Laundry Detergent Pods
A laundry detergent pod is a common household product which contains concentrated laundry detergent encapsulated in a dissolvable biodegradable polymer disc. The detergent is the soap (a mixture of surfactants with cleaning properties), whilst the water-soluble pouch it sits in is generally made from (polyvinyl alcohol) PVA or a derivative of PVA.
Over the past four years, a team from Durham University and Procter & Gamble have been studying the surface segregation behaviour of different types of surfactants (cationic, non-ionic and more recently, anionic) in water-soluble PVA films. The aim of their research is to use neutron technology to understand how to increase the lifetime and improve performance of these popular laundry products.
In liquitab detergents, glycerol is used to plasticize PVA - a non-toxic water soluble, biodegradable polymer. By doing so, it increases the polymer's flexibility, fluidity and durability, meaning that it is ideal for encapsulating detergents in the production of liquitabs.
The ability to predict the behaviour of surfactants in polymer films can be tricky, however, and the tendencies of small molecules to migrate and segregate remains a fundamental scientific challenge. With this in mind, the group would like to shed some light on this phenomenon and they're doing so with the help of neutrons.
In order to build a more complete picture of surfactant segregation in PVA, the team focused on the behaviour of a particular anionic surfactant called sodium dodecyl sulfate or SDS. This model was chosen because anionic surfactants like SDS are commonly used in liquitab formulations on the market. The three techniques of ion beam analysis, atomic force microscopy and neutron reflectometry allowed the team to investigate the blooming of the model surfactant (SDS) and the influence of the plasticizer (glycerol) on surfactant distribution in PVA films.
Why Neutron Reflectometry?
The team wanted to use neutron reflectometry (NR) to quantify surface segregation of surfactants in PVA film. Neutron Science allowed them to address the influence of surfactants on the PVA/liquid interface for the first time. Compared to nuclear reaction analysis (NRA), NR allows scientists to study adsorbate properties with much higher precision and even eliminates the concern that freezing samples, as is the case with NRA, might alter the properties of the material under investigation. As liquitab formulations contain PVA adjacent to a concentrated surfactant solution, the unique property of NR which allows for the quantification of surface segregation of surfactants in PVA film was particularly valuable in these experiments.
NR combined with deuterium labelling revealed details about the interface between the segregated layers and the PVA film, providing some insight into compatibility and interactions.
“Neutron-reflectometry has long been a key technique for studying the structural organisation of soft matter such as surfactants and polymers in solution. A significant advantage of this technique over most other surface characterization methods of comparable resolution is its ability to work at atmospheric pressure, which enables the study of samples under controlled atmosphere. In this study, neutron reflectometry revealed a surprising level of self-organisation in the surfactants that bloomed to the surface of plasticized PVA. Our ultimate aim going forward is to build models that will enable a broad range of industries to predict molecular migration phenomena in complex polymer formulations. This work forms part of an EPSRC-industry collaboration encompassing experiments, simulation and theory. Neutron reflectometry is vital to provide the necessary level of detail for the models and we are extremely fortunate to have had access to these facilities and the support that comes with it. "
Richard Thompson, Durham University
ISIS Neutron and Muon Source & Industry
Scientists at Procter & Gamble and Durham University have used the SURF and INTER instruments at ISIS Neutron and Muon Source to measure the migration of two deuterated surfactants (d-SDS and d-C12hE5) in PVA systems with and without glycerol. This allowed them to gain a greater understanding of the effect of common solvents on the lifetime of household liquitabs.
“As we aim to continuously improve our product s for consumers, it is critical to understand all parameters that can influence the shelf-life and the performance of our products. In Unit Dose, understanding how the detergent interacts with the film is therefore at the forefront of our research interests, and the level of detail revealed by Neutron Reflectometry has proven invaluable to describe surfaces at a molecular level."
Florence Courchay, P&G
Controlling the formation and stability of surfactant blooms on polymer films has significant implications for their surface properties and applications. The team's findings show that a simple plasticizer like glycerol can have a transformative effect on the properties of PVA, despite having no inherent surface activity on its own. As there are a large number of applications in which plasticized PVA films may encounter surfactant-rich environments, studying their behaviour will be very valuable for their future improved design in order to increase product performance and lifetime.
“Neutron reflectivity at ISIS enables the identification of nanostructures on the surface of films, which are undetectable using other techniques, and has allowed us to determine the segregation behaviour in mixtures of polymers and small molecules. The findings from these experiments can have implications for a wide range of systems, providing insight into the migration of small molecules in polymers, and enabling driving forces for segregation to be identified."
Rebecca Fong, Durham University
Research date: January 2018
Read the full research article: Arron Briddick, et al. “Blooming of Smectic Surfactant/Plasticizer Layers on Spin-Cast Poly (vinyl alcohol) Films" Langmuir, American Chemical Society Publications DOI:10.1021/acs.langmuir.7b04046
Learn more on ISIS Neutron and Muon Source instruments SURF and INTER.
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