Structural transition of bifunctional surfactants

Abstract  The effect of primary alcohols (butanol to hexanol) and n-hexylamine on the micellar properties of solutions of alkanediyl-α,ω-bis(dimethylcetylammonium bromide) (16-s-16, with s = 5, 6) in the absence and presence of KBr was investigated by viscometric studies at 30 ± 0.1°C. The viscosities were determined using an Ubbelohde viscometer. The viscosity values exhibit the order s = 5 > 6, i.e., the gemini with short spacer produces the more viscous solution with hexanol > pentanol > butanol. The high viscosities observed in these systems are interpreted in terms of a micellar sphere to rod transition that takes place over a certain range of concentration of the surfactants with added salt or organic additives or both. Micellar growth was higher in case of C₆H₁₃OH when compared to C₆H₁₃NH₂. In the presence of C₆H₁₃NH₂, the relative viscosity becomes almost constant when the concentration of the additive is increased, which has also been discussed in terms of electrostatic and hydrophobic forces operating in the solution. The positive synergistic effects of additives also confirm pronounced micellar growth. Graphical abstract        

Morphological Changes of Cationic Gemini Surfactants 14-s-14 (s=4,5,6) in the Presence of Additives

The effect of adding aliphatic alcohols (C₄OH, C₅OH, C₆OH) and corresponding amines (C₄NH₂, C₅NH₂, C₆NH₂) on a series of dicationic gemini surfactants with the general formula C₁₄H₂₉(CH₃)₂N⁺?C(CH₂)_s?CN⁺(CH₃)₂C₁₄H₂₉, 2Br^? (14-s-14; s=4,5,6), in the absence and presence of KNO₃, has been studied by viscosity measurements at 303.15?K. As the chain length of the additive increased, the viscosity increased with increasing additive concentration and the extent of the effect followed the sequence: C₆OH>C₅OH>C₄OH; C₆NH₂>C₅NH₂>C₄NH₂. The simultaneous presence of salt and additives showed an increase in ?? _r values due to a synergistic effect. However, for equal chain lengths in the additives, the effect was greater for the n-alcohols. The tendency for the micelles to grow from spherical to rod-like structures is mainly influenced by the spacer chain length. At 303.15?K, the micellar growth was more pronounced for the shorter spacer, i.e. s being 4, which can be interpreted in terms of the short spacer having a higher tendency for micellar growth. Contrary to the cationic geminis, no effect was observed with a conventional surfactant of equal chain length, TTAB, even in the presence of KNO₃ at the same concentration used for the geminis.     

A viscometric study of tuning micellar morphology by organic additives

The micellar morphology in aqueous 0.2 M sodium dodecyl sulfate (SDS) solutions has been studied in the simultaneous presence of organic salts (anilinium hydrochloride, AHC; ortho-toluidine hydrochloride, oTHC; para-toluidine hydrochloride, pTHC) and aliphatic alcohols (n-butanol, C₄OH; n-pentanol, C₅OH; n-hexanol, C₆OH; n-heptanol, C₇OH), aliphatic amines (n-butylamine, C₄NH₂; n-pentylamine, C₅NH₂; n-hexylamine, C₆NH₂; n-heptylamine, C₇NH₂), or hydrocarbons (n-hexane, C₆H; n-heptane, C₇H) by viscosity measurements under Newtonian flow conditions at 30 °C. Addition of alcohols and amines causes micellar growth which is found to be dependent upon chain length of the additive and nature of organic salt counterion. It is observed that amines are more effective in increasing the viscosity of the system if added in pure 0.2 M SDS solution, while SDS + pTHC system was found versatile when alcohols were added to this system. The increased viscosity or the micellar growth is explained in terms of the site of solubilization of the respective additive and the interaction of the additive with micellar surface and salt counterion present in the head group region. Hydrocarbons are nearly ineffective in inducing micellar growth and can be used as ‘micellar destroyer’ for the grown micelles. The additives used are found effective in tuning the environment of the micelle which is reflected in viscosity behavior. This work may find use in micellar ultrafiltration as well as in mimicking the natural cell, which has several aspects common with the micelle.     

Synergism in cationic gemini – additive systems

A series of dicationic gemini surfactants with the general formula C₁₆H₃₃(CH₃)₂N⁺?(CH₂)_s?N⁺(CH₃)₂C₁₆H₃₃, 2Br^? (where s?=?4–6), designated as 16-s-16, were synthesised. Their interaction with organic additives: n-alcohols (C₃H₇OH, C₇H₁₅OH, C₈H₁₇OH) and the corresponding amines (C₃H₇NH₂, C₇H₁₅NH₂, C₈H₁₇NH₂) in the absence and presence of KNO₃ at 30°C was studied viscometrically to observe their effect on assembly formation and micellar transition. The simultaneous presence of KNO₃ and organics induced rich aggregates morphologies in the gemini micellar systems by giving high viscosity values. On comparing the behaviour of the gemini surfactant series for a given alkyl chain length of the organic additive, the spacer is found to markedly influence the behaviour; shorter the spacer, earlier the sphere-to-rod transition. In the case of the conventional surfactant, CTAB, the concentration of KNO₃ used with the geminis was insufficient to induce any transition.     

Adsorption and thermodynamic properties of dissymmetric gemini imidazolium surfactants with different spacer length

A series of dissymmetric gemini imidazolium surfactants with different spacer length ([C_mC_sC_nim]Br₂, m + n = 24, m = 12, 14, 16, 18; s = 2, 4, 6) were synthesized and characterized by ¹H NMR and ESI-MS spectroscopy. Their adsorption and thermodynamic properties were investigated by the surface tension and electrical conductivity methods. Consequently, the surface activity parameters (cmc, γ_cmc, π_cmc, pC₂₀, cmc/C₂₀, Γ_max, A_min) and thermodynamic parameters (ΔG_m^θ, ΔH_m^θ, ΔS_m^θ) were obtained. The effects of the dissymmetry (m/n) and the spacer length (s) on the surface activity and micellization process of surfactants have been discussed in detail.     

疏水相互作用引起Gemini表面活性剂柔性联接链的弯曲

为了理解gemini表面活性剂柔性烷基联接链在自组织过程中的特殊作用,我们合成了三种gemini表面活性剂烷基-a,w-二（二-十二烷基甲基溴化铵）（记为2C12-s-2C12×2Br (s=3, 6, 8)）。2C12-s-2C12×2Br在水表面构成铺展膜后,由于每个分子带有4根烷烃链,它们形成了稠密的烷烃尾链层。增强的烷烃尾链与联接链间的疏水相互作用促使联接链弯曲朝向空气一端,可发生弯曲的联接链长度要小于吸附在水溶液表面上的gemini表面活性剂C12-s-C12×2Br,后者每个分子只有2根烷烃链。由此可见,增强的烷烃尾链与联接链间的疏水相互作用可以有效地促进联接链的弯曲。     

Effect of dicationic gemini surfactants 16–s‐16 (s = 4, 5, 6) on the ninhydrin‐dipeptide (glycyl‐tyrosine) reaction

The effect of dicationic gemini surfactants H₃₃C₁₆(CH₃)₂N⁺‐(CH₂)_s‐N⁺(CH₃)₂ C₁₆H₃₃, 2Br^? (s= 4, 5, 6) on the reaction of a dipeptide glycyl–tyrosine (Gly–Tyr) with ninhydrin has been studied spectrophotometrically at 70°C and pH 5.0. The reaction follows first‐ and fractional‐order kinetics, respectively, in [Gly–Tyr] and [ninhydrin]. The gemini surfactant micellar media are comparatively more effective than their single chain–single head counterpart cetyltrimethylammonium bromide (CTAB) micelles. Whereas typical rate constant (k_Ψ) increase and leveling‐off regions, just like CTAB, are observed with geminis, the latter produces a third region of increasing k_Ψ at higher concentrations. This subsequent increase is ascribed to the change in the micellar morphology of the geminis. The pseudophase model of micelles was used to quantitatively analyze the k_Ψ ? [gemini] data, wherein the micellar‐binding constants K_S for [Gly–Tyr] and K_N for ninhydrin were evaluated. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 800–809, 2012     

Micellar effects upon the reaction of hydroxide ion with 2-phenoxyquinoxaline

Cationic micelles of alkyltrimethylammonium chloride and bromide (alkyl = n? C₁₂H₂₅, n? C₁₄H₂₉, and n? C₁₆H₃₃) catalyze and anionic micelles of sodium dodecyl sulfate inhibit the reaction of hydroxide ion with 2-phenoxyquinoxaline (1). Inert anions such as chloride, nitrate, mesylate, and n-butanosulfonate inhibit the reaction in CTABr by competing with OH^? at the micellar surface. The overall micellar effects on rate in cationic micelles and dilute electrolyte can be treated quantitatively in terms of the pseudo-phase ion-exchange model. The determined second-order rate constants in the micellar pseudo-phase are smaller than the second-order constants in water. © 1994 John Wiley & Sons, Inc.     

Synthesis and Surface Properties of Novel Cationic Gemini Surfactants

A series of novel cationic gemini surfactants, p-[C _n H_2n+1N⁺(CH₃)₂CH₂CH(OH)CH₂O]₂C₆H₄·2Cl^? [A(n = 12), B(n = 14) and C(n = 16)], containing a spacer group with two flexible and hydrophilic groups (2-hydroxy-1,3-propylene) on both sides of a rigid and hydrophobic group (1,4-dioxyphenylene) has been synthesized by the reaction of hydroquinone diglycidyl ether with N,N-dimethylalkylamine and N,N-dimethylalkylamine hydrochloride. Their surface-active properties have been investigated by surface tension measurement. The critical micelle concentration (cmc) values of the synthesized cationic gemini surfactants are one order of magnitude lower than those of their corresponding monomeric surfactants (C _n H_2n + 1N⁺(CH₃)₃·Cl^?). Both the cmc and surface tension at the cmc (γ_cmc) of A are lower than those of p-[C₁₂H₂₅N⁺(CH₃)₂CH₂]₂C₆H₄·2Cl^? (D). The novel cationic gemini surfactants A and B also show good foaming properties.     

Small-angle neutron-scattering study and micellar model of the gemini (phenylenedimethylene)bis(n-octylammonium)dibromide surfactant micelles in water

The microstructure of the micelles formed in aqueous solution by gemini surfactants with aromatic spacers, [Br(CH₃)₂N⁺(C _m H_{2 m +1})-(Ph)-(C _m H_{2 m +1})N⁺(CH₃)₂Br, m=8 and Ph = o-, m- or p-phenylenedimethylene] has been examined by small-angle neutron scattering. Aggregation of the gemini surfactants with an o-phenylenedimethylene spacer brings about formation of premicelles and small micelles at concentrations below the second critical micelle concentration, while above this concentration marked micellar growth and variation in shape occurs. It is suggested that the minimum aggregate formed at this critical micelle concentration may be the trimer or tetramer and that this result supports the mechanism of “gemini → submicelle → assembly” for micellar growth. Received: 8 September 1998 Accepted in revised form: 27 November 1998     

Role of added counterions in the micellar growth of bisquaternary ammonium halide surfactant (14-s-14): 1H NMR and viscometric studies

The change in the morphology of a series of dicationic gemini surfactants C(14)H(29)(CH(3))(2)N(+)-(CH(2))(s)-N(+)(CH(3))(2)C(14)H(29), 2Br(-) (14-s-14; s=4-6) on their interaction with inorganic (KBr, KNO(3), KSCN) and organic salts (NaBenz, NaSal) have been thoroughly investigated by means of (1)H NMR spectral analysis and the results are well supported by viscosity measurements. The presence of salt counterions results in structural transition (spherical to nonspherical) of gemini micelles in aqueous solution. With an increase in salt concentration all the three gemini surfactants showed changes in their aggregate morphology. This change is dependent on the nature and size of the added counterion. The effect of inorganic counterions on the micellar growth is observed to follow the Hofmeister series (Br(-) < NO(3)(-) < SCN(-)). The roles of organic counterions are discussed on the basis of probable solubilization sites of the substrate molecule in the gemini micelles, showing more growth in case of Sal(-) than Benz(-). The results are confirmed in terms of the obtained values of chemical shift (δ), line width at half height (lw), and relative viscosity (η(r)). Also, the growth of micelles was most pronounced for the gemini surfactant with the shortest spacer (s=4). This was attributed to the unique molecular structure of gemini surfactant micelles having flexible polymethylene spacer chain linking the twin polar headgroups.     

Surface and Solution Properties of Cationic Gemini Surfactants with Primary Linear Alkanols

Using surface tension and fluorescence methods, the surface and solution properties of two cationic gemini surfactants {pentanediyl-1,5-bis(dimethylcetylammonium bromide) and hexanediyl-1,6-bis(dimethylcetylammonium bromide)} (referred to as 16-5-16 and 16-6-16) have been studied in the presence and absence of primary linear alkanols. Parameters studied include the critical micelle concentration (CMC), C ₂₀ (the surfactant concentration required to reduce the surface tension of the solvent by 20 mN·m^?1), Г _max (maximum surface excess), and A _min (minimum surface area per molecule). These parameters indicate mixed micelle formation and, therefore, surfactant-additive interaction parameters in mixed micelles and mixed monolayers, as well as activity coefficients, were calculated. A synergistic effect was observed in all instances and was found to be correlated with the chain length of the alkanols. The CMC values of 16-s-16 (s = 5, 6) decrease with increasing alkanol concentration and the extent of this effect follows the sequence: 1-octanol (C₈OH) > 1-heptanol (C₇OH) > hexan-1-ol (C₆OH) > 1-pentanol (C₅OH) > butanol (C₄OH). The micelle aggregation number (N _agg) of mixed micelles has been obtained using the steady state fluorescence quenching method. The micropolarity of gemini/alkanol systems has been evaluated from the ratio of intensity of peaks (I ₁/I ₃) of the pyrene fluorescence emission spectra. Results are interpreted on the basis of the structure of mixed micelles and monolayers.     

Micellization of monomeric and dimeric (gemini) surfactants in polar nonaqueous-water-mixed solvents

Dimeric or gemini surfactants are novel surfactants that are finding a great deal of discussion in the academic and industrial arena. They consist of two hydrophobic chains and two polar head groups covalently linked by a spacer. Data on critical micelle concentration (cmc) and degree of counterion dissociation (α) are reported on bis-cationic C₁₆H₃₃N⁺(CH₃)₂–(CH₂)_s–N⁺(CH₃)₂C₁₆H₃₃, 2Br⁻, referred to as 16-s-16, for spacer lengths s=4, 5, 6 in aqueous and in polar nonaqueous (1-propanol, 2-methoxyethanol or methyl cellosolve, dimethyl sulfoxide, acetonitrile)-water-mixed solvents. The behavior is compared with conventional monomeric surfactant cetyltrimethylammonium bromide (CTAB). Thermodynamic parameters are obtained from the temperature dependence of the cmc values. It is observed that micellization tendency of the surfactants decreases in the presence of polar nonaqueous solvents. However, detailed studies with dimethylsulfoxide (DMSO) show that the geminis nearly outclass the micellization-arresting property of this solvent. Also, within geminis, higher spacer length is found suitable for showing micellization even with high DMSO content (50% v/v). The implications of these results of gemini micellization may be useful in micellar catalysis in polar nonaqueous solvents.     

Study of the Interaction Between Promazine Hydrochloride and Surfactant (Conventional/Gemini) Mixtures at Different Temperatures

In the present paper, the micellization of an amphiphilic drug, promazine hydrochloride, and gemini surfactants (16-s-16) with s = 4–6 and the monomeric hexadecyltrimethylammonium bromide (CTAB) counterparts has been examined conductometrically in the pure and mixed states in aqueous solutions at different compositions and temperatures (298.15–308.15 K). Dicationic gemini surfactants provide much better environment for the micellization behavior than the corresponding monocationic counterpart CTAB. The critical micelle concentration (cmc) values are lower than the cmc for ideal mixing, cmc ^id, suggesting attractive interactions between the two components in mixed micelles. The micellar mole fractions of surfactants, evaluated by different models, show greater contributions of surfactants in mixed micelles and increase with increasing concentrations of these surfactants. The negative values of β suggest synergism in the mixtures, which is highly beneficial as it reduces the total amount of surfactants required in a particular application, leading to reductions of cost and environmental impact. Activity coefficients (f ₁ and f ₂) are always less than unity showing nonideality in the systems. The data have been also used for evaluation of thermodynamic parameters.     

Thiol‐responsive micelles based on nonionic gemini surfactants with a cystine disulfide spacer

Thiol‐responsive micelles consisting of novel nonionic gemini surfactants with a cystine disulfide spacer are reported. The gemini surfactants, (C₁₈‐Cys‐mPEG)₂ and ((C₁₈)₂‐Lys‐Cys‐mPEG)₂, were synthesized from polyethylene glycol, cysteine, and stearic acid, and their structures were confirmed by ¹H NMR and gel permeation chromatography. (C₁₈‐Cys‐mPEG)₂ and ((C₁₈)₂‐Lys‐Cys‐mPEG)₂ formed micelles with average diameters of 13 and 22 nm above the critical micelle concentration of 6.5 and 4.7 µg mL^?1, respectively. The micelles of ((C₁₈)₂‐Lys‐Cys‐mPEG)₂ containing more stearoyl groups showed encapsulated more hydrophobic indomethacin (IMC) with higher entrapment efficiencies than those of (C₁₈‐Cys‐mPEG)₂. The gemini surfactant micelles exhibited an accelerated release of encapsulated IMC with the concentration of the reducing agent, glutathione (GSH), whereas they were unaffected by the presence of reduced GSH (GSSG). The 3‐(4,5‐dimethylthiazol‐2‐yl)‐5‐(3‐carboxymethoxyphenyl)?2‐(4‐sulfophenyl)?2H‐tetrazolium studies revealed the noncytotoxic nature of the gemini surfactant micelles. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 582–589     

Wormlike Micelle Formation and Rheological Behavior in the Aqueous Solutions of Mixed Sulfate Gemini Surfactant without Spacer Group and Dodecyltrimethylammonium Bromide

The rheological behavior of the aqueous solutions of mixed sulfate gemini surfactant with no spacer group, referred to as d‐C₁₂S, and dodecyltrimethylammonium bromide (C₁₂TABr) at a total concentration of 100 mmol·L⁻¹ but different molar ratios of C₁₂TABr to d‐C₁₂S (α₁) was investigated using steady rate and frequency sweep measurements. The wormlike micelles were formed over a narrow α₁ range of 0.20–0.27. The viscoelastic solutions exhibited Maxwell fluid behavior. At the optimum molar ratio of 0.25, the zero‐shear viscosity was as high as 600 Pa·s and the length of the mixed wormlike micelle was about 0.45–0.85 µm. The present result provides an example to construct long wormlike micelles by anionic gemini surfactant.     

Synthesis and Surface Activity of Anionic Gemini Surfactants with Alkyl Spacers

A series of anionic Gemini surfactants called alkanediyl-α,ω-bis(m-octylphenoxy sulfonate) with different length of (CH₂)_x spacer, C₈C_xC₈ (x = 2, 4, 6, 8), have been synthesized from 4-n-octylphenol and their basic physicochemical properties are investigated. The results indicate that they are different from cationic Gemini surfactants called alkanediyl-α,ω-bis(dimethyldodecylammonium bromide), 12-(CH₂)_s-12, in the literature. It is found that as the carbon atom number of the spacer increases, the cmc (critical micelle concentration) decreases gradually, and the surface area per molecule (A_min) decreases initially and then increases. The breakpoints appear at number 4 of carbon atom in the spacer. Though the length of the spacer is different for the Gemini surfactants from C₈C₂C₈ to C₈C₆C₈, there is no obvious change on the micropolarity.     

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

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

Small-angle neutron-scattering study of bis(quaternaryammonium bromide) surfactant micelles in water. Effect of the long spacer chain on micellar structure

The microstructure of the normal micelles formed by dimeric surfactants with long spacers, [Br⁻(CH₃)₂N⁺(C _m H_{2 m +1})-(CH₂) _S  -(C _m H_{2 m +1})N⁺(CH₃)₂Br⁻, m = 10 and s = 8, 10 and 12], has been investigated by small-angle neutron scattering and compared with previously reported results for micelles of the same dimeric surfactants with shorter spacers (m = 10 and s = 2, 3, 4 and 6). It was found that for dimeric surfactants with long spacers (s = 8 and 10), both micellar growth and variation in shape occur to only a small extent, if at all, compared with dimeric surfactants with short spacers. However, for the dimeric surfactant with the longest spacer, s = 12, the extent of micellar growth and shape variation is also large. These results are due to the differences in conformation of dimeric surfactants with short spacers (s = 2–6) compared with that of the surfactants with long spacers (s = 8–12). Received: 15 June 1998 Accepted: 22 July 1998     

Studies on the Binding of Bis–Quaternary Ammonium Surfactants to Bovine Serum Albumins by Microcalorimetry and Circular Dichroism

The interactions of bovine serum albumin (BSA) with two cationic gemini surfactants, (C _n N)₂Cl₂ (n = 12, 14), in buffer solutions (pH = 7.0) were investigated by isothermal titration calorimetry (ITC) and circular dichroism (CD). CD spectra showed that the two surfactants change the secondary structure of BSA. The thermodynamic results suggest that there exist two binding types (high affinity/low affinity) in the interacting process of (C _n N)₂Cl₂ micelles with BSA. The high affinity binding is an endothermic process driven by entropy, in which the synergistic effect among weak interactions plays an important role. The low affinity binding is an exothermic process accompanied by positive entropy effect, in which hydrophobic interaction is dominant in all driving forces. Furthermore, corresponding binding site number of BSA for (C₁₄N)₂Cl₂ is much smaller than that for (C₁₂N)₂Cl₂, indicating that the hydrophobic chain length of surfactant plays a key role in low affinity binding process.