Influence of Glycerol and Temperature on the Phase Behavior and Micellization of CTAB and SDS in Aqueous Solutions

In this work the micellization of sodium dodecylsulfate (SDS) and cetyltrimethyammonium bromide (CTAB) in water-glycerol mixed solvent have been investigated at 25, 35, and 45°C, respectively. The micellization of both surfactants in pure water at different temperatures has also been studied. The phase diagrams of the surfactants in water-glycerol mixed solvent were also established. From the conductivity measurements, the critical micelle concentration (CMC) and the degree of counterion dissociation (β) were obtained as a function of glycerol-water ratio and temperature. Standard free energy of micellization (ΔG°_mic) as a function of glycerol contents and temperature was calculated and discussed. It has been found that the micellization of the two surfactant in solutions with glycerol at 25°C and in pure water at higher temperatures are not the same although they have equal dielectric constants values.     

乙二醇对烷基三甲基溴化铵胶团化行为的影响

利用电导法研究了烷基三甲基溴化铵表面活性剂(CnTAB,n=12,14,16),即十二烷基三甲基溴化铵(DTAB),十四烷基三甲基溴化铵(TTAB)和十六烷基三甲基溴化铵(CTAB),在混合极性溶剂乙二醇/水(体积分数0～40%)中的胶团化行为。考察了温度对胶团形成的影响,应用相分离模型估算了三个表面活性剂的胶团热力学参数。结果表明临界胶团浓度(cmc)和反离子解离度(α)都随乙二醇组分的增加而增大。在乙二醇/水混合溶剂中胶团形成的标准吉布斯自由能相差很小,混合焓都是负值,而混合熵都为正值,说明焓-熵补偿效应在胶团形成中起主导作用。     

Studies on the Effect of Organic Solvents and Temperature on the Micellar Solution of Pentamethylene-1,5-bis(tetradecyldimethylammonium bromide) Gemini Surfactant

Effects of three organic solvents, viz. methyl cellosolve, acetonitrile, and formamide, on the micellization process of Gemini surfactant pentamethylene-1,5-bis(tetradecyldimethylammonium bromide) aqueous solutions, with the volume percentages of the organic solvents up to 50%, have been investigated conductometrically. The studies were made at different temperatures and the data were used to find out different micellization parameters. From the study, it was observed that, although an increment in the amount of the organic solvents delays the micellization, the increase in the critical micelle concentration (cmc) is comparatively less below 20%(v/v) showing the predominance of water character in the bulk phase at lower compositions of the organic solvents. Applying equilibrium model for micelle formation, various thermodynamic parameters were also calculated from the temperature dependence of the cmc values and the results show that the micellization process becomes less spontaneous as the volume % of the organic solvent increases in the system due to the action of water-organic solvent mixed media as better solvent than pure water (solvophobic effect) for the studied Gemini molecules.     

Conductometric studies of hexadecyltrimethylammonium bromide in aqueous solutions of ethanol and ethylene glycol

The effect of cosolvent on micellization of hexadecyltrimethyl ammonium bromide (CTAB) in aqueous solutions was studied. The conductivity of a mixture (cosolvent + water) as function of CTAB concentration was measured at different temperatures. Ethylene glycol and ethanol were used as a cosolvent. The conductivity data were used to determine the critical micelle concentration (CMC) and the effective degree of counterion dissociation of micelle in the temperature range 303.2 to 313.2 K. In all the cases studied, a linear relationship between log([CMC]_mix/mol dm⁻³) and the mass fraction of cosolvent in solvent mixture has been observed. The free energy (ΔG _mic ⁰ ), enthalpy (ΔH _mic ⁰ ), and entropy (ΔS _mic ⁰ ) of micellization were determined using the temperature dependence of CMC. The dependence of these thermodynamic parameters on solvent composition was determined. The standard free energy of micellization was found to be negative in all cases and becomes less negative as the cosolvent content increases. The enthalpy and entropy of micellization are independent of temperature in pure water, while ΔH _mic ⁰ and ΔS _mic ⁰ decrease dramatically with temperature in mixed cosolvents. Furthermore, the entropic contribution is larger than the enthalpic one in pure water, while in the mixed solvents, the enthalpic contribution predominates. The text was submitted by the authors in English.     

Triton X-100/CTAB在乙二醇/水混合溶剂中的热力学性质和胶团化行为

利用表面张力法, 研究了非离子表面活性剂Triton X-100和离子表面活性剂十六烷基三甲基溴化铵(CTAB)混合体系在混合极性溶剂乙二醇/水(乙二醇的体积分数分别为5%、10%和20%)中的热力学性质和胶团化行为. 结果表明, 混合体系在乙二醇水溶液中存在协同效应, 临界胶束浓度随乙二醇含量的增加而增大. 利用Rubingh和Maeda模型计算了混合物中各组分在胶团相中的组成、相互作用参数以及自由能的贡献. 在实验研究的乙二醇浓度范围内, 发现该非离子/离子混合体系在离子组分摩尔分数约为0.3时, 协同效应最强.     

Micellization of Cationic Surfactant Cetyltrimethylammonium Bromide in Mixed Water-Alcohol Media

The effect of methanol, ethanol, and 1-propanol on cmc and degree of counter ion binding, β, of cetyltrimethylammonium bromide (CTAB) has been studied conductometrically. The micellization of CTAB in these water-alcohol media have been found to be both dependent on nature as well as the concentration of alcohol in water. The cmc values shift toward higher concentration with increase in alcoholic content up to certain concentration beyond which decrease in cmc is registered in case of all the alcohols. However, the maximum in the cmc versus concentration of alcohol shifts to lower concentration with the increase in number of carbons in alcohol. β shows the inverse peaked behaviour in conformity with the cmc variation. The effects viz. solvent modifying tendency of alcohols and their tendency of penetration into the micelles have been used to interpret their effect on micellization of CTAB.     

Hydrogels from phospholipid vesicles

It is shown that phospholipid dispersions with a few percent of diacylphosphocholine PC in water can be swollen to single-phase lyotropic liquid crystalline L_α-phases by the addition of co-solvents like glycerol, 1,3-butyleneglycol BG or 1,2-propyleneglycol PG. The birefringent L_α-phases contain small unilamellar and multilamellar vesicles if the temperature of the samples is above the Krafft-Temperature T_m of the phospholipid. When such transparent birefringent viscous samples are cooled down below T_m the samples are transformed into birefringent gels. Cryo-TEM and FF-TEM measurements show that the bilayers of the vesicles are transformed from the liquid to the crystalline state during the transformation while the vesicle structure remains. The bilayers of the crystalline vesicles form adhesive contacts in the gel. Pulsed-field gradient NMR measurements show that two different kinds of water or co-solvent can be distinguished in the gels. One type of solvent molecules can diffuse like normal solvent in a continuous bulk phase. A second type of water diffuses much more slowly. This type of solvent is obviously trapped in the vesicles. The permeability of the crystalline vesicles for water and solvent molecules is much lower in the crystalline state than in the fluid state.     

Stripping study of U(VI) from loaded TBP/n-paraffin using ammonium nitrate bearing waste as strippant

Stripping studies of U(VI) from loaded solvent TBP/n-paraffin was carried out using ammonium nitrate solution as strippant. Effects of various stripping parameters such as concentration of ammonium nitrate solution, U(VI) concentration in organic phase, initial pH of strippant, temperature etc. have been investigated in detail. Kinetics of the stripping process by ammonium nitrate was found to be slower than that of stripping with water. It was observed that with the increase in ammonium nitrate concentration in aqueous solution, stripping of U(VI) decreased. With the increase in U(VI) loading in the organic phase, there was an increase in uranium stripping for ammonium nitrate whereas for distilled water it becomes reverse. With the increase in pH of the aqueous ammonium nitrate solution, stripping increased up to a certain pH of 8.5 and after that precipitation of uranium started. Increase in temperature of the biphasic system shows an enhancing effect of U(VI) stripping. Evaluation of thermodynamic data such as ?H indicated that the process is endothermic. Based on the optimized conditions, McCabe–Thiele diagram was constructed for U(VI) stripping using ammonium nitrate solution at room temperature.     

Influence of organic solvents and temperature on the micellization of conventional and gemini surfactants: a conductometric study

The micellar behaviour of similar hydrophobic chain length conventional (cetyltrimethyl ammonium bromide, CTAB; cetyl pyridinium chloride, CPC; cetyldimethylbenzyl ammonium chloride, C16BCl) and gemini surfactant (16-2-16) in water and polar non-aqueous solvents has been investigated in the temperature range 288.15–318.15 K with the help of conductivity measurements. The method proposed by Carpena et al. has been used to analyse the conductivity–concentration to determine the micellization parameters using critical micelle concentration (CMC) and degree of counter-ion dissociation (α) of the micelle. It shows much better performance than the conventional methods and the effect of experimental errors on the evaluation of the micellization parameters has been shown to be minimal by using this procedure. It was observed that the micellization tendency of the surfactant decreases in the presence of solvents. Thermodynamic parameters were also evaluated from the temperature dependence of the CMC values.     

表面张力法研究温敏性接枝共聚物的微胶束化行为

用表面张力法研究了以水溶性可生物降解的葡聚糖为主链 ,具有温敏相变特性的聚 (N 异丙基丙烯酰胺 )为接枝链的葡聚糖 接枝 聚 (N 异丙基丙烯酰胺 ) (Dextran g PNIPAM)共聚物在水溶液中的胶束化行为 .研究结果表明Dextran g PNIPAM体系的微胶束化行为与共聚物结构和溶液体系的温度密切相关 ,接枝共聚物中PNIPAM含量越大 ,水溶液体系的温度越高 ,形成胶束的临界胶束浓度 (CMC)越小 .特别值得指出的是 ,无论水溶液的温度是否高于PNIPAM接枝链段的相变温度 (LCST) ,即PNIPAM链段由亲水性转变为疏水性的温度 ,Dextran g PNIPAM均呈现一个临界胶束浓度大 ,对该现象给予了解释 .     

Formation of ordered phases of (hydroxypropyl)cellulose in miscible and immiscible isobutyric acid-water mixtures

(Hydroxypropyl)cellulose (HPC) is known to form birefringent liquid-crystalline phases at elevated polymer concentrations in either water or isobutyric acid (IBA). The HPC concentration at which the polymeric phase exhibits birefringence decreases as the IBA content in mixed H₂O/IBA solvents decreases, even though the concentration ?ⁱ_c for the formation of an ordered phase of HPC in water is greater than that in IBA. Water is a spectator component and apparently does not participate in the formation of a birefringent phase when IBA is present. A birefringent phase forms once the concentration of HPC in the solution omitting the H₂O equals the ?ⁱ_c of binary HPC/IBA solutions for temperatures from 23 to 95°C. The strong preferential affinity of HPC for IBA is visually evident as an HPC coagulate separates from dilute solution when the solvent mixture contains as little as 5% IBA. The coagulate dissolves to give a monophasic isotropic solution as the IBA content in the solvent is increased. A heterogeneous system in which a clear supernatant fluid covers a pearly white polymeric phase forms when the solvent mixture is immiscible and the HPC content is less than 50%. At high HPC content, the classical appearance associated with concentrated HPC solutions is seen. The optical and rheological properties of the heterogeneous systems are compared with those of homogeneous solutions at several HPC concentrations.     

Partition and water/oil adsorption of some surfactants

Adsorption isotherms have been determined at the water/oil interface for five biphasic systems involving surfactants (non-ionic and ionic) present in both phases at partition equilibrium. The systems studied were polyoxyethylene(23)lauryl ether (Brij35) in water/hexane and four ionic surfactants, hexadecyltrimethylammonium bromide (CTAB), and a series of three tetraalkylammonium dodecylsulfate (TEADS, TPADS, and TBADS) in water/CH 2Cl 2. Interfacial tension measurements performed at the water/air and water/oil interfaces provided all the necessary information for the determination of the adsorption parameters by taking partition into account. These measurements also allowed the comparison of the adsorption properties at both interfaces which showed an increase of the adsorption equilibrium constant and a decrease of the maximum surface concentration at the water/oil interface compared to water/air. The values of the critical aggregation concentration showed, in all cases, that only the surfactant dissolved in the aqueous phase contribute to the decrease of the water/oil interfacial tension. In the case of the four ionic surfactants, the critical aggregation concentration obtained in biphasic conditions were lowered because of the formation of mixed surfactant-CH 2Cl 2 aggregates.     

Mutual influence of cetyltrimethylammonium bromide and Triton X-100 on their adsorption at the water–air interface

On the basis of surface tension values of the aqueous solution of cetyltrimethylammonium bromide (CTAB) and Triton X-100 (TX-100) mixtures measured at 293 K as a function of CTAB or TX-100 concentration at constant TX-100 or CTAB concentration, respectively, the real surface area occupied by these surfactants at the water–air interface was established which is inaccessible in the literature. It appeared that at the concentration of the CTAB and TX-100 mixture in the bulk phase corresponding to the unsaturated monolayer at the water air-interface this area is the same as in the monolayer formed by the single surfactant at the same concentration as in the mixture. In the saturated mixed monolayer at this interface the area occupied by both surfactants is lower than that in the single surfactant monolayer corresponding to the same concentration in the aqueous solution. However, the decrease of the CTAB adsorption is lower than that of TX-100 and the total area occupied by the mixture of surfactants is also lower than that of the single one. The area of particular surfactants in the mixed saturated monolayer changes as a function of TX-100 and CTAB mixture concentration and at the concentrations close to CMC or higher the area occupied by both surfactants is the same. The changes of the composition of the mixed surface monolayer are connected with the synergetic effect in the reduction of the water surface tension by the adsorption of CTAB and TX-100 at the water–air interface. This effect was confirmed by the values of the standard Gibbs free energy of adsorption of both individual surfactants and their mixtures with different compositions in the bulk phase determined by using the Langmuir equation if RT instead of nRT was applied in this equation.     

SEP嵌段共聚物胶束化过程中溶液的粘度行为

用粘度法主要研究了聚苯乙烯和氢化异戊二烯二嵌段共聚物在选择性溶剂中的粘度行为, 并计算得到其流体力学半径. 测得在胶束化过程中溶液粘度出现异常变化, 溶液在胶束化时粘度迅速增大, 并与用光散射法研究共聚物溶液胶束化过程得封的结果相比较.     

Gelation of poly(n-butylisocyanate) in benzene

Poly (n-butylisocyanate)-benzene solutions prepared by solubilization at 45°C, followed by aging at room temperature were found to be metastable for months, although, eventually, they separated into a birefringent polymer-rich phase and an isotropic solution. These metastable solutions, as well as isothermally phase-separated biphasic samples, flowed and exhibited dynamic moduli indicative of low polymer connectivity. By contrast samples prepared by a freeze-thaw cycle were uniformly and highly birefringent and showed network (gel) behavior at room temperature. The mechanism of gel formation is most likely the exclusion of the polymer from the benzene crystal during crystallization, forcing the polymer to align and exist at grain boundaries. Films formed from solutions have different moduli than those formed from gels, and are consistent with the proposed mechanism.     

Interfacial and bulk behavior of sodium dodecyl sulfate in isopropanol-water and in isopropanol-poly(vinylpyrrolidone)-water media

The surface activity of isopropanol (IP) and poly(vinylpyrrolidone) (PVP) at the air/water interface has been studied. The self-aggregation of sodium dodecyl sulfate (SDS) in IP-water as well as in IP-PVP-water media has been investigated using physical methods, viz., tensiometry, conductometry, calorimetry, and viscometry. The interaction of SDS with PVP in IP-water medium as well as its self-aggregation (or micellization) in the presence of PVP has been assessed. The results reveal a fair degree of surface activity of IP in aqueous medium, which is only moderate for PVP. The critical micellar concentration (CMC) of SDS passes through a minimum at (v/v) % IP = 6.62. SDS interacts with PVP, yielding a critical aggregation concentration (CAC) at a low [SDS], independent of IP content in the medium. At a higher [SDS], free micelle formation takes place in solution, which is lower in mixed solvent than in water and is independent of solvent composition by tensiometry, but not by conductometry and calorimetry. The viscosity of micelle-interacted PVP in solution takes a long time to stabilize, whereas, for non-interacting additives, such as NaCl and cetyltrimethylammonium bromide (CTAB), it is time independent.     

The Self-Association and Mixed Micellization of an Anionic Surfactant,Sodium Dodecyl Sulfate,and a Cationic Surfactant,Cetyltrimethylammonium Bromide: Conductometric,Dye Solubilization,and Surface Tension Studies

In the present study, we investigate the self-association and mixed micellization of an anionic surfactant, sodium dodecyl sulfate (SDS), and a cationic surfactant, cetyltrimethylammonium bromide (CTAB). The critical micelle concentration (CMC) of SDS, CTAB, and mixed (SDS + CTAB) surfactants was measured by electrical conductivity, dye solubilization, and surface tension measurements. The surface properties (viz., C₂₀ (the surfactant concentration required to reduce the surface tension by 20 mN/m), Π_CMC (the surface pressure at the CMC), Γ_max (maximum surface excess concentration at the air/water interface), and A_min (the minimum area per surfactant molecule at the air/water interface)) of SDS, CTAB, and (SDS + CTAB) micellar/mixed micellar systems were evaluated. The thermodynamic parameters of the micellar (SDS and CTAB), and mixed micellar (SDS + CTAB) systems were evaluated.

A schematic representation of micelles and mixed micelles.     

Thermodynamics of micellization of tetradecyltrimethylammonium bromide in ethylene glycol–water binary mixtures

The aggregation behaviour of tetradecyltrimethylammonium bromide in ethylene glycol–water mixtures across a range of temperatures has been investigated by electrical conductivity measurements. The critical micelle concentration (cmc) and the degree of counterion dissociation of micelles were obtained at each temperature from plots of differential conductivity, (κ/c) _{T , P} , versus the square root of the total concentration of the surfactant. This procedure not only enables us to determine the cmc values more precisely than the conventional method, based on plots of conductivity against total concentration of surfactant, but also allows straightforward determination of the limiting molar conductance and the molar conductance of micellar species. The equilibrium model of micelle formation was applied to obtain the thermodynamics parameters of micellization. Only small differences have been observed in the standard molar Gibbs free energies of micellization over the temperature range investigated. The enthalpy of micellization was found to be negative in all cases, and it showed a strong dependence on temperature in the ethylene glycol poor solvent system. An enthalpy–entropy compensation effect was observed for all the systems, but whereas the micellization of the surfactant in the solvent system with 20 wt% ethylene glycol seems to occur under the same structural conditions as in pure water, in ethylene glycol rich mixtures the results suggest that the lower aggregation of the surfactant is due to the minor cohesive energy of the solvent system in relation to water. Received: 13 December 1998 Accepted in revised form: 25 February 1999     

甲酰胺与正负离子表面活性剂有序溶液的研究

对羧酸钠与烷基三甲基溴化铵1：1混合体系的研究表明：常温下各体系在不同比例甲酰胺（FA）／水混合溶剂中，表面张力随浓度变化均有明显的转折点，显示了混合体系中胶团的存在．实验中发现随混合溶剂中FA比例增加，各体系的临界胶团浓度（cmc）增大．在较高温度下发现在甲酰胺中亦存在着因胶团形成而产生的表面张力-浓度对数（γ－logc）曲线的转折点，利用相分离模型对体系胶团热力学参数进行了计算．并探讨了FA对正负离子表面活性剂囊泡的影响．     

Formation of block copolymer micelles in solutions of a linear homopolymer in a good solvent

The existence of micelles of polystyrene-block-poly(ethylene/propene) in solutions of polystyrene in toluene was investigated. Toluene is a good solvent of both copolymer blocks whereas polystyrene and poly(ethylene/propene) are immiscible polymers. The presence of homopolystyrene at high enough concentration can induce the micellization of polystyrene-block-poly(ethylene/propene) in solution of a good solvent such as toluene. The thermodynamics of this new micelle system at a given polystyrene concentration was studied. Light scattering measurements were carried out in order to determine the critical micelle temperature (CMT) of different micellar solutions. Standard Gibbs energy, enthalpy and entropy of micellization were estimated from CMT and concentration data. The numerical values found were less negative than those found for micelle systems consisting in a block copolymer dissolved in a single selective solvent.