Structural Change of an α-Gel (α-Form Hydrated Crystal) Induced by Temperature and Shear Flow in an Oleic Acid Based Gemini Surfactant System

Abstract

We studied the effects of temperature and shear flow on the structures of α-gel bilayers and domains. The α-gel samples were prepared by a carboxylate-type gemini surfactant synthesized from oleic acid and a long-chain alcohol (1-tetradecanol) with water. The structural change as a function of temperature was investigated using small-and wide-angle X-ray scattering (SWAXS) measurements, spin−spin relaxation time (T 2 ) measurements, and optical microscopy observations. SWAXS measurements suggested that the decreased temperature yielded the α-gel phase from a lamellar liquid-crystal phase. We also found that the lamellar d-spacing drastically decreased at the phase transition temperature. The T 2 measurements suggested that two kinds of protons with different mobilities coexisted in amphiphiles consisting of lamellar bilayers. The abundance of the protons with low mobility increased with decreasing temperature. Optical microscopy results indicated that the size of the α-gel domains increased with decreasing temperature. We assumed that the increased abundance of the low-mobility protons, indicating low flexibility of lamellar bilayers, led to a decreased lamellar d-spacing and increased size of the α-gel domains. Shear-induced structural changes in the α-gel were also studied using simultaneous small-angle neutron scattering and rheological measurements. The α-gel can maintain bilayer structures even at high shear rates. We also found that the lamellar d-spacing was independent of the shear rate.

Rheology of α-Gel Formed by Amino Acid-Based Surfactant with Long-Chain Alcohol: Effects of Inorganic Salt Concentration

“…Similar inflection points were observed previously in the α-gel system . Recently, we performed simultaneous small-angle neutron scattering and rheological (Rheo-SANS) measurements in an α-gel system consisting of an anionic gemini surfactant, 1-tetradecanol, and water . In this earlier work, we found that the bilayer structure remained even at high shear rates .…”

“…36 In this earlier work, we found that the bilayer structure remained even at high shear rates. 36 It seems, therefore, that the inflection points result from a change in the domain-to-domain network structure, although the exact mechanism for the observed behavior is not clear at present.…”

An aqueous two-phase system formed in single-component solution of α-ketooctanoic acid

“…22)) indicates that a lot of water is trapped between the lamellar bilayers, similar to the previous reports. 14,25,26 Based on the above results along with our previous work, 22 it can be concluded that, microscopically, a concentration-driven stepwise aggregation occurs in the KOCOOH solution (Fig. 5).…”

Morphological Transformation of SDS Aggregates in a Deep Eutectic Solvent Above the Fracto-eutectogel to Fluid Transition Temperature

“…For instance, a family of lysine-based surfactants formed a range of tubule structures below their Krafft points (Tk = 25 -49 o C depending on surfactant architecture); however, a tubule-to-vesicle transition was observed as the temperature was increased [24]. Of specific relevance to gelation, the transition between a lamellar liquid-crystalline phase and an -gel phase was observed at T ~ 40 o C as a ternary gemini surfactant (synthesised from oleic acid), 1-tetradecanol, and water system was cooled [25].…”