Assembly of a model hydrophobic drug into cationic bilayer fragments

Catanionic surfactants result from the pairing of oppositely charged amphiphilic molecules, forming a new class of surfactant molecules with various interesting lyotropic and thermotropic properties. With the aim of probing the role of both headgroup chemical nature/structure and molecular shape, a series of catanionic surfactants were synthesized. The cationic portion of the molecule is kept constant, being the dioctadecyldimethylammonium double chain. Different single-chained surfactants with varying headgroups and chain lengths are used as the anionic pair. The thermotropic behavior has been studied by DSC and the mesophase structural investigated by polarized light microscopy. The results indicate that, for a given chain length, parameters such as headgroup polarity and charge density, as well as volume, influence the catanionic surfactant behavior. The thermodynamic parameters are qualitatively evaluated, considering the headgroup chemical nature and the overall molecular structure.     

Oil Solubilization Capacity,Liquid Crystalline Properties,and Antibacterial Activity of Alkanolamine‐Based Novel Cationic Surfactants

A series of monomeric and dimeric cationic surfactants with tuned polarity was synthesized. Oil solubilization capacity, thermotropic liquid crystalline properties, and minimum inhibitory concentration (MIC) of novel hydroxylated cationic surfactants using selected gram positive and gram negative bacteria were examined. Antibacterial activity and the propensity of gemini surfactants for oil solubilization were observed to be better than those of corresponding monomeric surfactants. Pseudo ternary phase diagrams for these surfactants, methyl methacrylate (MMA), and water clearly showed, that microemulsions can be easily formulated with all these surfactants. Solubilization and foam studies of mixed surfactant systems were also examined. Molecular architecture like the tail length, head group area, and presence of ethanolic goups in the surfactant affect the performance properties. Unlike conventional gemini surfactants the synthesized gemini surfactants also show thermotropic liquid crystalline properties (smectic‐A, L_α phase).     

Structure/property relationships for the thermotropic behavior of lysine-based amphiphiles: from hexagonal to smectic phases

Amino acid-derived gemini surfactants arise as a potentially good alternative to the more conventional lipid and synthetic catanionic systems in view of their enhanced interfacial properties, increased chemical stability, and low toxicity. The presence of an amino acid as the polar headgroup allows toxicity reduction, with the simultaneous increase of biodegradability. For these compounds, the establishment of structure/function relationships from the assessment of their basic aggregation properties is therefore of the utmost interest, e.g., in the design of operative self-assembled systems (e.g., liposomes, nanotubes, etc). In this context, the study of the thermal phase behavior of the dry surfactants is a natural, straightforward first step, the more so as thermotropic liquid crystals are also relevant for practical applications. In this work, several lysine-based amphiphiles with a gemini-like configuration have been synthesized, with the amino acid side chain as the spacer group. The molecules are either esters (neutral, with C6-C12 even chains) or sodium carboxylates (anionic, with C6-C12 even chains). Upon increasing the temperature, different crystalline (cr) and liquid-crystalline (lc) phases have been detected and the corresponding thermodynamic and structural parameters determined by a combination of differential scanning calorimetry, polarizing light microscopy and small-angle X-ray scattering. The phase behavior of the amphiphiles is highly dependent on both the chain length and the presence of charge on the headgroup, with significant differences occurring within and between each group of molecules. The C6 and C8 esters form reverse hexagonal cr and lc phases, while C10 and C12 self-assemble into smectic cr and lc structures, with C10 showing also a reverse hexagonal lc phase prior to isotropization. All the carboxylate derivatives form smectic lc phases at high enough temperature prior to isotropization. The rationalization of the phase behavior and phase transition energetics of the compounds has been put forth on the basis of the intermolecular interactions at stake (van der Waals, H-bonding, electrostatic, and packing) and the molecular shape of the amphiphile.     

Phase behavior of cyclic siloxane-based liquid crystalline compounds

A series of 24 cyclic siloxane-based liquid crystalline compounds was synthesized using conventional hydrosilation chemistry. Variables examined included ring size, spacer group length, and type and composition of pendant mesogeric groups. Both pentamethylhydro- and tetramethylhydrosiloxane rings were reacted with mesogens based on cholesterol, biphenyl, or equimolar mixtures of both. Four different length spacer groups containing terminal vinyl groups were used to attach the mesogens to the cyclic siloxane core. The thermotropic liquid crystalline phase behavior was studied using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). A lengthening of spacer groups resulted in lower crystallization temperatures for the biphenyl-based compounds and lower glass transition temperatures for the cholesterol- and mixed mesogen-based compounds. The tetramethyl ring derivatives exhibited higher glass transition or crystallization temperatures than their pentamethyl counterparts. Biphenyl-based compounds exhibited low temperature crystalline phases while the cholesterol-based compounds exhibited low temperature glassy phases. © 1994 John Wiley & Sons, Inc.     

Polymorphism and mesomorphism of oligomeric surfactants: effect of the degree of oligomerization

A series of cationic oligomeric surfactants (quaternary dodecyldimethylammonium ions with two, three, or four chains connected by an ethylene spacer at the headgroup level, abbreviated as dimer, trimer, and tetramer) were synthesized and characterized. The influence of the degree of oligomerization on their polymorphic and mesomorphic properties was investigated by means of X-ray diffraction, polarizing optical microscopy, thermogravimetry, and differential scanning calorimetry. All compounds display layered arrangements with interdigitated dodecyl chains. The increase in the degree of oligomerization increases the interlayer distance and decreases the ordering in the solid phase; whereas the dimer sample is fully crystalline with well-developed 3D ordering and the trimer and tetramer crystallize as highly ordered crystal smectic phases. The number of thermal phase transitions and sequence of phases are markedly affected by the number of dodecyl chains. Anhydrous samples exhibit polymorphism and thermotropic mesomorphism of the smectic type, with the exception of the tetramer that displays only transitions at higher temperature associated with decomposition and melting. All hydrated compounds form lyotropic mesophases showing reversible phase transitions upon heating and cooling. The sequence of liquid-crystalline phases for the dimer, typical of concentrated ionic surfactant systems, comprises a hexagonal phase at lower temperatures and a smectic phase at higher temperatures. In contrast, the trimer and tetramer reveal textures of the hexagonal phase.     

Positional isomers of linear sodium dodecyl benzene sulfonate: solubility, self-assembly, and air/water interfacial activity

Commercial linear alkyl benzene sulfonates (ABS) are a very important class of anionic surfactants that are employed in a wide variety of applications, especially those involving wetting and detergency. Linear ABS surfactants generally consist of a complex mixture of different chain lengths and positional isomers. This diversity and level of complexity makes it difficult to develop fundamental structure-property correlations for the commercial surfactants. In this work, six monodisperse headgroup positional isomers of sodium para-dodecyl benzene sulfonate (Na-x-DBS, x = 1-6) have been studied. The influence of headgroup position and added electrolyte (NaCl) on the solubility and self-assembly (micellar and vesicular aggregation and lyotropic liquid crystalline phase behavior) in the temperature range from 10 to 90 degrees C have been investigated. Additionally, the air-aqueous solution interfacial adsorption at 25 (no added NaCl) and 50 degrees C (from 0 to 1.0 M added NaCl) has been examined. The observed physicochemical behavior is interpreted in terms of local molecular packing constraints, and in the case of the lyotropic liquid crystalline behavior global aggregate packing constraints as well.     

A series of azo-type side-chain liquid crystalline polysiloxane–palladium complexes

A series of azo-type side-chain liquid crystalline polysiloxanes (AZLCPs) were synthesized, starting from organic polysiloxane and azo-type mesogenic compounds having an end allyl group. The AZLCPs were further used to coordinate with palladium dichloride and potassium chloride, by which a series of palladium complexes of AZLCPs (Pd–AZLCPs) were prepared. The mesogenic properties of all of the liquid crystalline polymers were characterized by using differential scanning calorimetry, polarized microscope and wide-angle X-ray diffraction. It was found that all of the polymer ligands and their palladium complexes showed thermotropic liquid crystallinity and that the incorporation of the palladium ions gave positive effects to the mesogenic properties of their polymer ligand counterpart. Compared with the corresponding AZLCPs, the Pd–AZLCPs have higher isotropization temperatures and a broader mesophase temperature range. The mesogenic properties of the liquid crystalline polymer ligands and their palladium complexes were also varied gradually by changing the length of the alkoxy groups on the side chain. The polymers that have a color emissive group and a highly flexible polysiloxane main chain may potentially be used as nonlinear optical materials.     

Enhanced interfacial properties of novel amino acid-derived surfactants: Effects of headgroup chemistry and of alkyl chain length and unsaturation

Amino acid-derived surfactants have increasingly become a viable biofriendly alternative to petrochemically based amphiphiles as speciality surfactants. Herein, the Krafft temperatures and critical micelle concentrations (cmc) of three series of novel amino acid-derived surfactants have been determined by differential scanning microcalorimetry and surface tension measurements, respectively. The compounds comprise cationic molecules based on serine and tyrosine headgroups and anionic ones based on 4-hydroxyproline headgroups, with varying chain lengths. A linear dependence of the logarithm of cmc on chain length is found for all series, and in comparison to conventional ionic surfactants of equal chain length, the new amphiphiles present lower cmc and lower surface tension at the cmc. These observations highlight their enhanced interfacial performance. For the 18-carbon serine-derived surfactant the effects of counterion change and of the presence of a cis-double bond in the alkyl chain have also been investigated. The overall results are discussed in terms of headgroup and alkyl chain effects on micellization, in the light of available data for conventional surfactants and other types of amino acid-based amphiphiles reported in the literature.     

Comparative studies on the crystalline to fluid phase transitions of two equimolar cationic/anionic surfactant mixtures containing dodecylsulfonate and dodecylsulfate

In this work, a cationic surfactant, dodecyltrimethylammonium bromide (DTAB), and an anionic surfactant, sodium dodecylsulfonate (SDSO(3)) or sodium dodecylsulfate (SDSO(4)), were mixed in an equimolar ratio to prepare SDSO(3)-DTAB and SDSO(4)-DTAB binary mixtures. The phase behavior, structure, and morphology of these two surfactant mixtures were investigated by differential scanning calorimetry, synchrotron X-ray scattering, freeze-fracture electron microscopy, and Fourier transform infrared spectroscopy. It was found that upon heating, both of the two systems transform from multilamellar crystalline phase to liquid crystalline (or fluid) phase. It is interesting to find that, although SDSO(3) has a lower molecular weight, the crystalline phase of SDSO(3)-DTAB shows much higher thermostability as compared with that of SDSO(4)-DTAB. Other than this, we observed a large difference in the repeat distances of the two crystalline phases. More interestingly, at 60 °C in the fluid phases, cylindrical micelles formed in the SDSO(3)-DTAB system, while spherical micelles were observed in the SDSO(4)-DTAB system. Our present work demonstrates that a subtle difference in the headgroup structure of the anionic component markedly affects the thermostability, packing structure, and morphology of the surfactant mixtures, which suggests the importance of the match of the head-head and tail-tail interactions between the cationic and anionic surfactants.     

正、负离子表面活性剂混合体系溶致液晶生成的相行为

研究了烷基(C8,C12,C14)三甲基溴化铵、烷基(C12,C14)溴化吡啶与烷基(C8,C12)硫酸钠混合体系溶致液晶形成的条件与结构的变化.在高浓度的水溶液中,随着正、负离子表面活性剂摩尔比接近于1,液晶结构由六角相过渡为层状相.表面活性剂非极性链长改变,对相行为影响显著,短碳链的正、负离子表面活性剂混合体系,在等摩尔比时,体系为层状液晶或立方液晶为主,夹杂少许沉淀.随碳链增长,两类表面活性剂间的静电吸引效果表现为生成沉淀的摩尔比例范围变宽,沉淀量增多,共存的液晶相减少,甚至消失.若只改变正离子的极性头基,季胺盐比吡啶盐与烷基硫酸盐的作用要强,形成不溶物的混合摩尔比例范围更宽.     

DSC and Raman studies of the side chain length effect of ubiquinones on the thermotropic phase behavior of liposomes

Differential scanning calorimetry and Raman spectroscopy have been used to examine the effects of ubiquinones (UQn) on the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) in multilamellar vesicles, for UQ/DPPC molar ratios ranging from 0.01 to 0.1. The influence of the side chain length has been investigated by comparing the effect of a series of UQ with 2 (UQ2), 4 (UQ4), 6 (UQ6) and 10 isoprene units (UQ10).In the presence of increasing amount of UQ2 or UQ4, concomitant shift of the gel to liquid crystalline phase transition towards lower temperatures and vanishing of the pretransition are observed. Short-chain ubiquinones are thus inserted parallel to phospholipid chains, their benzoquinone ring being close to the DPPC polar headgroups. In addition, broadening and skewing of the main transition peak support the fact that UQ2 and UQ4 are laterally self-organized in highly concentrated regions located at the boundary of lipid domains. The lipid thermotropic behavior is not affected by the presence of other analogues of the series, UQ6 and UQ10. They remain homogeneously dispersed within the midplane of the phospholipid bilayer. Such a chain length dependence on the location and the organization of ubiquinones analogues may be correlated with their biological activities in biomembranes.     

几种表面活性剂与DNA的相互作用

用循环伏安、紫外－可见光谱和交流阻抗等方法，以电活性小分子亚甲基蓝（ MB）为探针，研究了几种表面活性剂与DNA的相互作用。研究发现，阴离子、阳离 子和非离子表面活性剂均可通过疏水和静电作用与固定在电极表面的DNA分子结合 ，改变电极表面DNA的状态，进而影响电活性小分子的电化学行为。阴离子表面活 性剂与DNA之间以静电排斥为主，也有部分疏水性结合，它使MB的氧化还原峰峰电 流减小。阳离子表面活性剂十六烷基三甲基溴化铵、十二烷基三甲基氯化铵均在一 定浓度范围内对MB的电化学响应有增敏作用，而代十六烷基吡啶、溴代十八烷基吡 啶表现出抑制效应，它们与DNA间既有疏水性作用，也有静电吸引。非离子表面活 性剂与DNA的结合较弱，其主要是通过改变溶液的性质（如粘度、极性和介电常数 等）影响DNA的构象，从而导致MB电化学参数的微弱变化。此外，表面活性剂疏水 链的长短及极性头基的大小对作用过程也有一定影响。     

The synthesis of mesogenic polymers provoked by molecular mobility. polysiloxanarylenes

An approach to the synthesis of potentially thermotropic liquid crystalline polymers based on parallel investigation of their molecular mobility was realized. The initial idea was provoked by the observation that there exists some correspondence of molecular mobility data and the ability of a polymer to form a liquid crystalline phase. Previously this phenomenon was demonstrated on the example of a series of thermotropic main chain polymers with flexible dimethylsiloxane spacers of variable length. The relation between the structure of the main chain and local molecular mobility of different fragments was investigated in a series of regular polysiloxane-silarylenes containing rigid aromatic sequences. Molecular mobility was studied by dielectric spectroscopy in solution and in solid state. The structure of the main chain has been changed by variation of the repeated fragments' length, substituents and joint groups. The data of molecular mobility and their conformity with the chain structure were used for directed synthetic search of desired mesogenic polymers.     

Thermotropic behavior of poly(oxyethylene) cholesterol ethers

The thermotropic behavior of poly(oxyethylene) cholesterol ether surfactants was studied by differential scanning calorimetry and small-angle X-ray scattering. Contrary to what is usually observed in conventional poly(oxyethylene)-type surfactant systems, poly(oxyethylene) cholesterol ether surfactants show a change of the fusion mechanism as the chain length is varied. For long chain lengths (n > or = 15) the usual solid-liquid transition is found, but for short chain lengths (n < or = 10) the transition goes through a birefringent lamellar phase. The appearance of this liquid crystal (LC) phase seems to be related with the predominance of the cholesterol part in the short chain polyoxyethylene surfactants. On the contrary, for long polyexyethylene chains the polymer gains in importance and only a solid crystalline structure is observed at low temperatures. An antiparallel packing structure with totally overlapped chains is found for both, the solid and the LC phase. The chains seem to be in a zigzag configuration, and only for the longest surfactant here studied (n = 30) a change of the chain configuration to a much shorter meander configuration is observed.     

烷基-α-D-吡喃葡萄糖苷的合成与性能

以D-葡萄糖为原料,经全乙酰化、在SnCl_4催化下与脂肪醇糖苷化、脱保护3步反应合成了7种不同碳链长度的烷基-α-D-吡喃葡萄糖苷。利用核磁共振、表面张力仪和偏光显微镜等对其进行结构、表面张力和热致液晶等性能测试,结果表明,当烷基-α-D-吡喃葡萄糖苷烷基链长(n)为6~9时,均有发泡和乳化性能,其中正壬基-α-D-吡喃葡萄糖苷具有最佳的发泡和乳化性能;烷基糖苷(n=6~9)的表面张力(γ_(CMC))及临界胶束浓度(CMC)均比较低;饱和吸附量(Γ_(max))随烷基链的增长而减小,饱和吸附面积(A_(min))随烷基链增长而增大;形成胶束时的标准自由能(ΔG_(mic))和吸附自由能(ΔG_(ads))均为负值,其绝对值随烷基链增长而越来越大,其中正辛基-α-D-吡喃葡萄糖苷的表面活性最好;烷基糖苷(n=4~9)对皮肤均无急性刺激作用;所合成的烷基糖苷均具有热致液晶行为,随烷基链长的增加,液晶相的温度范围变宽,液晶相的稳定性越好。     

Effect of headgroup on DNA-cationic surfactant interactions

The interaction behavior of DNA with different types of hydroxylated cationic surfactants has been studied. Attention was directed to how the introduction of hydroxyl substituents at the headgroup of the cationic surfactants affects the compaction of DNA. The DNA-cationic surfactant interaction was investigated at different charge ratios by several methods like UV melting, ethidium bromide exclusion, and gel electrophoresis. Studies show that there is a discrete transition in the DNA chain from extended coils (free chain) to a compact form and that this transition does not depend substantially on the architecture of the headgroup. However, the accessibility of DNA to ethidium bromide is preserved to a significantly larger extent for the more hydrophilic surfactants. This was discussed in terms of surfactant packing. Observations are interpreted to reflect that the surfactants with more substituents have a larger headgroup and therefore form smaller micellar aggregates; these higher curvature aggregates lead to a less efficient, "patch-like" coverage of DNA. The more hydrophilic surfactants also presented a significantly lower cytotoxicity, which is important for biotechnological applications.     

Surfactant adsorption at the salt/water interface: comparing the conformation and interfacial water structure for selected surfactants

We report in situ spectroscopic measurements monitoring the adsorption of a series of carboxylate surfactants onto the surface of the semisoluble, ionic solid fluorite (CaF2). We employ the surface-specific technique, vibrational sum-frequency spectroscopy (VSFS), to examine the effect that surfactant adsorption has on the bonding interactions and orientation of interfacial water molecules through the alteration of the electric properties in the interfacial region. In addition, we report on the chain length and headgroup dependence of the formation of hydrophobic self-assembled monolayers on the surface of the solid phase. Differences in chain length and headgroup functionality lead to large changes in the adsorption behavior and structuring of the monolayers formed and the interactions of interfacial water molecules with these monolayers. Fundamental studies such as these are essential for understanding the mechanisms involved in the surfactant adsorption process, information that is important for industrially relevant processes such as mineral ore flotation, waste processing, and petroleum recovery.     

Substitution effects on the liquid crystalline properties of D,L-xylitol amphiphiles

In this article we describe the self-assembling properties of alkyl substituted xylitols in relation to both thermotropic and lyotropic liquid crystalline mesophases. Three series of substituted xylitols were prepared where aliphatic chains of varying length were attached to a xylitol moiety via ether, thioether and ester linking groups. The thermotropic properties were investigated by thermal polarized light microscopy and differential scanning calorimetry, and evaluated as a function of chain length and linking group. The lyotropic phase behaviour was investigated via the addition of water to each material at room temperature. The efficiency for forming thermotropic phases was found to be reversed for the lyotropic phases in respect of the three series, i.e. as a function of the linking unit.     

Dimesogenic compounds consisting of cholesterol and azobenzene-based moieties: dependence of liquid crystal properties on spacer length and fluorination of the terminal chain

The liquid crystalline (LC) properties of two series of non-symmetric dimesogenic compounds consisting of cholesterol and azobenzene-based moieties interconnected by ω-oxyalkanoyl spacers of varying length are compared: one series (AOC-n) has an octyloxy chain attached to the azobenzene mesogen unit while the other (AOCF-n) has a perfluoroheptylmethyloxy chain. In general, compounds bearing the fluorinated alkoxy chain exhibited LC properties over a much broader temperature range than those with the alkoxy chain. In addition, the AOC-n series exhibited the chiral smectic C (SmC*), smectic A (SmA) and cholesteric (N*) phases depending on the length of the central spacer, whereas the AOCF-n series favoured the formation of only the SmA phase with the N* phase completely suppressed. Both series showed an odd-even dependence of the isotropization temperature on spacer length.     

Electrokinetic chromatographic characterization of novel pseudo-phases based on N-alkyl-N-methylpyrrolidinium ionic liquid type surfactants

MEKC and linear solvation free energy relationships (LSFERs) have been used to characterize the solute distribution between water and self-assemblies formed from ionic liquid type mono-chain cationic surfactants containing a cyclic pyrrolidinium head group. Several features of the solvation environment afforded by these micellar solutions were found to be quite different from that of CTAB, a structurally analogous cationic surfactant with a conventional, acyclic quaternary ammonium head group. None of the LSFER coefficients were found to vary in any systematic way with increasing alkyl chain length for these unique surfactants. Overall, however, these surfactants display different behavior than do all known cationic detergents such as CTAB. In chemical terms, pseudo-phases formed by these N-alkyl-N-methylpyrrolidinium bromides interact more strongly with polar compounds and less strongly with compounds having nonbonding or pi-electrons and are more cohesive compared to the well-studied CTAB.