Fluorescence Probe Studies of Ionic Micelles in Concentrated Salt Solutions

We have investigated the effect of salt concentration and temperature on the average aggregation number and micro-polarity of the interior of micelles of sodium dodecyl sulfate (SDS). sodium tetradecyl sulfate (STDS) and lithium dodecyl sulfate (LiDS). The transient fluorescence decay of micelle-solubilized pyrene has been measured and analyzed. An exponent weighted average aggregation number <n>_e was obtained by this technique. For SDS and STDS in NaCl solution, <n>_e increases as the temperature is lowered or salt concentration is increased <n>_e increased from ～ 50 to ～ 250 over [NaCl] = 0 to 0.8 M. Due to the strong counterion binding of lithium in the micellar solution, the LiDS micelle is much, smaller and does not increase appreciabily even at [LiCl] =0.8 M. From the fluorescence spectrum fine structure of pyrene and the fluorescence decay of the monomer and excimer, we can understand the local polarity and the water penetration to the interior of the micelle upon addition of salts and with changing temperature. The interior of the micelle becomes more nonpolar as the salt concentration is increased and the temperature is lowered. A complete kinetic analysis of the time–dependence of the fluorescence is given. The kinetic analysis is in agreement with the results reached by fluorescence spectral analysis.     

稳态荧光猝灭法确定胶束聚集数的研究

选用芘作为荧光探针, 二苯酮作为猝灭剂, 以稳态荧光猝灭法测定了十二烷基硫酸钠、十二烷基磺酸钠和十六烷基三甲基演化铵的胶束聚集致, 并对其测定胶束聚集数方法的有效范围进行了讨论. 实验结果表明, 选择的探针-猝灭剂体系适用于稳态荧光猝灭法确定阴离子和阳离子表面活性剂的胶束浓度和聚集数.     

Salt influence in the polymer-surfactant interaction in solution. A fluorescence probe investigation of the EHEC/SDS/ water system

 The effect of small amounts of salt on the interaction between two fractions of ethyl(hydroxy)ethyl cellulose, EHEC, and sodium dodecyl sulfate, SDS, has been investigated by means of steady-state fluorescence measurements. The two polymer fractions display different properties in hydrophobicity expressed as different cloud points. The results are discussed in relation to hydrodynamic (viscosity) and thermodynamic (equilibrium dialysis) properties. The micropolarity as sensed by the probe pyrene shows that the polymers begin to interact with SDS at a lower concentration in the presence of salt. The average aggregation numbers of polymer-bound clusters, N _p, were obtained by fluorescence-quenching data in combination with equilibrium dialysis experiments. N _p was found to increase in the presence of salt for the EHEC fraction with a high cloud point (CP). The polymer with a low CP displays higher N _p in the presence of salt at low SDS concentrations, but exhibit lower N _p at higher SDS concentrations than in the salt-free system. The microviscosity index as determined by intramolecular excimer formation of 1, 3-di(1-pyrenyl)propane (P3P) is highest for the lowest N _p and there is a corre-lation with N _p in the presence as well as absence of salt for both EHEC fractions. It is found that when the same fractional amount of SDS is bound to the polymers, 10–20% of the value of saturation, the increase in macroviscosity occurs and the microviscosity shows high rigidity. Received: 3 March 1997 Accepted: 23 May 1997     

Photoinitiation. II. Kinetics of the acrylonitrile polymerization photoinitiated by aromatic hydrocarbons

The kinetics of acrylonitrile polymerization photoinitiated by aromatic hydrocarbons have been studied. For the acrylonitrile polymerization photoinitiated by naphthalene the rate of polymerization depends on the square root of incident light intensity, on the square root of naphthalene concentration, and on the 1.5 power of acrylonitrile concentration. In the system acrylonitrile-1-methoxynaphthalene the rate of acrylonitrile polymerization depends on the first power of acrylonitrile concentration. The monoradical character of this polymerization process has been established. For the interpretation of experimental results a reaction mechanism involving the formation of the exciplex between the first singlet or triplet of aromatic hydrocarbon and acrylonitrile in the ground state as a precursor of polymerization reactions is suggested. The photoinitiating efficiency of various aromatic hydrocarbons in acrylonitrile polymerization increases in the order: fluoranthene (zero efficiency) ? pyrene < phenanthrene, fluorene ≈ 2-methoxynaphthalene ≈ biphenyl < anthracene < 2-methylnaphthalene < 1-methoxynaphthalene < 2,3,6-trimethylnaphthalene < 2,3-dimethylnaphthalene ≈ naphthalene < 1-methylnaphthalene < 2,6-dimethylnaphthalene < p-terphenyl < acenaphthene, provided that the systems absorb the same amount of the incident light. The explanation of this result ensues from the study of the effect of concentration on the rate of polymerization and from the quenching of hydrocarbon fluorescence by acrylonitrile. The photoinitiating efficiency of a given aromatic hydrocarbon is mainly determined by the value of the rate constant k_q for the formation of exciplex as well as the self-quenching efficiency of aromatic hydrocarbon. By using the literature data for the lifetime of fluorescence τ the values of k_q were calculated from the Stern-Volmer equation expressing the quenching of hydrocarbon fluorescence by acrylonitrile. The order of aromatic hydrocarbons according to increasing values of k_q is as follows: pyrene < phenanthrene < anthracene ≈ naphthalene < 2-methylnaphthalene ≈ 1-methylnaphthalene ≈ 2,3-dimethylnaphthalene < 2,6-dimethylnaphthalene < acenaphthene < p-terphenyl < 1-methoxynaphthalene. The study of the concentration effect reflecting the self-quenching of aromatic hydrocarbons during polymerization has given the following sequence for decreasing self-quenching efficiency of aromatic hydrocarbons: 2-methoxynaphthalene ≈ pyrene > anthracene > 1-methoxynaphthalene > fluorene > 2,6-dimethylnaphthalene, phenanthrene, acenaphthene > 2,3,6-trimethylnaphthalene > 2,3-dimethylnaphthalene > 1-methylnaphthalene > naphthalene. It has been shown that the photoinitiating efficiency of a given aromatic hydrocarbon in the polymerization of acrylonitrile can be roughly predicted from the position of that aromatic hydrocarbon in the above-mentioned sequences.     

Self aggregation of binary mixtures of sodium dodecyl sulfate and polyoxyethylene alkyl ethers in aqueous solution

The mixed micelles of sodium dodecyl sulphate (SDS) with Brij35 and Brij 97 were studied separately by fluorescence measurement using pyrene as fluorescent probe. In the range of 0–1.0 mole fraction (X) of added SDS to Brij solutions, the cmc value of the mixed micelles varies from 0.085 to 8 mmol with Brij 35 and 0.04 to 8 mmol with Brij 97. The aggregation number also changes. A measure of the stability of mixed micelles is also presented. The interaction parameter ₁₂ and the chain–chain contribution parameter (B₁) are extracted from the analysis of the results. This parameter B₁ is related to the standard free energy change associated with the introduction of one ionic species into a nonionic micelle coupled with the release of one nonionic species from the micelle. The clouding behaviour of Brij 97 in the presence of SDS was investigated and the associated thermodynamic parameters of clouding were generated and discussed.     

Preparation of substituted ionic carbohydrate polymers and their interactions with ionic surfactants

The formation of micelles of hexadecyltrimethylammonium chloride (CTAC) and sodium dodecylsulfate (SDS) in aqueous solutions containing charged polysaccharides was studied by steady-state and time-resolved fluorescence measurements using pyrene as a photophysical probe. Micropolarity studies using the I₁/I₃ ratio of the vibronic emission bands of pyrene and the behaviour of the I_E/I_M ratio between the excimer and monomer emissions show the formation of hydrophobic domains. The interactions between the polyelectrolytes and surfactants of opposite charge lead to the formation of induced pre-micelles at surfactant concentrations lower than the critical micellar concentration (cmc) of the surfactants. At similar concentrations, the I_E/I_M ratio shows a peak. This aggregation process is assumed to be due to electrostatic attractions. At higher surfactant concentrations, near the critical micellar concentration, micelles with the same properties as those found in pure aqueous solution are formed. On the other hand, systems containing polyelectrolytes and surfactants of the same charge do not show this behaviour at low concentrations. The presence of long alkyl chains bound to the polyelectrolytes also induces the formation of free micelles at concentrations somewhat below the aqueous cmc.     

Properties of Surface and Micelle in a pH‐Mediated Ternary Surfactant Mixture in Salt Solution

We have investigated the mixing behavoir of a pH‐mediated ternary surfactant mixture at constant ratio of dodecyldimethylamine oxide (DDAO) and Triton X‐100 (9:1). From the equilibrium surface tension measurements at different pHs, the critical micelle concentration (cmc) data were obtained as functions of the pH. Values of the cmc and composition of the micelles were predicted using the regular solution approximation. To some extent, the experimental cmc values agree with the predicted cmc. The average degree of ionization of dodecyldimethylamine oxide in the mixed surfactant systems was estimated using potentiometric titrations. The surface electric potential of the micelles (Ψ_o) was determined using two methods, one by hydrogen ion titration and the other by the dissociation constants of an acid‐base indicator. In a high degree of ionization of DDAO in the micelles phase (a_m), Ψ_o estimated from acid‐base indicator is much higher than that from hydrogen ion titration. In the protonated dodecyldimethylamine oxide/TX‐100 binary surfactant system, Ψ_o estimated from hydrogen ion titration was as high as 89 mV. The micellar aggregation numbers evaluated by the steady‐state fluorescence probe method increase with pH except at pH=5.03. At pH=5.03, the maximum micelle aggregation number was observed.     

Photophysical characterization of mixed micelles of n-butanol/SDS and n-hexanol/SDS. A study at low alcohol concentrations

 The effect of the addition of n-butanol (BuOH) and n-hexanol (HexOH) on the micellization of sodium dodecylsulfate (SDS) has been investigated using fluorescence quenching methods. The binding constants were calculated using an expression which relates the total concentration of alcohols and the micelle concentration. The values of K were 4.67 and 17.6 M^-1 for BuOH/SDS and HexOH/SDS, similar to values obtained by other methods. The cmc of SDS decreases on addition of alcohols and goes through a minimum for the BuOH/SDS system. Micellar aggregation numbers (N) were determined from linear plots of Ln (I ⁰/I) against [Quencher] at low alcohol concentrations. For 15 mM SDS, in the presence of BuOH the N values decrease on addition of alcohol up to 0.2 M. For HexOH, N can be assumed to be constant up to 4.8 mM, after which N decreases. The polarity of the micellar core containing alcohol was evaluated from the I ₁/I ₃ ratio of monomeric pyrene. The effect of addition of the alcohol causes a decrease in the I ₁/I ₃, which corresponds to a decrease in the polarity of the pyrene solubilization site. Received: 28 October 1996 Accepted: 10 January 1997     

Bending Energetics of Tablet‐Shaped Micelles: A Novel Approach to Rationalize Micellar Systems

A novel approach to rationalize micellar systems is expounded in which the structural behavior of tablet‐shaped micelles is theoretically investigated as a function of the three bending elasticity constants: spontaneous curvature (H₀), bending rigidity (k_c), and saddle‐splay constant (k?_c). As a result, experimentally accessible micellar properties, such as aggregation number, length‐to‐width ratio, and polydispersity, may be related to the different bending elasticity constants. It is demonstrated that discrete micelles or connected cylinders form when H₀>1/4ξ, where ξ is the thickness of a surfactant monolayer, whereas various bilayer structures are expected to predominate when H₀<1/4ξ. Our theory predicts, in agreement with experiments, a transition from discrete globular (tablet‐shaped) micelles to a phase of ordered, or disordered, connected cylinders above a critical surfactant concentration. Moreover, a novel explanation for the mechanism of growth, from small globular to long rodlike or wormlike micelles, follows as a consequence from the theory. In accordance, polydisperse elongated micelles (large length‐to‐width ratio) form as the bending rigidity is lowered, approaching the critical point at k_c=0, whereas monodisperse globular micelles (small length‐to‐width ratio) are expected to be present at large k_c values. The spontaneous curvature mainly determines the width of tablet‐shaped or ribbonlike micelles, or the radius of disklike micelles, whereas the saddle‐splay constant primarily influences the size but not the shape of the micelles.     

Study of mixed micellar aqueous solutions of sodium dodecyl sulfate and amino acids

Thermodynamic properties of sodium dodecyl sulfate (SDS) in micellar aqueous solutions of L-serine and L-threonine were determined by fluorescence spectroscopy and dynamic light scattering techniques. The values of Gibbs free energy, enthalpy and entropy of the process of micelle formation were calculated using the critical micelle concentration and degree of dissociation. Changes in critical micelle concentration of SDS with the addition of amino acids were examined by both conductivity and pyrene I ₁/I ₃ ratio methods at different temperatures. The pyrene fluorescence spectra were used to study the change of micropolarity produced by the interaction of SDS with amino acids. The aggregation behavior of SDS was explained in terms of structural changes in mixed solutions. The data on dynamic light scattering suggest that size of SDS micelles was influenced by the presence of amino acids.     

New Design of Thiol‐Responsive Degradable Polylactide‐Based Block Copolymer Micelles

A new design to synthesize thiol‐responsive degradable polylactide (PLA)‐based micelles having a disulfide linkage in the middle of triblock copolymers is reported. They were synthesized by a new method that centers on the use of a disulfide‐labeled diol as an initiator for ring‐opening polymerization, followed by controlled radical polymerization. These well‐controlled copolymers with monomodal and narrow molecular weight distribution (M _w/M _n < 1.15) self‐assembled to form aqueous micellar aggregates with disulfide‐containing PLA cores, which is not toxic to cells. Central disulfide linkages were cleaved in response to thiols; such thiol‐triggered degradation enhanced the release of encapsulated anticancer drugs.     

The aggregation of the sodium dodecyl sulfate –n - octanol–water system at low concentration

Micelle formation in sodium dodecyl sulfate (SDS)–n-octanol mixtures was studied by several techniques and the results were interpreted using regular solution theory for mixed-micelle formation. Octanol was considered as a nonionic surfactant. The composition of micelles at the critical micelle concentration (cmc) was computed together with the interaction parameter and the activity coefficient of the components of the micelles. The fluorescence quenching technique with pyrene was employed to obtain the SDS and octanol aggregation numbers at the cmc. The results were in agreement with similar studies on other alcohol–SDS systems. At the cmc spherical, almost fully ionized micelles formed, while at a higher concentration there was a transition to anisometric (probably rodlike) micelles which pushed sodium counterions into their Stern double layer. Mixed anisometric micelles were more ionized than pure SDS micelles. When the octanol:SDS total ratio exceeded 0.85:1, an emulsion of octanol appeared in equilibrium with the micelles. Received: 23 December 1998 Accepted in revised form: 3 March 1999     

Solubilization of 1-butanol in a sodium dodecyl sulfate-poly(ethylene oxide) system by NMR and conductivity at 298.1 and 283.1 K

 The effects of adding 0.1 molal 1-butanol to the aqueous SDS system at 298.1 K and the aqueous PEO–SDS system at 298.1 and 283.1 K have been studied. NMR NOESY experiments on the PEO– SDS–1-butanol system in D₂O were obtained. NMR self-diffusion experiments and measurements of NMR chemical shifts and specific conductivity were carried out on the samples, i.e. on samples with PEO and without PEO. The addition of 1-butanol to an aqueous SDS–PEO system decreases the critical aggregation concentration (c.a.c). Determination of the second critical concentration (c ₂) depends on the method of measurements, i.e. the molecular species monitored. Conductivity measurements will give c ₂ as the SDS concentration where free micelles (micelles not bound to the polymer) are formed. PEO self-diffusion measurements, on the other hand, determine c ₂ as the SDS concentration where the polymer is saturated with SDS. Both the c.a.c and the c ₂ decrease upon 1-butanol addition. However, the c ₂ value exhibits a larger decrease than the c.a.c value. Thus, the amount of polymer bound surfactant molecules decreases upon addition of 1-butanol. Micellar solubilization of 1-butanol starts at c.a.c., but the solubilization capacity is low until the surfactant concentration reaches c ₂, where the increase in solubilization is significant. Thus, solubilization data can be used to detect c ₂, the concentration where free micelles form. Received: 21 July 1997 Accepted: 9 February 1998     

十二烷基硫酸钠对两性咪唑类离子液体表面活性剂1-磺丙基-3-十二烷基咪唑内盐界面聚集行为的影响

利用界面扩张流变技术,研究了两性咪唑类离子液体表面活性剂1-磺丙基-3-十二烷基咪唑内盐(C₁₂imSP)的界面聚集行为,探讨传统表面活性剂十二烷基硫酸钠(SDS)对C₁₂imSP界面聚集行为的影响机制。 结果表明,少量SDS的加入可以填补界面上疏松的C₁₂imSP分子间的空位,界面上形成表面活性剂混合吸附膜,界面张力显著降低;提高SDS的浓度,其分子从体相向界面层的扩散交换占优势,界面层分子逐渐达到饱和吸附,此后体系中有混合胶束形成。 体相胶束中富集的SDS分子对C₁₂imSP分子的“收纳”作用及进一步的“挽留”作用,加之C₁₂imSP分子本身相对较大的空间位阻效应导致界面上的C₁₂imSP分子一旦通过扩散作用被交换至体相,其很难再回复到表面层,即界面膜以SDS分子为主。 通过调节体系中SDS的含量,可以实现对混合体系SDS/C₁₂imSP/NaCl(0.1 mol/L)界面聚集行为的调控,进而实现对界面膜性质的调控。     

稳态荧光探针法测定临界胶束聚集数

以芘为荧光探针、二苯甲酮为猝灭剂,用稳态荧光探针法测定了SDS和AS的胶束聚集数(Nm).以芘的饱和水溶液为溶剂配制表面活性剂溶液,二苯甲酮的适宜浓度取小于1.00 mmol•L－1时,可以获得满意的实验结果.当表面活性剂溶液浓度为5～9倍cmc时,Nm随表面活性剂浓度增大而线性增大,而随温度的变化略有波动.cSAA=cmc时,Nm值为一本征值,定义为临界胶束聚集数[Nm].[Nm]值可从Nm－cSAA实验曲线外延得到.25℃时SDS的临界胶束聚集数[Nm]为57;40℃时SDS的[Nm]为49,AS的[Nm]为55.     

Interaction of slightly cross-linked gels of poly(diallyldimethylammonium bromide) with sodium dodecyl sulfate. Diffusion of surfactant ions in gel

The dynamics of the changing microenvironment of the fluorescent probe pyrene in slightly cross-linked networks of poly(diallyldimethylmmonium bromide) during diffusion of sodium dodecyl sulfate (SDS) in the gel phase has been investigated by fluorescence spectroscopy. Values of the spectral ratio I₃/I₁ for pyrene monomer included in SDS micelles in the swollen networks fall between the corresponding values for pyrene in water and for pyrene dissolved in SDS micelles in aqueous solution. In the narrow interval of the surfactant concentrations in the gel phase, the formation of pyrene excimers is observed. The values of the critical micelle concentration in the gel phase (ca. 5 × 10^?4 to 8 × 10^?4 mol/L) are tenfold lower than in aqueous solutions of the surfactant. The effective micellar diffusion coeffecient D in the gel phase increases with increasing swelling of the network. © 1993 John Wiley & Sons, Inc.     

Micellization of Sodium Dodecyl Sulfate in Glycerol Aqueous Mixtures

The effect of glycerol on both micellar formation and the structural evolution of the sodium dodecyl sulfate (SDS) aggregates in the context of the action mechanism of the cosolvent has been studied. The critical micelle concentration and the degree of counterion dissociation of the surfactant over a temperature range from 20°C to 40°C were obtained by the conductance method. The thermodynamic parameters of micellization were estimated by using the equilibrium model of micelle formation. The analysis of these parameters indicated that the lower aggregation of the surfactant is mainly due to a minor cohesive energy of the mixed solvent system in relation to the pure water. The effect of glycerol on the mean aggregation number of the micelles of SDS was analyzed by the static quenching method. It was found that the aggregation number decreased with the glycerol content. This reduction in the micellar size seems to be controlled by an increase in the surface area per headgroup, which was ascribed to a participation of glycerol in the micellar solvation layer. Studies on the micropolarity of the aggregates, as sensed by the probe pyrene, indicated that this microenvironmental parameter is almost unaffected by the presence of glycerol in the mixture. However, an increase in the micellar microviscosity at the surface region was observed from the photophysical behavior of two different probes, rhodamine B and auramine O. These results suggest a certain interaction of the cosolvent in the micellar solvation of SDS micelles.     

Spectroscopic Investigations on the Interaction of Crystal Violet with Nonionic Micelles of Brij and Igepal Surfactants in Aqueous Media

The behavior of the triphenylmethane dye crystal violet in aqueous solutions containing polyoxyethylene nonionic surfactants was investigated using absorption and fluorescence spectroscopic techniques. The interactions of the dye were examined in micellar media in order to prevent dye aggregation and to ensure maximum dye and surfactant interaction. The relative fluorescence enhancements and the binding constants of the dye to the surfactant micelles were determined. The micropolarities of the micellar environment sensed by the pyrene probe were estimated from the I ₁/I ₃ intensity ratios of the fluorescence spectra of pyrene. The fluorescence quenching of pyrene by hexadecylpyridinium chloride was investigated in aqueous surfactant mixtures at a fixed concentration of surfactant in order to determine the aggregation numbers. Attempts were made to correlate the binding constants obtained in this investigation to various micellar parameters.     

Small-angle neutron scattering from micelles of alkylpolyoxyethylene sulfate: Effect of chain length

Small-angle neutron scattering (SANS) data have been obtained for (i) a series of solutions of C _m H_2m+1(OCH₂–CH₂)₂SO₄Na, for m=18, 16, and 14; (ii) an approximately 0.07M solution of C₁₄H₂₉(OCH₂–CH₂)₂SO₄Na to which different amounts of NaCl were added; and (iii) a series of solutions of variable concentration of C₁₂H₂₅(OCH₂–CH₂)SO₄Na. The increase of the number of carbon atoms of the hydrocarbon chain produces a noticeable increase of the aggregation number of the micelles, while the salt tolerance decreases with increasing m. All the data can be described in terms of a monodispersed, charged, hard-spheres model interacting via a screened Coulombic potential, except the run at highest salt concentration, for which an ellipsoid model gives better results.