and stability determination of aqueous and colloidal-based formulations. The technique is more sensitive to instability than conventional light scattering techniques and allows measurement in a shorter time frame. This property may lead to reducing a product’s time to market.

Results shown in this article demonstrate that ultrasound measurement is capable of providing microscopic and macroscopic data. Furthermore, ultrasound scanning of colloidal systems has the potential to be an inline, in-process quality-monitoring tool. It can also provide a means of determining product shelf life. This final benefit may be extremely important to cosmetic marketers and formulators because deformation of a product over time can change a product’s odor, appearance or feel, and negatively affect the consumer’s confidence in the product. 

References

1. MJW Povey, Is there such a thing as a stable colloid? An ultrasonic view, In Frontiers in Particle Science and Technology 2, M Fairweather, DT Goddard, S Lawson and J Young, eds, University of Leeds (2007) pp 23–28 

2. E Dickinson, Structure and rheology of simulated gels formed from aggregated colloidal particles, J Colloid Int Sci 225 2–15 (2000) 

3. H Tanaka, Viscoelastic model of phase separation in colloidal suspensions and emulsions, Phys Rev E 59 6842–6852 (1999) 

4. H Tanaka, Viscoelastic phase separation, J Phys Condens Matter 12 R207–R264 (2000)

5. Y Wang, Characterization of Emulsions by the Ultrasound Profiling Method, a PhD thesis, University of Leeds (2000)