pH-sensitive surfactants are becoming increasingly important in many novel applications in various fields including medicine, fabrication, sensing, optics, and oilfield. For upstream industry, it can be used in drilling operation, hydraulic fracturing, acidizing treatments, and sand control. The widespread applications of viscoelastic surfactants (VES) originate from the intrinsic duality in their molecular characteristics. Through many years of experience, various classes of VES have been introduced including cationic, zwitterionic (betaine) and especially, alkylamine oxide surfactants which represent a promising and well-characterized class of responsive fluid. These materials have considerable influence on acid placement in oil well stimulation. As the acid reacts with carbonate minerals in particular, the pH rises and the concentration of calcium chloride, magnesium chloride, or both will increase. The presence of salts and the increase in pH will cause the surfactant molecules to form long rod-like micelles. The behavior of wormlike micelles have drawn considerable interest over the past two decades, both from a theoretical viewpoint as well as for industrial and technological applications, the range of which keeps expanding.
The objective of this paper is to focus on the interaction of different types of surfactant with water using density functional theory (DFT) and classical molecular dynamics (MD) simulations. The structural and energetic properties of five-cationic, four-amidoamine oxide, and six-betaine head groups, (all of which are well known importance for the design of efficient VES acid diverters), are calculated with Hartree-Fock and DFT methods in gas phase and water solution. The conformational behavior of the molecules is studied using the analysis of thermodynamic properties. The information obtained may serve to understand the electronic origin of the factors that determine the properties of these simple molecules and to get insight into the more complex ones in which the tail is not a single methyl group rather a chain of C18-C22, and how they are affected by the water environment.