Probing Sub-microscopic Particles
Particle Size Distribution in Colloids
Colloids are complex physical systems, but understanding their properties is important in the development of of products in a variety of sectors. The Bristol Colloid Centre at the UK's University of Bristol has been using two different instruments from Brookhaven in their research into colloidal systems
Use of Brookhaven's Bl200SM dynamic and static light scattering goniometer and Bl-9010AT correlator as a photon correlation spectroscopy system has helped the Bristol Colloid Centre, a commercial enterprise founded by the University of Bristol, probe the properties of a variety of complex colloidal systems. The research has important consequences for industries ranging from petrochemicals to pharmaceuticals and personal care products.
The world of microscopic particles is complex and difficult to probe. One of Bristol Colloid Centre's main concerns is the study of minute particles suspended in liquids. The team there has found that its Brookhaven photon correlation spectroscopy equipment-the Bl-200SM goniometer and the Bl9010AT correlator - has helped to expand its capabilities, earning it valuable research contracts and helping to push back the boundaries of this area of work.
The BCC is part of the School of Chemistry's colloid and condensed phase sector. Recognised as a world centre of excellence in its field, the University's colloid researchers focus on areas such as the properties of colloidal dispersions, inch~ding those which conduct electricity and are important to the environment, for example interactions within colloids and concentrated colloids.
The company carries out contract research work and offers industrial training courses and specialist consultancy to clients in a wide range of markets, including agrochemical, cosmetics, food, paints and coatings, personal health care, petrochemical and pharmaceutical industries. Much of this vork, carried out by its permanent staff researchers and PhD students, is on the cutting edge of the client companies' product development, and is herefore covered by strict commercial confidentiality.
The centre's current research is just as diverse as the sectors it serves. Current projects include an investigation into how colloidal semiconductor crystals grow and develop in complex surfactant media, which could have great importance in the design of inks and other coloured compounds.
A related project is looking at the development of extremely tough inks, capable of resisting heat, solvents, corrosion and abrasion and which can be used for printing on glass, ceramics and metal surfaces. Surfactants are another interest, from the development of new surfactant molecules to the design of new shampoos that enhance the shininess of hair.
Contract research was the driving force for BCC to buy the Brookhaven instruments. Its laboratories already had equipment for light and X-ray scattering techniques, pulsed-field-gradient NMR, advanced electrokinetic methods, theological techniques, advanced electron and optical microscopy with image-analysis and calorimetry. However, in the autumn of 1996, a demanding project on a series of microemulsions required the use of a high-performance photon correlation spectroscopy instrument. The conventional use of this equipment is to size particles, but the team needed to analyse the properties of microemulsions, which contain very small droplets of dispersed material and study their structure under different conditions.
The BI-200SM dynamic and static light scattering goniometer
"The centre's current research is just as diverse as the sectors it serves. Current projects include the development of new surfactant molecules to the design of new shampoos that enhance the shininess of hair, and also an investigation into how colloidal semiconductor crystals grow in complex surfactants"
There are generally two goals for this: optimising formulations and characterising the microstructure of the colloid,' explains Dr Cheryl Flynn, one of the Centre's research scientists. 'Microemulsions contain very small species, and it's for that reason the performance of the correlator at high speeds was important.'
Assessing instruments
BCC assessed several competitive instruments before settling on the machines from Brookhaven. The system was bought after submitting samp1es for competitive tender. "We sent three candidates' samples of microemulsions which were, as far as we could establish, typical of the type of system which we wanted to study and compared the data," says Flynn.
Brookhaven won through because of the performance of its machines Flynn continues, "This system complements our other photon correlation spectroscopy (PCS) and angular light scattering techniques. It is a more powerful system than other PCS instruments available, in terms of laser power and correlation."
The two instruments form a powerful and versatile suite for studying particulate systems. The Bl-200SM goniometer, whose flexible design allows any laser to be used, is generally used for particle size analysis, the investigation of the structure of complex fiuids, micelles, microemulsions and colloids It allows the use of tnulti-angle measurements yielding more information on particles and molecules than a sfilgle-angle system. Despite this, its simple alignment is easy to learn, adjust and maintain.
The Bl-9010AT correlator is equally versatile, allowing sampling times as low as 25ns and as high as 40ms, with delays ranging from 25ns to 1,310s, It can process two different signals simultaneously, for example to crosscorrelate two signals from different positions in a liquid or from two detector angles.
PCS analyses light scattered by the particles of an emulsion or colloid within the liquid in which they are dispersed (the continuous phase) The colloid scatters the light randomly, producing a 'noisy' signal at the spectrometer's light-sensitive detector. However, the particles are moving constantly, both because of the energy from the laser and because of Brownian motion. The intensity of the signal at the detector varies over time and the frequency of this variation is related to the size of the particles.
The Bristol team s research stretches the capabilities of the Brookhaven machines. "Since these instruments arrived at the centre, they have been used extensively for a wide variety of specialised research applications. They have proved particularly useful with Difficult systems" such as small silica particles and microemulsions,' says Flynn 'We have used them to study both oil-continuous and water-continuous microemulsions, both in their native state and when swollen with active ingredients. There is also an ongoing study of bicontinuous microemulsion phases."
Further applications of the technology have included the study of several other types of colloid under investigation at the centre, polymeric molecular species, biological complexes and inks are just three examples.
Flynn adds, 'We use the system to operate in a multi-angle mode, both by analysing the correlation data and the total intensity or count rate. We have also carried out correlation and total intensity measurements simultaneously, to enable us to get a more detailed characterization of the material under investigation. Before they bought the Brookhaven equipment, the Centre had to perform two complete series of measurements on two separate insttuments to offer this type of analysis.
Brookhaven Bl-9010AT correlator can be used in a photon correlation spectroscopy system
She concludes, "Good after-sales service is always an important consideration for us when choosing equipment. We have had no major complaints about the service we have had from Brookhaven and would certainly consider it the next time we need to update instrumentation."