Study on the adsorption and aggregation of dilute gemini surfactant solutions

As a function of temperature in aqueous solutions, the adsorption and aggregation of N,N’-bis (tetradecyl dimethyl)-1,2-dibromide-ethanediyl ammonium salt (GS14-2-14) and N,N’-bis (hexadecyl dimethyl)-1,2-dibromide-ethanediyl ammonium salt (GS16-2-16), were researched with drop-volume technique and conductometry, respectively. The results of surface tension measurements, which were analyzed by originally developed thermodynamic equations, illustrate that GS14-2-14 has a better surface activity and arranges more tightly in the adsorbed film than GS16-2-16. The data of conductivity were used to find critical micelle concentration (cmc) and counterion binding degree of micelle (β). Thermodynamic parameters of micellization were also obtained from the temperature dependence of cmc values. From the study, it is discovered that the micellization process is spontaneous and exothermic in nature and it is mainly driven by entropy.     

Cationic starch derivatives as dynamic coating additives for protein analysis in capillary electrophoresis

Positively charged starch derivatives were used to modify the inner surface of fused-silica capillaries by addition to running buffer, which were subsequently employed in capillary electrophoresis (CE). Capillaries coated with the cationic starch derivatives were shown to generate a stable, reversed electroosmotic flow (EOF) in the investigated pH range of 3-9. The presented coating procedure was fast, based on a simple rinsing protocol where the polymer created a physically adsorbed, cationic polymer layer. Among the additives studied, a quaternary ammonium starch derivative showed a fast EOF mobility and effectively suppressed the adsorption of proteins. The intra- and inter-day reproducibility of the coating referring to the EOF mobility were satisfactory with relative standard deviation (RSD) of 0.27 and 1.67%, respectively. The coating enabled separation of some protein mixtures including basic proteins within l3 min with efficiencies up to 280,000 plates/m. In addition, this cationic starch derivative possessed a good solubility (about 100mg/mL), and it does not significantly contribute to the background adsorption in the UV region of 190-400 nm.     

Salt effect on microstructures in cationic gemini surfactant solutions as studied by dynamic light scattering

A cationic gemini surfactant, dodecanediyl-1,12-bis(dodecyldiethylammonium bromide) (C12C12C12(Et)), in aqueous solutions with varying NaBr concentration was studied by dynamic light scattering (DLS). As a comparison, its single-chained counterpart, dodecyl triethylammonium bromide (DTEAB), was also investigated under the same conditions. Similar to the case of a polyelectrolyte, C12C12C12(Et) underwent a typical "ordinary-to-extraordinary (o-e) transition" with decreasing salt concentration to zero. At higher salt concentration, a single relaxation mode, corresponding to the diffusion of regular micelles, was observed. While in the "extraordinary regime", DLS detected two characteristic relaxation modes with the values of the diffusion coefficient being different by at least 2 orders. The fast mode was consistent with the polyion-small ion coupled-mode theories, as well as the direct polyion-polyion repulsion interactions. Because the slow mode disappeared at elevated salt concentrations and generated negligible scattered intensity, we attributed it to multimacroion domains.     

Organic and inorganic di-cations for capillary silica coating and EOF modulation in CE: Example of application in PEG analysis

EOF measurements, by using 1,4-di-(4-aza-1-azonia-bicyclo[2.2.2]octane)butane diiodide, barium and strontium tetraborate as silica wall modifiers, are reported and, as an example of application, analysis of PEG (PEG 400-2000) polydisperse preparations in free solution CZE is shown. PEGs have been derivatized with phthalic anhydride so as to form singly or doubly charged derivatives with strong UV absorbance at 214 nm. Whereas separations in plain tetraborate buffer, pH 9.0, without any EOF control, did not lead to good resolution of all-size oligomers and suffered from long analysis times, excellent resolution of all oligomers up to 40 ethylene oxide (EO) units could be obtained under EOF control. Such EOF modulation was engendered by addition of 1 mM M7C4M7, a doubly charged organic cation able to stick tenaciously to the silica wall. Further modulation of EOF and silica surface modification could be achieved also by addition of inorganic cations, notably those of group II, whereas monovalent cations did not seem to affect much the EOF flux. Among the doubly charged cations investigated, Ca++, Mg++, Sr++ and Ba++, the latter did seem to offer best EOF control and reproducible runs. A judicious blend of M7C4M7 (0.33-1 mM range) with barium (10-20 mM range) allowed baseline resolution of all PEG oligomers investigated up to PEG 2000 and >40 EO units in length. In this last case, best results in terms of reproducibility and separation efficiency of the more heavy homologues were obtained using Li+ salt in small amounts.     

Cationic ester-containing gemini surfactants: adsorption at tailor-made surfaces monitored by SPR and QCM

Adsorption of a series of ester-containing cationic surfactants at a surface containing 90% methyl groups and 10% carboxyl groups was studied by two surface analysis techniques, surface plasmon resonance (SPR) and quartz crystal microbalance (QCM). Such a surface, which is at the same time hydrophobic and negatively charged, is of interest as a model for many polymeric surfaces. Two different types of ester gemini surfactants and their monomeric counterparts were included together with nonester containing surfactants of similar structure. The results show that the gemini surfactants give the same adsorbed amount at the surface as the monomeric surfactants when compared at the same bulk concentration normalized to the critical micelle concentration (cmc) in bulk. Since the cmc of the geminis is around 20 times lower than the cmc of the corresponding monomeric surfactants, the gemini surfactants are much more effective in covering the surface. The two techniques gave similar relative values but the QCM values were always higher than those from SPR, which is due to the former method taking also adsorbed water into account. The adsorption, as measured by both methods, was found to follow closely the Langmuir adsorption model.     

Assessment of polyelectrolyte coating stability under dynamic buffer conditions in CE

Dynamic buffer conditions are present in many electrophoretically driven separations. Polyelectrolyte multilayer coatings have been employed in CE because of their chemical and physical stability as well as their ease of application. The goal of this study is to measure the effect of dynamic changes in buffer pH on flow using a real-time method for measuring EOF. Polyelectrolyte multilayers (PEMs) were composed of pairs of strong or completely ionized polyelectrolytes including poly(diallyldimethylammonium) chloride and poly(styrene sulfonate) and weak or ionizable polyelectrolytes including poly(allylamine) and poly(methacrylic acid). Polyelectrolyte multilayers of varying thicknesses (3, 4, 7, 8, 15, or 16 layers) were also studied. While the magnitude of the EOF was monitored every 2 s, the buffer pH was exchanged from a relatively basic pH (7.1) to increasingly acidic pHs (6.6, 6.1, 5.5, and 5.1). Strong polyelectrolytes responded minimally to changes in buffer pH (<1%), whereas substantial (>10%) and sometimes irreversible changes were measured with weak polyelectrolytes. Thicker coatings resulted in a similar magnitude of response but were more likely to degrade in response to buffer pH changes. The most stable coatings were formed from thinner layers of strong polyelectrolytes.     

Effect of dynamic surfactant adsorption on emulsion stability

The effect of dynamic surfactant adsorption on the stability of concentrated oil in water emulsions is studied. For this purpose, a modification of the standard Brownian dynamics algorithm (Ermak, D.; McCammon, J. A. J. Chem. Phys. 1978, 69, 1352) previously used to study the behavior of bitumen emulsions assuming instantaneous adsorption (Urbina-Villalba, G.; García-Sucre, M. Langmuir 2000, 16, 7975) was employed. In the present case, dynamic adsorption (DA) was accounted for through a time-dependent electrostatic repulsion between the drops, a function of the surfactant surface excess. The surface excess was allowed to evolve with time according to well-established analytical expressions which depend parametrically on the surfactant diffusion constant (Ds) and the total surfactant concentration (C). The investigation required appropriate incorporation of hydrodynamic interactions in concentrated systems. This was achieved through a novel methodology, which expresses the diffusion constant of each particle as a function of its local concentration and the shortest distance of separation between nearest neighbors. In model systems, the variation of the number of drops as a function of time was followed for different magnitudes of the apparent diffusion constant D(app) of the surfactant. For each of these values, the effect of C and the volume fraction of internal phase (phi) was considered. DA was found to influence emulsion stability appreciably at moderately high phi. In this case, the average collision time between drops is comparable to the time required for the occurrence of a substantial surfactant adsorption, but the interdrop separation is sufficiently large to prevent a considerable slowdown of particle movement due to hydrodynamic interactions.     

Control of the EOF in CE using polyelectrolytes of different charge densities

The control of the EOF direction and magnitude remains one of the more challenging issues for the optimization of separations in CE. In this work, we investigated the possibility to use variously charged polyanions for a fine-tuning of the EOF using polyelectrolyte multilayers. For that purpose, polyanions of poly(acrylamide-co-2-acrylamido-2-methyl-1-propanesulfonate) (PAMAMPS) with different chemical charge rates varying between 3 and 100% were used. These copolymers are statistic hydrophilic copolymers of acrylamide (AM) and 2-acrylamido-2-methyl-1-propanesulfonate (AMPS). The study of the influence of the chemical charge rate (AMPS molar proportion in the copolymer) on the electroosmotic mobility (mu(eo)) of a capillary modified by a polyelectrolyte bilayer (polycation/PAMAMPS) revealed that the fine-tuning of the EOF was possible, at least for cathodic or slightly anodic EOF (micro(eo) from -5 x 10(-5) to +35 x 10(-5) cm(2)V(-1)s(-1)). Electroosmotic mobility values were compared with the free-draining electrophoretic mobilities of the PAMAMPS constituting the last layer of the capillary coating. The stability of the EOF is discussed in detail on the basis of successive determinations of electroosmotic mobility and migration times. The application to the separation of a model peptide mixture demonstrated the interest (and the simplicity) of this approach for optimizing resolution and analysis time. Experimental resolutions were compared to the theoretical ones that we would obtain on a fused-silica capillary having the same EOF as the coated capillary.     

Cationic polymer coatings for design of electroosmotic flow and control of DNA adsorption

A difficulty with the design and operation of an electrokinetically operated DNA hybridization microfluidic chip is the opposite direction of the electroosmotic flow and electrophoretic mobility of the oligonucleotides. This makes it difficult to simultaneously deliver targets and an appropriate hybridization buffer simultaneously to the probe sites. In this work we investigate the possibility of coating the inner walls of the microfluidic system with hexadimentrine bromide (polybrene, PB) and other cationic polymers in order to reverse the direction of electroosmotic flow so that it acts in the same direction as the electrophoretic transport of the oligonucleotides. The results indicated that the electroosmotic flow (EOF) in channels that were coated with the polymer could be reversed in 1× TBE buffer or 1× SSC buffer. Under these conditions, the DNA and EOF move in the same direction, and the flow can be used to deliver DNA to an area for selective hybridization within the channel. The effects of coating the surface of a nucleic acid microarray with polybrene were also studied to assess non-selective adsorption and stability. The polybrene coating significantly reduced the extent of non-selective adsorption of oligonucleotides in comparison to adsorption onto a glass surface, and the coating did not alter the extent of hybridization. The results suggest that use of the coating makes it possible to achieve semi-quantitative manipulation of nucleic acid oligomers for delivery to an integrated microarray or biosensor.     

Catalytic role of gemini surfactant micelles in the ninhydrin-L-isoleucine reaction

Effect of dicationic gemini surfactants C₁₆H₃₃(CH₃)₂N⁺-(CH₂) _s -N⁺(CH₃)₂C₁₆H₃₃, 2Br⁻ (where s = 4, 5, 6) on the reaction of ninhydrin with L-isoleucine has been investigated spectrophotometrically as a function of [gemini], [L-isoleucine], [ninhydrin], and pH. The reaction follows first- and fractional-order kinetics, respectively, in [L-isoleucine] and [ninhydrin]. The gemini surfactant micellar media are found more effective for the reaction than their conventional monomeric counterpart CTAB. Furthermore, whereas typical rate constant (k _ψ) increase and leveling-off regions are observed with CTAB and geminis, the latter produce a third region of increasing k _ψ at concentrations ≥ 60 cmc’s. ¹H NMR studies reveal that this unusual third-region effect of the geminis is due to changes in their micellar morphologies. Quantitative kinetic analysis has been performed on the basis of modified pseudo-phase model.     

Adsorption and association in aqueous solutions of dissymmetric gemini surfactant

A gemini surfactant with two hydrocarbon chains differing in length and with an ethylene spacer N,N-dimethyl-N-(2-(N',N'-dimethyl-N'-dodecylammonio) ethyl) tetradecylammonium dibromide, 12-2-14, was synthesized and its physicochemical properties were studied by surface tension, conductometry, potentiometry, viscosimetry, and light scattering measurements, as well as by optical microscopy. Surface properties and thermodynamic parameters lie between those obtained for its symmetric counterparts, while association in solution exhibited peculiar properties, i.e., high polydispersity and the coexistence of three populations of differently sized aggregates.     

双子表面活性剂溶液的表面活性的研究

研究了阳离子型双子表面活性剂,二溴化-N,N'-二(二甲基烷基)乙(已)二铵,以及它们与阴离子表面活性剂十二烷基苯磺酸钠(SDBS)复配体系的表面活性,测定上述体系的平衡态表面张力。结果表明：双子表面活性剂的表面活性大大高于十二烷基三甲溴化铵(DTAB);对于两种双子表面活性剂,其表面活性和表面张力时间效应受其联接基团的影响远大于其烷基链的影响。双子表面活性剂与SDAB复配,其协同效应不如DTAB。动表面张力测定得到它们的各种参：t~i,t~m,γ~m,t*和n等值,结果表面双子表面活性剂的瞬时活性也高于DTAB。     

Synthesis and surface properties of PDMS–acrylate emulsion with gemini surfactant as co-emulsifier

Composite latex particles of acrylate and polydimethylsiloxane (PDMS) with high PDMS content was prepared by emulsion copolymerization and characterized by particle size analyzer, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). With gemini surfactant as the co-emulsifier in the system, the PDMS content in the system reached 50%, which was far higher than the other reported values. Through the characterization of the particle size analyzer, the particle size augmented with the increase of the amount of PDMS, which could be said that the polysiloxane had participated into the reaction and had been introduced into the colloid particle. The results of FTIR indicated that almost all the monomer had been exhausted in the reaction because there was no C=C and D₄ characteristic peaks in the spectrum. Besides the surface properties also were measured by surface tension analysis, water absorption, and the static contact angle, it could be found that with the increase of polysiloxane content, the excellent properties acquired by PDMS were clearly revealed by the findings, such as the decrease of surface tension and water absorption, and the increase of static contact angle. All the measurements were consistent with the conclusion that the composite latex particles of polysiloxane and acrylate with high siloxane content had been prepared successfully.     

Thermodynamics of micelle formation of the counterion coupled gemini surfactant Bis(4-(2-dodecyl)benzenesulfonate)-Jeffamine salt and its dynamic adsorption on sandstone

A novel counterion-coupled gemini (cocogem) surfactant, DBSJ, was synthetized via the 2:1 coupling reaction between 4-(2-dodecyl)benzenesulfonic acid (Lutensit A-LBS) and polypropyleneglycol-bis(2-aminopropyl) ether (Jeffamine D230). The surfactant had a polydispersity index of Mw/Mn = 1.04, as determined by electrospray-ionization mass spectrometry. The micellar properties of DBSJ in water were investigated in the temperature range 283-348 K by conductometry and titration microcalorimetry. The critical micelle concentration (cmc) of the cocogem was found to be more than 1 order of magnitude less than that of monomeric sodium 4-(2-dodecyl)benzenesulfonate (SDBS). The mean degree of dissociation in the temperature range studied proved to be alpha = 0.39. The calorimetric enthalpies of micelle formation agreed well with the enthalpies calculated via the van't Hoff relation. The cmc versus T curve passes through a minimum just below room temperature, after which the micelle formation changes from endothermic to exothermic. The Gibbs free energy of micelle formation was nearly constant as the temperature was increased, due to enthalpy/entropy compensation. The isotherm for DBSJ adsorption from aqueous solution onto sandstone was determined by continuous flow frontal analysis solid/liquid chromatography at 298 K and 60 bar. The adsorption of DBSJ on sandstone followed an S-type isotherm. Surface aggregation occurred over an extended range of concentration. Surface saturation was reached at a solution concentration more than 1 order of magnitude less than for monomeric SDBS. This finding is a point of concern in the chemical flooding of oil reservoir rocks to enhance oil recovery.     

A novel cationic triblock copolymer as noncovalent coating for the separation of proteins by CE

A novel noncovalent adsorbed coating for CE has been prepared and explored. This coating was based on quaternized poly(2-(dimethylamino)ethyl methacrylate)-block-poly(ethylene oxide)-block-poly(2-(dimethylamino)ethyl methacrylate) (QDED) triblock copolymer which was synthesized by atomic transfer radical polymerization (ATRP) in our laboratory. The polycationic polymer and the negatively charged fused-silica surface attracted each other through electrostatic interactions and hydrogen bonds. It was demonstrated that the coated capillaries provided an electroosmotic flow with reverse direction, and the magnitude of the electroosmotic flow can be modulated by varying the molecular mass of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) block and pH value of the buffer. The effects of the molecular mass of PDMAEMA block in QDED triblock copolymer and pH value of the buffer on the separation of basic proteins were investigated in detail. The triblock copolymer coatings showed higher separation efficiency, better migration time repeatability and would apply to wider range of pH than bare fused-silica capillary when used in separating proteins. Proteins from egg white were also separated through this QDED triblock copolymer-coated capillary. These results demonstrated that the QDED triblock copolymer coatings are suitable for analyzing biosamples.     

Effect of hydrocarbon parts of the polar headgroup on surfactant aggregates in gemini and bola surfactant solutions

Cationic gemini surfactant homologues alkanediyl-alpha,omega-bis(dodecyldiethylammonium) bromide, [C12H25(CH3CH2)2N(CH2)SN(CH2CH3)2C12H25]Br2, where S = 4, 6, 8, 10, or 12, referred to as C12CSC12(Et), and cationic bolaamphiphiles BPHEAB (biphenyl-4,4'-bis(oxyhexamethylenetriethylammonium) bromide), PHEAB (phenyl-4,4'- bis(oxyhexamethylenetriethylammonium) bromide) were synthesized, and their aggregation behaviors in aqueous solution were studied and compared by means of dynamic light scattering, fluorescence entrapment, and transmission electron microscopy. Spherical vesicles were found in the aqueous solutions of these gemini and bola surfactants, which can be attributed to the increase of the hydrocarbon parts of the polar headgroup of the surfactants. In combination with the result of the other gemini with headgroup of propyl group, the increase of the hydrophobic parts of the surfactant polar headgroup will be beneficial to enhance the aggregation capability of the gemini and bola surfactants. Both of the vesicles formed in the gemini and bola systems showed good stabilities with time and temperature, but different stability with salt due to the different membrane conformations of surfactant molecules in the vesicles.     

Lyotropic and interfacial behaviour of an anionic gemini surfactant

The sodium salt of N,N'-hexane-bis (1-dodecen-1-ylsuccinamic acid) is an anionic dimeric (gemini) surfactant. A flooding penetration scan of this surfactant in water demonstrates a sequence of lyotropic phases at room temperature (20 degrees C). Preparation of surfactant-water mixtures has resulted in a phase diagram which shows that the same sequence of phases exists up to 100 degrees C. These phases are tentatively assigned to the sequence: micellar to normal hexagonal (H1) to cubic (V1) to lamellar (Lalpha). The interfacial tension at the n-heptane/water interface has been determined in the presence of this surfactant. The surfactant head group area at the interface is large (2.8+/-0.3 nm2 at 298 K) and the interfacial tension above the critical micelle concentration is low (7 mN m(-1)), but considerably higher than the ultra-low values that have been reported for cationic dimeric surfactants at various hydrocarbon-water interfaces.     

Cationic double-chained surfactant as pseudostationary phase in micellar electrokinetic capillary chromatography for drug separations

The cationic double-chained surfactant didodecyldimethylammonium bromide (DDAB) was used as pseudostationary phase (PSP) in micellar electrokinetic capillary chromatography (MEKC). Its performance on the three kinds of drugs, i.e., basic, acidic, and neutral drugs, was systematically investigated. Nicotine, cotinine, caffeine, lidocaine, and procaine were selected as the model basic drugs. Good baseline separation and high efficiency were obtained under the optimal separation condition that consisted of 50mM phosphate (pH 4.0) and 0.08 mM DDAB. Three basic phenylenediamine isomers can also be well separated with DDAB in buffer. In addition, DDAB can form cationic bilayer on the capillary wall, thus the wall adsorption of basic analytes was greatly suppressed. Compared with commonly used CTAB, the separation of basic drugs was significantly improved with a much lower amount of DDAB present in the buffer. The DDAB-involved MEKC also showed superiority to CTAB upon the separation of acidic drugs, amoxicillin and ampicillin. In the case of neutral compounds, a good separation of resorcinol, 1-naphthol and 2-naphthol was achieved with 0.1mM DDAB and 30% (v/v) acetonitrile (ACN) present in buffer. Hence, it was concluded that the double-chained cationic surfactant DDAB can be a good substitute for traditional single-chained surfactant CTAB in MEKC.