Mesoscopic simulation of alkylimidazoline self-aggregation in aqueous solution

A dissipative particle-dynamics method was used to simulate the self-aggregation behavior of 15 alkylimidazoline surfactants of different structures. The effects of concentration and structure of hydrophilic and lipophilic groups of the alkylimidazolines on the configuration and aggregation number (N_agg) of the micelles were also explored. Results show that the concentration of the alkylimidazoline has a significant influence on the configuration of the micelles generated. More specifically, alkylimidazolines of different concentrations are found to generate different structures (spherical, rod-shaped, layered, and interlaced) in aqueous solution. At low (1–10?wt%) and medium (40?wt%) concentrations, the micelles generated in aqueous solution are spherical and rod-shaped, respectively. At higher concentrations (80?wt%), the micelles generated present interlaced shapes with rod-shaped and layered micelles when the length of the lipophilic chain is greater than C₁₃ or the hydrophilic group is H_c. N_agg is strongly dependent on alkylimidazoline concentration and increases as concentration increases. N_agg is also greatly dependent on the structure of the hydrophilic and lipophilic groups present: it increases as the chain length of the lipophilic group increases but decreases as that of the hydrophilic group increases.     

醇-可聚合表面活性剂混合胶束法合成微嵌段缔合聚合物及其流变性

低碳醇与可聚合表面活性剂(surfmer)可以形成混合胶束, 本文通过稳态荧光猝灭法(SSFQ)测定了该混合胶束中可聚合表面活性剂的聚集数, 并通过绝热聚合和后水解工艺合成出不同微嵌段长度的较高分子量(M_w)缔合聚合物聚(丙烯酰胺-丙烯酸钠-十六烷基二甲基烯丙基氯化铵)[P(AM-NaAA-C₁₆DMAAC)]. 聚合物结构经傅里叶变换-红外(FT-IR)光谱和核磁共振碳谱(¹³C NMR)表征证实, 并通过流变性实验研究了微嵌段长度对增粘性能、抗盐性能、粘弹性能的影响规律. 实验结果表明: 该方法可以有效调节缔合聚合物中微嵌段的长度, 随着微嵌段长度增加, 临界缔合浓度(CAC)逐渐降低, 增粘性能、粘弹性能先逐步增加后逐步降低, 并且发现其对聚合物的性能的影响规律中存在最佳嵌段长度.     

Rheological properties of multisticker associative polyelectrolytes in semidilute aqueous solutions

Multisticker associative polyelectrolytes of acrylamide (≈86 mol %) and sodium 2‐acrylamido‐2‐methylpropanesulfonate (≈12 mol %), hydrophobically modified with N,N‐dihexylacrylamide groups (≈2 mol %), were prepared with a micellar radical polymerization technique. This process led to multiblock polymers in which the length of the hydrophobic blocks could be controlled through variations in the surfactant‐to‐hydrophobe molar ratio, that is, the number of hydrophobes per micelle (N_H). The rheological behavior of aqueous solutions of polymers with the same molecular weight and the same composition but with two different hydrophobic block lengths (N_H = 7 or 3 monomer units per block) was investigated as a function of the polymer concentration with steady‐flow, creep, and oscillatory experiments. The critical concentration at the onset of the viscosity enhancement decreased as the length of the hydrophobic segments in the polymers increased. Also, an increase in the N_H value significantly enhanced the thickening ability of the polymers and affected the structure of the transient network. In the semidilute unentangled regime, the behavior of the polymer with long hydrophobic segments (N_H = 7) was studied in detail. The results were well explained by the sticky Rouse theory of associative polymer dynamics. Finally, the viscosity decreased with an increase in the temperature, mainly because of a lowering of the sample relaxation time. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1640–1655, 2004     

Effects of stereocomplexation on the physicochemical behavior of PLA/PEG block copolymers in aqueous solution

A series of polylactide/poly(ethylene glycol) (PLA/PEG) block copolymers were synthesized by ring‐opening polymerization of L ‐ or D ‐lactide in the presence of mono‐ or di‐hydroxyl PEG. The effects of stereocomplexation on the physicochemical behavior of PLA/PEG copolymers in aqueous solution were investigated by varying the degree of stereocomplexation or PLLA/PEG to PDLA/PEG ratio. In mixture solutions of insoluble and soluble copolymers, stereocomplexation strongly affects the solubility of the copolymers. In mixture solutions of soluble copolymers, both the size and aggregation number (N_agg) of the aggregates vary as a function of the degree of stereocomplexation. It is suggested that the size variation of the aggregates with increasing the degree of stereocomplexation is dependent on N_agg changes which are determined by two effects: the self‐adjusting of the aggregates so as to minimize the free energy and thus to increase the N_agg, and the kinetics of aggregation which tend to form more aggregates and thus to decrease the N_agg. Combination of the two opposite effects well explains the diverse variations of N_agg and size of the aggregates as a function of the degree of stereocomplexation. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Interfacial and Thermodynamic Properties of Anionic‐Nonionic Mixed Surfactant System: Influence of Hydrophobic Chain Length of the Nonionic Surfactant

The influence of hydrophobic chain length in nonionic surfactants on interfacial and thermodynamics properties of a binary anionic‐nonionic mixed surfactant was investigated. In this study, nonionic surfactants lauric‐monoethanolamide (C₁₂ MEA) and myrisitic‐monoethanolamide (C₁₄ MEA) were mixed with an anionic surfactant, α‐olefin sulfonate (AOS). The critical micelle concentration (cmc), maximum surface excess (Γ_max), and minimum area per molecule (A_min) were obtained from surface tension isotherms at various temperatures. The thermodynamic parameters of micellization and adsorption were also computed. Micellar aggregation number (N_agg), micropolarity, and binding constant (K_sv) of pure and mixed surfactant system was calculated by fluorescence measurements. Rubingh's method was applied to calculate interaction parameters for the mixed surfactant systems.     

Self-Aggregation of Cationic Dimeric and Anionic Monomeric Surfactants with Nonionic Surfactant in Aqueous Medium

Spectrofluorometric measurements have been used to elaborate the self-aggregation of mixture of anioinic, sodium dodecylbenzenesulfonate (SDBS), and cationic gemini, alkanediyl-α, ω-bis (tetradecyldimethylammonium bromide) (14-4-14) with nonionic surfactant, polyoxyethylene 10 cetyl ether (Brij-56). The critical micelle concentration (cmc) of the binary mixtures has been investigated. Application of the regular solution theory (RST) to the experimental data yield the interaction parameter at mixed micelles (β), indicate an attractive interaction and reflect the synergistic behavior in both Brij-56/SDBS and Brij-56/14-4-14 systems. The micelle aggregation number (N _agg) was measured using a steady state fluorescence quenching method. The N _agg values of the mixed surfactant system were larger than those of pure components. The micropolarity of various combinations and the binding constants (K _sv) were determined from the ratio of intensity of peaks (I ₁/I ₃) of pyrene fluorescence emission spectrum and its quenching, respectively.     

Aggregation behaviors of amine-oxide gemini surfactants

The micellar aggregation of a series of gemini surfactants [N, N’-dimethyl-N, N’-bis(2-alkylamideethyl)-ethylenediamine oxide (alkyl?=?C₁₁H₂₃, C₁₃H₂₇, C₁₅H₃₁)] in aqueous media has been investigated. The results show that there is an excellent agreement among the critical micelle concentration (CMC) values obtained by surface tension and steady-state fluorescence methods. Because of the occurrence of self-coiling or the formation of pre-micellization, the CMC values, the I₁/I₃ values, and the micelle aggregation numbers (N_agg) at CMC (N_m) increase with the hydrophobic alkyl chain length increasing. Besides, vesicles are observed above the CMC for all these surfactants.     

Monitoring the Discontinuous Dodecamer–Icosamer Transition of a Calix[4]arene-Derived Surfactant by Time-Resolved Small-Angle X-ray Scattering

Calix[4]arene-derived surfactants form monodisperse micelles with a well-defined aggregation number (N_agg) of 4, 6, 8, 12, or 20, corresponding to the Platonic solids. This feature is in strong contrast to conventional micelles. In this study, a transition from a dodecamer (N_agg=12) to an icosamer (N_agg=20) was induced by a rapid increase in the NaCl concentration (C_NaCl) using a stopped-flow device and directly observed by time-resolved small-angle X-ray scattering. The N_agg remained unchanged during the first 60 s after the increase in C_NaCl , and then abruptly increased to 20. This feature resembles phase transitions in supersaturated or supercooled states, or highly cooperative phenomena. We surmise that this finding may be due to the fact that only a few N_agg values are thermodynamically allowed when N_agg is sufficiently small. This is the first observation of such an induction time in micellar aggregation.     

Kinetic behavior in self-assembly process of associative polymer solutions

Using associative polymers with different micro-block length (N_H), their kinetic behaviors in the self-assembly process have been discussed with steady flow, step shear rate and dynamic light scattering (DLS) experiments. The results showed that the N_H is the most important parameter to affect rheology of associative polymers, and the equilibrium time and viscosity recovery are an obvious difference from seconds to hours after violent pre-shearing. With DLS data, the hydrophobic micro-zone size is always a polydispersion distribution, and gradually concentrated three scales of size, the associative polymers with higher N_H values have a longer time to reach equilibrium state. The kinetic behavior of associative polymers has a greater potential to apply in enhanced oil recovery (EOR) field, we conclude that the polymers with medium associative strength have a smaller shear thinning index and higher viscosity recovery, and may satisfy the polymer flooding requirements of displacement efficiency and injectivity.     

Interfacial and aggregation properties of aqueous sodium <Emphasis Type="Italic">N</Emphasis>-dodecanoylsarcosinate solutions at different pH

The pH dependence of an anionic surfactant, sodium N-dodecanoylsarcosinate (SLAS), has been studied by measuring interfacial tension, fluorescence, dynamic light scattering, etc., in aqueous solutions with phosphate and borate buffers. The interfacial tension (γ) of SLAS decreases remarkably with a pH decrease and is constant at pH > 7.3. The observed values for the critical micelle concentration (cmc) and the surfactant concentration at which its γ value is reduced by 20 mN/m from that of pure water (C ₂₀) decrease with a pH decrease, while those also become constant at pH > 6.5 and >7.3, respectively. On the other hand, the interfacial excess of SLAS increases at pH < 7.3. These interfacial behaviors have been further investigated by the addition of Tl⁺ which replaces Na⁺ of SLAS. The observed γ values of LAS⁻ with the different counter cations are in the order of H⁺ < Tl⁺ < Na⁺. In order to reveal aggregation properties of SLAS, the aggregation number (N _agg), the micropolarity, the hydrodynamic radius (R _h) of micelle, and the fluorescence anisotropy of Rhodamine B (r) have been evaluated at various pHs. The N _agg value shows a decreasing tendency with a pH increase. The I ₁/I ₃ ratio and the R _h values do not strongly depend on pH. The r value decreases until pH 7 and remains constant at pH > 7.0. These interfacial and micelle properties have been discussed in detail considering the electrostatic interaction and the molecular structures of the hydrophilic headgroup.     

Thermodynamic and spectroscopic studies on cationic surfactant tetradecyltrimethylammonium bromide in aqueous solution of trisubstituted ionic liquid 1, 2-dimethyl-3-octylimidazolium chloride at different temperatures

In this work, the effects on micellar behavior of long chain cationic surfactant tetradecyltrimethylammonium bromide (TTAB) upon the addition of trisubstituted ionic liquid (IL), 1, 2-dimethyl-3-octylimidazolium chloride [odmim][Cl] at temperatures, 298.15–318.15 K has been studied. Different techniques such as conductance, surface tension, fluorescence and ¹H NMR have been employed to understand the interactional mechanisms. The values of critical micelle concentration (cmc) and various thermodynamic parameters have been calculated from conductivity measurements. The surface parameters like effectiveness of decrease in surface tension (Π_cmc), minimum surface area occupied per surfactant monomer (A_min), maximum surface excess concentration (Γ_max), and adsorption efficiency (pC₂₀) have been evaluated by surface tension measurements. Micellar aggregation number (N_agg) has been determined by quenching of pyrene. Further to understand interactions in post micellar region, ¹H NMR measurements have been performed. It has been observed that the lipophilicity of interacting ion modified the thermodynamic and aggregation properties of TTAB.     

Surface activities and thermodynamic properties of novel cationic surfactants with hydroxymethyl group

A new series of cationic surfactants, N–alkyl–N,N–dimethyl–N–(p–(hydroxymethyl) benzyl) ammonium chlorides (p-DHBA-m), were synthesized and the structures were characterized by ¹HNMR, ¹³CNMR, FT–IR and ESI–MS. The surface activities, thermodynamic properties and aggregation behaviors of p-DHBA-m in aqueous solutions were respectively studied by means of surface tension, isothermal titration calorimetry and steady-state fluorescence methods. Thermodynamic parameters show that the micellization is an entropy-driven process. According to the fluorescence quenching method, the micelle aggregation numbers (N_agg) of p-DHBA-m were calculated and found that the increase of temperature or the elongation of alkyl chain length could lead to the reduction of the N_agg, respectively.     

Synthesis and characterization of hydrophobically modified polyacrylamides and some observations on rheological properties

Hydrophobically modified polyacrylamide (PAM) polymers were synthesized by means of an aqueous micellar copolymerization, the di-alkyl substituted acrylamides di-n-propylacrylamide (DPAM) and di-n-octylacrylamide (DOAM) being used as hydrophobic comonomers. The number of hydrophobic blocks was varied using three different comonomer contents (f=1, 3 and 5 wt.%). The length of hydrophobic blocks, i.e. the number of hydrophobes per micelle, N_H, was controlled by the sodium dodecyl sulphate surfactant concentration. The effect of the type of hydrophobic comonomer and the number and length of hydrophobic units on the composition of the copolymer and its solubility and rheological properties were studied. The results indicate that the average copolymer composition is independent of the degree of conversion and surfactant concentration. Solubility and rheological measurements lead to a number of conclusions. First, DPAM is a weak hydrophobic monomer, all DPAM/AM (AM=acrylamide) copolymers being water soluble. Second, there is no strong hydrophobic interaction between DPAM units, in particular for low polymer concentration and N_H, and thus no strong associative thickening behaviour. Third, DOAM is a strong hydrophobic comonomer. Incorporation of a small amount of DOAM into PAM causes a dramatic enhancement in viscosity due to hydrophobic interactions. The properties of the copolymers are strongly dependent on the N_H values, most DOAM/AM copolymers being insoluble in water.     

Electrochemical and chemical polymerization of acrylamide

The electrochemical and chemical polymerization of acrylamide (AA) has been studied. The electrolysis of the monomer in N,N-dimethylformamide (DMF) containing (C₄H₉)₄NClO₄ as the supporting electrolyte leads to polymer formation in both anode and cathode compartments. The cathodic polymer dissolves in the reaction mixture and the anodic polymer precipitates during the course of polymerization. A plausible mechanism for the anodic and cathodic initiation reaction has been given. The chemical polymerization of acrylamide that has been initiated by HClO₄ is analogous to its anodic polymerization. The polymer yield increases with an increase in concentration of the monomer and HClO₄. Raising the reaction temperature also enhances the polymerization rate. The overall apparent activation energy of the polymerization was determined to be ca. 19 kcal/mole. The copolymerization of acrylamide was carried out with methyl methacrylate (MMA) in a solution of HClO₄ in DMF. The reactivity ratios are r₁ (AA) = 0.25 and r₂ = 2.50. The polymerization with HClO₄ appears to be by a free radical mechanism. When the polymerization of acrylamide is carried out with HClO₄ in H₂O, a crosslinked water-insoluble gel formation takes place.     

Studying Effect of Typical Nonplanar Cyclic Alcohols (n = 5‐7) on Micellization of Sodium Dodecyl Sulfate (SDS) in Aqueous Solution and Locating Their Solubilization Site in SDS Micelles

Cyclic alcohols (n = 5‐7) are compounds of distinctive nonplanar structure. Effect of the alcohols on micellization of sodium dodecyl sulfate (SDS) in aqueous solution are examined by determining the critical micelle concentration (CMC) by conductometry and the micelle aggregation numbers (N_agg) by fluorometry, respectively. In general, the CMC of SDS decreases with increase in volume of a cyclic alcohol in water and increases further after attaining a minimum value. The N_agg of SDS varies little with small addition of a cyclic alcohol, but decreases when added in sufficient volume. Both the changes of the CMC and N_agg with carbon number in the ring of the alcohols occur irregularly due to their steric reasons and nonplanar nature. The irregularity makes a difference between the cyclic alcohols and their chain counterparts. Based on ¹H NMR chemical shift measurements, the cyclic alcohols are found to be solubilized in the palisade layer in SDS micelles.     

Interaction Between an Active Pharmaceutical Ingredient Ionic Liquid Benzalkonium Salicylate and Small Biomolecules in Aqueous Solution: UV Absorption,Conductivity, and Volumetric Study

UV absorption spectroscopy, electrical conductivity and density experiments have been used to investigate the interactions of some small biomolecules (amino acids/dipeptides) with an active pharmaceutical ingredient in ionic liquid form (API-IL), benzalkonium salicylate (BaSal), in aqueous solution. A number of useful parameters, such as critical micellar concentration (cmc), aggregation number (N_agg) and limiting molar conductivity (Λ₀) of BaSal, standard partial molar volumes (\(V_{2,\phi }^{ \circ }\)), corresponding volumes of transfer from water to aqueous BaSal solutions (Δ_trV^o), standard partial molar expansibilities (\(E_{\phi }^{ \circ }\)), hydration number (n_H) of small biomolecules, as well as the binding constants (K_b) for small biomolecule–BaSal complexes have been evaluated. The dependence of the properties on concentration, temperature and alkyl chain length of amino acids/dipeptides is examined. The results are used to identify the solute–solvent physicochemical interactions occurring in the studied systems.     

Interactions of Ionic Surfactants With PEO-PBO-PEO Triblock Copolymers in Aqueous Solutions

The interactions of triblock copolymers (TBP) with ionic surfactants were studied employing surface tensiometry, electrical conductivity, steady-state fluorescence (SSF), and dynamic light scattering (DLS) techniques. An increasing trend in the critical micelle concentration (CMC) of SDS/CTAB in the presence of triblock copolymers was observed especially at higher polymer to surfactant ratio. The delay in the CMC of surfactants was more pronounced in the presence of E₄₈B₁₀E₄₈ possibly due to its less hydrophobic nature. The negative values of free energy of micellization (ΔG_m) both in case of SDS and CTAB confirmed the spontaneity of the processes. The aggregation number (N_agg) and hydrodynamic radius (R_h) of polymer/surfactant mixed systems were determined by SSF and DLS. The suppression of the surfactant micelle size in the presence of TBP was confirmed by SSF and DLS studies.     

Monitoring the Discontinuous Dodecamer–Icosamer Transition of a Calix[4]arene‐Derived Surfactant by Time‐Resolved Small‐Angle X‐ray Scattering

Calix[4]arene‐derived surfactants form monodisperse micelles with a well‐defined aggregation number (N _agg) of 4, 6, 8, 12, or 20, corresponding to the Platonic solids. This feature is in strong contrast to conventional micelles. In this study, a transition from a dodecamer (N _agg=12) to an icosamer (N _agg=20) was induced by a rapid increase in the NaCl concentration (C _NaCl) using a stopped‐flow device and directly observed by time‐resolved small‐angle X‐ray scattering. The N _agg remained unchanged during the first 60 s after the increase in C _NaCl , and then abruptly increased to 20. This feature resembles phase transitions in supersaturated or supercooled states, or highly cooperative phenomena. We surmise that this finding may be due to the fact that only a few N _agg values are thermodynamically allowed when N _agg is sufficiently small. This is the first observation of such an induction time in micellar aggregation.     

Micellization behavior of [C16-12-C16], 2Br gemini surfactant in binary aqueous-solvent mixtures

The micellization behavior of bis cationic gemini surfactant, N,N′-dihexadecyl-N,N,N′,N′-tetramethyl-1,12-dodecanediammonium dibromide [C₁₆H₃₃N⁺(CH₃)₂-(CH₂)₁₂-N⁺(CH₃)₂C₁₆H₃₃, 2Br⁻] has been studied in binary aqueous mixtures of dimethyl sulfoxide, methanol, 1,4-dioxane, glycerol and ethylene glycol by conductivity and surface tension measurements at 300 K. The critical micellar concentration, degree of micelle ionization (α), surface excess concentration (Г_max), minimum surface area per molecule of surfactant (A_min), Gibbs free energy of micellization (ΔG_m°), the surface pressure at cmc (π_cmc), and the Gibbs energy of adsorption (ΔG_ad°) of the gemini surfactant have also been determined. The cmc, α, A_min increases where as (ΔG_m°), Г_max, and π_cmc decreases with increasing volume percentage of the solvents in the solvent–water binary mixture. The interfacial properties of the gemini surfactant, solute–solute, solvent–solute interactions and the effectiveness of a surface-active molecule in binary solvent systems have been discussed.     

Micellization of anionic gemini surfactants and their interaction with polyacrylamide

A series of anionic gemini surfactants have been synthesized. The surface properties and micellization process of as-prepared sulfonate gemini surfactants (SGS) and carboxylate gemini surfactant (CGS) have been studied by surface tension measurement and isothermal titration microcalorimetry. Meanwhile, the interaction of these five surfactants with polyacrylamide (PAM) was investigated using surface tension, steady-state fluorescence measurement, and isothermal titration microcalorimetry. The results show that the critical micelle concentrations (CMCs) of above-mentioned surfactants are more than 1 order of magnitude lower than those of corresponding single chain surfactants. Moreover, the enthalpy of micelle formation (ΔH _mic) for the investigated gemini surfactants is negative. In the surfactant–PAM systems, the thermodynamic parameters of binding have also been determined. The conclusion may be drawn that the binding strength of SGS onto PAM is stronger than that of CGS, resulting from more compact structure of SGS aggregates. With increasing surfactant hydrophobicity, the values of ΔH _agg become more exothermic and a ΔS _agg decrease was observed. Therefore, the interaction between SGS and PAM is enthalpy-driven.