Surfactants – Making Water Wetter
It may sound funny, but water does not always wet well….
Remember, as a kid, when you poured water on the dry soil in the yard and it just beaded up and did not seem to wet the dust and go into the soil?
That was because the drops in the water had high surface tension (caused by internal electrical charges that hold them together as drops and don’t let them spread), and so the individual drops would sit there practically as little beads and not really wet the soil until we stomped on them, then they finally wet the ground.
This is great when the wax finish on your car is still good since the rain droplets will “bead-up” on the hood and not really wet the surface, as they are repelled by the wax. The impermeability of the wax on the car’s body, and the surface tension in the rain droplets come together and really keep the water from “wetting”.
Water is a polar molecule: it has both positive and negative ends.
When these ends are linked by an electrical charge, a chain forms and droplets occur. This is called Hydrogen Bonding and is the cause of surface tension.
Mind you, this is not the tension of the surface that the droplet is to land on, but the tension on the outer surface of the actual droplet that, in effect, holds it together and keeps it from collapsing and spreading on the surface it has landed.
Breaking down this surface tension allows the droplet to collapse and spread over surface, wetting it thoroughly without running off, and that precisely is what we want our sprays doing to our crops.
There are many different types of products developed not only for agriculture but many other industries that need to get the most out of their water’s wetting abilities, that will break down the surface tension and make the water “wetter”.
These are called by many names, such as:
- Wetting agents
These are all formulated from a wide spectrum of chemistry, from all natural vegetable oil emulsifiers, detergents, and fatty acids, all the way to organo-silicones, emulsifiable oxidized polyethylenes, ethers and even alcohols.
First of all, let’s get the names right.
An Adjuvant is defined as any substance added to a pesticide in order to improve its performance.
A Surfactant is a substance added to the spray solution to reduce the surface tension so that droplets spread out and adsorb to a greater surface area.
Now, to make things simpler, we are going to use the word Surfactant as a general definition, because what we are most interested in is getting our sprays to spread and therefore work better at lower a cost.
Surfactants overcome the effects of beading or surface tension. The surfactant molecule has one end that is soluble in oily or waxy substances, and a second that is water soluble. When a surfactant is added to water and oil, its molecules align themselves at the appropriate ends of the interface and pull the layers together, reducing the beading, or surface tension.
This is visible on the leaf surfaces as the surfactant molecules pull the water and the wax (on the leaf surface) together, causing the droplets to spread out.
Aside from trying to get the best coverage and spread on the crops, we also use surfactants to keep our tank mixes in suspension. This is especially true with Wettable Powders, which usually do not dissolve in the water, such as instant coffee, but remain in their original form:
tiny, micro-pulverized particles that become surrounded by water, which then transports them to the target in the spray and deposits them on the surface, after which, when the water evaporates, these particles go to work.
In other words, water is the carrier.
If that water is not “conditioned” and made wetter, the wettable powders do not readily mix with the tank solution and many times bunch up in tiny groups of many particles which then, when sprayed out on the crop, settle in clumps and do not get spread evenly on the target surfaces. Then you have poor coverage, because the chemical is not evenly distributed.
The basic rule on Wettable Powders is that a Surfactant must be used every time, even if it is only a laundry or dish washing , soap or detergent. (By the way, detergents such as Tide, Dawn, Ivory, Lux, etc. make excellent surfactants, especially in hot weather).
The Jar Test
Labels and literature will refer to a compatibility test system called the “Jar Test”. Try this before you mix your next batch of pesticides:
- Fill a glass jar with water and sprinkle some wettable powder into it
- Shake it well and set it down on a table
- Now, take another jar, fill it with water, sprinkle the same amount of a wettable powder and add a drop of dishwashing liquid
- Shake well and set it down on the table
Come back and look at the jars some five to ten minutes later:
- The first jar will show the powder settled at the bottom
- The second jar will show the contents mixed as they were after you shook them
The dishwashing liquid’s surfactants have “made the water wetter” and have suspended the particles of the powder, not dissolving them but making them part of the solution.
Use this test also to make sure the different materials you are mixing into the tank do not have some adverse reaction among themselves.
Reactions between materials will show up as “layering” in the water where the incompatible materials reject each other and separate into layers. (this is great if you are a bartender making exotic drinks, but a no-no for tank mixes.)
When you have a “layering” condition, try adding a surfactant. If layering does not occur after minutes, you can safely mix the formula, as the reaction was not evidently chemical, but a problem of solubility.
Factors Affecting Surfactants
Surfactants cannot be used across the board, especially in warm sub-tropical and tropical climates, such as Florida, the Caribbean and Central America. There are several factors that have to be considered because of temperature, humidity, and leaf surfaces.
Temperature, along with solar intensity is a prime culprit in spray crop damage. Many publications, texts, labels and crop advisor’s insist on avoiding spray applications during the heat of the day.
This is especially critical on delicate crops in high temperature growing areas. It can also be aggravated by the use of a surfactant with pore sealing characteristics such as oils, silicones and polys, as these do not readily break down or evaporate, but on the contrary, seal the stomas, inhibiting transpiration.
Consequently, logical and careful selection of a surfactant will prevent potential crop damage and/or phytotoxicity.
Humidity is another important factor that affects the performance of the spray on the leaf surface. Especially when that humidity is intense, such as rain.
A soluble surfactant, such as detergent will wash away and carry the chemicals with it. However, a very non-soluble surfactant with “sticking” characteristics, may be too sticky and plug up the stomas.
This is really no problem with herbicides, on the contrary, we are killing and want to keep the material on the leaf, no matter what. So a real Sticker will not only prevent wash-off, but also allow us to reduce our formulation and get more coverage for our dollars.
However, with pesticides and nutritionals we should try to schedule our sprays in cooler times where we can safely use surfactants with organo-silicones, polys and/or ethyls that will not wash off.
The best times to schedule these sprays is towards the evening, which also works well if the wind dies down, preventing excessive drift.
Leaf Surfaces are an often overlooked factor in surfactant selection. Leaves can be smooth, brittle, waxy, hairy, or rough. This is Nature’s way of protecting the plant’s production areas, and sprays will react differently when trying to land on these varied surfaces.
Here again, common sense is probably the best method to select the best surfactant to use, depending on what the target is presenting as a landing pad.
Detergents will work well with most surfaces, but are susceptible to washoff. Fatty acids and oils, both vegetable and petroleum, are not the best for waxy surfaces, yet do well on all others. Organo- silicones, Polys and esters will give you good coverage across the leaf spectrum but must be used with extreme care in heat, especially with delicate foliage.
Good applications are possible using inexpensive surfactants such as household soaps, detergents, fatty acids and oils. A natural approach to what we put on our crops is always a plus, and in this case: simple, effective and economical.
The more specific surfactants being marketed by a host of ag chemical companies are just that: more specific, for more specific applications, more expensive and with a certain amount of risk if not used properly, especially in our extreme climates.
These products are formulated for the entire cross section of the US growing areas, of which Florida, some parts of the Southeast US, the Caribbean and Central America are not representative. What works in Ohio may burn our crops in Ruskin, and so on.
So my advice to you is use your common sense, your experience as a grower, when deciding what to put in your spray water. Salesmen, advisor’s, even this writer are not growing your crop – you are, and you will have to live with the results.
Be conservative and logical and your plants will thrive.