Preferential adsorption of cationic surfactant mixture studied by bromide ion selective total-reflection XAFS measurement

The total-reflection XAFS measurement possessing bromide ion selectivity at the interfacial region was applied to the adsorbed film of hexadecyltrimethylammonium chloride (HTAC) and dodecyltrimethylammonium bromide (DTAB) mixture. The surface compositions XjH of individual ions j ( j = HTA+, Cl(-), DTA+, and Br (-)) were evaluated by combining the surface excess concentration of Br(-) estimated from the XAFS with the surface composition of the respective surfactants from the surface tension results. It is clearly shown that HTA+ and Br(-) are preferentially adsorbed to DTA+ and Cl(-) at the air/water interface. The preferential adsorption was estimated numerically in terms of activity coefficient fi+/-(H,p) of component i and excess Gibbs energy of adsorption ?prH,E. Then, the magnitude of ?prH,E was compared with that of ?prH,E attributable to intrinsic interaction between ions.     

Salt effect on the complex formation between polyelectrolyte and oppositely charged surfactant in aqueous solution

The complex formation between sodium carboxymethylcellulose (NaCMC) and dodecyltrimethylammonium bromide (DTAB) at various sodium bromide concentrations (C(NaBr)) has been studied by microcalorimetry, turbidimetric titration, steady-state fluorescence measurements, and the fluorescence polarization technique. The addition of salt is found to influence the formation of NaCMC/DTAB complexes markedly. At C(NaBr) = 0.00, 0.01, 0.02, 0.10, and 0.20 M, DTAB monomers form micelle-like aggregates on NaCMC chains to form NaCMC/DTAB complexes above the critical surfactant concentration (C1). At C(NaBr) = 0.23 M, DTAB molecules first form micelles above a 2.46 mM DTAB concentration prompted by the added salt, and then, above C1 = 4.40 mM, these micelles can aggregate with NaCMC chains to form NaCMC/DTAB complexes. However, at C(NaBr) = 0.25 M, there is no NaCMC/DTAB complex formation because of the complete salt screening of the electrostatic attraction between DTAB micelles and NaCMC chains. It is also surprisingly found that the addition of NaBr can bring out a decrease in C1 at C(NaBr) < 0.20 M. Moreover, the addition of NaBr to a mixture of 0.01 g/L NaCMC and 3.6 mM DTAB can directly induce the formation of NaCMC/DTAB complexes. This salt-enhancing effect on the complex formation is explained as the result of competition between the screening of interaction of polyelectrolyte with surfactant and the increasing of polyelectrolyte/surfactant interaction owing to the growth of micelles by added salt. When the increasing of polyelectrolyte/surfactant interaction exceeds the screening of interaction, the complex formation can be enhanced.     

Studies on an ester-modified cationic amphiphile in aqueous systems: behavior of binary solutions and ternary mixtures with conventional surfactants

The aqueous behavior of an ester-modified cationic amphiphile with the molecular structure CH3CH2O(C=O)(CH2)6(C=O)O(CH2)8N+(CH3)3Br-, in the following referred to as A, has been investigated. Systems with A as the only solute, as well as different aqueous mixtures with conventional cationic surfactants, primarily dodecyltrimethylammonium bromide (DTAB), were included in the study. Isotropic solution samples were characterized using 1H NMR, 13C NMR, NMR diffusometry, and conductivity measurements, whereas liquid crystalline samples were investigated by optical polarization microscopy and small-angle X-ray diffraction. The results are compared to the behavior of the binary system of DTAB and water. A does not exhibit a typical surfactant behavior. When it is present as the only solute in a binary aqueous system, it forms neither conventional micelles nor liquid crystalline phases. However, there is clear evidence that it assembles with lower cooperativity into loose clusters at concentrations above 25-30 mM. When A is mixed with DTAB in solution, the two amphiphiles form mixed assemblies, the structure of which varies with the total amphiphile concentration. In concentrated mixtures with alkyltrimethylammonium surfactants, A can participate in hexagonal liquid crystalline phases even when it constitutes a significant fraction of the total amphiphile content.     

Estimation of degree of counterion binding and thermodynamic parameters of ionic surfactants from cloud point measurements by using triblock polymer as probe

Cloud point (C(P)) was measured for ternary mixtures of different ionic surfactants such as sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide (DTAB), and dimethylene bis(dodecyldimethylammonium bromide) (12-2-12) plus triblock polymer (TBP) ((PEO)(2)(PPO)(15.5)(PEO)(2)) plus water, keeping the concentration of TBP constant and varying the surfactant concentration from pre- to postmicellar regions. These experiments were also performed in the presence of different fixed amounts of NaBr to evaluate the salt effect on the clouding behavior of these ternary mixtures. The C(P) value of TBP exhibits a drastic change at the cmc of each surfactant. The cmc values thus obtained both in the absence and in the presence of NaBr were used to evaluate counterion binding (beta) with the Corrin-Harkins method. beta values were also used to evaluate the thermodynamic parameters of these ionic surfactants. The results suggest that the beta values evaluated using this method, especially at low [TBP], are in good agreement with those reported in the literature.     

Giant vesicles of a single-tailed chiral cationic surfactant, (1R,2S)-(-)-N-dodecyl-N-methylephedrinium bromide, in water

Self-assembly properties of a single-tailed chiral cationic surfactant, (1R,2S)-(-)-N-dodecyl-N-methylephedrinium bromide (DMEB), have been studied in water. The molecular self-assemblies of the amphiphile have been characterized by surface tension, fluorescence probes, light scattering, and microscopic techniques. The results have been compared with those of dodecyltrimethylammonium bromide (DTAB) surfactant. The critical aggregation concentration of DMEB was found to be much less than that of DTAB. Surface tension and fluorescence probe studies have suggested formation of micellar structures at low temperature (<28 degrees C) and spontaneous formation of giant vesicles in water above 28 degrees C. The mean size of the aggregates has been measured by a dynamic light scattering method. The micropolarity and microviscosity of the self-assemblies were determined by fluorescence probe technique. The (1)H NMR and FTIR spectra were recorded to elucidate the role of the hydrophobic head group towards the formation of bilayer structures. The phase transition temperatures of the vesicular aggregates were determined by measurement of fluorescence anisotropy at various temperatures.     

Selective determination of surface density of bromide ion through XAFS and its application to verification of a criterion of an ideal mixing of surfactant mixture

The mole fraction of chloride ion of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium chloride (DTAC) mixture in the adsorbed film XHC was estimated not only by the thermodynamic analysis of the surface tension data but also by analyzing the Br K-edge jump of the XAFS spectrum under the total reflection condition (TRXAFS method). The phase diagrams of adsorption (PDA) at several surface tensions from the two methods were in good agreement. On the basis of the PDA obtained, it was clearly shown that the criterion of an ideal mixing for the DTAB-DTAC system is not given by the linear relation between the total molality of surfactant mixture m and XHC, m = m0B + (m0C - m0B)XHC, but by the one between m2 and XHC, m2 = (m0B)2 + [(m0C)2 - (m0B)2]XHC. Furthermore, it was demonstrated that the theoretical approach that provides the latter relation draws a distinction between the criteria for an ionic surfactant mixture without a common ion and that for an ionic surfactant mixture with a common ion.     

Mesomorphic complexes of DNA with the mixtures of a cationic surfactant and a neutral lipid

Polyanionic DNA binds to cationic lipids to form electrostatic complexes exhibiting rich self-assembled structures. These types of complexes have been considered as a nonviral carrier in gene therapy and as a template for nanostructure construction. For the latter application where biocompatibility is not the key issue, replacement of cationic lipid by cationic surfactant is advantageous due to the wide availability of surfactant. Here we report the self-assembly behavior of the complexes of DNA with a cationic surfactant, dodecyltrimethylammonium bromide (DTAB), mixed with a neutral lipid, dioleoylphosphatidylethanolamine (DOPE), in fully hydrated state as a function of DTAB-to-DNA base pair molar ratio (x), DOPE-to-DTAB molar ratio (m) and temperature. The binary complexes of DNA with DTAB microphase separated to form hydrophilic and hydrophobic domains without long-range order. Incorporating DOPE into the complexes effectively strengthened the hydrophobic interaction and hence promoted the formations of long-range ordered mesophases, including a condensed multilamellar phase (L(alpha)(c)) at small to intermediate m (m approximately 6). The lyotropic mesophase transition with respect to the change of m was properly predicted by a formula for calculating the packing parameter of amphiphile mixture. In addition to the lyotropic transition, an unusual thermotropic order-order transition (OOT) between L(alpha)(c) and H(II)(c) phases was revealed for the isoelectric complex with m = 3. This OOT was thermally reversible and was postulated to be driven by the reduction of the effective headgroup area due to the release of trapped water molecules.     

Spontaneous vesicle formation of single chain and double chain cationic surfactant mixtures

The concentration vs composition diagram of aggregate formation of the dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) mixture in aqueous solution at rather dilute region was constructed by analyzing the surface tension, turbidity, and electrical conductivity data and inspected by cryo-TEM images and dynamic light scattering data. Although the aqueous solution of DTAB forms only micelles, the transition from monomer to small aggregates and then to vesicle was found at 0.1 < X2 相似文献   

Study on the distribution of binary mixed counterions in surfactant adsorbed films by total reflection XAFS measurements

The total reflection X-ray absorption fine structure (TR-XAFS) technique was applied to adsorbed films at the surface of aqueous solutions of surfactant mixtures composed of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium tetrafluoroborate (DTABF₄). The obtained XAFS spectra were expressed as linear combinations of two specific spectra corresponding to fully hydrated bromide ions (free-Br) and partially dehydrated bromide ions adsorbed to the hydrophilic groups of surfactant ions (bound-Br) at the surface. The ratio of free- and bound-Br ions was determined as a function of surface tension and surface composition of the surfactants. Taking also the results in our previous studies on the DTAB - dodecyltrimethylammonium chloride (DTAC) and 1-hexyl-3-methylimidazolium bromide (HMIMBr) - 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF₄) mixed systems into consideration, the relation between counterion distribution and miscibility of counterions at the solution surface was deduced for the surfactant mixtures having common surfactant ions but different counterions.     

Salt effect on complex formation of neutral/polyelectrolyte block copolymers and oppositely charged surfactants

The salt effect on the complex formation of poly(acrylamide)- block-poly(sodium acrylate) (PAM- b-PAA) as a neutral-anionic block copolymer and dodecyltrimethylammonium bromide (DTAB) as a cationic surfactant at different NaBr concentrations, CNaBr, was investigated by turbidimetric titration, steady-state fluorescence spectroscopy, and dynamic light scattering. At C NaBr < 0.25 M, DTAB molecules may form micelle-like aggregates on PAM- b-PAA chains to form a PAM- b-PAA/DTAB complex above the critical surfactant concentration C critical for the onset of complex formation. In the region of relatively high turbidity, a larger complex is likely to form a core-shell structure, of which the core is a dense and disordered microphase made of surfactant micelles connected by the PAA blocks. The corona was a diffuse shell of PAM chains, and it ensured steric stability. At CNaBr = 0.25 M, a higher electrostatic intermicellar repulsion and intercomplex repulsion induced by a large amount of bound DTAB micelles may lead to a redissolution of large colloidal complexes into intrapolymer complexes. Moreover, a salt-enhancing effect on the complex formation was observed in the PAM- b-PAA/DTAB system; the critical surfactant concentration decreased with increasing salt concentration at CNaBr < 0.10 M. The salt-enhancing effect is due to the larger increase of interaction in comparison to the screening of the interaction.     

Structure of mesh phases in a cationic surfactant system with strongly bound counterions

We report the observation of an intermediate mesh phase with rhombohedral symmetry, corresponding to the space group Rm, in a mixed surfactant system formed by the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the organic salt 3-sodium-2-hydroxy naphthoate (SHN). It occurs between a random mesh phase (L(alpha)(D)) and a lamellar phase (L(alpha)) at low temperatures; at higher temperatures, the (L(alpha)(D)) phase transforms continuously into the (L(alpha)) phase with an increasing surfactant concentration (phi(s)). To separate the effects of salt and phi(s) on the phase behavior, the ternary system consisting of cetyltrimethylammonium 3-hydroxy-naphthalene-2-carboxylate (CTAHN), sodium bromide (NaBr), and water was studied. The intermediate mesh phase is found in this system at high NaBr concentrations. The micellar aggregates, both in the intermediate and random mesh phases, are found to be made up of a two-dimensional network of rod-like segments, with three rods meeting at each node. The average mesh size increases with phi(s), and the transition from the random mesh phase to the intermediate phase is found to occur when it is approximately 1.5 times the lamellar periodicity. The intermediate mesh phase is absent in the equimolar dodecyltrimethylammonium bromide (DTAB)-SHN system, indicating the role of the surfactant chain length in the formation of this phase. This system exhibits a random mesh phase over a very wide range of water content, with the average mesh size decreasing upon an increasing phi(s), contrary to the trend seen in the CTAB-SHN system.     

Effects of alkyl chain length on synergetic adsorption and micelle formation in homologous cationic surfactant mixtures

The surface tensions (gamma) of the aqueous solutions of tetradecyltrimethylammonium bromide (TTAB) and dodecyltrimethylammonium bromide (DTAB) were measured as a function of the total molality of surfactants (m) and the relative proportion (composition) of DTAB (X(2)) at 298.15 +/- 0.05 K under atmospheric pressure. The effect of the difference in the hydrophobic chain length between hexadecyltrimethylammonium bromide (HTAB) and DTAB on the synergism was examined. This synergism was observed in the miscibility at the surface of a mixture of these two compounds. The excess Gibbs energy of adsorption of the TTAB-DTAB system was positive in contrast to the HTAB-DTAB system. This indicates that there are certain restrictions on the difference in the hydrophobic chain length for the synergism to be brought about in homologous cationic surfactant mixtures. This mechanism was explained by the theory of a staggered structure formation at the air/water interface. A similar argument successfully applied to the hexadecyltrimethylammonium chloride (HTAC)-dodecyltrimethylammonium chloride (DTAC) and tetradecyltrimethylammonium chloride (TTAC)-DTAC mixtures also.     

粘度法研究胶团的球-棒转变

本文基于粘度测量求得了溴化十四烷基吡啶胶团水溶液的特性粘度随盐量(0.5—2.0 mol L~(-1)NaBr), 温度(20°—40 ℃)和活性剂浓度(3-25 g L~(-1))的变化。利用Missel等提出的热力学模型,由此求算了支配球-棒转变的平衡常数K和多分散胶团的重均聚集数, 当盐浓度为0.5和1.0 mol L~(-1)时, 由粘度法在不同温度下得到的K值以及在30 ℃下随活性剂浓度的变化, 与光散射的相应结果相当一致。此外, 在不同温度下求得的参数K在一定的胶团浓度范围内保持为常量。这符合模型的要求。盐量增至2 mol L~(-1)时, K明显地具有浓度依赖性,并且两法给出的K和值都相差较大。上述模型不再适用。对于符合球-棒转变的胶团体系, 从特性粘度具有浓度依赖性出发的粘度方法是研究胶团长大的简单而可靠的方法, 能够提供很多重要的信息。     

Effect of preferential adsorption on the synergism of a homologous cationic surfactant mixture

To evaluate the effect of preferential surface adsorption of bromide ions on the synergism of homologous cationic surfactant mixtures reported previously, the surface tension of the aqueous solutions of the hexadecyltrimethylammonium chloride (HTAC)-dodecyltrimethylammonium bromide (DTAB) system was measured as a function of the total molality of surfactants and the relative proportion of DTAB at 298.15 +/- 0.05 K under atmospheric pressure. The excess Gibbs energies calculated from them were -2.6 kJ mol(-)(1) in the mixed adsorbed film and -2.0 kJ mol(-)(1) in the mixed micelle, respectively. A useful analytical procedure to evaluate the composition of individual ions (hexadecyltrimethylammonium, dodecyltrimethylammonium, chloride, and bromide ions) in the adsorbed film and micelle was developed and applied.     

Surfactant adsorption onto interfaces: measuring the surface excess in time

We propose a direct method to measure the equilibrium and dynamic surface properties of surfactant solutions with very low critical micellar concentrations (CMC) using a pendant drop tensiometer. We studied solutions of the nonionic surfactant hexaethylene glycol monododecyl ether (C(12)E(6)) and of the ionic surfactant hexadecyl trimethyl ammonium bromide (CTAB) with concentrated sodium bromide (NaBr). The variation of the surface tension as a function of surface concentration is obtained easily without the need for complex models and compares well with the result obtained using the Gibbs adsorption equation. The time-dependent surface concentration of each surfactant was also measured, and the adsorption process was found to be diffusion-controlled. The diffusion coefficients of the two surfactants can be extracted from the data and were found in very good agreement with literature values, further validating the method.     

Studies of mixed surfactant solutions of cationic dimeric (gemini) surfactant with nonionic surfactant C12E6 in aqueous medium

The interaction between the alkanediyl-alpha,omega-type cationic gemini surfactant, [(C(16)H(33)N(+)(CH(3))(2)(CH(2))(4)N(+)(CH(3))(2)C(16)H(33))2Br(-)], 16-4-16 and the conventional nonionic surfactant [CH(3)(CH(2))(10)CH(2)(OCH(2)CH(2))(6)OH], C(12)E(6) in aqueous medium has been investigated. The critical micelle concentrations of different mixtures have been measured by surface tension using a du Nouy tensiometer in aqueous solution at different temperatures (303, 308, and 313 K). Maximum surface excess (Gamma(max)) and minimum area per molecule (A(min)) were evaluated from a surface tension vs log(10)C (C is concentration) plot. The cmc value of the mixture was used to compute beta(m), the interaction parameter. The beta(sigma), the interaction parameter at the monolayer air-water interface, was also calculated. We observed synergism in 16-4-16/C(12)E(6) system at all concentration ratios. The micelle aggregation number (N(agg)) has been measured using a steady state fluorescence quenching method at a total surfactant concentration approximately 2 mM at 25 degrees C. The micropolarity and the binding constant (K(sv)) of mixed systems were determined from the ratio of intensity of peaks (I(1)/I(3)) of the pyrene fluorescence emission spectrum. The micellar interiors were found to be reasonably polar. We also found, using Maeda's concept, that the chain-chain interactions are very important in this system.     

Hydration structures of bromides on cationic micelles

The peripheral structures of bromides on dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (HTAB) micelles have been studied by X-ray absorption fine structure (XAFS) at the Br-K edge. The XAFS spectra indicate that water is a dominant scattering group for Br- even in these micellar solutions. However, the oscillation intensity decreases with increasing micellar concentration, suggesting that the bromides are dehydrated to some extent when they are bound to the micelles. A XAFS analysis routine gives unusually short Br-O (water) distances and is inapplicable to the present systems. This comes from the structure of the first coordination shell, in which two or more scattering paths are involved. The second scattering group is obviously the head group of the surfactants forming the micelles. The detailed analysis has allowed us to estimate the hydration number of the bromides bound on the DTAB and HTAB micelles (N = ca. 4.2). The assumption that all of the bromides form direct ion-associates with the head groups causes the contradiction to the results of the XAFS analyses. This strongly implies that some of the bromides partitioned into the micelle are completely hydrated as far as their first coordination shell is concerned. Assuming that the maximum hydration number of the bromides bound to the head groups of the micelle is three, 40% of the bromides partitioned into the micelle are completely hydrated.     

Effect of surfactant type on surfactant-maltodextrin interactions: isothermal titration calorimetry, surface tensiometry, and ultrasonic velocimetry study

Isothermal titration calorimetry (ITC), surface tensiometry, and ultrasonic velocimetry were used to characterize surfactant-maltodextrin interactions in buffer solutions (pH 7.0, 10 mM NaCl, 20 mM Trizma base, 30.0 degrees C). Experiments were carried out using three surfactants with similar nonpolar tail groups (C12) but different charged headgroups: anionic (sodium dodecyl sulfate, SDS), cationic (dodecyl trimethylammonium bromide, DTAB), and nonionic (polyoxyethylene 23 lauryl ether, Brij35). All three surfactants bound to maltodextrin, with the binding characteristics depending on whether the surfactant headgroup was ionic or nonionic. The amounts of surfactant bound to 0.5% w/v maltodextrin (DE 5) at saturation were < 0.3 mM Brij35, approximately 1-1.6 mM SDS, and approximately 1.5 mM DTAB. ITC measurements indicated that surfactant binding to maltodextrin was exothermic. Surface tension measurements indicated that the DTAB-maltodextrin complex was more surface active than DTAB alone but that SDS- and Brij35- maltodextrin complexes were less surface active than the surfactants alone.     

DTAB-月桂酸钠体系表面活性剂双水相研究

绘制和分析了十二烷基三甲基溴化铵（DTAB） 月桂酸钠（SL）组成的表面活性剂混合体系相图,系统研究了温度和无机盐等因素对该体系表面活性剂双水相 (ASTP) 影响的规律性.结果表明:与富含负离子表面活性剂双水相 (ASTP A) 相比较,富含正离子表面活性剂双水相(ASTP C)区域更大、更稳定;温度升高,无机盐(NaBr)的浓度增大,都引起 ASTP 体系上相体积的减小和下相体积的增加;囊泡普遍存在于ASTP的上下相之中.     

Kinetic investigations on the alkaline hydrolysis of tris-(1,10-phenenthroline)Fe(II) with guar gum–surfactant interactions

The kinetic investigations on the alkaline hydrolysis of tris-(1,10–phenanthroline)iron(II) has been explored spectrophotometrically in microheterogeneous environment at 301?K and ionic strength of 0.13?mol?L^?1. Guar gum, cationic amphiphiles, and their mixtures are used as the reaction environments to carry out the reaction. Guar gum decreases the rate of reaction, which indicates that Fe(II) complex may be trapped in the hydrophobic region of gum. Cationic amphiphile decreases the rate in the presence of guar gum. The extent of interaction between guar gum and amphiphile increases with the hydrophobic carbon chain length. The critical aggregation concentration (CAC) and critical micelle concentration (CMC) of the amphiphiles (cetyl trimenthyl ammonium bromide (CTAB), tetradecyl trimenthyl ammonium bromide (TTAB), dodecyl trimenthyl ammonium bromide (DTAB)) in the presence of guar gum have been determined with conductometry and tensiometry. All observations support either weak or strong interaction of cationic amphiphiles with guar gum. Activation parameters of the reaction in different environments have been determined which corroborate the rate data.