The concept of 2-in-1 shampoo is to balance the formulation by first cleaning the hair during shampoo and second deposit a protective conditioning film on the hair surface during rinse. The first 2-in-1 shampoo was introduced by P&G in 1987.  It used dispersed dimethicone as the conditioning agent [3, 4]. Other conditioning agents, such as polyquaternium, polyvinylpyrrolidone and cationic surfactants are also often used. Cleansing and foaming agents are primarily anionic in nature while the conditioning agents are primarily cationic and simply mixing these ingredients together will cause separation. Conditioning silicones also requires emulsifying/dispersing agents to keep them from separating. To put all the ingredients of a 2-in-1 shampoo together in a stable system and deliver a balanced cleansing and conditioning performance is not straight forward. Recent developments even used nonionic gel network as conditioning agents [5, 6].

There are a few techniques to help formulators to achieve the ultimate goal of a 2-in-1 shampoo. Some use a nonionic or amphoteric as the primary cleansing surfactants to achieve compatibility with conditioning agents. When anionic surfactants are used as primary cleansing agents, control of the molar ratio between anionic to the cationic conditioning agents is crucial. Higher amounts of anionic surfactants are usually required to disperse the conditioning agents or solubilize them through micellization. This will also ensure the product’s cleansing function, compatibility and stability. For such systems, the cleansing surfactants are able to emulsify the dirt and sebum and lift them off the hair at a concentrated stage. During rinsing, the surfactant concentration is diluted and causes the coacervation of conditioning agents, which ultimately allows attachment to the hair surface through electronic interaction and hydrophobic affinity. Depending on the anionic surfactants used in formulation and the chemistry of conditioning agents, the compatibility of a system can be quite different. For example, amino acid surfactants are more compatible with cationic surfactants versus alpha olefin sulfonate. The coacervation concentration can also be different between different chassis, such as water soluble cationic which is different from water insoluble conditioning agents. US patent 6306805 employed a bridging surfactant to help compatibilize anionic and cationic mixture, which can be another tactic [7]. Adding a secondary and tertiary surfactant can also help design a proper coacervation region to deliver the ultimate conditioning performance [8, 9]. Proper selection of anionic and cationic surfactants not only increases product stability but also can create a strong synergistic effect to improve cleansing and reduce build-up. Understanding the physical properties of each ingredient and their interactions are keys for successful formulation.  

Even though it’s not called out as a 2-in-1 shampoo, modern shampoos have fully incorporated the 2-in-1 concept into the conditioning shampoo, hydration shampoo, repairing shampoo, etc. The advances in chemistry and technology also made it possible to use complex formulation to incorporate other aesthetic and functional additives into a single product.

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