Key surfactant characteristics that define a surfactant's behaviour and performance and are to be considered during a formulation design are:
𝟭. 𝗖𝗿𝗶𝘁𝗶𝗰𝗮𝗹 𝗠𝗶𝗰𝗲𝗹𝗹𝗲 𝗖𝗼𝗻𝗰𝗲𝗻𝘁𝗿𝗮𝘁𝗶𝗼𝗻 (𝗖𝗠𝗖)
𝟮. 𝗖𝗹𝗼𝘂𝗱 𝗣𝗼𝗶𝗻𝘁 (𝗖𝗣)
𝟯. 𝗛𝘆𝗱𝗿𝗼𝗽𝗵𝗶𝗹𝗶𝗰-𝗟𝗶𝗽𝗼𝗽𝗵𝗶𝗹𝗶𝗰 𝗕𝗮𝗹𝗮𝗻𝗰𝗲 (𝗛𝗟𝗕)
𝟰. 𝗠𝗼𝗹𝗲𝗰𝘂𝗹𝗮𝗿 𝗪𝗲𝗶𝗴𝗵𝘁 (𝗠𝗪).
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𝗖𝗿𝗶𝘁𝗶𝗰𝗮𝗹 𝗠𝗶𝗰𝗲𝗹𝗹𝗲 𝗖𝗼𝗻𝗰𝗲𝗻𝘁𝗿𝗮𝘁𝗶𝗼𝗻 (𝗖𝗠𝗖):
The CMC is the concentration above which surfactant molecules start to form aggregate in a solution called micelles. At concentrations below the CMC, surfactants exist as individual molecules (monomers), but, once CMC is achieved, they aggregate into micelles encapsulating dirt and oil.
𝗙𝗮𝗰𝘁𝗼𝗿𝘀 𝗜𝗻𝗳𝗹𝘂𝗲𝗻𝗰𝗶𝗻𝗴 𝗖𝗠𝗖
• 𝙎𝙪𝙧𝙛𝙖𝙘𝙩𝙖𝙣𝙩 𝙨𝙩𝙧𝙪𝙘𝙩𝙪𝙧𝙚: Increasing the hydrophobic tail length decreases the CMC, while increasing the hydrophilic head size raises the CMC.
• 𝙏𝙚𝙢𝙥𝙚𝙧𝙖𝙩𝙪𝙧𝙚: Higher temperatures generally decrease the CMC, as thermal energy helps overcome repulsive forces between hydrophilic heads.
• 𝙋𝙧𝙚𝙨𝙚𝙣𝙘𝙚 𝙤𝙛 𝙚𝙡𝙚𝙘𝙩𝙧𝙤𝙡𝙮𝙩𝙚𝙨: Adding salts can reduce the CMC by screening charges on ionic surfactant heads.
𝗜𝗺𝗽𝗮𝗰𝘁 𝗼𝗻 𝗧𝗲𝘅𝘁𝗶𝗹𝗲 𝗔𝗽𝗽𝗹𝗶𝗰𝗮𝘁𝗶𝗼𝗻𝘀
• 𝙒𝙚𝙩𝙩𝙞𝙣𝙜: As surfactant concentration increases, surface tension decreases significantly until the CMC is reached. This reduction is critical because lower surface tension allows water to penetrate surfaces and fabrics more effectively, enhancing wetting and cleaning power.
• 𝘿𝙚𝙩𝙚𝙧𝙜𝙚𝙣𝙘𝙮: At concentrations above the CMC, detergency increases significantly as micelle interact, trap, solubilize and remove oil, grease, and soil resulting improved detergency in textiles. Conversely, at concentrations below the CMC, surfactants may be ineffective, as they cannot form micelles to solubilize dirt effectively.
• 𝙁𝙤𝙖𝙢 𝙎𝙩𝙖𝙗𝙞𝙡𝙞𝙩𝙮: Above CMC, micelles stabilize foam, which enhances the cleaning action by allowing the surfactant to remain in contact with the surface longer (e.g., in laundering).
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