Effect of the Hydrophilicity of Aromatic Counterions on the Structure of Ionic Micelles

Abstract

Small-angle neutron scattering experiments have been carried out on aqueous micellar solutions of cationic surfactant cetyltrimethylammonium bromide (CTABr) in the presence of different aromatic salts. The salts with different hydrophilicity of the counterions that have been used are sodium benzoate (NaB), sodium 2-hydroxy benzoate (Na2-HB), sodium 2,6-dihydroxy benzoate (Na2,6-DHB), and sodium 4-hydroxy benzoate (Na4-HB). It is found that cationic CTABr micelles grow strongly with the addition of small amount of NaB, Na2-HB, and Na2,6-DHB salts. The effect of addition of salts on the growth of the micelles is significantly higher for Na2-HB and Na2,6-DHB as compared to that for NaB. On the other hand, the addition of salt Na4-HB is relatively much less effective similar to the effect of inorganic salts (e.g., NaBr) on the growth of the micelles. We explain the propensity of strong growth of the micelles in the presence of NaB, Na2-HB, and Na2,6-DHB because of the high charge neutralization by these salts as the aromatic counterions are adsorbed on the surface of the micelle. In these counterions, the hydrophilicity seems to be important in deciding the location of the charged group of the counterions with respect to the level of the oppositely charged micellar surface. The counterions of Na4-HB are not adsorbed on the micellar surface, and hence, it behaves differently than the above other salts.

Contrasting the Effects of Hydrophobicity and Counterion Size on Anionic Wormlike Micelle Growth

“…Thus the structural parameter of the surfactant can be tuned favorably towards the formation of wormlike micelles. The superiority of hydrophobic counterions over simple ones is mainly attributed to the contribution of hydrophobic interaction to micellar growth in cationic surfactant systems [6][7][8][9].…”

“…Although the aromatic organic counterion has been ubiquitously acknowledged for its higher effectiveness in promoting micelle growth than inorganic ones in cationic wormlike-micelle systems [6][7][8][9], the anionic wormlike micelles are mainly induced by inorganic salts [10]. The hydrophobic salts have rarely been employed in anionic wormlike micelles probably due to the poor resources of cationic hydrotropes.…”

Investigation of the properties of decaoxyethylene n‐dodecyl ether, C12E10, in the aqueous sugar‐rich region

“…SANS measurements. SANS is well-known as an ideal technique for studying micellar morphology (28,29), and this also has been demonstrated for surfactant micelles in the presence of various additives (30)(31)(32)(33)(34)(35). SANS experiments were performed on the SANS spectrometer at the Dhruva reactor, Trombay, Mumbai, India (36).…”

Effects of oilfield injection water component on rheological characteristics of CTAC/NaSal aqueous solution

“…There have been some studies on the effects of the salt added to the ionic surfactant 19–28 . For ionic surfactants, the chemical nature of the counterion, such as chemical structure, electron valence, counterion radius, or charge density, affects the degree of salt effect or some specific ion effect 22,24,29–34 ; the ion opposite to the charge of the surfactant head group ion plays a key role; these counterions first neutralize the surfactant head group ions in the aqueous solution, reduce the electrostatic repulsion between the surfactant molecules, and arrange orderly the molecules to form longer micelles. Secondly, the electrolyte ions through electrostatic interaction compress the diffused electric double layer of micelles results into tighter micelles and more complicated network structure.…”