Preparation of a neutral porous monolith and its evaluation in pressurized capillary electrochromatography with neutral and charged solutes

A neutral, nonpolar monolithic capillary column was evaluated as a hydrophobic stationary phase in pressurized CEC system for neutral, acidic and basic solutes. The monolith was prepared by in situ copolymerization of octadecyl methacrylate and ethylene dimethacrylate in a binary porogenic solvent consisting of cyclohexanol/1,4‐butanediol. EOF in this hydrophobic monolithic column was poor; even the pH value of the mobile phase was high. Because of the absence of fixed charges, the monolithic capillary column was free of electrostatic interactions with charged solutes. Separations of neutral solutes were based on the hydrophobic mechanism with the pressure as the driving force. The acidic and basic solutes were separated under pressurized CEC mode with the pressure and electrophoretic mobility as the driving force. The separation selectivity of charged solutes were based on their differences in electrophoretic mobility and hydrophobic interaction with the stationary phase, and no obvious peak tailing for basic analytes was observed. Effects of the mobile phase compositions on the retention of acidic compounds were also investigated. Under optimized conditions, high plate counts reaching 82 000 plates/m for neutral compounds, 134 000 plates/m for acid compounds and 150 000 plates/m for basic compounds were readily obtained.     

Separation of basic, acidic and neutral compounds by capillary electrochromatography using uncharged monolithic capillary columns modified with anionic and cationic surfactants

A mode of capillary electrochromatography (CEC), based on the dynamical adsorption of surfactants on the uncharged monolithic stationary phases has been developed. The monolithic stationary phase, obtained by the in situ polymerization of butyl methacrylate with ethylene dimethacrylate, was dynamically modified with an ionic surfactant such as the long-chain quaternary ammonium salt of cetyltrimethylammonium bromide (CTAB) and long-chain sodium sulfate of sodium dodecyl sulfate (SDS). The ionic surfactant was adsorbed on the surface of polymeric monolith by hydrophobic interaction, and the ionic groups used to generate the electroosmotic flow (EOF). The electroosmotic mobility through these capillary columns increased with increasing the content of ionic surfactants in the mobile phase. In this way, the synthesis of the monolithic stationary phase with binary monomers can be controlled more easily than that with ternary monomers, one of which should be an ionic monomer to generate EOF. Furthermore, it is more convenient to change the direction and magnitude of EOF by changing the concentration of cationic or anionic surfactants in this system. An efficiency of monolithic capillary columns with more than 140000 plates per meter for neutral compounds has been obtained, and the relative standard deviations observed for to and retention factors of neutral solutes were about 0.22% and less than 0.56% for ten consecutive runs, respectively. Effects of mobile phase composition on the EOF of the column and the retention values of the neutral solutes were investigated. Simultaneous separation of basic, neutral and acidic compounds has been achieved.     

Enantioselectivity of vesicle-forming chiral surfactants in capillary electrophoresis. Role of the surfactant headgroup structure

Two vesicle-forming single-tailed amino acid derivatized surfactants sodium N-[4-n-dodecyloxybenzoyl]-L-leucinate (SDLL) and sodium N-[4-n-dodecyloxybenzoyl]-L-isoleucinate (SDLIL) have been synthesized and used as pseudo-stationary phase in micellar electrokinetic chromatography to evaluate the role of steric factor of amino acid headgroup and hydrophobic/hydrophilic interactions for enantiomeric separations. The aggregation behavior of the surfactants has been studied in aqueous buffered solution using surface tension and fluorescence probe techniques. Results of these studies have suggested formation of vesicles in aqueous solutions. Microenvironment of the vesicle, which determines the depth of penetration of the analytes into vesicle was determined by fluorescence probe technique using pyrene, N-phenyl-1-naphthylamine (NPN), and 1,6-diphenyl-1,3,5-hexatriene (DPH) as probe molecules. Atropisomeric compounds (+/-)-1,1'-bi-2-naphthol (BOH), (+/-)-1,1'-binaphthyl-2,2'-diamine (BDA), (+/-)-1,1'-binaphthyl-2,2'-diylhydrogen phosphate (BNP) and Tr?ger's base (TB) and chiral compound benzoin (BZN) has been enantioseparated. The separations were optimized with respect to surfactant concentration, pH, and borate buffer concentration. SDLL was found to provide better resolution for BOH, BNP, and BZN. On the other hand, SDLIL offers better resolution for BDA. The chromatographic results have been discussed in the light of the aggregation behavior of the surfactants and the interaction of the solutes with the vesicles.     

胶束溶液体系对毛细管电动色谱酸性化合物分离的影响

以阴离子表面活性剂十二烷基硫酸钠（SDS）、非离子表面活性剂吐温20（ Tween 20）及两者组成的混合胶束体系作为毛细管胶束电动色谱（MECC）的分离介 质，进行4种结构相似的酸性化合物的MECC分离研究，考察了胶束的类型、表面活 性剂的浓度、缓冲溶液的pH值及有机改性剂乙醇对分离的影响。结果表明各因素对 酸性药物的MECC分离有不同的影响规律。SDS胶束体系对溶质的保留值最大， Tween 20体系的保留值最小，二者的分离选择性正好相反，混合胶束体系的分离行 为则介于两者之间；在SDS和Tween 20体系中，表面活性剂浓度增加，溶质的保留 时间均随之递增，混合胶束体系中，总浓度一定，随Tween 20配比的增加，溶质的 保留时间先减少后增加；缓冲溶液的pH值增大，使溶质的分离效果均能变差；有机 改性剂乙醇的加入对容量因子的影响主要与溶质的疏水性有关，并对分离作用机理 进行了探讨。在SDS和Tween 20 MECC体系下，分别进行了实样测定，取得了满意的 结果。     

Natural surfactants

The ever-increasing environmental concern about surfactants triggers an interest in natural surfactant. This review, which has an emphasis on work published since 1998, covers three categories of natural surfactants: amphiphiles produced by yeast or bacteria, amphiphiles containing a natural polar headgroup and amphiphiles containing a natural hydrophobic tail. Microorganisms produce both high molecular weight and low molecular weight surfactants. Only the low molecular weight compounds are included in the review. Sugars and amino acids are the two most important examples of surfactant polar headgroups of natural origin. The research is particularly intense in the area of sugar surfactants and the review covers three types: alkylglucosides, alkylglucamides and sugar esters. Surfactants based on two types of natural hydrophobic tails are included: fatty acid monoethanolamides and sterol ethoxylates. Routes of preparation as well as physico-chemical properties are discussed for the surfactants prepared by organic synthesis.     

Molecular recognition by chiral cationic micellar and micelle-like aggregates in electrokinetic capillary chromatography

We examined the enantiomer separation with micelles and a micelle-like polymer made with trimethylammonium-terminated surfactants all of whose hydrocarbon chains contain hydrogen bonding valinediamide moieties in electrokinetic chromatography (EKC). The surfactants used were 3-(N-dodecanoyl-L-valylamino)-propyltrimethylammonium bromide (surfactant 1) and 6-(N-nonanoyl-L-valylamino)hexyl-trimethylammonium bromide (surfactant 2); the micelle-like polymer was derived from 3-(N-10-undecenoyl-L-valyl)aminopropyltrimethylammonium bromide (surfactant 3). N-Acylamino acids and their isopropyl esters were separated with enantiomers with the same configuration as the chiral surfactant and which were retained to a greater extent than the counterparts in micelles. The micellar hydrophobic environment, in which amides function as hydrogen bonding sites with solutes, and ceased micellar kinetic association-dissociation with polymerization are discussed.     

N-Alkylnicotinium halides: A class of cationic matrix additives for enhancing the sensitivity in negative ion Fast-Atom bombardment mass spectrometry of polyanionic analytes

The addition of some surfactants to the fast-atom bombardment (FAB) matrix previously has been demonstrated to enhance analyte signals in fast-atom bombardment mass spectrometry. In particular, cationic surfactants appear to enhance the negative ion FAB detectability of analytes that exist as anionic species in the matrix solution. It has been proposed that the charged surfactant concentrates the oppositely charged analyte near the surface, which results in larger signals for the analyte. Cationic surfactants that contain a fixed positive charge and an additional basic site were prepared with different hydrophobic moieties and were evaluated for their effectiveness as FAB matrix additives. The compound N-octylnico-tinium bromide (ONBr) is shown to improve greatly the analyte-related signals in negative ion fast-atom bombardment mass spectrometry for a variety of polyanionic analytes, relative to other surfactants (e.g., cetylpyridinium salts). This surfactant not only enhances detectability, but also simplifies the pseudomolecular ion region of the resulting spectra by reducing or eliminating metal cation adduct peaks. The simple mechanism of enhancement via surface activity is evaluated, and alternative mechanisms are considered. It is clearly shown that ONBr, as a FAB matrix additive, will allow mass spectrometry to be used for the analysis of anionic compounds that normally exhibit very low responses.     

Novel anionic copolymerized surfactants of mixed achiral and chiral surfactants as pseudostationary phases for micellar electrokinetic chromatography

One disadvantage of amino acid-based chiral selectors for micellar electrokinetic chromatography (MEKC) is that either they have very low solubility or are insoluble at acidic pHs. In order to increase solubilities at lower pHs, we have synthesized a highly water-soluble achiral surfactant and copolymerized it with an amino acid-based chiral surfactant. These two surfactants were polymerized either separately or at various molar rations of binary solutions, yielding pure molecular or copolymerized surfactant (CoPS), respectively. All surfactants were characterized by use of several analytical techniques prior to using them as novel pseudostationary phases in MEKC. The chromatographic performance of the CoPS in MEKC was tested with chiral and achiral analytes. The highly soluble sulfate head group significantly increased the solubility of amino acid-based CoPS over a wide range of pH. Three chiral binaphthyl derivatives were tested and each surfactant system was found to have different selectivity.     

Polymeric sulfated surfactants with varied hydrocarbon tail: I. Synthesis, characterization, and application in micellar electrokinetic chromatography

The influence of surfactant hydrocarbon tail on the solute/pseudostationary phase interactions was examined. Four anionic sulfated surfactants with 8-, 9-, 10-, and 11-carbon chains having a polymerizable double bond at the end of the hydrocarbon chain were synthesized and characterized before and after polymerization. The critical micelle concentration (CMC), polarity, and aggregation number of the four sodium alkenyl sulfate (SAIS) surfactants were determined using fluorescence spectroscopy. The partial specific volume of the polymeric SAIS (poly-SAIS) surfactants was estimated by density measurements and capillary electrophoresis (CE) was employed for determination of methylene selectivity as well as for elution window. The CMC of the monomers of SAIS surfactants decrease with increase in chain length and correlated well when fluorescence method was compared to CE. The physicochemical properties (partial specific volume, methylene selectivity, electrophoretic mobility, and elution window) increased with an increase in chain length. However, no direct relationship was found between the aggregation number and the length of hydrophobic tail of poly-SAIS surfactants. These polymeric surfactants were then used as pseudostationary phases in micellar electrokinetic chromatography (MEKC) to study the retention behavior and selectivity factor of 36 benzene derivatives with different chemical characteristics. Although variation in chain length of the polymeric surfactants significantly affects the retention of nonhydrogen bonding (NHB) benzene derivatives, these effects were less pronounced for hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) benzene derivatives. Therefore, hydrophobicity of poly-SAIS surfactants was found to be a major driving force for retention of NHB derivatives. However, for several benzene derivatives (NHB, HBA, and HBD) significantly higher selectivity factor was observed with longest chain polymeric surfactant (e.g., poly(sodium 10-undecenyl sulfate), poly-SUS) compared to shorter chain polymeric surfactant (e.g., poly(sodium 7-octenyl sulfate), poly-SOcS). In addition, the effect of the surfactant hydrophobic chain was also found to have some impact on migration order of NHB, HBA, and HBD benzene derivatives.     

Molecular rulers: new families of molecules for measuring interfacial widths

Homologous series of solvatochromic neutral alcohols and ionic sulfates are synthesized and characterized. Each surfactant series consists of hydrophobic, p-nitroanisole-based chromophores attached to polar or ionic headgroups by n-alkyl spacers. UV absorption measurements show that the optical properties of surfactant chromophores closely track those of the parent chromophore. Interfacial tension measurements are used to calculate surface excess concentrations of ionic surfactants adsorbed to an aqueous-cyclohexane interface. With a hydrophobic chromophore, a hydrophilic headgroup, and a variable-length, alkyl spacer, these surfactants have the potential to function as molecular rulers: probes of molecular-scale variation in solvation forces across condensed-phase interfaces. Changing the separation between the hydrophobic, solvatochromic probe and the hydrophilic headgroup should enable different members of a homologous series to span different interfacial widths, thus exposing the chromophore to different chemical environments. This idea is explored by using surface-specific, nonlinear optical spectroscopy. Resonant second harmonic spectra of p-nitroanisole and the surfactant product 4a adsorbed to an aqueous-cyclohexane interface show the surfactant spectrum blue-shifted 9 nm relative to the spectrum of adsorbed p-nitroanisole. On the basis of chromophore solvatochromism, these results are consistent with a less polar environment surrounding the surfactant chromophore. Significant differences in interfacial solvation resulting from a approximately 5 A separation between the surfactant headgroup and chromophore support recently proposed models of molecularly sharp, microscopically flat aqueous-alkane interfaces.     

The use of novel water-in-oil microemulsions in microemulsion electrokinetic chromatography

We describe the novel use of water-in-oil (W/O) microemulsions to achieve unique separations in microemulsion electrokinetic chromatography (MEEKC). The choice and concentration of the buffer type, surfactant and co-surfactant were all examined and optimized. Separations of a range of neutral and acidic analytes was shown to be markedly different to that obtained by (oil-in-water) O/W MEEKC. Neutral solutes are separated by virtue of their solubility (log P) values in O/W MEEKC with the more water-insoluble solutes migrating last. This separation process does not occur in W/O, as neutral solutes are not separated in order of log P.     

Microcalorimetric study on micellization of nonionic surfactants with a benzene ring or adamantane in their hydrophobic chains

The micellization of a novel family of nonionic surfactants poly(oxyethylene) glycol alkyl ethers has been studied by microcalorimetry. One of the surfactants has adamantane, and the other nonionic surfactants have a benzene ring in their hydrophobic chains, which moves from the terminal of the hydrophobic chain toward the headgroup. Moreover, the alkyl chain of the nonionic surfactants is straight or branched. Both the critical micelle concentration (cmc) and the thermodynamic parameters associated with the micelle formation have been obtained. The cmc decreases and the enthalpy of micelle formation (deltaH(mic)) becomes less positive gradually as the length of hydrophobic chain increases, whereas the values of cmc and deltaH(mic) tend to increase for the surfactants with a longer ethylene oxide chain. However, the deltaH(mic) value of the surfactant with seven carbon atoms in a hydrophobic chain is more positive than that of the surfactant with six carbon atoms in a hydrophobic chain. Comparing with the nonionic surfactant with a methylene hydrophobic chain, the surfactants with benzene rings and adamantane groups have larger cmc values and the cmc values increase with the size of the groups. Furthermore, moving the phenyl group from the terminal of the hydrophobic chain to the neighbor of the hydrophilic headgroup leads to the decreased cmc. Both the variation of hydrophobic interaction from the movement of phenyl group and pi-pi interaction among adjacent phenyl groups affect deltaH(mic) values.     

Application of a chemical equilibrium model to thermodynamic functions of transfer of alcohols from water to aqueous surfactants

In ternary aqueous solutions, hydrophobic solutes such as alcohols tend to aggregate with surfactants to form mixed micelles. These systems can be studied by meas of the functions of transfer of hydrophobic solutes from water to aqueous solutions of surfactant. These thermodynamic functions often go through extrema in the critical micellar concentration (CMC) region of the surfactant. A simple model based on interactions between surfactant and hydrophobic solute monomers, on the distribution of the hydrophobic solute between water and the micelles and on the shift in the CMC induced by the hydrophobic solute, can simulate the magnitude and trends of the transfer functions using parameters which are mostly derived from the binary systems. In order to check the model more quantitatively, volumes and heat capacities of transfer of alcohols from water to aqueous solutions of a nonionic surfactant, octyldimethylamine oxide, were measured. A quantitative agreement was achieved with three adjustable parameters. Good fits are also obtained for the transfers to the ionic surfactants, octylamine hydrobromide and sodium dodecylsulfate. When the equilibrium displacement contribution is small, the distribution constants and the partial molar properties of the alcohols in the micellar phase agree well with the parameters obtained with similar models.     

Influence of ester and amide-containing surfactant headgroups on selectivity in micellar electrokinetic chromatography

The selectivity differences between six anionic surfactants in micellar electrokinetic chromatography (MEKC) are presented and the structural influence of the surfactant head-group is investigated. It was determined that the surfactant structure can have a significant impact on retention and selectivity. Linear solvation energy relationships (LSERs) are used to study the role of solute size, polarity/polarizability, and hydrogen bonding characteristics in determining retention and selectivity. While both the solute size and hydrogen bond accepting ability were found to be the most important factors in solute retention, the hydrogen bonding characteristic of the solutes have the largest influence on selectivity differences between surfactants.     

高效液相色谱槲皮素键合硅胶固定相分离极性化合物

槲皮素是一种植物体中含量丰富、价格较便宜的黄酮类化合物,本研究以γ-[(2,3)-环氧丙氧]丙基三甲氧基硅烷(KH-560)为偶联剂,将其化学键合到硅胶上,得到一种含天然配体的槲皮素键合硅胶固定相(QUSP)。 采用红外光谱、热重分析、元素分析及固体核磁对其结构进行表征,测得硅胶表面槲皮素的键合量为0.139 mmol/g。 采用不同结构的溶质作探针,在评价固定相反相液相色谱疏水作用性能的基础上,侧重研究新固定相对极性芳香族化合物的分离能力,探讨了新固定相的色谱分离机理。 研究表明,仅采用甲醇或乙腈-水简单流动相,无需用缓冲液精确控制pH值,QUSP就能分别实现吡啶类、芳胺类、苯酚类、苯甲酸类和黄酮类等极性化合物的快速基线分离。 QUSP键合的槲皮素除含疏水性的C6-C3-C6骨架外,黄酮环还能为溶质提供氢键、偶极、π-π、电荷转移等多种作用位点,各种协同作用有利于提高色谱分离选择性,尤其对极性的可离子化的酸性和碱性化合物。     

Metal-organic frameworks for reverse-phase high-performance liquid chromatography

Metal-organic framework MIL-53(Al) is explored for reverse-phase high-performance liquid chromatographic separation of a wide range of analytes from non-polar to polar, and acidic to basic solutes with high resolution, good selectivity, stability and reproducibility.     

Retention behavior of basic compounds on alkylphosphonate-modified magnesia-zirconia composite stationary phase in RPHPLC

Summary The chromatographic properties of an alkylphosphonate-modified magnesia-zirconia composite stationary phase have been investigated by reversed-phase high-performance liquid chromatography with basic compounds as probes. The influence of organic modifier composition and mobile phase pH was studied. The new stationary phase, similar to a silica-based reversed-phase stationary phase, has hydrophobic properties, but greater pH stability. Use of the phase results in more symmetric peaks for basic compounds. A possible mechanism of retention of basic solutes on the new stationary phase is discussed. The chromatographic behavior of the basic solutes depends mainly on hydrophobic interactions between the solutes and the hydrophobic moiety of the stationary phase. Br?nsted acidic and basic sites on the surface of the new stationary phase play an important role in the retention of ionized solutes by ion-exchange interaction. Promising separations of some basic compounds have been achieved by use of methanolic TRIS buffer, pH 10.0, as the mobile phase.     

Chromatographic behavior of a new hybrid type RP material containing silica bonded 1,3‐alternate 25,27‐bis‐[cyanopropyloxy]‐26,28‐bis‐[3‐propyloxy]‐calix[4]arene

A novel 1,3‐alternate 25,27‐bis‐[cyanopropyloxy]‐26,28‐bis‐[3‐propyloxy]‐calix[4]arene‐bonded silica gel stationary phase (CalixPrCN) was prepared and its structure was confirmed by ATR‐FTIR spectroscopy and elemental analysis. The CalixPrCN phase was characterized in terms of its surface coverage, hydrophobic selectivity, aromatic selectivity, shape selectivity, hydrogen bonding capacity, residue metal content, and silanol activity based on Tanaka, Lindner, and SMR 870 test protocols. The effect of the acetonitrile content on the retention and selectivity of the selected neutral, basic, and acidic solutes was studied. The neutral and acidic analytes exhibited classical RP behavior, in which retention time decreases with increasing acetonitrile content. In contrast, basic analytes showed an increase in retention at low and high percentages of acetonitrile, forming “U‐shaped” retention profiles. The new calixarene phase was compared with previously reported 1,3‐alternate 25,27‐bis‐[propyloxy]‐26,28‐bis‐[3‐propyloxy]‐calix[4]arene stationary phase and commercial cyanopropyl column. The results indicate that the CalixPrCN stationary phase behaves like RP packing; however, inclusion complex formation, dipole–dipole, and π–π interactions seem to be involved in the separation process. The selectivity of this phase was demonstrated in separation of polynuclear aromatic hydrocarbons, non‐steroidal anti‐inflammatory drugs, and sulfonamides as analytes.     

厚朴酚键合硅胶液相色谱固定相的制备、表征与性能评价

通过γ-巯丙基三甲氧基硅烷(KH-590)的作用, 将具有抗菌功能的中草药厚朴的主要药用成分厚朴酚键合在硅胶表面上, 制备了厚朴酚键合硅胶液相色谱固定相. 采用红外光谱、元素分析和热重分析对该固定相进行了表征. 以苯同系物、5种吡啶、6种苯胺和8种芳香羧酸类化合物为溶质探针, 初步考察了该新型固定相的基本色谱性能, 研究了其对这些化合物的保留机理. 结果表明, 该固定相的反相色谱性能类似于十八烷基键合硅胶固定相(ODS), 分离原理与疏水性作用有关; 另外, 该固定相包含有别于疏水性作用的氢键作用、π-π电荷转移作用和偶极-偶极等作用, 多种作用力使其在分离某些可电离的碱性和酸性化合物时表现出更好的选择性和分离效果. 厚朴酚配体的多种作用位点对快速分离极性芳香化合物有重要贡献.