Cool characterization methods and where to find them
By Fiona Case
- The performance of commercially relevant soft materials is controlled by their internal structure. Understanding this structure is important for both product development and quality control.
- Structural characterization requires a range of specialized techniques and expertise. Not even the largest companies have all of the equipment in-house.
- Identifying the links between formulas and processing, structure, and consumer perceivable physical properties is challenging. This article summarizes relevant characterization methods and identifies where to find them.
The common feature of most soft materials—foods, cosmetics, personal and home care products—is the existence of an internal structure created by particle aggregation or the self-assembly of molecules, and by processing. This structure can change over time. It can change as the material is spread or kneaded or applied to skin, diluted, heated, cooled, chewed, or squeezed out of a bottle. It controls the performance and consumer perceivable attributes of soft materials based products.
If a development team wants to move beyond iterative line-extensions and develop a completely new product (to use new types of bio-sourced ingredients or to meet a new consumer need, for example), they begin by understanding this structure. Structural characterization can also solve quality control mysteries. When a product goes out of specifications, it may be because the internal structure has changed unexpectedly.
Not even the largest consumer products, cosmetics, or foods company has all the advanced characterization equipment that may be relevant, and many restrict their in-house capabilities to techniques that will be used regularly for quality control. For researchers interested in forging new links between formulation, structure, and performance, an obvious destination might be their local university-based characterization facility. In recent years many US-based academic institutions have “put out a shingle” and will provide a materials characterization service, for a price.
US-based university characterization facilities
“An advantage of going to a good university-based characterization facility is the depth and breadth of their experience with the analytical equipment. The researchers are focused on method development and, if you choose the correct location, you will benefit from their experience with sample preparation,” says Greg Haugstad, the director of the Characterization Facility (CharFac) at the University of Minnesota.
Imaging and scattering techniques using electrons or X-rays are fairly common offerings of university-based analytical services. Transmission Electron Microscopy (TEM) and scanning electron microscopy (SEM) images of micelles, vesicles, colloidal, and liquid-crystal systems can reveal the structure of soft materials. Techniques that rely on X-ray and electron scattering can provide information about the composition of a material and characterize crystallization and aggregation behavior (identifying crystal structures in lipid systems, for example). But the complex soft materials found in many consumer products and foods present challenges for these methods, both for sample preparation and data analysis. It is rare to find user facilities that have experience with these materials.
“High-energy beam techniques, such as electron microscopy, break organic bonds and can alter the structure of soft materials,” explains Haugstad. “Both electron beam and X-ray scattering methods rely on variations of electron density but the variations in organic materials are meager, requiring selective staining with heavy elements. The lack of electrical conductivity can result in surface charging. You often need a thin layer of metal coating on the sample. Sample preparation can be very challenging for these soft organic materials”
Haugstad and his colleagues at the CharFac have worked with well over 100 companies and a considerable amount of their work has been with soft materials, gels and suspensions. “There are the additional issues with materials that have liquid phases, such as colloidal suspensions,” explains Haugstad. “Vacuum-based methods such as electron microscopy require solid samples. The most common approach is to freeze the sample using cryo-vitrification. Liquid is wicked away on a metal mesh scaffold (the TEM grid), and then the sample is plunged into liquid ethane.”
These methods are quite well developed for simple aqueous systems, but not for non-aqueous systems, and for more complex or gel phase systems Haugstad sounds a particular caution.
“There are open questions regarding the veracity of imaged structures; I’ve seen published examples where the structure is induced by the cryo-vitrification process rather than being a characteristic of the material itself.”
It’s a reason to choose your characterization facility with care!
Bring in the Feds
Alternative, and potentially free, options for accessing sophisticated analytical equipment in the United States are provided by the US Department of Defense (DOE)-funded Nanoscale Science Research Centers (NSRCs) and by other government funded facilities such as NIST. These world-class facilities can be used by both academic and industrial researchers at no cost—if the proposed studies are deemed sufficiently interesting by a panel of expert (generally academic) reviewers. Although the majority of the techniques at the NSRCs are focused on characterizing and controlling the nanoscale structure of hard materials, such as next generation alloys and ceramics, there are some methods that are applicable to foods, cosmetics, or personal and home care products. For example, researchers from PepsiCo recently used 3D X-ray imaging capabilities at the Berkeley Lab in Berkeley, California, to study changes in structure as starch pellets are transformed into an aerated popped snack. And there is a very good reason to consider some of these research centers: They have neutron sources.
You may need neutrons
Neutron scattering and neutron imaging are good at revealing structure in soft matter. X-rays or electron beams cannot easily reveal structures that are comprised of lighter elements such as carbon, oxygen, and hydrogen. But neutrons are even scattered by hydrogen.
The Oak Ridge National Laboratory in Tennessee has two of the most powerful neutron science facilities in the world—the High Flux Isotope Reactor, and the Spallation Neutron Source.
“We have a range of instruments that can be used to study soft matter,” says Volker Urban, an instrument scientist at Oak Ridge. “Small angle neutron scattering, SANS, explores the length scale from 1nm to 100nm; that is where you find molecular assembly. If you are interested in emulsions, micelles, or colloidal systems, this is the length scale you need.” (Fig. 1)
Fig. 1 Overview of Bio-SANS Sample Area
Image credit, https://neutrons.ornl.gov/biosans/gallery
Neutron scattering experiments reveal the shape and conformation of nano to micron-scale structures. For example, in a microemulsion, SANS can measure the average droplet size, and also the size of the corona. It can reveal the number of lipids or surfactants stabilizing the emulsion droplets and study this over time to get exchange kinetics. In a surfactant solution, SANS can differentiate between spherical micelles, worm-like micelles, or lamella. In a lipid membrane, it can be used to quantify membrane fluctuations and lipid diffusion.
Neutron imaging provides a complementary picture of a specific part of the system. “It’s like X-ray radiography but while X-ray shows the heavy elements we can show where the hydrogens are which is important for soft matter,” explains Urban.
The technique depends on how different materials attenuate the neutron beam; the image is the shadow that is cast under the neutron illumination. Of particular value is the ability to use neutrons for contrast—to differentiate between different materials based on the relative percentage of hydrogen.
“Lipids or surfactants with long alkyl tails have lots of hydrogen,” says Urban. ‘Proteins also have hydrogen but also carbon, oxygen, and nitrogen. You can tune the neutron imaging or scattering experiment to see one or the other, for example to characterize lipid/protein complexes. In a food product you may want to know about the distribution of oil or water. In a system with carbohydrate, protein and fat you can see where the fat is.”
X-rays or electron beams can damage soft samples, but the non-destructive nature of SANS and Neutron imaging means that the same samples can be characterized using several different techniques.
“Neutrons are low energy so you don’t get radiation damage. After the sample is scanned to check that it has not picked up any radioactivity it can be handed back and the same material can be used in other characterization methods. We have found it particularly valuable to combine spectroscopic methods—IR and Raman—with neutron scattering,” says Urban.
Urban and his colleagues are interested in studying commercially relevant soft materials, such as foods, personal/home care products, cosmetics, and so on. He encourages industrial researchers to contact the Instrument Scientists at Oak Ridge before submitting a proposal. “The reviewers recommend proposals based on the novelty of the research or the material,” he notes. “And unfortunately, not many industrial proposals are accepted based on these criteria. But, we can provide assistance with proposal writing, and while 75% of our time is devoted to those projects that pass the peer review, 25% of our time is discretionary. We use that time to develop our techniques but we can also work on our own projects, with industry. Contact us and talk to us!”
Options in Europe
Peter Schurtenberger is a professor of physical chemistry at Lund University in Sweden who has considerable experience in characterization of colloids and complex soft matter systems. In particular, he is known for the development of dynamic light scattering techniques that can characterize very turbid colloidal suspensions such as milk (commercialized by LS Instruments in Fribourg, Switzerland). In tune with all the experts we interviewed for this feature he emphasizes the value of using multiple characterization methods.
“Basic understanding and rational design of soft matter—liquids, colloids, polymers, foams, gels, granular materials, liquid crystals, and biological materials—requires access to a variety of advanced, non-standard equipment, and a broad spectrum of expertise,” says Schurtenberger. “You need static and dynamic characterization over different length and time scales.”
Schurtenberger’s Institute participates in the European Soft Matter Infrastructure (EUSMI) program that provides both industrial and academic users free access to a wide range of state-of-the-art characterization facilities across Europe (including funding for their travel and accommodation).
“We want researchers from industry or academia anywhere in the EU to be able to access the world-class facilities that have been established for this purpose," he says.
The EUSMI program includes optical and electron microscopy tools (University of Edinburgh, UK, University of Bayreuth, Germany, and the University of Antwerpen, Belgium), light scattering (Lund University, Sweden, FZ Jülich, Germany, and FORTH Crete, Greece), neutron scattering (Jülich Center for Neutron Scattering, Munich, Germany), and X-ray scattering (Paul Scherrer Institute, Switzerland). There are also supercomputers for modeling work (FZ Jülich, Germany). In all these locations, the staff members whot work with researchers are experts in handling soft materials. And this wealth of characterization methods is not exclusively for EU-based companies.
“It is important to point out that 20% of the access time is accessible to groups where a majority of the users are not working in the EU,” says Schurtenberger.
Call the concierge?
With so many choices in Europe, identifying the most appropriate techniques and the people with the most appropriate expertise can be challenging. This is where Marc Obiols-Rabasa, the Industrial Liaison Officer (ILO) for the EUSMI program, comes in.
“My role is to help industrial users to identify projects that could benefit from EUSMI, and look for the most suitable infrastructure to characterize their material,” he says. “I provide support as they write the proposal, make the introductions, and get their visit to the chosen facility settled. The industrial user never has to take a decision alone. The ILO is there to assist.” The EUSMI program is only a year old but it has already attracted some industrial users. “There have been eight companies that have already performed studies,” says Obiols-Rabasa.
But what does it mean for my product?
If you are looking to go beyond characterization of your material, and to develop a model to relate structure to performance, then a longer term relationship with an academic research group may be a good approach.
There are a number of groups that would be interested in industrial funding to support their work discerning the fundamental relationships between structure and physical properties in soft materials. These groups typically use simple model systems, and want to obtain data that is publishable. Companies may retain the PI as a consultant, and value the laboratory as a training ground for future hires. But to characterize commercial products, and solve real-world problems, companies may wish to target the smaller number of groups that are interested in this kind of work.
“When companies are looking for a partner to work with, experience of a characterization method is not enough. The group needs to be interested in working with complex materials, interested in solving industrial problems,” says Paschalis Alexandridis, a professor at the University at Buffalo in New York State, USA.
Alexandridis is known for his work characterizing and manipulating the self-assembly of amphiphiles, such as surfactants, lipids, proteins, and block copolymers—the building blocks for soaps and detergents, foods, coatings, and ink.
“My most common interactions with industry occur when an established product is not performing well, often due to a change in ingredients. We look at the nanostructure—the structure that comes from molecular interactions and from processing. Understanding this will enable you to understand whatever challenges you face with your product. If you are creating new materials, or introducing new ingredients, you can design based on knowledge of this structure,” he says.
Commercial products are necessarily complex, with broad molecular weight distributions, side-products, and additives (fragrances, flavors, preservatives) that introduce many variables. Characterization results are challenging to interpret and publication can be difficult. Like most applications-focused research groups, Alexandridis’s team uses a range of analytical techniques, starting with the simplest.
“I assign great value to optical microscopy and rheology,” he says. “These techniques, if used correctly, can reveal information about the sizes and textures of the internal structures and their response during processing or consumer use. We also use X-ray diffraction to characterize the crystallinity in materials such as lipids. Commercial materials are always multi-component systems, but you can still get insight if you apply multiple characterization methods to the same system.”
Alexandridis is interested in establishing long term relationships with industrial groups. He appreciates the funding, but also values the interesting challenges and industrially relevant experience it provides for his students.
“Academic and industrial researchers can learn together how characterize the specific types of material in the commercial product, and to understand the link between structure and product, “he says. “I can use industrial money to pay the students who work on the industrial project (in addition to other projects). Publication is not essential; it is not the motivation for me.”
Julian McClements, a professor in the Department of Food Science at UMass Amherst, is also interested in working on longer-term products studying commercially- relevant materials. McClements specializes in food biopolymers and colloids, and in the development of food-based delivery systems for bioactive components. He and his team have considerable experience in characterizing colloidal systems, such as emulsions, nanoemulsions, starch granules, and biopolymer nanoparticles using scattering methods and microscopy. For example, in a recent work they characterized the surface texture and nanoscale structures of potato granules using confocal laser scanning microscopy (CLSM), and scanning electron microscopy (SEM) and correlated this with the pasting properties of the starch.
“We tend to work with industry on specific contracts (typically 6 months to 2 years). My laboratory rarely does one-off characterization projects,” he says.
A longer term project may make it easier to justify the time and effort required to set up the collaboration, which can be considerable in the United States.
“The business aspects of setting up collaboration can be challenging,” says Alexandridis. “It’s easier to bring your expertise into corporate research in Europe. US universities view industrial research as a moneymaking opportunity, and there are challenges about assigning intellectual property. It has become increasingly complex in recent years and this can add significant time and cost.”
McClements agrees, although he does see some recent improvements as universities recognize the barriers they have erected to collaboration.
“UMass has revamped its office dealing with academic-industry affairs making it much quicker to get contracts started,” he says.
Alejandro G. Marangoni, a professor at the University of Guelph in Canada, is known for his work on characterizing and manipulating lipid systems—in particular for applying X-ray diffraction methods to the characterization of foods, biolubricants, and cosmetics. He faces a somewhat easier climate for industrial collaboration than his US colleagues.
“We do a lot of powder xray diffraction for companies. Their needs are pretty complicated,” he says (Fig. 2).
Lipid crystallization and melting behavior is strongly influenced by other components in the system, for example the gluten in wheat flour in baked goods. In a recent example, Marangoni used powder X-ray diffraction to elucidate the crystal structure in a croissant, assessing the solid fat content by pulsed nuclear magnetic resonance (NMR), and the thermal behavior by differential scanning calorimetry (DSC). The work provided insight into the structures that determine baking behavior and mouth-feel and predicted the consequences of ingredient changes in laminated bakery products.
FIG. 2. TEM image of a fully hydrogenated canola oil nanocrystal with liquid oil removed using cold isobutanol. Source: Alejandro Marangoni
“A university-based analytical service laboratory may have the equipment, but not the personal and experience to characterize these materials effectively,” says Marangoni. “In an academic setting many professors would prefer to work on simpler model systems which can provide publishable data. There is a pretty big disconnect between the need and the supply for characterization services.”
The full service
The best approach, particularly if you need information fairly quickly, may be to work with a characterization consultant or consulting company with experience in your specific industry.
Dalia Yablon runs the SurfaceChar AFM measurement service in Sharon, MA.
“I've recently characterized both food materials and personal care products on behalf of industrial clients,” she says.
Atomic Force Microscopy (AFM) is often called on to measure sample adhesion and modulus in addition to providing a topological map of the sample surface (no other technique can do this on such a small length scale).
“I’ve studied properties of hair as a function of conditioner use or treatment type,” says Yablon. “I’ve also identified the nano to micron scale structures responsible for unwanted stickiness during food processing, and characterized the adhesion of edible food coatings. These types of material are challenging because the samples tend to be heterogeneous and can be difficult to handle. Even relatively simple AFM topography imaging can give misleading results if it is not conducted correctly.” (Fig. 3)
FIG. 3. Atomic Force Microscopy (AFM) image of nano-sized domains of rubber embedded in a thermoplastic matrix creating an impact copolymer designed to absorb impact but maintain stiffness in applications from appliance linings to sneakers and yogurt cups. Source: Dalia Yablon, SurfaceChar.
CR Competence AB in Lund, Sweden, works with about 20 companies each year, and is focused on characterization of commercially relevant soft materials using a wide range of techniques including scattering techniques (light, X-rays, and neutrons), surface imaging and characterization, and advanced NMR methods.
“We have worked with companies in Japan, Germany, France, Italy, UK, Denmark, Switzerland, Netherlands, Spain, Finland, Norway, Indonesia, and Turkey. We’ve only worked with one US company so far, but our relationship with P&G goes back more than 10 years,” says Anna Stenstam, the co-founder and CEO of the company.
A typical project for CR compares different formulations to assess how structure relates to product performance or stability. Changes in surface deposition behavior can be linked to structural information obtained from small angle X-ray scattering (SAXS) and to aggregation behavior characterized by NMR.
“Analysis of the effects of charge densities, incorporation of surfactants, and processing steps on active release, product stability, and film formation on a specific surface are typical requests,” says Stenstam.
Most of the work done by CR is on specific characterization projects with deliverables and go/no go points, but they also provide consultancy on an Ad hoc basis with a monthly retainer or on an hourly basis.
“This is useful so a team can call us without asking a manager first, or when we are discussing a project with our clients, or reshaping the project based on new findings,” says Stenstam. “We can be super agile.”
According to Eric Johnson, a research fellow, with P&G Beauty, “CR has brought great insights and value to real world problems in our product development cycle. They have a great balance of being focused on the end product and also bring best in class scientific thinking and approaches.”
“Our objective is to provide a scientifically sane basis for business decisions. That basis can come from structural characterization set in context,” says Stenstam.
Fiona Case is a freelance writer based in San Diego California. She can be reached at Fiona@casescientific.com.
Three approaches to characterizing soft materials
There are three general classes of characterization methods that can reveal structure in commercially relevant soft materials:
- Scattering methods can characterize structure within a soft material. Soft materials are somewhat transparent to radiation such as electrons, X-rays, neutrons, and in many cases to light (visible, UV or Infrared). The radiation is scattered by structures within the material and the scattering pattern can be fitted to models to reveal the sizes and shapes of ordered nanoscale to micron sized structures (the existence of characteristic distances). Depending on the type of radiation that is being scattered by the sample (light, electrons, X-rays, neutrons) and the angle at which the radiation hits the object and is scattered, the ordered arrangements of atoms (crystals) or the prevalence, sizes and shapes of larger structures such as micelles and colloidal particles can be deduced.
- Microscopy techniques use reflected radiation (light, X-rays, electrons, neutrons) to provide an image of specific portion of the material - the outside surface, or of a slice extracted from the interior of a frozen sample. Atomic force microscopy (AFM) measures surfaces attributes such as height and friction with a very sharp probe testing the mechanical properties of the material or providing a topographical map as the probe is rastered across the surface.
- Rheological methods measure how easy it is to deform (squeeze or shear) a material, but in the hands of an expert the subtle details of that response can suggest much about the internal structure.
Commercially relevant soft materials can also be studied using the wide range of spectroscopy methods designed to reveal chemical structure (which molecules are present) – and this can be useful in situations where chemical degradation is suspected. Special spectroscopy techniques (e.g., diffusion and spin echo methods in NMR) can be used to deduce larger scale structures.
List of characterization services
We expect this list to grow. The most recent version can be found at https://docs.google.com/spreadsheets/d/1E23hPKRyV1cWWUUIW6d0Y1EVKjfcpl2rpMYYOeATlcA/edit?usp=sharing. Also, if you provide structure characterization services and have a proven track record working with commercially relevant soft materials (foods, cosmetics, personal, or home care materials) please let us know at https://goo.gl/forms/Bg2GBD21OATE3kee2.
US university characterization facilities:
A list of characterization facilities supported by the National Science Foundation's Material Research Science and Engineering Centers initiative can be found here: https://mrfn.org/
There are 23 characterization facilities, offering 1,155 instruments and 262 expert users! But the vast majority of the work carried out in these facilities is not focused on complex soft materials such as those found in foods, personal or home care products or cosmetics. The following facilities have specialized equipment and expertise for soft matter characterization:
- The University of Minnesota Characterization Facility provides analytical services, machines for independent use (with training). http://www.charfac.umn.edu/about.html. Contact Greg Haugstad: firstname.lastname@example.org.
- The Center for Particulate and Surfactant Systems at the University of Florida and Columbia University in New York includes optical microscopy, light scattering, AFM, and rheometry and has a number of industrial members. http://cpass.mse.ufl.edu/, contact Hollie Starr: email@example.com.
- GeorgeTown University Institute for Soft Matter Synthesis and Metrology provides microscopy, rheometry, and light scattering. https://softmatter.georgetown.edu/; contact Daniel Blair: firstname.lastname@example.org.
- Stanford University Soft & Hybrid Materials Facility (SMF) does not provide analytical services, but they do provide machines for independent use (with training), https://snsf.stanford.edu/equipment/smf/index.html; contact email@example.com.
- The Soft Matter Facility, Texas A&M University will be opening shortly. It will provide analytical services and machines for independent use (with training). https://somf.engr.tamu.edu/; contact Svetlana Sukhishvili: firstname.lastname@example.org.
US government facilities for soft matter characterization using neutrons:
- Neutron scattering and imaging facilities at Oak Ridge National Laboratory: https://neutrons.ornl.gov/; contact, Volker S. Urban, email@example.com, or firstname.lastname@example.org.
- NIST Center for Neutron Research: https://www.nist.gov/ncnr, contact Robert M Dimeo, email@example.com.
- The NIST nSoft Consortium: https://www.nist.gov/nsoft/about; contact Ronald L. Jones, firstname.lastname@example.org.
European Soft Matter Infrastructure (EUSMI) program:
Free access to characterization facilities focused on soft materials at 15 top-level institutions (available for users outside Europe) https://eusmi-h2020.eu/ ;contact Marc Obiols-Rabasa, the Industrial Liaison Officer (ILO), email@example.com.
Consultants and characterization services with specific interest/expertise in characterizing commercially relevant soft materials:
SurfaceChar; contact Dalia Yablon: firstname.lastname@example.org.
CR; contact Anna Stenstam: email@example.com.