Colloid and Surface Science Aspects of Disinfectants
Reginald Jacques
Garret Lau
Carla Ng
Pintu Saha
University at Buffalo, Department of Chemical Engineering
Introduction
Product and Consumer Considerations
Marketing
Components and Compositions of Disinfectants for Household Use
General Components of Cleaning Solutions
Common Disinfecting Chemicals
Colloids in Disinfectants: Surfactants
Structure-Property Relationships
Disinfectants of the Future: Current Research
Introduction
Disinfectants represent a wide range of substances that are used in various applications. The food industry requires the use of disinfectants to sanitize food preparation areas, and serve preservative functions. Chlorine and other organic oxidizers are employed for the purification of drinking water. Hospitals and clinics rely on disinfectants to sanitize their medical facilities. Disinfectants are even used for their preservative abilities in paints, inks, cosmetics, and other industries. And of course, the disinfectants most people are familiar with, household disinfectants, serve to help us with controlling germ and bacteria levels in our kitchens, bathrooms, and bodies. Despite the variety in disinfectant materials, they all strive for one desired characteristic: selective toxicity. Disinfectants are engineered to kill bacteria, viruses, and mildew, yet be safe to possible human contact.
In the United States, the primary regulations that disinfectants must abide by are established by the Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA). The EPA regulates drinking water purification, finding ways to limit or replace chlorine as a disinfectant, fearing the biological contamination of organic chlorinated byproducts (w4). Household disinfectants found in soaps and other cleaners are regulated by the FDA, where products are tested and examined for public health and safety (w6). While many disinfectants pass through government agencies for safety approval, we chose to focus our scope on common household disinfectants.
Product and Consumer Considerations
Household disinfectants must meet several criteria. Besides having anti-microbial properties and being safe to use around humans and the environment, household disinfectants are almost exclusively incorporated into a cleaner that provides multipurpose capabilities, aside from just killing germs. A 1996 Hard Surface Cleaner Market Study determined that consumers desire cleaners that disinfect, cut grease, deodorize, and at the same time be cost effective (14). Bathroom cleaners were expected to remove soap scum and stains while leaving no film or residue (14). Cleaners must possess a dispersive quality – the ability to be sprayed or spread onto a surface for the necessary time required for disinfection and cleaning. This often means the incorporation of surfactants and the use of colloidal properties. Soaps and detergents frequently contain moisturizers to alleviate the harsh conditions on the skin. Despite these many considerations in household cleaners, disinfectant and antibacterial properties are among the most important aspects of cleaners consumers use most often.
While consumers worry about the sterilization capability of cleaners, a distinction must be made between the degrees of sterilization. Many common household cleaners are simply antibacterial. Hand soaps and liquid dish detergents contain triclosan, also known by its trade name Microban, which is strictly an antibacterial (5). Salmonella, E. coli, and bacteria that cause strep throat and staph infections are typically the common targets of triclosan. A true disinfectant also kills viruses and other pathogens along with bacteria. Alcohols, bleach (sodium hypochlorite), quaternary ammonium compounds, and certain oils comprise the most familiar household disinfectants. All disinfectants used in household cleaners must meet EPA or FDA approval. Regulations outlined by the EPA include stringent tests against Salmonella choleraesuis, Staphylococcus aureus and other bacteria, along with a performance mandate that in order to be termed a disinfectant for use on hard materials, the disinfectant must kill on 59 out of each set of 60 carriers and is required to provide effectiveness at the 95% confidence level (w5).
Marketing
The increasing popularity of disinfectants in common household products is a recent trend. Between 1997 and 1999, manufacturers introduced 700 everyday products claiming antibacterial or disinfectant properties (5). Disinfectant cleaners make up half of the $2.1 billion cleanser market in the United States (15). Marketing household disinfectants is not about convincing consumers to sterilize their homes – studies show consumers already have this fear. Rather, the effectiveness of the cleaner in cleaning and disinfecting as compared to other brands is important, along with its relative price compared to other brands. Brand name cleaners may not necessarily provide a better product – often generic brands contain the same concentration of active ingredients.
Final product considerations reside in packaging and sales factors. Typical cleaners sell for $2-$4 for 32 ounces, representing the smaller sample size for household use (5). Packaging of disinfectants mostly involves the chemically inert plastic bottles that cleaners are supplied in. Variations exist in the aluminum spray cans for aerosol cleaners, and now technology is finding ways to introduce disinfectants into other mediums. N-halamine structures have been polymerized with a grafting process into cellulose containing and nylon fabrics, giving everyday materials anti-microbial properties within themselves (16). Triclosan is also being incorporated into a polymer for use in fabric seat covers, tables, chairs, and clothing sanitation (9). With so many methods of incorporating disinfectants into the household and the relative inexpensiveness of processing these materials, there seems no immediate decline to the increasing market of disinfectant products.
Components and Compositions of Disinfectants for Household Use
General Components of Cleaning Solutions
There are seven main ingredients found in most household cleansers. These are (in order of decreasing amount) (w12):
(1)Surfactants: these are amphiphilic molecules which serve several purposes in a disinfecting cleaner:
a)they adsorb to surfaces, where they aid in loosening and removing soils
b)they hold particles in suspension and prevent redeposition on the surface
c)they cause “wetting” be reducing the surface tension of water and allows it to spread over the surface
d)anionic (negatively charged) surfactants are best against particulate dirt and oily soils, but can react with minerals in hard water to form scum
e)cationic (positively charged) surfactants are effective as germicides on hard surfaces
f)nonionic surfactants have no charge and therefore are able to work even in hard water, are low foaming and well-suited for no-rinse applications ().
Surfactants will be treated in greater detail in later sections of this report.
(2)Builders: these compounds are primarily responsible for reacting with hardness chemicals (such as calcium and magnesium) to keep them from interfering with surfactant action; in other words, they are water softeners. They can also aid in keeping soil particles in suspension, and are especially useful in all-purpose cleaners. These are found in three types:
(a)sequestering builders form tightly-bound, water-soluble complexes with Ca and Mg ions
(b)precipitating builders form insoluble calcium compounds, which will then need to be removed from the surface as it is cleaned
(c)ion exchange builders “neutralize” hardness mineral through electrical charge exchange.
(3)Abrasives: these particles are added to increase the mechanical cleaning ability of a product. They usually consist of small hard particles. Some examples are silica, calcite and feldspar.
(4)Acids: these are used to dissolve calcium and metal salts. They are common in “tub-and-tile” type cleaners used in the bathroom.
(5)Alkalis: these chemicals give cleaning solutions a high pH and help remove solid grease, as well as providing some building action. Mild alkaline chemicals, such as baking soda, may be used in products formulated for contact with skin.
(6)Antimicrobial agents: these are the focus of our report, and are chemicals that destroy bacteria and viruses, as well as (in some cases) fungi. These agents must be registered with the EPA before they can be sold. A description of the most common disinfecting agents follows this section.
(7)Bleaching agents: these components attack dirt by chemical breakdown, normally via oxidation. Stains are broken down into smaller, colorless forms that are easier to remove. The most common bleaching agent is sodium hypochlorite, which is capable of destroying bacteria, viruses and mold.
In addition to these major components, household disinfectant products may contain: colorants, which provide a purely aesthetic effect; enzymes, which are capable of breaking down specific organic soils; fragrances, to cover the base odor of the cleaning solution as well as leave a pleasant, “clean” scent behind; polymers, which can be used in floor cleaners to provide a shiny, dirt-repelling film after drying, or in more general cleaners as building or thickening agents; processing aids, which keep the product from separating during storage and helps give it the desired dispensing characteristics; preservatives, which protect against bacterial attack; and solvents, which are particularly useful in the removal of grease without leaving a residue, such as is desired with glass cleaners.
In the following sections we will describe in greater detail disinfecting chemicals and surfactants, the two key components in household disinfectants.
Common Disinfecting Chemicals
Disinfectants for household use are divided into four main subcategories, as follows: alcohol, chlorine compounds, iodine compounds and quaternary ammonium compounds. These disinfectants are active at many pH levels. Iodines, for example, are most effective in the lower pH ranges, from ~2-6, whereas chlorines work best at the higher end, from 6-10. Quaternary ammonium compounds are the most versatile, with a working range between 3 and 10.5 (2, Fig 23.1, p.477). For this reason, household products containing disinfectants are similarly solutions at various pH levels.
Alcohols
One of the most common chemicals present in the average person’s medicine cabinet is rubbing alcohol. It is often used as an antiseptic to clean minor wounds, and also as a hard surface cleaner. Like many chemical disinfectants, alcohols are generally considered to be nonspecific antimicrobials. They show a multiplicity of toxic effect mechanisms. This has important implications for the spectrum, speed and overall effectiveness of alcohol as a disinfectant. Not all alcohols show bactericidal effect; the amount of inhibition increases with the chain length of the alcohol (see 10, Table1, p.800).
Chlorine Compounds
Chlorine compounds are some of the most active ingredients in disinfectants. Use of chlorinated lime as a deodorant for sewage goes as far back as 1854 in Great Britain. Calcium hypochlorite has now largely replaced the older chlorinated lime, and sodium hypochlorite is th emost active principle of many household products. Various types of algae, bacteria, fungi, protozoa and viruses have shown resistance to hypochlorites (2, Table 7.1, p.143). The bactericidal action of hypochlorites is caused by the release of hypochlorous acid and contributions of hypochlorite ions (OCl-). Hypochlorites are subject to gradual deterioration over a period of time, which depends on three main factors. The most important factors are the pH and temperature of the environment. The lower the pH, the less stable the solution, but the more germicidal its action.
Chlorine dioxide is used a great deal for drinking water and wastewater treatment (2). It has the ability to break down phenolic compounds and removes phenolic tastes and odors from water. There are numerous antimicrobial chlorine compounds, but a major advantage of this particular formulation is that is does not form trihalomethanes (THMs) or chlorophenols, which are both harmful to the environment and have been coming under scrutiny by environmentalists making groundwater studies.
The major advantages of chlorine compounds are that they have very fast reaction times and are effective biocides for a broad spectrum of microorganisms. They are inexpensive compounds that do not foam, are not temperature dependent, and can be used in liquid or powder form. The main disadvantages are that chlorine is unstable in concentrate. It reacts strongly with organic materials and is corrosive to metals. More importantly, it is unfriendly to the environment.
Iodines
Iodine, the heaviest of the common halogens (126.9 g.mol), melts at 113.5ºC to a black liquid, and is a valuable ingreadient in antiseptics. Iodine is a highly reactive substance combining with proteins partly by chemical reations and adsorption. Iodine-based disinfectants can be divided into three main groups according to the solvent and substances interacting with the iodine species: pure aqueous solutions, alcoholic solutions and iodophoric preparations. They exhibit essential differences in their chemical and microbiocidal properties. The iodine compounds not only kill microorganisms but also interact with the materials to be disinfected. To understand these interactions, knowledge about the particular species, solvent, equilibrium concentrations and individual reactivity is essential.
Iodine ions are often added to increase the solubility of iodine in water. This increase takes place by the formation of triiodide, I3-. Pure aqueous solutions, for the iodine-water system, produce at least ten iodine species:
I-, I2, I3-, I5-, I6-, HOI, OI-, HI2O-, I2O2-, H2OI+, and IO3-.
The ratio of their formation depends on the concentration of iodine.
Iodophors are polymeric organic molecules, such as alcohols, amides and sugar, which are capable of forming iodine species. This results in reduced equilibrium concentrations of species compared with those of pure aqueous solutions with the same total iodine and iodide concentrations. Since iodophoric preparation always contains appreciable iodide, the relevant species tat must be considered are restricted to I-, I2 and I3-, for the following simplified reactions (2, p.168):
I2 + R R.I2
I3- + RR.I3-
I-+ R R.I-
R represents the structural regions of the iodophor molecule capable of forming complexes by electronic effects.
An important solubilizing agent and carrier for iodine is poly(vinylpoyrrolidinone) (PVP). PVP-iodine is externally used on humans as an antiseptic. Some commercial brands are Betadine and Isodine.
Quaternary Ammonium Compounds
Quaternary ammonium compounds are often used in contact lens solutions for cleaning and preservative purposes.
The antibacterial precursors of the quaternary ammonium compounds (“quats”) are aliphatic long-chain ammonium salts. The direct counter part of soap may be considered as a primary ammonium salt. Both are surface-active substances. In soap, the anion contributes the hydrophobic part and the primary ammonium salt (the cation) is hydrophobic.
The primary long-chain ammonium salts are derived from the weakly basic aliphatic amines. Their aqueous solutions require a pH low enough to counteract hydrolysis and partial liberation of the amine base. Because quats ae salt bases, they remain in solution in acidic as well as in basic media. Quaternary ammonium salts produce bacteriostasis in very high dilutions. This property is associated with the inhibition of certain bacterial enzymes, especially those involved in respiration and glycolysis.
Among the many quaternary ammonium salts available, only a small number are of interest as antibacterial agents. Among them are: benzalkonium chloride, alkylbenzyldimethylammonium chloride, methydimethyl ammonium chloride, methylbenzethonium chloride, hexadecylpyridinium chloride, and alkylisoquinolinium bromide (10).
It is evident from the varied examples of disinfectant chemicals above that we as consumers have a wide range of products to choose from when it comes to ridding our homes of germs. Each has specific properties, advantages and disadvantages and it is important to keep this in mind, as well as the intended use, when choosing an appropriate cleaning product.
Colloids in Disinfectants: Surfactants
Colloid science is concerned with the study of materials that exist as dispersions in a medium of some other material. They are sometimes defined as particles that would remain suspended in water for an extended amount of time. A colloid differs from a true solution in that the dispersed particles are larger than normal molecules, though they are too small to be seen with a regular microscope. The typical size of a dispersed particle is from a few nanometers to several micrometers. One consequence of this small size is a high surface area, so that the properties of the interfaces may become important. The common element among all the types of colloids is the fact that they are held in suspension by electrostatic interaction with water molecules. Another important parameter is the thermal motion, which dominates the dynamic properties. Crucial examples include, food, paint, and household products (12).
Colloids in which the continuous phase is water are classified as follows: hydrophilic colloids, hydrophobic colloids and association colloids. The first two types differ from each other by their chemical configuration and/or composition.
Hydrophilic colloids are large molecules that contain functional groups as an integral part of their structure. The functional groups form hydrogen bonds with water molecules. Common examples of hydrophilic colloids are proteins and synthetic polymers. Two examples of hydrophilic colloids are depicted below (11):
Figure 1: Hydrophilic colloids
Hydrophobic colloids are substances that have charged surfaces in water, and form an electrical "double layer" that holds them in suspension. Clays form a negative charge on their surface when placed in water, and remain in suspension by the electrostatic interaction between the negative surface charge and positive charges from cations in the water. The figure below depicts the colloidal clay particles that are suspended in solution by electrostatic interaction (11).