Interfacial tension (using a drop-shape analysis technique), fluorescence (of a widely used spectroscopic molecular probe, pyrene), and ¹H NMR measurements were used to monitor the adsorption at the aqueous solution–air interface and self-aggregation behaviour (critical micelle concentration, CMC) of room-temperature ionic liquids (ionic liquids) of the 1-alkyl-3-methylimidazolium family of cations, [C_nmim]⁺, with different linear alkyl chain lengths, C_nH_2n+1 (½n = 1–7), and different counter-ions, namely [C_nmim]Cl (n = 2–14), [C_nmim][PF₆] (n = 4 or 10), and [C₁₀mim][NTf₂]. Only [C_nmim]Cl with n > 8 unambiguously form aggregates in solution and the nature of this self-aggregation is discussed in terms of the electrostatic vs. hydrophobic contributions of the isolated cation. In contrast, the shortest chains behave, as anticipated, as simple salts. In turn, the transitional ionic liquid, [C₆mim]Cl is able to develop a monolayer at the aqueous solution–air interface but shows no noticeable self-aggregation in the bulk fluid. Moreover, the micellar characteristics of the well-studied sodium dodecyl sulfate (SDS) aqueous solutions as a function of the total concentration of [C_nmim]Cl (½n = 1–7) showed a clear change in the behaviour of the mixtures [C_nmim]Cl + SDS for n ≈ 6–8, with a characteristic mixed-micelle formation for the longer and a pure salt effect for the shorter chain lengths of [C_nmim]Cl.

Anionic and cationic alkyl-chain effects on the self-aggregation of both neat and aqueous solutions of 1-alkyl-3-methylimidazolium alkylsulfonate salts ([C_nH_2n+1mim][C_mH_2m+1SO₃]; n = 8, 10 or 12; m = 1 and n = 4 or 8; m = 4 or 8) have been investigated. Some of these salts constitute a novel family of pure catanionic surfactants in aqueous solution. Examples of this class of materials are rare; they are distinct from both mixed cationic–anionic surfactants (obtained by mixing two salts) and gemini surfactants (with two or more amphiphilic groups bound by a covalent linker). Fluorescence spectroscopy and interfacial tension measurements have been used to determine critical micelle concentrations (CMCs), surface activity, and to compare the effects of the alkyl-substitution patterns in both the cation and anion on the surfactant properties of these salts. With relatively small methylsulfonate anions (n = 8, 10 and 12, m = 1), the salts behave as conventional single chain cationic surfactants, showing a decrease of the CMC upon increase of the alkyl chain length (n) in the cation. When the amphiphilic character is present in both the cation and anion (n = 4 and 8, m = 4 and 8), novel catanionic surfactants with CMC values lower than those of the corresponding cationic analogues, and which exhibited an unanticipated enhanced reduction of surface tension, were obtained. In addition, the thermotropic phase behaviour of [C₈H₁₈mim][C₈H₁₈SO₃] (n = m = 8) was investigated using variable temperature X-ray scattering, polarising optical microscopy and differential scanning calorimetry; formation of a smectic liquid crystalline phase with a broad temperature range was observed.

A set of 1-alkyl-3-methylimidazolium alkanesulfonate ionic liquids, [C_nmim][C_kSO₃], formed by the variation of the alkyl chain lengths both in the cation and the anion (n = 1–6, 8, or 10; k = 1–4, or 6), was synthesised, with sixteen of them being novel. The ionic liquids were characterised by ¹H and ¹³C NMR spectroscopy, and mass spectrometry. Their viscosities and densities as a function of temperature, as well as melting points and decomposition temperatures, were determined. The molecular volumes, both experimental and calculated, were found to depend linearly on the sum (n + k).