The Temperature Paradox By Aziz Ullah Ph.D. It has long been considered as fact that increased heat leads to increased cleaning ability and efficiency. In the following article Dr Aziz Ullah, one of the cleaning indusrtry's leading chemists, explodes some of the myths surrounding the "more heat is better" argument and provides proof of the loss of cleaning ability at certain high temperatures on certain surfaces. The conclusions drawn by Dr Aziz may well suprise you and challenge established concepts held in the cleaning industry The Cleaning Process The three principal mechanisms of soil removal are: surface detersive processes, mechanical processes, and chemical actions or reactions. Detergency involves the use of surfactants (surface active agents) to remove soil through surface detersive processes which involve solubilization and emulsification. Even though surface chemistry is a distinct and recognized field in the annals of science. The surface detersive action is purely physical, as no chemical bonds are created or broken and no new chemical compounds are formed. This distinction is Important as we will later relate in this article. Mechanical processes involve the use of mechanical action, such as agitation, the impinging action of pressure sprays, or the buffing action in bonnet cleaning. To a much lesser degree, some soil removal relies on chemical actions or reactions where the soil is chemically degraded. This can happen when a) high alkalinity is used to solubilize natural oils and fats by a process known as saponification, b) bleaches are used to oxidize stains, and c) enzymes are used to break down protein soils and stains. In each case, chemical bonds are broken and new ones formed. Out of the three, the detersive and the mechanical processes are recognized as the two most important factors in practical detergency, and most practical operations depend on them. In most applications, soil removal through the detersive process is preferred because it offers a more cost effective and versatile approach and usually does not permanently affect the textile surface. In 1948, Bacon and Smith found that within reasonably wide limits the following relationship was valid: S=K(CFT)", where S is the soil removal, C is the detergent concentration, F is force (mechanical) and T is time (contact or dwell time). K and n are constants, and n is always positive and less than unity. According to this equation, soil removal will increase by increasing any of the variables (time, force or detergent concentration) and likewise decrease by decreasing these variables. Energy Plateau In 1951, Dickinson and Palmer studied the effects of mechanical action in the cleaning of wool and found that mechanical action reached a plateau after which further increases in mechanical action did not cause any more soil removal. In cleaning of textiles, controlling the energy (heat, agitation) is important. Too large an energy input for too long a time can damage the textile fibres. All surface detersive processes involve wetting to some degree, but mechanisms vary depending on the soil. Liquid oil soils are removed via a "roll-up" mechanism. Solid organic soils (waxes, greases) also involve processes that liquefy the soil (heat or solubilization). For particulate soils that do not liquefy, the surfactant molecules adsorb onto the surface of the particulate matter and the surface to be cleaned. This confers a similar charge on both surfaces, which repel each other and help lower adhesion, suspending the particulate matter for its removal. Temperature Effects The effect of temperature depends on the type of soil and cleaning chemical. A first consideration is to use enough temperature to melt soil that can be melted, such as those containing wax, fats or grease, which are the common components of soil. This can make a significant difference in the rate of cleaning. In many cases, the soil (fat or wax) melts to a low viscosity liquid, and further increase in temperature has a limited effect. For most oils, an increase in temperature reduces viscosity, making it more mobile and, therefore, more easily removed. Most reactions are faster at higher temperatures. Even fireflies flash (luminescence, a biochemical reaction catalysed by the enzyme luciferase) at a faster rate on warmer nights. The Arrhenius Equation The acceleration of chemical reactions by increased temperature is an everyday experience. A well-established principle states that the rate of chemical reactions is doubled for each 18" F increase in temperature. Reactions of larger molecules, such as proteins, increase even more - fiftyfold for every 18"F rise in temperature. An example is the rate of reaction (denaturation) of albumin (the process that occurs when an egg is boiled). The Swedish chemist Svante Arrhenius, for whom the equation is named, showed that the relationship is applicable to almost all kinds of chemical reactions. The Arrhenius equation, the mathematical expression that describes the effect of temperature on the velocity (speed) of chemical reactions, was originally formulated by J. J. Hood on the basis of studies of the variation of rate constants of reactions with temperature. The Arrhenius temperature dependence is quite simple. Not only do the reaction species meet in order to undergo a chemical reaction, but they also must have enough energy to break and reorganise their chemical bonds. The Equation Does Not Fit Let us examine the temperature/cleaning dogma being advocated. Folklore teaches that "every 18"F in temperature above 118"F will double the molecular activity and hence the cleaning." Both these assumptions are wrong. The 118"F arbitrary limitation, that is being orchestrated as the temperature at which the Arrhenius rule becomes applicable, is itself a contradiction of the Arrhenius rule. When a graph of the reaction rate is plotted against the temperature, the Arrhenius equation gives a straight line. (See graph below.) A temperature difference of I 8"F on any point on either side of this line either doubles or halves the reaction rate. The Arrhenius equation dictates that it is applicable at all temperatures. Even at very low temperatures, some reaction will occur, although it may be immeasurably slow or hardly detectable. This means that the reaction or increase of reaction not only will be constant with change in temperature but so will be applicable throughout the temperature range, not just over 118"F. Also, most steam cleaning chemicals contain nonionic surfactants that exhibit separation of the surfactant rich phase with increasing temperature, which is known as the cloud point. At the cloud point, the surfactant begins to lose sufficient solubility to perform some or all functions as a surfactant. These nonionic surfactants are known to clean best just above the cloud point. Any further increase in temperature will cause the cleaning efficiency to rapidly drop off. In spite of significant heat "during the steam cleaning process, increasing temperature too high above the cloud point (some surfactants have cloud points as low as 32"F, the freezing point of water) will rapidly reduce detergency. Hence the 118"F limitation at which the Arrhenius equation is supposed to apply is only groundless but fallacious as well. As stated earlier, the Arrhenius rule is applicable to virtually all-chemical reactions. Some chemical reactions are possible in cleaning, but they are of minor consequence. Chemical reactions can take place because of bleach, enzyme or the alkalinity of the cleaning solution. Because bleaches are able to adversely affect dyes, they have limited use in the steam extraction process. Enzymes function within a narrow temperature range and are deactivated at higher temperatures. Alkaline materials can chemically react with soils composed of natural oils, fats, and greases through saponification. Neutralization of acid soils probably does take place; however, with the amount of alkali present in the cleaning solution and the limited contact time during the steam cleaning process, the occurrence of any saponification is doubtful. Research Studies Now let us look at some of the many studies on the effect of temperature on cleaning, using different soils, cleaning agents, and surfaces. Cleaning is usually poorer at lower temperatures; however, in 1967, Spangler and co-workers reported better cleaning of polyester at 600F than at 120"F. In 1967, and again in 1969, Gordon and co-workers found similar effects. These studies were published in the Journal of American Oil Chemists Society. In another study presented at the American Oil Chemists Society meeting in New Orleans in 1970, radio-labeled soils, both polar (ionic type) and non-polar (nonionic type), were used. Gamma ray counting methods to determine soil removal were employed. (See table below.) The cleaning was done under care fully controlled conditions. Results show detergency of soiled cotton was less efficient at 600F than at 1200F, performing at approximately 70% to 80% of the 120"F level. Detergency of an anionic-based product was found to decrease with an increase in water hardness. Surprisingly, polyester and nylon behaved quite differently than cotton. A reverse temperature sensitivity was found for nylon when using a nonionic surfactant: at 1200 detergency was 68% to 96% of the 60"F level. Using an anionic surfactant, it ranged only 20% to 38% of the 60" level. The non polar soil was as much as three times more readily removed than the polar soil. The researchers suggest that low temperature cleaning of synthetics may be advantageous. Under test conditions, the nonionic surfactant was found to be twice as effective in removing soil from polyester and even more effective on nylon. As previously stated, a well-known fact is that nonionic surfactants exhibit separation of the surfactant-rich phase with increasing temperature. This is aptly called the cloud point. In a paper presented at the World Surfactant Congress in Munich, Germany, in 1984, Benson and co-workers found that optimum cleaning occurs just above the cloud point, while at still higher temperatures, detergency rapidly declines due to coalescing and separation of the surfactant phase. Using polyester/cotton blend swatches soiled with radio-labeled oil/lampblack, they found 20% soil removal at 680F, 25% soil removal at 1400 and only 18% soil removal at 176"F. In a paper presented in Chicago at the 1979 mid-year meeting of leading experts from the soap and detergent industry. Scharer and associates using different surfactant systems on linoleum panels coated with oily pigment soils, found that cleaning at 400F ranged from 56% to 76% of the cleaning efficiency at 74"F. In yet another study FMC, a producer of cleaning chemicals, using china dishes with baked-on food soil consisting of eggs, milk gravy and starch and a built detergent system, found that 85'~/0 of the soil was re moved at SOOF and only 55% of the soil was removed at 1200. Of course, in this case study easier-rinsing dishes were used. Conclusions A very large number of detergent studies have been published. Some studies are concerned with the effects of variables such as pH, salt content, builder content, water hardness, temperature and the nature of the substrate textile. The results are frequently expressed as curves of detergency surfactant versus concentration. In some cases, where another variable is being studied, the concentration may be held constant. For example, in studying the effects of temperature variation on detergency, the results may be expressed as curves of detergency versus temperature at constant concentration. The data are completely reliable. In fact, they are unchallengeable; however, they can seldom be prorated or used to deduce the properties of another detersive system. In other words, these studies are difficult or impossible in constructing broad generalizations of detergency. In some cases, the data become inapplicable whenever minor changes are made in the soil/ cloth system or in the mechanical and physical conditions of cleaning. However, we can safely make the following conclusions: 1 ) Even though the cleaning action is by the chemical, the action it self is essentially physical and not chemical action or reaction. 2 ) Higher temperature, as expected, helps the cleaning process; however, studies also show that as the solution (detergent) temperature increases, the cleaning activity does not necessarily increase. It can also decrease with increasing temperature. 3 ) In the studies described and numerous other studies carried out, radioactive tracers indicate that soil residues can be detected even after good detergency, although other methods of measurement indicate the surface to be clean. 4 ) The belief that at 180F the cleaning or molecular activity doubles for every 18"F increase in temperature has no basis. Clearly, the Arrhenius equation does not apply to the cleaning process. This is not to say, how ever, that cleaning is necessarily unaffected by temperature. A scientifically valid equation is being miss-applied. Not only is it inconsistent with the best available data, it is misleading as well. For those who believe that in creased temperature affects the cleaning process exponentially, this information may not be very inspiring. The unfortunate truth is that anecdotal conclusions are being drawn. These conclusions are a warm home too much loved theories that just do not pass scientific muster. Ambiguous connections are being made between legitimate science and speculation in order to make ideas that are largely built on unsound reasoning sound convincing. An amazing number of incredulous myths have stayed on for so long that they have become a part of the vocabulary. Myths can harm us all in the long run; the result will be further erosion of credibility and respect. We are not talking about rocket science but the very fundamental aspects of cleaning. Those who have been espousing these tall tales perhaps do not clearly understand the cleaning process. To make cleaning part of main stream science, we have to venture out of the proverbial cocoon by expunging the myths. Perhaps, we have been too deluded to do that. If we truly wish to make cleaning a science, we must stop perpetuating unfounded theories and take the time to delve deeper into scientific principles.