The Effect of NaCl on the Krafft Temperature and Related Behavior of Cetyltrimethylammonium Bromide in Aqueous Solution

Abstract

This paper presents the effect of NaCl on the Krafft temperature (TK), surface adsorption and bulk micellization of cetyltrimethylammonium bromide (CTAB) in aqueous solution. The critical micelle concentration (CMC) of CTAB in the presence of NaCl increased and then decreased with increasing temperature. Thus, the CMC–temperature data can be represented by a bell‐shaped curve. The micellar dissociation (fraction of counterion binding) and energetic parameters (free energy, enthalpy and entropy changes) of both adsorption and micellization were calculated. The processes were found to be both enthalpy and entropy controlled and appeared to be more and more enthalpy driven with increasing temperature. An enthalpy–entropy compensation rule was observed for both adsorption and micelle formation. The TK of the surfactant decreased significantly in the presence of NaCl, which is a sharp contrast to the usual behavior of the effect of electrolytes on the TK of classical ionic surfactants. The surface excess concentrations decreased with increasing temperature. However, the values were much higher in the presence of NaCl compared to the corresponding values in pure water. The solubilization behavior of a water‐insoluble dye, Sudan red B (SRB), in the micellar system was studied by the UV–visible spectrophotometric technique. The molar solubilization ratio in the presence of NaCl was found to be about three times higher than that in pure water, indicating that the solubilization of SRB in the CTAB micelles significantly increases in the presence of NaCl.

Counterion Effect on Krafft Temperature and Related Properties of Octadecyltrimethylammonium Bromide

“…The T K of pure OTAB was found to be 36.7°C. This result is consistent with observation that the T K increases linearly with increasing the hydrocarbon chain length [1] and that the T K values of the C14 and C16 homologues are found to be 13.2 [25] and &24.8°C [1,7,23], respectively. Figure 2 depicts the influence of the concentration of different counterions on the T K of the surfactant.…”

“…Ions with relatively high charge density such as SO 4 2-, F -, Cl -, NO 3 -, benzoate and salicylate lower the T K while those with low charge density such as Br -, SCNand Iraise the T K of the surfactant. Previous studies reveal that the T K increases gradually for CTAB [7,23], CPB [24] and SDS [7] in the presence of their respective common ions in aqueous systems. Therefore, the decrease in the solubility with a consequent increase in the T K of OTAB in the presence of Brcan be attributed to the common ion effect.…”

“…Although these studies provide useful information about the effect of electrolytes on the adsorption and micellar properties of ionic surfactants over a wide range of temperature, relatively less attention has been paid to the influence of a series of added counterions on the T K and CMC of ionic surfactants. In this work, we have investigated the effect of a number of counterions of the Hofmeister series as well as two hydrotropes on the T K , and micellar behavior of octadecyltrimethylammonium bromide (OTAB) in aqueous solution, which is an extension to our previous work [21][22][23][24]. The main objective of the present work is to lower the T K of the surfactant so that it can readily form micelles in the bulk of the aqueous solution, which is an important criterion of surfactants from the viewpoint of both household and industrial applications.…”

Molecular Dynamics Simulations of Cetyltrimethylammonium Bromide (CTAB) Micelles and their Interactions with a Gold Surface in Aqueous Solution

“…5 The minimum concentration required for surfactant aggregation is defined as the critical micellar concentration (CMC), and most of the characteristics of these aggregates are controlled by factors such as the solvent type, chemical structure of the surfactant, and solution conditions (e.g., concentration, temperature, presence of additives, and ionic strength).…”

“…3,4 Above a certain temperature in solution (the Krafft temperature), surfactants tend to aggregate to minimize unfavorable interactions between the surfactants and the surrounding environment. 5 The minimum concentration required for surfactant aggregation is defined as the critical micellar concentration (CMC), and most of the characteristics of these aggregates are controlled by factors such as the solvent type, chemical structure of the surfactant, and solution conditions (e.g., concentration, temperature, presence of additives, and ionic strength). 6 Variations of these factors yield aggregates with different morphologies such as spherical or ellipsoidal micelles, cylindrical or thread-like micelles, disk-like micelle, membranes and vesicles.…”