Gordon Alastair Bell

Model-based computer-aided design for controlled release of pesticides

In the field of controlled release technology for pesticides or active ingredients (AI), models that can predict its delivery during application are important for purposes of design and marketing of the pesticide product. Appropriate models for the controlled release of pesticides, if available, can be used to study and analyze some of the important issues related to the design/application of the pesticide. This paper highlights the needs for predictive models and proposes the use of a computer aided modelling framework through which a collection of reliable and predictive constitutive (property) models can be combined with various types of release models. Use of a group-contribution based property model for one of the constitutive variables (AI solubility in polymers) and a free-volume theory based model for another (diffusion coefficient), has been proposed and the corresponding extended models have been developed and implemented into a computer-aided system. The total model consisting of the property models embedded into the release models are then employed to study the release of different combinations of AIs and polymer-based microcapsules.

The application of ToF-SIMS to the analysis of herbicide formulation penetration into and through leaf cuticles.

Understanding the movement of the active ingredient in relation to the other formulation components following application is crucial to an overall understanding of herbicide performance. We describe the novel use of time-of-flight secondary ion mass spectrometry (ToF-SIMS) as a tool for following the movement of herbicide formulation components into and across plant cuticles. This technique provides new insights since it provides both high (sub-micron) spatial resolution combined with the chemical specificity associated with organic mass spectrometry. The components studied include the oligomeric ethoxylate surfactants Synperonic A7 and A20 and active ingredient Sulfosate (trimesium glyphosate). The movement of these molecules, both separately and when combined in a simple formulation, into the surface of Prunus laurocerasus leaves and across the isolated plant cuticle was investigated and clear differences in penetration/diffusion behaviour were identified. ToF-SIMS was uniquely able to (simultaneously) spatially resolve all the species involved, including the anion and cation components of the active ingredient. Also, using spectral reconstructions from the imaging raw data streams, the behaviour of individual oligomers within the surfactant distributions, could be assessed. The observations are discussed with reference to the action of surfactants identified in parallel micro-structural studies and the current understanding of herbicide uptake.

Cu(0)-mediated living radical polymerisation in dimethyl lactamide (DML); an unusual green solvent with limited environmental impact

The synthesis of poly-acrylates, methacrylates and styrene derivatives by SET-LRP is reported in a user and environmentally friendly “green” solvent, dimethyl lactamide (DML). The occurrence of a SET-LRP mechanism in DML was demonstrated via UV-Vis spectroscopy measurements following the disproportionation of Cu(I) in the presence of N-containing ligands. The synthesis of hydrophobic and hydrophilic poly acrylate and methacrylate (methyl, n-butyl, lauryl, poly(ethylene glycol), 2-hydroxyethyl and 2-(dimethyamino)ethyl derivatives) and styrene was investigated. The controlled behaviour of the polymerisation was observed via kinetic experiments. Finally the possibility to produced well-defined polymers with functional chain-ends was demonstrated with the SET-LRP of poly(ethylene glycol) methyl ether acrylate.

AFM relative stiffness measurement of the plasticising effect of a non-ionic surfactant on plant leaf wax.

An AFM relative stiffness technique was applied to reconstituted Beta vulgaris L. wax films. Consecutive force arrays (n=100) made on the waxy surface at the same locations showed that there was no relative change in surface elasticity and this information was used as a reference to further experimental measurements. A surfactant solution was subsequently dropped on the waxy surface and the same array of indents was made at the same location as the reference test. The plant wax surface showed a reduction in its surface elasticity properties. The study has demonstrated that the AFM technique could be used to undertake a systematic assessment of the plasticising effects of agrochemicals on native and reconstituted plant wax films.

Computer-aided and predictive models for design of controlled release of pesticides

Abstract In the field of pesticide controlled release technology, a computer based model that can predict the delivery of the Active Ingredient (AI) from fabricated units is important for purposes of product design and marketing. A model for the release of an AI from a microcapsule device is presented in this paper, together with a specific case study application to highlight its scope and significance. The paper also addresses the need for predictive models and proposes a computer aided modelling framework for achieving it through the development and introduction of reliable and predictive constitutive models. A group-contribution based model for one of the constitutive variables (AI solubility in polymers) is presented together with examples of application and validation.

Structural features of reconstituted wheat wax films

Cuticular waxes are essential for the well-being of all plants, from controlling the transport of water and nutrients across the plant surface to protecting them against external environmental attacks. Despite their significance, our current understanding regarding the structure and function of the wax film is limited. In this work, we have formed representative reconstituted wax film models of controlled thicknesses that facilitated an ex vivo study of plant cuticular wax film properties by neutron reflection (NR). Triticum aestivum L. (wheat) waxes were extracted from two different wheat straw samples, using two distinct extraction methods. Waxes extracted from harvested field-grown wheat straw using supercritical CO2 are compared with waxes extracted from laboratory-grown wheat straw via wax dissolution by chloroform rinsing. Wax films were produced by spin-coating the two extracts onto silicon substrates. Atomic force microscopy and cryo-scanning electron microscopy imaging revealed that the two reconstituted wax film models are ultrathin and porous with characteristic nanoscale extrusions on the outer surface, mimicking the structure of epicuticular waxes found upon adaxial wheat leaf surfaces. On the basis of solid–liquid and solid–air NR and ellipsometric measurements, these wax films could be modelled into two representative layers, with the diffuse underlying layer fitted with thicknesses ranging from approximately 65 to 70 Å, whereas the surface extrusion region reached heights exceeding 200 Å. Moisture-controlled NR measurements indicated that water penetrated extensively into the wax films measured under saturated humidity and under water, causing them to hydrate and swell significantly. These studies have thus provided a useful structural basis that underlies the function of the epicuticular waxes in controlling the water transport of crops.

Analysis of leaf surfaces using scanning ion conductance microscopy

Leaf surfaces are highly complex functional systems with well defined chemistry and structure dictating the barrier and transport properties of the leaf cuticle. It is a significant imaging challenge to analyse the very thin and often complex wax‐like leaf cuticle morphology in their natural state. Scanning electron microscopy (SEM) and to a lesser extent Atomic force microscopy are techniques that have been used to study the leaf surface but their remains information that is difficult to obtain via these approaches. SEM is able to produce highly detailed and high‐resolution images needed to study leaf structures at the submicron level. It typically operates in a vacuum or low pressure environment and as a consequence is generally unable to deal with the in situ analysis of dynamic surface events at submicron scales. Atomic force microscopy also possess the high‐resolution imaging required and can follow dynamic events in ambient and liquid environments, but can over exaggerate small features and cannot image most leaf surfaces due to their inherent roughness at the micron scale. Scanning ion conductance microscopy (SICM), which operates in a liquid environment, provides a potential complementary analytical approach able to address these issues and which is yet to be explored for studying leaf surfaces. Here we illustrate the potential of SICM on various leaf surfaces and compare the data to SEM and atomic force microscopy images on the same samples. In achieving successful imaging we also show that SICM can be used to study the wetting of hydrophobic surfaces in situ. This has potentially wider implications than the study of leaves alone as surface wetting phenomena are important in a range of fundamental and applied studies.

Structural Features of Reconstituted Cuticular Wax Films upon Interaction with Nonionic Surfactant C12E6

The interaction of nonionic surfactant hexaethylene glycol monododecyl ether (C12E6) with a reconstituted cuticular wheat wax film has been investigated by spectroscopic ellipsometry and neutron reflection (NR) to help understand the role of the leaf wax barrier during pesticide uptake, focusing on the mimicry of the actions adjuvants impose on the physical integrity and transport of the cuticular wax films against surfactant concentration. As the C12E6 concentration was increased up to the critical micelle concentration (CMC = 0.067 mM), an increasing amount of surfactant mass was deposited onto the wax film. Alongside surface adsorption, C12E6 was also observed to penetrate the wax film, which is evident from the NR measurements using fully protonated and chain-deuterated surfactants. Furthermore, surfactant action upon the model wax film was found to be physically reversible below the CMC, as water rinsing could readily remove the adsorbed surfactant, leaving the wax film in its original state. Above the CMC, the detergency action of the surfactant became dominant, and a significant proportion of the wax film was removed, causing structural damage. The results thus reveal that both water and C12E6 could easily penetrate the wax film throughout the concentration range measured, indicating a clear pathway for the transport of active ingredients while the removal of the wax components above the CMC must have enhanced the transport process. As the partial removal of the wax film could also expose the underlying cutaneous substrate to the environment and undermine the plants health, this study has a broad implication to the roles of surfactants in crop care.

Non-ionic surfactant phase diagram prediction by recursive partitioning.

A model has been designed to predict the phase which forms in water for a non-ionic surfactant, at a given concentration and temperature. The full phase diagram is generated by selecting enough data points to cover the region of interest. The model estimates the probability for each one of 10 possible phases and selects the one with the highest likelihood. The probabilities are based on the recursive partitioning of a dataset of 10 000 known observations. The model covers alkyl chain length and branching, ethoxylate head length and number, and end capping of one or more of the ethoxylate chains. The relationship between chemical structure, shape and phase behaviour is discussed. This article is part of the themed issue ‘Soft interfacial materials: from fundamentals to formulation’.

A framework for product analysis: Modelling and design of release and uptake of pesticides

This paper presents a framework for chemical product (pesticide) design and analysis. The framework consists of a set of computer-aided methods and tools that have been integrated to tackle the needs with respect to solution of chemical product design problems related to pesticide formulations. Two of the mathematical models (controlled release and pesticide uptake) that provide the principal calculation options are highlighted together with selected results from case studies.