Crystal phases of dodecyl sulfates obtained from aqueous solutions: structure of the hexaaquamagnesium salt

Tailored synthesis of molecularly thin platinum nanosheets using designed 2D surfactant solids

The assembly of the surfactants has been utilized as unique templates for the controlled synthesis of metal nanosheets. However, current strategies for metal nanosheets have mainly focused on the liquid-phase surfactant assembly. Herein, we found the solid-state surfactants as designable crystals suitable for nanostructural control and proposed a novel synthetic route for molecularly thin Pt metal nanosheets using solid surfactant crystals as a precursor. The 2D surfactant crystals containing planarly arranged Pt complexes were prepared, and the subsequent UV-ozone treatment and reduction process allowed us to obtain Pt metal nanosheets. Pt metal nanosheets had a distinct morphology with various thicknesses (from 1.5 nm to 3.0 nm), characteristic of 2D surfactant crystals.

Molecular Behavior of Linear Alkylbenzene Sulfonate in Hydrated Crystal, Tilted Gel, and Liquid Crystal Phases Studied by Molecular Dynamics Simulation

The lamellar phase produced by surfactants with water exhibits several subphases, such as hydrated crystal (L_c), gel (L_β), tilted gel (L_β'), and liquid crystal (L_α) phases, depending on temperature, pressure, and hydration. The dynamics of the surfactant molecules in these phases are still unclear. In the present study, we investigate the translational and conformational dynamics of sodium linear alkylbenzene sulfonate (LAS) molecules in the L_c, L_β', and L_α phases. In the L_α phase, the lateral diffusion of LAS is as fast as that found for phospholipid bilayers in the L_α phase. The diffusion coefficient was undetectably small for the L_c and L_β' phases. The conformation of LAS in the L_α phase relaxes very rapidly, whereas those in the L_c and L_β' phases relax very slowly. The time scale of the relaxations greatly depends on the segment of the LAS molecule for the latter two phases. The relaxation time for the SO₃^- head group and benzene ring of LAS was much longer than that for alkyl chains. Conformational pattern analyses of LAS alkyl chains revealed that the high fraction of the gauche conformation for the odd-numbered C-C bonds aligns the chain parallel to the bilayer normal and is the main origin of the different relaxation times for different segments in the chain. In the L_c, L_β', and L_α phases, the orientations of the SO₃^- group and the benzene ring are locked by the salt bridge among SO₃^- groups and sodium ions. As a result, the orientational order found for the C-C bonds in the LAS alkyl chains is kept even in the L_α phase.

Structure of Hydrated Crystal (Lc), Tilted Gel (Lβ'), and Liquid Crystal (Lα) Phases of Linear Alkylbenzene Sulfonate (LAS) Studied by X-ray Diffraction and Molecular Dynamics Simulation

Precipitation of crystals in aqueous surfactant solutions hampers their use as liquid detergents. Understanding the molecular structure that causes the crystal formation is of key importance in designing a new surfactant. In this study, we first identified the hydrated crystal (L_c) structure of linear alkylbenzene sulfonate (LAS) at ambient temperature by combining X-ray diffraction (XRD) with all-atom molecular dynamics (MD) simulation. It is known that the L_c domain involves only a stoichiometrically small number of water molecules, and the water number of existing interlamellar layers may change when the system shows the thermally induced phase transition. Therefore, we explored possible crystal structures with five different hydration levels of 0, 0.5, 1, 2, and 4 water per LAS using MD simulations with the d-spacing measured by XRD. Among them, only the diffraction pattern calculated for the monohydrate LAS well coincided with the experimental diffraction pattern at 300 K. A structural change of the monohydrate from the L_c phase to the tilted gel (L_β') phase was also observed by heating from 300 to 360 K in the MD simulations, where a cross-sectional structure of the alkyl groups of LAS molecules changed from a face-centered rectangular lattice to a hexagonal one. Further heating to 400 K resulted in a disordered liquid crystal (L_α) phase. Thus, configurational changes of the detergent during thermally induced phase transitions were well explored and characterized by MD simulations with the aid of XRD data. This approach can be broadly applied where we explore the phase behavior of hydrated crystals of any surfactant species.

Crystallisation of sodium dodecyl sulfate–water micellar solutions with structurally similar additives: counterion variation

Kinetic and morphological insight into the crystallisation of micellar SDS–H₂O solutions using small quantities of structurally similar additives.

PagP crystallized from SDS/cosolvent reveals the route for phospholipid access to the hydrocarbon ruler

Enzymatic reactions involving bilayer lipids occur in an environment with strict physical and topological constraints. The integral membrane enzyme PagP transfers a palmitoyl group from a phospholipid to lipid A in order to assist Escherichia coli in evading host immune defenses during infection. PagP measures the palmitoyl group with an internal hydrocarbon ruler that is formed in the interior of the eight-stranded antiparallel β barrel. The access and egress of the palmitoyl group is thought to take a lateral route from the bilayer phase to the barrel interior. Molecular dynamics, mutagenesis, and a 1.4 A crystal structure of PagP in an SDS / 2-methyl-2,4-pentanediol (MPD) cosolvent system reveal that phospholipid access occurs at the crenel present between strands F and G of PagP. In this way, the phospholipid head group can remain exposed to the cell exterior while the lipid acyl chain remains in a predominantly hydrophobic environment as it translocates to the protein interior.

Crystal structure analysis of [Ca(O3SC18H37)2(DMSO)2], a lamellar coordination polymer and its relevance for model studies in biomineralization

Single crystals of a one-dimensional Ca coordination polymer of the surfactant octadecyl sulfonate (C(18)H(37)SO(3)(-)) have been grown from hot DMSO solution. The X-ray structure analysis of the compound [Ca(O(3)SC(18)H(37))(2)(DMSO)(2)] (1) shows a lamellar interdigitated arrangement of hydrophobic tails of the amphiphilic ligands. Each Ca ion is coordinated by four different sulfonate groups, and its nearly octahedral coordination environment is completed by two dimethyl sulfoxide (DMSO) ligands. The octadecyl sulfonate ligand coordinates to Ca ions in a micro(2)-bridging mode, which contrasts to information from literature suggesting a micro(3)-bridging coordination mode. Since the growth of highly oriented calcite single crystals underneath Langmuir monolayers of this particular surfactant is often regarded as textbook example of a heteroepitaxy ("template") mechanism in biomineralization, we present a critical discussion of the crystal structure of the title compound in this context.