Quantifying soapiness; there's pH, pKa and pO2, is there a p_soap or p_surfactance?

Question

Yes, $\mathrm{pH}$ is a concentration, $\mathrm{p}K_\mathrm{a}$ is a dissociation constant, and $\mathrm{pO_2}$ is a partial pressure. These are (roughly speaking) ways to indicate how much of a key ingredient is in a mixture or how active it is.

When I go away for a long weekend and have unwashed dishes, I just soak them in extra-soapy water; arguably to do some pre-cleaning but mostly as a lame attempt at preventing life from taking hold and multiplying exponentially in the hot summer weather before I get back.

All soaps are not created equal

I want to ask in Biology SE about the level of soapiness necessary to prevent this from happening, but first I want to ask here if there is a way to quantify the soapiness of soapy water on some recognized or at least recognizable scale.

There is a wide range of soaps available in a household and when there's no dish detergent per se available and nobody is looking I've been known to use other products. A gram of laundry powder, window cleaner, bar soap, and dish detergent could potentially have very different levels of soapiness and therefore ability to strip living cells of their protective lipid membranes, either in bulk or just key, vulnerable constituents.

Question

For purposes of quantifying soapiness, is there a recognized, or at least recognizable parameter, something like p_soap or p_surfactancein like there's $\mathrm{pH},$ $\mathrm{p}K_\mathrm{a}$ and $\mathrm{pO_2}$ for other situations?

Answer 1

A concept that captures how effective a detergent is at doings its job is aptly called "detergency." As might be expected this is a complex property and difficult to describe unambiguously with a single parameter. Quoting Ref. 1 :

Detergency is difficult to evaluate because it depends on a multitude of variables that in most cases are elusive to monitor and measure.

Given its practical importance it should not be surprising that a lot of effort has been expended to characterize this property, which in technical lingo is referred to as "detersive efficiency". Ref. 1 explains that standardized detergency-testing methods have been developed by the American Society for Testing and Materials (ASTM) or the International Standardization Organization (ISO). The method codes for various tests are listed in Table 1 of that publication. The AISE (International Association for Soaps,Detergents and Maintenance Products) has also developed laundry detergent testing guidelines.

In Ref. 1 detergency is quantified as a parameter $De$, the ratio of mass of soil suspended in the bath after treatment to the total mass of soil in the system.

References

1. E. Jurado Alameda, V. Bravo Rodrıguez, R. Bailon Moreno, J. Nunez Olea, and D. Altmajer Vaz. Bath-Substrate-Flow Method for Evaluating the Detersive and Dispersant Performance of Hard-Surface Detergents. Ind. Eng. Chem. Res. 2003, 42,4303-4310.

Answer 2

Soapiness or anything like that cannot be represented by a single number. Hence no point in inventing such a quantity. Just like we cannot associate a plain number to odors, soapiness is scientifically meaningless because it will be an umbrella term. Just like the term polarity is misused, soapiness could be even worse. The only common property of surfactants is the critical micellar concentration (CMC) but that has nothing to do with how clean our dishes or clothes look after a wash cycle. CMC would be useful if there were a single component in dishwashing or laundry soaps. Alas, our synthetic laundry detergents are a mix of really fancy chemicals.

To give you an idea, I quote from a monograph on how the detergents are tested so one can feel the complexity and understand why quoting soapiness with a single number is not useful. Take an example of a laundry detergent. See where does soapiness fit in? You can extend the same ideas to a dishwashing liquid.

Single wash cycle performance (soil and stain removal and bleaching)

• Multiple wash cycle performance, e.g., after 25 or 50 washes (soil antiredeposition properties, degree of whiteness, buildup of undesirable deposits, fiber damage, stiffness, color change, fluorescent whitening)
• Special characteristics (powder characteristics such as density, free flowability, dispensing in a washing machine, homogeneity, dusting properties, solubility, foaming, rinse behavior, and such storage characteristics as chemical and physical stability, hygroscopicity, color, odor, and tendency to form lumps)

The literature describes numerous methods for testing according to the above criteria, some of which are standardized. Standardization is a concern not only of national bodies (e.g., ANSI, the American National Standards Institute; JISC, the Japanese Industrial Standards Committee; DIN, Deutsches Institut für Normung [559]; AFNOR, Association Française de Normalisation; BSI, British Standards Institution), but also of international groups (e.g., ISO, International Organization for Standardization). The above national organizations are all members of ISO and can, therefore, exercise influence on questions of international standardization [560]. Another particularly important organization concerned with international standardization of test methods is the CID (Comité International des Derivés Tensio-Actifs) with its subcommittee, the CIE (Commission Internationale d'Essai). This organization was disbanded in 1978 , but in the meantime, its activities have been taken over and carried forward by the Working Group TMS (Test Methods for Surfactants) of the CESIO (Comité Européen d'Agents de Surface et Intermédiaires Organiques).

Reference: Laundry Detergents. E. Smulders Wiley-VCH Verlag GmbH & Co. KGaA,

If there detergent properties could be summarized by a single number, all these govt. and regulatory agencies would have come up with a single number. To the best of my knowledge, there is none! I would say that synthetic detergent chemists and those who analyze them (analytical chemists) are certainly very smart and creative people.

Come to Nature's detergents like soapwort seeds. These "fruits" when rubbed or boiled make a good "detergent" solution. Got a chance to use them a couple of times as they are pretty common in South Asia. Their detergent action comes from saponins and even that is not a pure compound but a class of compounds which foam in water naturally but there are not typical surfactants. Again soapiness is not useful even for natural detergents.

Caution: A small note of caution of soaking dishes long term for days in dishwashing liquid solution is not a good idea. Depending on the dishware, small cracks, chips, poor coatings can lead to absorption of detergent in the dishes. Of course, this should exclude all wooden items and even plastic items.