Solubilisation of dyes by surfactant micelles. Part 3; A spectroscopic study of azo dyes in surfactant solutions†

A spectroscopic study (UV–vis and adsorption) has been made of the interactions of select model azo dyes with a range of surfactant types or their mixtures both above and below their respective critical micelle concentrations. All surfactants inhibit adsorption of the dyes to cotton above their critical micelle concentrations due to incorporation in micelles. However, formation of 1;1 complexes between dyes and cationic or zwitterionic surfactants in sub‐micellar regions results in enhanced deposition on cotton. It is shown that attractive or repulsive electrostatic interactions play a key role in dye binding to micelles. Unusually, spectra of complexes formed between the dye and cationic surfactant are typical of those of the azo tautomeric form as opposed to the hydrazone form that is prevalent in aqueous media. Addition of anionic surfactant to micellar solutions of nonionic or zwitterionic surfactants results in successive displacement of dye from the respective micelles, i.e. binding is competitive.     

Solubilisation of dyes by surfactant micelles. Part 1; Molecular interactions of azo dyes with nonionic and anionic surfactants

An ultraviolet-visible spectroscopic investigation has been made of the interactions of a specially synthesised series of substituted, model arylazonaphthol dyes with nonionic and anionic surfactants. Changes in spectral features were recorded above the critical micelle concentrations, suggesting specific interactions of dyes with micelles of the respective surfactants. The affinity of the dye for the surfactant micelles increased when various p -substituent were incorporated in to the dyes. Similarly, there was a shift in azo–hydrazone tautomeric equilibria and an increase in measured dye p K _a values. Models are proposed for the location of dyes in nonionic or anionic micelles. Unlike earlier studies, it is concluded that the solubilised dye experiences only one environment in nonionic micelles but the specific location, i.e. whether preferentially incorporated in the hydrophobic micellar interior or in the more hydrophilic, outer polyoxyethylene layer, depends upon the nature of the substituent.     

Adsorption of dyes to cotton and inhibition by surfactants, polymers and surfactant–polymer mixtures

A wide-ranging investigation has been made of the adsorption of direct dyes to cotton and of inhibition by surfactants, polymers and polymer–surfactant mixtures. Generally, the selected polymers are extremely effective at inhibiting adsorption of most of the direct dyes to cotton but are less effective at inhibiting adsorption of small, model azo dyes. Micellar solutions of zwitterionic and cationic surfactants can inhibit adsorption of both small dyes and commercial dyes. It is shown that anionic surfactants at sub-micellar concentrations can inhibit polymer-dye interactions due to displacement of dye and/or relocation into micelle-like polymer–surfactant complexes. New insights have been obtained into the interactions of dyes with cotton and with polymers, surfactants or their mixtures, particularly into observed dye selectivities.     

Adsorption of dyes to cotton and inhibition by polymers

This paper addresses some key factors that control the transfer of dyes between garments during detergency. It is shown that adsorption of a series of substituted arylazo-2-naphthol dyes to cotton under simulated detergency conditions is influenced by the log P fragment value of the dye substituent; this suggests that hydrophobic interactions make an important contribution to the binding free energy. The comparative effectiveness of nonionic, zwitterionic and cationic polymers in inhibiting adsorption of dye to cotton was also investigated. Increase in polymer concentration reduces dye adsorption to cotton; increase in polymer molecular weight at constant polymer concentration also inhibits dye adsorption up to a molecular weight of ca. 20000, above which there is no further change. Anionic surfactants reduce the efficacy of polymers by displacing dyes from polymers. Surprisingly, certain dyes become relocated in polymer/surfactant complexes; binding is much more effective than in corresponding surfactant micelles.     

A spectroscopic and modelling study of polymer/dye interactions†

A spectroscopic study has been made of the comparative effectiveness of nonionic, zwitterionic and cationic polymers in binding model dyes. Addition of polymer produces smaller changes in the UV–vis spectra than observed in micellar solutions. Upon binding to polymers, the measured pK_A values of the model dyes decrease. The results of modelling and spectroscopic studies of the interaction between the model arylazonaphthol dyes are discussed in this paper. Addition of anionic surfactants, e.g. SDS, below their critical micelle concentrations disrupts polymer/dye binding, resulting in relocation of model dye to new sites formed from polymer/surfactant interactions. These sites are more apolar and produce spectra similar to those in corresponding micelles but with higher dye pK_A values and binding affinities. For the previous paper in this series see page 140; for parts 1 and 2 see refs 1 and 2, respectively.     

Synthesis and Surface Active Properties of Gemini Cationic Surfactants and Interaction with Anionic Azo Dye (AR52)

A homologous series of new gemini cationic surfactants were synthesized and characterized using micro elemental analysis, FTIR, ¹H-NMR and mass spectra. The surface activities of these amphiphiles were determined based on the data of surface tension. Critical micelle concentration, effectiveness of the surface tension reduction, efficiency of adsorption, maximum surface excess, minimum surface area and critical packing parameter were evaluated. The effect of cationic micelles on solubilization of anionic azo dye, sulforhodamine B (Acid Red 52) in aqueous micellar solution of the synthesized gemini cationic surfactants was studied at pH 6.9 ± 0.5 and 25 °C. The results showed that the solubility of dye rose with increasing surfactant concentration as a consequence of some association between the dye and the micelles. It was also observed that the aggregation of surfactant and dye takes place at a surfactant concentration below the CMC of the individual surfactant. The partition coefficients between the bulk water and surfactant micelles as well as the Gibbs energies of distribution of dye between the bulk water and surfactant micelles were calculated using a pseudo-phase model. The effect of the hydrophobic chain length of Gemini cationic surfactants on the distribution parameters was also reported. The results show favorable solubilization of dye in cationic micelles.     

Nuclear Magnetic Resonance Investigation of the Effect of pH on Micelle Formation by the Amino Acid‐Based Surfactant Undecyl l‐Phenylalaninate

Micelle formation by the anionic amino acid‐based surfactant undecyl l ‐phenylalaninate (und‐Phe) was investigated as a function of pH in solutions containing either Na⁺, l ‐arginine, l ‐lysine, or l ‐ornithine counterions. In each mixture, the surfactant's critical micelle concentration (CMC) was the lowest at low pH and increased as solutions became more basic. Below pH 9, surfactant solutions containing l ‐arginine and l ‐lysine had lower CMC than the corresponding solutions with Na⁺ counterions. Nuclear magnetic resonance (NMR) diffusometry and dynamic light scattering studies revealed that und‐Phe micelles with Na⁺ counterions had hydrodynamic radii of approximately 15 Å throughout the investigated pH range. Furthermore, l ‐arginine, l ‐lysine, and l ‐ornithine were found to bind most strongly to the micelles below pH 9 when the counterions were cationic. Above pH 9, the counterions became zwitterionic and dissociated from the micelle surface. In und‐Phe/l ‐arginine solution, counterion dissociation was accompanied by a decrease in the hydrodynamic radius of the micelle. However, in experiments with l ‐lysine and l ‐ornithine, micelle radii remained the same at low pH when counterions were bound and at high pH when they were not. This result suggested that l ‐arginine is attached perpendicular to the micelle surface through its guanidinium functional group with the remainder of the molecule extending into solution. Contrastingly, l ‐lysine and l ‐ornithine likely bind parallel to the micelle surface with their two amine functional groups interacting with different surfactant monomers. This model was consistent with the results from two‐dimensional ROESY (rotating frame Overhauser enhancement spectroscopy) NMR experiments. Two‐dimensional NMR also showed that in und‐Phe micelles, the aromatic rings on the phenylalanine headgroups were rotated toward the hydrocarbon core of micelle.     

Effect of pH on the Binding of Sodium,Lysine, and Arginine Counterions to <Emphasis Type="SmallCaps">l</Emphasis>-Undecyl Leucinate Micelles

Micelle formation by the amino acid-based surfactant undecylenyl l-leucine was investigated as a function of solution pH with NMR, dynamic light scattering, and fluorescence spectroscopy. NMR and dynamic light scattering showed that 50 mM undecylenyl l-leucine and 50 mM NaHCO₃ solutions contained micelles approximately 20 Å in diameter and that micelle radius and the mole fraction of surfactant molecules associated with micelles changed very little with solution pH. The binding of the amino acids arginine and lysine to the anionic micelles was also investigated from pH 7.0 to 11.5. Below pH 9.0, the mole fraction of arginine cations bound to the micelles was approximately 0.4. Above pH 9.0, the arginine counterions became zwitterionic, and the mole fraction of bound arginine molecules decreased steadily to less than 0.1 at pH 11. When arginine dissociated from the micelles, their radii decreased from 14 to 10 Å. Similar behavior was observed with lysine; however, when lysine dissociated from the micelle surface, little change in micelle radius was observed. Two-dimensional NMR experiments suggested that below pH 9.0, l-arginine bound perpendicular to the micelle surface primarily though its side chain amine while l-lysine bound parallel to the surface through both of its amine functional groups. Finally, the rate at which the amide protons on the surfactant headgoup exchanged with solvent was investigated with NMR spectroscopy. The exchange reaction was faster in solutions containing only surfactant monomers and slower when the surfactants were in micellar form and the headgoup amide protons were less exposed to solvent.     

Effect of tartrazine dye on micellisation of cationic surfactants: conductometric,spectrophotometric, and tensiometric studies

The interactions between anionic dye (tartrazine) and cationic surfactants (dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide) have been studied by conductometric, spectrophotometric, and tensiometric techniques. The conductance and surface tension of dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide in pure water as well as in aqueous tartrazine when plotted with surfactant concentration gave values of the critical micelle concentration at different temperatures. As well as increasing the length of the carbon chain of surfactants, the presence of tartrazine reduces the critical micelle concentration. From specific conductivity data, the counterion dissociation constant, standard free energy, enthalpy, entropy of micellisation, surface excess concentration, surface tension at critical micelle concentration, minimum area per molecule, surface pressure at critical micelle concentration, and Gibbs energy of adsorption were evaluated. Spectroscopic studies reveal that the binding of dye to micelles brings a bathochromic shift in dye absorption spectra that indicates dye–surfactant interaction.     

Solubilisation of dyes by surfactant micelles. Part 4: Influence of dye fixatives

Dye loss from unfixed dyed fabrics has been found to be insensitive to change in surfactant type or concentration. There was accompanying dye transfer to white fabric but this was reduced by Synperonic A7 in the case of fabrics dyed with CI Direct Green 26, due to solubilisation of the dye in nonionic micelles. The anionic surfactant, SDS, selectively displaced dye from fixed dyed fabrics, paralleling its behaviour with water soluble polymers. Similarly, dye loss was related to concentration of surfactant monomer, the effect increasing with SDS concentration up to its critical micelle concentration. Other anionic surfactants have been found to exhibit a similar trend, the effect increasing with their increasing surface activity. The commercial polymeric fixatives, Tinofix ECO and Indosol E50, were the most effective of those studied and the single-chain cationic surfactant, CTAB, was the least effective.     

Interactions between dyes and surfactants in inkjet ink used for textiles

Optimal preparation of inkjet ink should be possible through the elucidation of the relationship between dye/additive interactions and ink performance. In the present study, the interactions between the dyes and surfactant additives were investigated. To investigate the physical properties of the surfactants used, the critical micelle concentration (cmc) and the aggregation number (N) were determined using electron spin resonance, static light-scattering, and fluorescence spectroscopy. On the basis of the cmc and N values, the visible absorption spectra of aqueous acid dye solutions (C. I. Acid Red 88, 13, and 27) containing surfactants (i.e., Surfynol 465 (S465), octaethylene glycol monododecyl ether (OGDE), and sodium dodecyl sulfate (SDS)) were measured. From the dependence of the spectra on the surfactant concentration, the binding constants, K(bind), of the acid dyes with the surfactant micelles were calculated: the K(bind) values decreased in the order of C. I. Acid Red 88 > C. I. Acid Red 13 > C. I. Acid Red 27, which correlates with the number of sulfonate groups. For all the dyes, the K(bind) values with the nonionic surfactants, S465 and OGDE, were much larger than those with the anionic surfactant, SDS. The thermodynamic parameters of the binding, i.e., the enthalpy change, ΔH(bind), and entropy change, ΔS(bind), were determined via the temperature dependence of the binding constants. The positive ΔH(bind) value for S465 indicates an endothermic binding process, while the negative ΔH(bind) values for SDS and OGDE indicate exothermic binding processes.     

Mixtures of anionic and cationic surfactants with single and twin head groups: Solubilization and adsolubilization of styrene and ethylcyclohexane

Mixtures of anionic and cationic surfactants with single and twin head groups were used to solubilized styrene and ethylcyclohexane into mixed micelles and adsolubilize them into mixed admicelles on silica and alumina surfaces. Two combinations of anionic and cationic surfactants were studied: (i) a single-head anionic surfactant, sodium dodecyl sulfate (SDS), with a twin-head cationic surfactant, pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD), and (ii) a twin-head anionic surfactant, sodium hexadecyl-diphenyloxide disulfonate (SHDPDS), with a single-head cationic surfactant, dodecylpyridinium chloride (DPCl). Mixtures of SDS/PODD showed solubilization synergism (increased oil solubilization capacity) when mixed at a molar ratio of 1∶3; however, the SHD-PDS/DPCl mixture at a ratio of 3∶1 did not show solubilization enhancement over SHDPDS alone. Adsolubilization studies of SDS/PODD (enriched in PODD) adsorbed on negatively charged silica and SHDPDS/DPCl adsorbed on positively charged alumina showed that while mixtures of anionic and cationic surfactants had little effect on the adsolubilization of styrene, the adsolubilization of ethylcyclohexane was greater in mixed SHPDS/DPCl systems than for SHDPDS alone. Finally, it was concluded that whereas mixing anionic and cationic surfactants with single and double head groups can improve the solubilization capacity of micelles or admicelles, the magnitude of the solubilization enhancement depends on the molecular structure of the surfactant and the ratio of anionic surfactant to cationic surfactant in the micelle or admicelle.     

Adsorption of Anionic–Cationic Surfactant Mixtures on Metal oxide Surfaces

This research evaluates the adsorption of anionic and cationic surfactant mixtures on charged metal oxide surfaces (i.e., alumina and silica). For an anionic-rich surfactant mixture below the CMC, the adsorption of anionic surfactant was found to substantially increase with the addition of low mole fractions of cationic surfactant. Two anionic surfactants (sodium dodecyl sulfate and sodium dihexyl sulfosuccinate) and two cationic surfactants (dodecyl pyridinium chloride and benzethonium chloride) were studied to evaluate the effect of surfactant tail branching. While cationic surfactants were observed to co-adsorb with anionic surfactants onto positively charged surfaces, the plateau level of anionic surfactant adsorption (i.e., at or above the CMC) did not change significantly for anionic–cationic surfactant mixtures. At the same time, the adsorption of anionic surfactants onto alumina was dramatically reduced when present in cationic-rich micelles and the adsorption of cationic surfactants on silica was substantially reduced in the presence of anionic-rich micelles. This demonstrates that mixed micelle formation can effectively reduce the activity of the highly adsorbing surfactant and thus inhibit the adsorption of the surfactant, especially when the highly adsorbing surfactant is present at a low mole fraction in the mixed surfactant system. Thus surfactant adsorption can be either enhanced or inhibited using mixed anionic–cationic surfactant systems by varying the concentration and composition.

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Synergisms in Binary Mixtures of Anionic and pH‐Insensitive Zwitterionic Surfactants and Their Precipitation Behavior with Calcium Ions

This work aims to investigate synergy in anionic and zwitterionic surfactant mixtures, as they result in better interfacial properties and micellization behavior. Various mixtures of the pH‐insensitive zwitterionic surfactant 3‐(decyldimethylammonio) propanesulfonate (Zwittergent 3–10) and sodium dodecylsulfate (SDS) were prepared in aqueous solution at a range of pH values between 2 and 13. The thermodynamic parameters during mixed surfactant adsorption at the air/water interface are obtained and the results show the mixed surfactant systems having superior properties to the constituent surfactants. Experimentally, the mixed surfactant solutions clearly improve the surface activities by lowering the critical micelle concentration (CMC) and lowering the surface tension at the air/water interface. The synergisms are investigated through the interaction parameters estimated from regular solution theory that is used to quantitatively describe the nonideality of surfactant mixtures. High negative interaction parameters are obtained from these surfactant mixtures. Experimental precipitation phase boundaries of SDS in the presence of CaCl₂ were also investigated in mixtures containing pH‐insensitive zwitterionic surfactant at different pH levels from 2 to 13 and SDS mole fractions of 0.25, 0.50, 0.75, and 1.00. Changes in the precipitation phase boundaries are due to the changes in the speciation or activities of the major components both below and above the CMC. As a result, the precipitation phase boundaries are pH dependent. In addition, mixed micellization and counterion binding to the micelle also change the precipitation phase boundary above the CMC. The activity‐based solubility product of calcium dodecylsulfate is also determined from the precipitation phase boundaries below the CMC. X‐ray diffraction patterns and SEM images confirm that only calcium dodecylsulfate precipitates in the soap scum for all pH and surfactant compositions studied.     

Natural dye–surfactant interactions: thermodynamic and surface parameters

The interaction of two Indian natural dyes, namely madder (Rubia cordifolia) and mallow (Punica granatum), with cationic surfactant cetyl trimethyl ammonium bromide and anionic surfactant sodium lauryl sulphate, has been studied. Spectrophotometric data showed a strong interaction between the natural dyes and the surfactants. The critical micelle concentration of the surfactants, determined by measurement of specific conductance and surface tension, was found to decrease on the addition of natural dyes in an aqueous solution of surfactants. The thermodynamic and surface parameters for the interaction have been evaluated.     

Photophysics of the phenoxazine dyes resazurin and resorufin in direct and reverse micelles

The photophysics of the phenoxazin-3-one dyes resazurin and resorufin was studied in a micellar solution of cetyltrimethylammonium chloride and in reverse micelles of 1,4-bis(2-ethylhexyl)sulfosuccinate and benzylhexadecyldimethylammonium chloride. Absorption and fluorescence emission spectra, as well as fluorescence lifetimes and T–T transient absorption spectra were determined as a function of surfactant concentration. In the presence of direct micelles of cetyltrimethylammonium chloride, both dyes displayed red shifts in the absorption and fluorescence spectra together with a simultaneous fluorescence lifetime increase. The electrostatic attraction between the anionic dyes and the positive micellar interface favors the location of the dyes closer to the head groups of the surfactant molecules. In reverse micellar systems the spectral properties depended upon the charge of the surfactant and water content. In the case of 1,4-bis(2-ethylhexyl)sulfosuccinate, at low water content both dyes were incorporated into the interface; as the water content increased their spectral properties tended to those in pure water. In contrast, in the case of cationic surfactant, the dyes were located in the interfacial pseudophase as a result of electrostatic interactions.     

Solubilisation study of water‐insoluble dye in cationic single/dimeric surfactant micelles: effect of headgroup,non‐polar tail,and spacer chain in aqueous and salt solution

The solubilisation of hydrophobic azo dye Orange OT in aqueous/salt solution in several cationic surfactant micelles was studied using UV‐vis spectroscopy. An attempt was made to correlate dye solubilising strength with adsorption/micellar characteristics. In our experiments we determined the change in solubilisation of hydrophobic dye when added to an aqueous solution of oppositely charged quaternary‐salt‐based cationic surfactants (conventional and gemini) and remarked on the probable location of the solubilised dye in the surfactant micelle. Results highlight the onset of dye solubilisation around the critical micelle concentration of each surfactant, which is influenced by the non‐polar tail, spacer, and polar headgroup, while no dye could be solubilised at concentrations below the critical micelle concentration. Orange OT solubilised almost linearly with increase in surfactant concentration at and above the critical micelle concentration. The change in colour intensity of the dye (darker below the critical micelle concentration, lighter at and above the critical micelle concentration) could be attributed to dye–surfactant interactions. Further dye solubilisation was observed in the presence of salt.     

Surface active betaines as protective agents against denaturation of an enzyme by alkyl sulfate detergents

A homologous series of higher alkyl sulfate surfactants inactivate β-fructofuranosidase (invertase) at levels coinciding with their critical micelle concentrations. It was possible to renature the enzyme by passing it through an anion exchange column. This inactivation was prevented by surface active betaines present at equimolar or higher concentrations than those of the anionics. Effective surfactant betaines include those with carboxylate, sulfonate, or phosphate radicals in their zwitterions. Betaines lacking surface active properties did not prevent denaturation indicating that the effects are due to comicellization. Studies with enzymes may point to appropriate anionic/zwitterionic surfactant ratios in solubilization procedures or detergent applications where biological properties must be preserved and anionic surfactants are required as components.     

Kinetics of the self-assembly of gemini surfactants

A stopped-flow technique combined with pulsed-field-gradient spin-echo nuclear magnetic resonance (NMR) measurements was used to study the kinetics of exchange, size, and shapes in micellar systems of cationic surfactant dimers of the alkanediyl-α-ω-bis(dodecyldimethylammonium bromide) type, with alkanediyl being 1,2-ethylene, 1,3-propylene, and 1,4-butylene. By measuring the slow relaxation time for micelles, τ₂, the micelle lifetime as a function of spacer length was obtained and was further confirmed by micelle exchange measurements by NMR diffusometry. The micelle lifetimes for the gemini surfactants were found to be in orders of magnitude longer than for the corresponding conventional surfactants. All three cationic surfactant dimers showed an increase in micelle size in one direction, i.e., became prolates, as the concentration was increased. The growth of the micelles was most pronounced for the gemini surfactants with the shortest linker unit, i.e., ethylene.     

Adsorption of cationic/nonionic surfactant mixtures on polyester

Experiments were performed to characterize the adsorption of the cationic surfactant benzalkonium chloride (BZK) on polyester as well as measure the effect of the cationic surfactant on polyester surface charge. Additional studies were performed to examine the effect of adding nonionic surfactants on surface charge. In studies of adsorption of BZK on polyester, different behaviors were observed at pH values 6 and 10, with adsorption reaching a maximum at pH 10 but not at pH 6. In probing the zeta potential and isoelectric point (IEP) of polyester exposed to solutions composed of BZK (cationic surfactant) and an ethoxylated alcohol (nonionic surfactant), it was seen that the IEP could be shifted to higher pH levels by increasing the mole fraction of nonionic surfactant in a cationic/nonionic surfactant solution. A maximum in the IEP was obtained at a certain mole fraction for most cases. The shift in the IEP was hypothesized to be driven by increased deposition of the cationic, since the nonionic itself did not significantly change the IEP. The cooperative interactions between cationic and nonionic species were theorized to be driven not so much by attractive interactions, but other interactions, such as minimization of cationic charge repulsion.