Dispersants in pesticide formulations – function and significance

Pesticide formulations come in various physical forms. Some of them are suspension concentrates (SC). They form liquid products in which solid, insoluble active substances are suspended in water and supported by other additives. When added to a sprayer, they disperse to form a stable and homogeneous suspension for spraying ^[1].

Another type of formulation is oil dispersion (OD). This formulation does not contain water, and the continuous phase is usually oils in which the active ingredients do not dissolve but are only dispersed. The nature of this formulation requires the use of both non-aqueous dispersants, which stabilise the formulation, and aqueous dispersants, which ensure the appropriate quality of the spray liquid ^[2].

A rather interesting formulation is the concentrated suspoemulsion (SE), which is a combination of a suspension and an emulsion. It is one of the types of formulations that allows pesticide substances with different physicochemical properties to be combined, e.g. substances soluble in organic solvents with substances insoluble in both organic solvents and water. This type of formulation is more effective than SC due to the presence of oil as a built-in adjuvant. In this type of formulation, surfactants are often selected not only for their ability to disperse the suspension, but also for their ability to emulsify the oil phase ^[3].

Of course, we cannot overlook loose formulations such as: water-soluble powders (SP), water-soluble granules (SG), powders for preparing suspensions in water (wettable powder, WP), and granules for preparing suspensions in water (water dispersible granules, WG). These types of formulations also require the use of WP), and water dispersible granules (WG). These types of formulations also require the use of dispersants to accelerate dissolution or ensure adequate dispersion of active ingredients in spray liquids ^{[4, 5]}.

Liquid pesticide formulations have a number of advantages, including high bioefficacy, low cost and safety. However, they are thermodynamically unstable systems and are subject to agglomeration or flocculation of particles, and sometimes, in the case of partially water-soluble substances, Ostwald ripening occurs. As a result, this causes sedimentation and the formation of a compact sediment at the bottom of the container (cake) and leads to the separation of the upper layer of the solvent (top clearing). A common problem in this type of formulation is also the stability of the suspension in the finished spray liquid, which leads to sedimentation and, consequently, failure to deliver the substance to the plants. The use of appropriate dispersants eliminates these problems ^{[1, 2, 3]}.

The importance of dispersants in pesticide formulations

The use of dispersants is essential in both liquid and solid pesticide formulations. They prevent flocculation, aggregation, sedimentation and separation of the formulation, and also disperse the product properly after dilution with water and stabilise the resulting dispersion. They can also prevent Ostwald ripening. This effect depends on the temperature amplitude and consists in faster dissolution of smaller crystals at higher temperatures and recrystallisation on larger crystals at lower temperatures. As a result, the entire particle size distribution changes, with most crystals exceeding 10 µm in size. Dispersants are no longer able to hold such large crystals, leading to sedimentation and delamination. This effect can be inhibited by selecting an appropriate system of ionic and non-ionic dispersants, which will form a barrier on the crystals and prevent them from dissolving in the solution ^{[1, 6]}.

A suspension used as a spray liquid must have adequate stability, which means that the dispersed particles must not settle too quickly to the bottom. Otherwise, the entire substance will not be evenly distributed over the crop during spraying, but only part of it will settle at the bottom of the sprayer. Poor suspension stability can also contribute to clogging of the spray nozzles if too much sediment is suddenly drawn in by the pump. A properly selected dispersant or combination of dispersants prevents these problems ^{[1, 6]}.

The speed of dispersion during mixing is equally important. Suspensions and powder formulations should disperse quickly throughout the entire volume of water used to prepare the spray liquid. With unsuitable dispersers, it is often possible to observe the formation of difficult-to-mix fragments (similar to lumps) when adding the formulation to water. These usually require much longer mixing times and, if not mixed properly, will also clog the spray nozzles ^{[4, 5]}.

Mechanism of action of dispersants

Most dispersants are fairly large, complex molecules with various functional groups. In order to disperse effectively, the molecule must first “anchor” itself, i.e. adsorb onto the surface of the suspended particle, and it achieves this by means of functional groups designed for this purpose. Depending on the substance, it has a different surface, more or less hydrophobic or hydrophilic, depending on its structure and crystal structure. Therefore, dispersants for hydrophobic substances are usually equipped with alkyl and/or aryl groups, while dispersants for hydrophilic substances are equipped with hydroxyl, carboxyl, carbonyl, amino and/or amide groups. Once the dispersant molecules anchor themselves to the surface of the substance, they begin to disperse the suspensions in an appropriate manner using the remaining functional groups ^[7].

In the case of aqueous dispersants containing ionic groups, electrostatic repulsion occurs as a result of electrochemical processes taking place at the interfacial boundaries. Ionic groups in an aqueous environment undergo dissociation, i.e. they break down into positively charged cations and negatively charged anions. Furthermore, in the case of anionic dispersants, the anionic groups are immobile because they are bound to the dispersant molecules, and the same applies to cationic dispersants. The layer of adsorbed dispersant molecules creates a charge on the surface of the particle and leads to the formation of a so-called double electric layer. When particles with the same double layer come close to each other, they are electrostatically repelled. The dispersant molecules on the surface of the particles form a layer that prevents them from sticking together again and ensures colloidal stability. The effectiveness of dispersion is indicated by the zeta potential, a parameter that determines the difference in potential between the particle (including its dispersant layer) and the diffusion layer. Electrophoretic, electroacoustic or flow methods are used to determine the zeta potential. The zeta potential is influenced by electrophoretic mobility, viscosity and electrical conductivity of the medium ^[7].

Dispersers also utilise the phenomenon of spatial repulsion, or steric repulsion. This occurs as a result of the creation of a physical barrier between the grains covered with dispersant molecules. Spatial repulsion is characteristic of dispersants with long chains (usually polymeric) in their structure, often referred to as “tails”, which prevent the agglomeration of grains. These chains are usually made of meres that dissolve well in water, e.g. ethylene oxide or acrylic acid ^[7].

Non-aqueous dispersants for OD oil suspensions, where the system is quite sensitive to ions, are mainly based on spatial repulsion. In the case of aqueous dispersants, however, spatial repulsion is an ideal complement to electrostatic repulsion, which translates into the super efficiency of these dispersants. That is why these dispersants are also referred to as “electrostatic” ^[7].

In order to further stabilise the suspensions and enhance the performance of electrosteric dispersers, co-dispersers are added to the system, most often EO/PO polymers, linear or branched with medium molecular weights. Co-­ molecules of dispersants support the action of the main dispersants in several ways. Due to their mass and structure, they exhibit wetting properties, modify the polarity and ionic strength of the system, are more mobile and usually remain in solution. Some of the co-dispersant molecules may adsorb onto the surface of the granules and further seal the barrier. In addition, the co-dispersant molecules in solution interact with the dispersant molecules deposited on the granules and stabilise the suspension ^[7].

Dispersants used in pesticide formulations in the PCC Exol portfolio

Depending on their type, pesticide formulations use different dispersants with different chemical structures. PCC Exol meets the expectations of pesticide formulation manufacturers by offering a wide range of dispersants and co-dispersants. Some of the most important groups are presented and described below.

• EO/PO copolymers “ROKAmer series” – are the simplest group of compounds, often used as co­ nd dispersants. Depending on the “starter”, i.e. the initial molecule subjected to alkoxylation, their structure can be branched (ROKAmer G or NP) or linear (ROKAmer R or PP). This depends on the available­ OH groups that undergo polymerisation. The mass of these copolymers usually ranges between 3,000 and 8,000 Da, and the ethylene oxide content between 20 and 80%, depending on the required HLB. The structure of copolymers can also be block (e.g. ROKAmer 6500), random or mixed (e.g. ROKAmer B4000). The best properties are observed in mixed structures, as the block fragment adsorbs well on the surface of the grains and the random fragment is responsible for low foaming and low melting point. EO/PO copolymers also exhibit wetting properties, which is why they are often used as wetting agents in aqueous systems ^[8].
• Ethoxylated vegetable oils and sugars “ROKAcet R, ROKAcet OR, ROKwin and ROKwinol series” – these are most often compounds with a branched structure and oily consistency. They are also often esterified with fatty acids (ROKAcet OR) to further expand their structure and lower their HLB. These compounds are often used as non-aqueous dispersants in OD oil suspensions. In addition to ensuring adequate suspension stability, they emulsify the oil phase very well during the preparation of the spray liquid. These are naturally occurring compounds, easily biodegradable and harmless to the environment, in line with the principles of green chemistry. They have no toxic effect on microorganisms and are therefore widely used in Bacillus or Trichoderma-based biopesticides ^[8].
• Sulphates “SULFOROKAnol series” – typically alkoxylated sulphated alcohols with a molecular weight of 1000–­ –2000 Da, which, despite their small molecules, have dispersing properties, combining electrostatic and spatial repulsion. In most cases, these are alcohols with fairly branched structures, such as isotridecanol (SULFOROKAnol IT2030) or tristyrylphenol (SULFOROKAnol TSP95), thanks to which they are easily anchored by these groups and dispersed by long EO/PO chains terminated with charged sulphate groups. They most often occur in the form of sodium, potassium, ammonium or other salts. These dispersants are widely used in aqueous formulations ^[8].
• ­Phosphoric esters “EXOfos series” – as with sulphates, alkoxylated alcohols are also used here as anchoring groups. The phosphate group allows for the production of di­ e and even tri e esters, which significantly increases the molecular weight and expands its structure, which has a positive effect on the efficiency of dispersion. These compounds can be dispersants for both hydrophobic substances, and hydrophilic substances. The latter interact with the phosphate groups of EXOfos through hydroxyl or amino groups, with which they form hydrogen or coordination bonds. As a result, the phosphate groups (very good donors) are anchored to the surface of the substance, and the fatty alcohols or EO/PO copolymers attached to them form a steric shell and prevent grain agglomeration. This gives phosphate esters greater potential as dispersants than sulphates. Phosphate esters can also be neutralised to various salts depending on requirements. The most popular are potassium salts (EXOfos PT-K25 and PT-K60) and triethanolamine salts (EXOfos PT-A and PT-A75) ^[8].
• Naphthalene derivatives “Rodys series” – these are mostly condensates of alkylnaphthalene sulfonic acids with formaldehyde (ANS) with medium molecular weights, occurring in the form of sodium or potassium salts. They are very efficient electrosteric dispersants, which can adequately stabilise aqueous suspensions at a concentration of only 1%. In addition, they have a very beneficial effect on the rheology of suspensions. They are most commonly used for hydrophobic substances due to their alkyl tails, which anchor them well to the surface of the substance. Naphthalene sulfonic acid condensates with formaldehyde (NSF) are also known, also with medium molecular weights and in the form of sodium or potassium salts, but they are not as effective and usually require higher concentrations. They are readily used in powder and granule formulations as well as in aqueous suspensions. The disadvantage of these dispersants is their poor biodegradability and negative impact on the environment, which is why they are increasingly being replaced by dispersants based on lignin derivatives or polycarboxyethers ^[8].
• Lignin derivatives – most often lignosulphonates formed as a by-product in the sulphite pulp production process. These are quite complex structures containing both hydrophobic aryl and alkyl groups and hydrophilic sulphonate groups. They can also be modified to improve their properties. They are used as water dispersants, especially in powder formulations or granules. However, they are not as effective as naphthalene derivatives, and their effectiveness is particularly influenced by the source and quality of lignin. However, they are much more environmentally friendly and easily biodegradable.
• Polycarboxyethers (PCE) “EXOdis AG-13” are copolymers in which the main chain consists of acrylic, methacrylic, maleic or styrene monomers to which long polyether chains are attached. s with a star-shaped structure have very effective dispersing properties, where the main chain serves to anchor itself to the surface of the substance grains and the attached polyether fragments disperse sterically. Depending on the monomers used, such a dispersant may be suitable for hydrophobic substances (methacrylic acid, styrene) or hydrophilic substances (acrylic acid), depending on the proportion of each monomer. PCEs are typically used in aqueous suspension concentrates. These copolymers are much more biodegradable than naphthalene derivatives and are less harmful to the environment. However, the polymerisation process itself requires special conditions and control, which makes it more difficult and costly ^[8].

In summary: The market offers a wide selection of dispersants, and new ones are constantly being developed to meet the needs of manufacturers of pesticide formulations in the form of water-based suspensions, oil-based suspensions, or powders or granules. Due to the growing trend towards developing natural dispersants that are safe for the environment and consumers, manufacturers are facing new challenges. Natural alternatives are not always easy to develop so that they can match the effectiveness of conventional dispersants and be attractive in terms of price at the same time.