When Oil and Water Do Mix
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LUBRICATION FUNDAMENTALS Dr. Robert M. Gresham / Contributing Editor Editor’s Note: This article is reprinted from the March 2008 issue of TLT. When oil and water do mix Thanks to the fascinating world of surface chemistry, you might have more options than you think when it comes to combining fl uids. Key concePTs According to conventional wisdom, oil and • There are several materials that derive some or water don’t mix. While most oils and waters have some very minute solubility in each other (on a macro scale, for exam- all of their properties from the use of surfactants. ple), you really can’t mix the two to get a true solution. • The physical and chemical behavior of liquids I remember the old DuPont adage: “Better living through chemistry.” Both Mother Nature and Mankind have shown ad- cannot be understood without taking surface mirable skills in this regard. What I’m referring to, of course, tension into account. is the wonderful and fascinating world of surface chemistry. As tribologists we are concerned very much with certain • The difference between suspensions, dispersions aspects of surface chemistry such as wear processes, tribo- and emulsions is their inherent stability or chemical reactions, friction, etc.—the whole nanoworld where tendency not to separate or settle. certain atoms or molecules on surfaces come in contact with other chemicals to produce interesting results. In the case 22 • AUGUST 2011 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG Surface-active agents allow us to mix incompatible materials in a generally synergistic manner for some purpose. innovation starts with ™ of oil and water, I’m referring to the 3. colloid. A solid, liquid or gas- SpectraSyn Elite chemistry of surfactants or surface-ac- eous substance made up of mPAO tive agents. Surface-active agents allow very small, insoluble particles SpectraSyn Elite™ metallocene us to mix incompatible materials in a (as single large molecules or polyalphaolefi n (mPAO) base stock generally synergistic manner for some masses of smaller molecules) uses a proprietary catalyst process purpose. that remain in suspension in that results in a PAO with improved There are all kinds of materials a surrounding solid, liquid or properties, which enables the fl exibility around us that derive some or all of gaseous medium of different to develop fi nished lubricants that address formulators’ challenges, their properties from the use of sur- matter. Colloids usually are particularly in industrial applications. factants, including engine oils, met- pretty stable; thus a stable col- Benefi ts include: alworking fluids, certain lubricants, loidal suspension such as milk higher VI for low and high face creams, lotions, conditioners and consists of an immiscible liquid temperature performance cosmetics, shaving cream, water-based (butter fat) dispersed and held impr oved shear stability paints and coatings and food products in another liquid by substances for durability conjured by man. called emulsifiers. lower pour point for increased We all know chemical companies fl ow in cold environments can do some pretty amazing things, 4. Dispersion. A colloidal system but what about Mother Nature? One with its dispersed particles and example that we encounter virtually at the medium in which these are viscosity index (vi) 210 Conventional 100 cSt PAO birth is obviously the most common suspended. These are materi- T SpectraSyn Elite™ 150 cSt :10” and best formulated: mammalian milk. als that sort of fit in the mid- 200 190 To understand all of this, we first dle. Most dispersions need or 180 204 need to review the following defini- should be shaken or stirred be- 170 160 170 test method: tions and, yes, I know there is a bit to fore use. Some salad dressings iscosity Index (VI) V 150 ASTM D 2270 digest here. immediately come to mind. SpectraSyn Elite™ 150 cSt has a PRoPeRTies oF higher VI than conventional 5. surface Tension. The effect with- 100 cSt PAO. suRFace chemisTRy in the surface layer of a liquid 1. Wetting. The spreading and pen- that causes the layer to behave etrating properties of a liquid as an elastic sheet. Surface ten- low temperature performance caused by lowering its surface sion allows insects to walk on -28 Conventional 100 cSt PAO -29 tension—that is, the tendency water. It also allows small met- -30 SpectraSyn Elite™ 150 cSt of its molecules to adhere to al objects such as needles, razor -30 C o -33 each other. blades or foil fragments to float -31 -32 test method: on water, causing capillary ac- ASTM D 97 / Pour Point, ASTM D 5950 2. suspension. The condition of a tion. Whenever a rain drop falls -33 substance whose particles are or a cleaning agent is mixed SpectraSyn Elite™ 150 cSt has a dispersed through a fluid but with water or an alcoholic bev- lower pour point than conventional not dissolved in it; the condi- erage is stirred in a glass, the ef- 100 cSt PAO. tion of one having relatively fects of surface tension are read- large particles that will separate ily visible. Everyone is familiar exxonmobilsynthetics.com on standing. These are materi- with water droplets forming als that always say, “shake well large beads (because of water’s before using” on labels. In na- high surface tension ~72) on a ADVANCED Innovative SYNTHETIC lubricants ture, a muddy river might be an waxed surface (low surface ten- BASE STOCKS start here example—once it stops flowing, sion ~<22), like a clean, freshly the mud settles and the water is waxed automobile hood. The clear again. physical and chemical behavior WWW.STLE.ORG TRIBOLOGY & LUBRICATION TECHNOLOGY AUGUST 2011 • 23 ME MagNwp Slug Name: or shaking (mixing with a fair amount of energy). In addition, we can make an emul- sion with a different surfactant (a better one for our particular system—an emulsifier), of liquids cannot be understood mixed with energy, resulting in a stable withoutHydrocarbon taking surfaceTail tension Polar Head emulsion. So with a little magic, we have into account. It governs the Polar solvents and functional groups on molecules generally Figureshape 1. that small masses of liq- have some kind of ionic character. Now let’s strip away the alchemist’s uid can assume and the degree of contact a liquid can make with robes, cowl, incantations and crow feathers another substance. and look at what’s going on. First, the sur- For example, imagine motor oil with a dispersant. The engine dirt tends to be factant is a molecule that typically has a 6. Surfactant. A substance such addition, we can make a dispersion pasta on the end of it. long carbon chain tail with some kind of polar, soas thean emulsifier,polar heads detergent of the ordispersant by adding a surfactant and stirring or The polar head generally likes po- polar head attached, as shown in Figure 1. attach towetting the polaragent partsthat lowers of the the dirt toshaking keep (mixing with a fair amount of lar solvents such as water, while the You can think of it as kind of like a piece of it in suspensionsurface tension in the of oil the until solvent its timeenergy). for a In addition, we can make an hydrocarbon tail generally likes non- in which it is dissolved or the emulsion with a different surfactant (a polar solvents such as oil or other spaghetti with something very different oil change or the particles agglomerate to a large enoughtension sizeat the to interface catch in between the oil filterbetter (see one for our particular system— similarly long, straight-chain non- two immiscible liquids. Add just an emulsifier), mixed with energy, re- polar chemicals. Polar solvents and Figure 2). The polar head generally likes polar sol- a little soap to the water in the sulting in a stableThe emulsion. physical andSo with functional groups on molecules gener- vents such as water, while the hydrocarbon As shownexample in Figureabove, and3 (see the page water 26 ), imaginea little magic, we have mixed oil and ally have some kind of ionic character: tail generally likes nonpolar solvents such very polarno longer calcium beads carbonate on the car hood (CaCO 3water.with chemical behavior of plus or minus charges, partial plus or as oil or other similarly long, straight-chain positivebut charges forms a onsmooth calcium) sheet. dispersed inNow a let’s strip away the alchemist’s minus charges or just atoms with a lot robes, cowl, incantationsliquids cannot and becrow of loosely held electrons. nonpolar chemicals. Polar solvents and nonpolar solvent like oil, hexane, etc., and OPTIONS feathers and lookunderstood at what’s going without on. The polar heads of the surfactant functional groups on molecules generally containing a dispersant that has a polar sul- Okay, let’s try to cut- to the chase. If we First, the surfactant is a molecule that like water; they align themselves like a have some kind of ionic character: plus or fonatewant tohead mix oil(SO and3 water,with wea canminus do charge).typically has a takinglong carbon surface chain ten- tail ball in the water with the polar heads minus charges, partial plus or minus Thus,a poor the job nonpolar by making tails a suspension, are to the outsidewith some kind sionof polar into head account. attached, on the outside (because they like wa- charges or just atoms with a lot of loosely of whichthe micelle, depends onmaking the nature the of calcium the oil carbon-as shown in Figure 1. You can think of ter) and the long nonpolar tails to the ateor compatiblemaybe by just with shaking the it nonpolar up a lot solvent.it as kind of like a piece of spaghetti inside (because they don’t like water).