Why do detergents containing sulfonate groups work better than soaps at all pH ranges?

Answer 1

Note: Answering this question requires the assumption that by soaps, the person submitting the question meant typical carboxylic acid-containing products. Please read on to understand.

Background. (i) Recall that any time one discusses chemicals (molecules) in practical use, we are actually talking about large "populations"-- where the chemical population of even a few ounces of water in a cup with some added detergent and a drop or two of vegetable oil is literally many billions of billions larger than the entire human population of our planet. Water solutions and suspensions (e.g., respectively, a glass of salt water and of soapy water) are such enormous populations. Bottom line: Molecules are very small, and their practical populations in use are very, very big, almost inconceivably so.

(ii) Levels of acidity of water (aqueous) solutions are measured using the pH scale. The more acidic the solution the lower the pH (e.g., 0-1 is very acidic), while the higher the pH, the more basic/alkaline the solution (e.g., 13-14 is very basic). The pH scale is logarithmic, thus compressing large ranges of acidities into a small range of numbers. Hence, a one unit change in pH corresponds to a ten-fold change, a two unit to a 100-fold change, etc. in the concentration of the active acidifying agent.

(iii) Detergents are most often used to disburse water-insoluble substances and mixtures of substances. The cleansing properties of a population of detergent molecules depend on their ability to form structured aggregates in which many billions of detergent molecules surround a smaller number of molecules of the water-insoluble materials, enclosing them in a water-soluble shell.

(iv) Specifically, detergents are most often molecules that are amphiphilic ("loving both sides"): they contain a hydrophilic group ("water loving", a positively charged or other very water-soluble structure) and a hydrophobic group ("water fearing", e.g., a 8-16 carbon long hydrocarbon chain or a steroid) . This combination of groups works by forming large ("supramolecular") aggregates -- e.g., spherical "micelles" or other emulsifying structures. In these, the hydrophilic groups point outward toward the water, and the hydrophobic groups point inward and "dissolve" similarly hydrophobic substances (e.g., in a familiar sense, cooking grease, or hydrocarbons in oil).

(v) The actual "energetics" of why this works are actually quite complex, with some details of mechanism and driving forces being intriguing, and even counter-intuitive.

(vi) In terms of molecular structure, detergents and soaps are composed of charged amphiphiles. While some charged groups are charged at all pH values (e.g., the alkyltrimethylammonium group of the detergent CTAB), most others are charged only at particular pH values. This arises because of the particular chemical structures of the charged group of the detergent amphiphile.

(vii) The chemical structures of many common commercial detergents and soaps conjoin "greasy" alkyl chains derived from animal or plant fats with a group having an electrical charge of -1 unit at neutral pH values (pH=7, neither acidic nor basic). Here, the charged groups are salts of various kinds of acids. One typical type of acid used in detergent structures are termed carboxylates (salts of fat-derived carboxylic acids, or fatty acids), and another are termed sulfonates (salts of sulfonic acids).

(viii) Acids have a defined pH point of being 50% charged, termed a pKa. Carboxylic acids are considered weak acids, and their pKa = pH 4.5; the proportion of carboxylates in a population that are charged at a particular pH is altered above and below this point. Molecules in a population of carboxylate detergents are almost all negatively charged at pH 6.5 and above, while at pH 2.5 they are almost all uncharged/neutral. Sulfonates, on the other hand, are salts of a extremely strong acids; their pKa is actually < 0 (negative!); hence, they remain negatively charged across the full normal pH range of aqueous solutions. (Sulfonic acids can only be made neutral by acidifying the aqueous solution in a way that is impractical with regard to human handling.)

Question answer. A detergent containing a sulfonate group likely "works better ... at all pH ranges" than a typical carboxylate-containing soap at all pH ranges, because at some pH values, the carboxylates do not form the necessary charged structure to be efficient amphiphiles that can form hydrophobe- (e.g., oil and grease) solubilizing aggregates such as micelles. Sulfonate detergents on the other hand remain charged and so effectively amphiphilic at all accessible aqueous pH values.

Answer 2

Detergents containing sulfonate groups are synthetic and have a more stable chemical structure compared to soaps, which are natural products. This allows them to maintain their effectiveness across a wider pH range without undergoing saponification, which is the reaction that makes soaps less effective at high pH. Additionally, sulfonate groups have a stronger negative charge than the carboxylate groups in soaps, leading to better interaction with water and more efficient cleaning.