We have performed intra‐ and extra‐cavity microwave frequency (1–100 GHz) measurements on high quality Y₁Ba₂Cu₃O_7−x superconducting thin films on (100) LaAlO₃ substrates. The ∼0.3 μm thin films fabricated by the pulsed laser deposition technique exhibit superconducting transition temperatures >90 K, as determined by resistivity and ac susceptibility measurements, and critical current densities of 5×10⁶ A/cm² at 77 K. Moreover, ion beam channeling minimum yields of ∼3% were measured, indicating the extremely high crystalline quality of films grown on the LaAlO₃ substrate. Microwave surface resistance values at 77 K for these films are found to be more than one to two orders of magnitude lower than for copper at 77 K for almost the entire frequency range explored. We postulate that the reason we observe such low surface resistances in these films is the virtual absence of grain and phase boundaries coupled with the high degree of crystallinity. Furthermore, we believe that the residual resistance measured below T_c is at present dominated by losses occurring in the substrate and the cavities rather than by losses intrinsic to the Y‐Ba‐Cu oxide superconductor.

Series of quaternary ammonium-based gemini surfactants with long alkyl chains (C12 and C18) containing different spacers and substituents attached to the polar head group have been synthesized and their aggregation properties in aqueous solution examined. The effect of the hydrophobic chain, the nature and structure of the spacer group and the polarity of the head group on the aggregation behavior of such dimeric surfactants has been investigated. The critical micelle concentration (cmc) values of gemini surfactants in aqueous solution were determined by conductivity, steady state fluorescence and potentiometric measurements. The size of aggregates formed by investigated amphiphiles above the cmc in aqueous solution was examined by dynamic light scattering. Gemini surfactants show cmc values significantly lower than those of comparable single chain surfactants. The tendency of trimeric surfactants with a rigid spacer to form aggregates is higher than that of the corresponding dimeric surfactants. As occurs for monomeric ionic surfactants, the cmc of gemini surfactants decreases with the elongation of the hydrophobic chain. However, the effect of lengthening the alkyl chain on the cmc depends on the structure of the spacer. C12 gemini surfactants with a rigid hydrophobic spacer exhibit cmc higher than those containing a flexible hydrophobic spacer. For gemini surfactants with C18 alkyl chains this effect is even more pronounced and leads to differences in cmc values greater than one order of magnitude. The structure of the spacer, flexible or rigid chain, has been found to be a critical parameter on the self-assembly of long chain gemini surfactants. Spherical micelles are spontaneously formed above the cmc for C12 gemini surfactants, whereas trimeric and C18 gemini surfactants seems to form vesicle-like aggregates when self-aggregation occurs.This work has been supported by the Spanish Ministerio de Economia y Competitividad (CTQ2013-41514-P) and the National Center for Research and Development (TANGO1/266340/NCBR/2015).\ud \ud O.K. thanks European Commission for financial support within Erasmus + Programme.Peer reviewe

Aerobic biodegradability and aquatic toxicity of five types of quaternary ammonium-based gemini surfactants have been examined. The effect of the spacer structure and the head group polarity on the ecological properties of a series of dimeric dodecyl ammonium surfactants has been investigated. Standard tests for ready biodegradability assessment (OECD 310) were conducted for C12 alkyl chain gemini surfactants containing oxygen, nitrogen or a benzene ring in the spacer linkage and/or a hydroxyethyl group attached to the nitrogen atom of the head groups. According to the results obtained, the gemini surfactants examined cannot be considered as readily biodegradable compounds. The negligible biotransformation of the gemini surfactants under the standard biodegradation test conditions was found to be due to their toxic effects on the microbial population responsible for aerobic biodegradation. Aquatic toxicity of gemini surfactants was evaluated against Daphnia magna. The acute toxicity values to Daphnia magna, IC50 at 48 h exposure, ranged from 0.6 to 1 mg/L. On the basis of these values, the gemini surfactants tested should be classified as toxic or very toxic to the aquatic environment. However, the dimeric quaternary ammonium-based surfactants examined result to be less toxic than their corresponding monomeric analogs. Nevertheless the aquatic toxicity of these gemini surfactants can be reduced by increasing the molecule hydrophilicity by adding a heteroatom to the spacer or a hydroxyethyl group to the polar head groups.