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,
- 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)
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 ; 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 . 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
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.