A Structural Study of an Organogel Investigated by Small-Angle Neutron Scattering and Synchrotron Small-Angle X-ray Scattering

Abstract

We investigated structures of 12-hydroxystearic acid (12-HSA) gels in a wide temperature range from the gel to the sol states and the gelation process by means of small-angle neutron scattering (SANS) and time-resolved synchrotron small-angle X-ray scattering (SAXS), respectively. The SANS result shows that the size of the cross section of the crystalline fibers in the gel was not affected much by temperature change, although the density of the fibers decreased with the increase of temperature even in the gel state. The SANS profiles at various temperatures and compositions were well reduced by the product of the volume fraction of the fiber and the square of the scattering contrast. At temperatures above the melting point of the gels obtained from differential scanning calorimeter, the gel turned into a homogeneous solution. The time-resolved small-angle X-ray scattering experiments revealed the gelation process as mentioned below. At the early stage, the scattering behavior did not show any change (induction period). The induction period was longer at higher temperatures. In the second stage, an amorphous precursor appeared as an initial structure of crystalline nucleus. In the next stage, the fiber was developed via crystalline nucleation and growth. In this stage, the fibers grew with keeping the size of the cross section (the radius of gyration R ≅ 82 -84 Å) constant, and finally, the fiber growth was arrested because of gelation.

Relationship between helical aggregates and polymorphs in a 12-hydroxystearic acid gel: their thermal stability and formation kinetics

“…We obtained ΔH d =159 kJ/mol, ΔS d =427 J/mol from the plot. The values are close to those for 12-HSA/toluene gel or 12-HSA/dodecane gel investigated in the previous studies [19,21]. We show a time course of POM images of the selfassembling structures for 3 wt% 12-HSA/F5 gel after T-jump from a sol state (110°C) into T=48°C (a) and T=71.4°C (b) in Fig.…”

“…The sol-gel transition temperatures are near to the melting point of 12-HSA powder (80.9°C [18]), e.g., in the case of 3 wt% 12-HSA/F5 gel, it was 78.3°C. Above the transition temperature, the mixture becomes a homogeneous solution [19]. Therefore, the sol-gel transition temperature may be related to the dissolution temperature.…”

Practical applications of small-angle neutron scattering

“…35,36 For self-assembling organic species in solution, SANS is particularly useful because of the afforded high contrast between a 1 H-rich material and a 2 D-rich solvent. Consequently, one of the more common uses of SANS is for the characterisation of aggregate structures of surfactants, [37][38][39][40][41] 33,52,53 are also investigated, for a variety of purposes including mustard gas decontamination, 50 art restoration, 51 and drug delivery. 33 In this case, selective deuteration of the different components is frequently used to obtain a more detailed structural picture, as highlighted in Fig.…”

Effects of Cooling Rates on Self-Assembling Structures of 12-Hydroxystearic Acid in an Ionic Liquid

“…More recently, we reported that different polymorphs appeared at different temperatures [14], for example, three (001) peaks ( 1,(001) , 2,(001) , and 3,(001) ) corresponding to long spacings of 46.5, 42.4, and 39.7Å, respectively, were observed for 12-HSA gels in phenylmethyl silicone and for a neat HSA prepared by a temperaturejump into different temperatures, and each crystal form was termed form I, form II, and form III [14]. Contrary to it, a different behavior was observed for 12-HSA gel in a different solvent; for example, our small-angle neutron scattering (SANS) and synchrotron SAXS studies revealed that the selfassembled structure of 12-HSA in toluene was not affected by concentrations of the gelator and temperatures, [15][16][17].…”