By Geoff Wylie   

Before discussing hydrotropes, it is essential to address one of the more frequently overlooked properties in formulation chemistry: the cloud point.

The cloud point is defined as the temperature at which a nonionic surfactant, within a micellar or microemulsion system, separates into two distinct phases: a surfactant-rich layer and a surfactant-poor layer. Cloud point is influenced by several factors, including the type of surfactant, concentration, carbon chain length, degree of ethoxylation, and the presence of additives such as co-surfactants and, most significantly, electrolytes.

Cloud point considerations are particularly important for low-foam surfactants, which become insoluble at their cloud point and consequently lose their foaming properties. Temperature-dependent low-foam surfactants are often designed to operate at or near the cloud point to achieve minimal or no foam generation. A common example is rinse-aid formulations used in automatic dishwashing applications.

Cloud point determination is especially important when working with nonionic ethoxylate chemistries. A straightforward method for determining cloud point involves filling a 10 mL graduated cylinder with the product sample and inserting a glass thermometer into the liquid. The cylinder is then placed in a water bath, and the temperature is gradually increased until the product becomes visibly cloudy. At this stage, the cylinder should be removed from the bath and the solution gently agitated using the thermometer until clarity returns. The temperature at which the solution clears is recorded as the cloud point. For accuracy and reproducibility, it is recommended that the procedure be repeated three times and the results averaged.

Hydrotropes, also known as coupling agents, play an important role in cloud point adjustment. During laboratory formulation work, it is not uncommon for a finished product to separate into two layers at room temperature, indicating that the cloud point is below ambient conditions. In many cases, this issue initially appears as a hazy or cloudy formulation at room temperature. To restore homogeneity, the cloud point must be increased through the addition of a suitable hydrotrope. If the cloud point already exceeds room temperature, the formulation will typically remain homogeneous.

However, achieving homogeneity at room temperature alone may not be sufficient. The cloud point must also be optimized to withstand anticipated storage and transportation conditions. For example, products shipped to regions such as the southern United States, including Arizona, may encounter temperatures as high as 55°C (131°F) during transport, whereas products distributed in locations such as Toronto, Canada, may only be exposed to temperatures approaching 40°C (104°F).  The choice of hydrotrope should take into account all the conditions the product may experience.

Hydrotropes that are amphoteric in nature and are generally compatible with nonionic, anionic, and cationic surfactants. While amphoteric hydrotropes are low in cost, they may require higher concentrations to raise the cloud point to the desired level which adds cost to your overall formula.  Additionally, they do not contribute to detergency or foam enhancement for the cleaner.

Another commonly used chemistry is based on iminodipropionates. JK50™ is an example of this type of chemistry. This material is more active and effective than amphoteric hydrotropes, allowing lower use levels to achieve the desired cloud point adjustment. Although the raw material cost may be higher, the reduced dosage can provide an overall cost-effective solution. Furthermore, JK50™ does not adversely affect foam profile or detergency.

New innovation in hydrotrope development has produced a product, that, in addition to improving cloud point stability, can now enhance detergency performance and contribute positively to foam generation.  SDH™ is an example of this type of technology.  SDH™ requires a lower use level than the iminodipropionates at similar pricing which can lower overall formula cost.  Its advantages become particularly evident in formulations containing high concentrations of caustic soda, typically in the range of 5–10% active content as it was designed to accommodatethese types of formulations. This new technology raises the possibility of lowering overall formula costs by using raw materials that can contribute more than just cloud point adjustment to help achieve overall formula goals.