Aggregates of Ethoxylated Surfactant with Added n-Octanol: Details of Corona Structure from a Molecular-Thermodynamic Model

Abstract

Polar heads of many nonionic surfactants have complex chemical structure. In the micellar aggregates these heads are engaged in specific and nonspecific interactions with one another and with the hydration water. We present here a molecular-thermodynamic aggregation model that takes into account explicitly the hydrogen bonding and other specific interactions within the hydrated corona of a mixed nonionic aggregate. Apart from the aggregation characteristics that are usually obtained from the classical aggregation models – critical micelle concentration (CMC), the distribution of the aggregates over the size and shape and their composition - our model helps to establish a number of likely details of structure, including the corona thickness, hydration, and the partial penetration of the micelle components in the corona region. In this work the model is applied to describe the aggregation equilibrium in aqueous solutions of nonionic surfactants of the polyoxyethylene-p-(1,1,3,3-tetramethylbutyl)phenyl ether family that is closely related to Tritons, the commercial surfactants of an extensive practical use. The model gives reasonable estimates of the CMCs and predicts correctly the trends in CMC and in the aggregation numbers vs the number of oxyethylene groups in the surfactant molecule. The hydration numbers and the fraction of water engaged in hydrogen bonds are calculated. For the hydration of the aggregate coronae, the predicted trend is in line with that found for other ethoxylated nonionic surfactants: poly(ethylene oxide) alkyl ethers and neonols. For surfactant solutions with added n-octanol, the model suggests that the tails of the octanol molecules are deeply buried in the cores of the mixed micelles with only one methylene group penetrating the coronae. We predict co-surfactant effect of added n-octanol: the depression of the CMCs is progressively stronger for more hydrophilic surfactants with larger EO heads.