Effect of slight structural changes on the gelation properties of N-phenylstearamide supramolecular gels.

Supramolecular gels present several applications in which the gelator properties are closely dependent on their structure and solvent. Despite this, there are few studies on the effect of the gelation ability of gelators with slight molecular changes. Therefore, N-arylestearamides (in which aryl = phenyl (1), 4-tolyl (2) and 4-acetylphenyl (3)) were evaluated in different solvents. The critical gelefication concentration (CGC) values indicated that the substituents can significantly affect the concentration at which the supramolecular gels are formed, mainly in non-aromatic solvents (e.g. cyclohexane, acetonitrile and DMSO). From UV-Vis and DSC data, we verified that the gel-sol and sol-gel transitions (Tgel-sol and Tsol-gel) increase in the order of 1 < 2 < 3. Organogel strength was evaluated for 1-3 as a function of concentration and solvent type using rheology data. Gel strength is concentration-dependent and a strength order was found in acetonitrile, cyclohexane and DMSO, in which: 1 ∼ 2 > 3. Dynamic viscoelastic measurements as a function of temperature sweeps indicate a predominantly enthalpic contribution to the elasticity of the organogels formed from 1-3. Temperature-dependent 1H NMR indicates that NHO interactions may be responsible for the molecular association of molecules into 1D fibers, while 3D fibers were formed from van der Waals interactions.

Physicochemical properties of methylcellulose and dodecyltrimethylammonium bromide in aqueous medium.

Interactions between uncharged polymers and cationic surfactants are considered weaker than interactions with the anionic analogues. This work describes the binding occurring between methylcellulose (MC) and the cationic surfactant DTAB in aqueous medium. In the absence of salt, MC-DTAB exhibits a maximum in hydrodynamic radius, R(h,slow), with the increase in the surfactant concentration. Otherwise, in presence of salt the MC-DTAB system shows only a linear increase of R(h,slow). CAC is lower than the CMC, which is taken as an evidence of binding between the cationic surfactant and neutral polymer that induces the aggregation process. Static light scattering, rheology and micro-DSC results highlight the hydrophobic MC-DTAB association. Salt-out and the salt-in effects were observed in presence of DTAB, with a clear transition at concentration values close to the CMC, as judged from rheological and micro DSC measurements. Indeed, DTAB affects both the pattern of the sol-gel transition and the gel strength.