I've written for years about surfactants and their importance when using herbicides for killing weeds and grass. Specifically, post-emergent liquid herbicides.
Let me start with the basics: In Southeast Texas, surfactants help herbicides do their job. Our hard water will not stick to leaf surfaces without the help of a surfactant. A friend of mine, who has a chemical engineering degree from Texas A&M, actually explained some of this stuff for me.
Technically, a surfactant is a soluble compound that reduces the surface tension of liquids or reduces interfacial tension between two liquids or a liquid and a solid. In other words, it's a linear molecule with a hydrophilic (attracted to water) head and a hydrophobic (repelled by water) end.
That's obviously more than most of us need to know. So simply put, surfactants make water wetter.
Ultimately, they're important in almost any liquid herbicide. Whether you're killing weeds, unwanted grass or brush, a surfactant is always essential in the mix because most water in our area is "hard." And hard water tends to bead up and just roll off leaf surfaces.
Try this test: Spray a broadleaf weed killer on some clover, dollar weed or thistle. You'll see that, in most cases, the water beads up. Then, add a surfactant to the mix, and you'll see that it then forms a sheen on the leaf surface. That's the herbicide actually sticking to the leaf and doing its intended job.
There are two ways of adding a surfactant to most herbicides. Just adding a little dish soap to the mix is the simplest way. About a tablespoon per gallon of spray will do. To keep the suds down when using over-the-counter soaps with a trigger-spray bottle or a pump-up sprayer, load the herbicide into the sprayer first, then add the dish soap. Be warned, though, that adding too little soap won't provide the "surfactancy" needed, and too much soap will create excessive tiny bubbles, often making hard to see exactly where the product is going.
I've discovered, by the way, that the new-fangled, super-duper, highly infused "anti-bacterial soaps" tend to work against surfactancy.
I've always recommended synthetic or professional surfactants like Hi-Yield's Spreader Sticker or Bonide's Turbo. Those two are the most readily available at retail, and they won't generate suds. I've also used the feed store version of a gallon-sized product called Alligare's Surface.
If your herbicide claims that it already contains a surfactant, I suggest adding a bit more.
Professional surfactants from a garden shop used to be just few bucks more than dish soap, but the cost has risen since I first wrote about them. Today, they're more like $5-$9 a bottle. Household products like Lux, Palmolive, Dial and Dawn are cheaper, but even the commercial stuff is relatively inexpensive, especially when you consider how little is needed to make a pump-up sprayer of weed killer work effectively.
By the way ... if you use one of those ready-to-use bottles of herbicide that hook onto the end of a hose, just fill the void at the top of the bottle with surfactant and shake. If you use dish soap in a ready-to-spray bottle, you will definitely get lots and lots of suds!