Jože Špan

A study of dye-surfactant interactions. Part 1. Effect of chemical structure of acid dyes and surfactants on the complex formation

Abstract The study of the interactions between anionic azo dyes C.I. Acid Orange 7 (AO7) and C.I. Acid Red 88 (AR88), with different surfactants, i.e., the cationic surfactants dodecylpyridinium chloride (DPC) and cetylpyridinium chloride (CPC) and the anionic surfactant sodium dodecylsulphate (SDS), in submicellar concentration ranges has been researched by a potentiometric technique using a surfactant-cation-sensitive membrane electrode. The e.m.f. measurements versus surfactant concentration have been performed for the systems CPC-AO7, DPC-AO7, DPC-AR88 and SDS-AO7 in pure water at 25°C. Experimental measurements give the option to determine the concentration of free and of bound surfactant cations at a given stoichiometric concentration and calculation of the dye-surfactant complex formation constant, K, and the standard free enthalpy change, ΔG°. The results show that under the same experimental conditios the complex formation between CPC and AO7 takes place at much lower surfactant concentration than between DPC and AO7, accompanied by much higher values of K. It can be seen from the shapes of the binding isotherms that the binding of DPC to AO7 is most likely cooperative. The results indicate that in the DPC-AR88 system, where more hydrophobic dye AR88 is used, the dye-surfactant interactions are stronger compared to DPC-AO7 system. Such observations enable the conclusion, to be made that, beside the electrostatic attractive interactions, the noncoulonic interactions are also very important for the formation of the complex between the oppositely-charged dyes and surfactants. The study of the interactions between the same-charged dye AO7 and surfactant SDS shows that they do not interact with one another in the whole measured concentration range. The most logical explanation for this is that the repulsive electrostatic interactions prevent the formation of the complex between the same charged AO7 and SDS.

Anionic dye–cationic surfactant interactions in water–ethanol mixed solvent

Abstract The interactions between an anionic dye and two cationic surfactants were studied by conductometry. The specific conductance of dye–surfactant mixtures was measured at three different temperatures in water–ethanol mixed solvent, containing 5, 10, 15 or 20 wt.% of ethanol. The equilibrium constants and other thermodynamic functions for the process of dye–surfactant ion pair formation were calculated on the basis of two theoretical models. The results showed that the presence of ethanol decreases the tendency for ion pair formation. According to the results, long range as well as short range interactions are responsible for the formation of the ion pair. The importance of long range electrical forces is basically to bring the dye anion and the surfactant cation close enough to enable the action of short range interactions whose contribution represents the major part of the standard free enthalpy change for the formation of the anionic dye–cationic surfactant ion pair.

A study of dye–surfactant interactions. Part 3. Thermodynamics of the association of C.I. Acid Orange 7 and cetylpyridinium chloride in aqueous solutions

Abstract The association of C.I. Acid Orange 7 (D) and the surfactant cetylpyridinium chloride (S) was studied at 15, 25, 35 and 45°C in the presence of 0, 0.05 and 0.1 mol/kg NaCl, by potentiometric titration, using a surfactant cation sensitive membrane electrode. By using the association constant (K1) for the first step of the association [D−+S+ ↔(DS)°], the standard free energy change, standard enthalpy change, and standard entropy change of the association were calculated at low surfactant concentrations. The unitary entropy of association was negative, suggesting that hydrophobic interactions do not play a major role in the initial interaction between dye and surfactant. On the other hand, in the second step [S++(DS)° ↔(DS2)+], the unitary entropy change in the medium-level surfactant concentration domain was highly positive, indicating that water-structure contributions are appreciable in the stabilisation of the (DS2)+ species. It was shown that a further stepwise association is not very likely in the third domain, and that a neutral quadruple species of type (SD)2° may be present in the solution. Further increases in surfactant concentration led to a very steep increase in bound surfactant as the concentration of free surfactant ions approached the c.m.c. point. It was also shown that the addition of a simple electrolyte (e.g. NaCl) decreased K1, mainly due to interference with the interaction of dye and surfactant ions. Since long range Coulombic interactions are absent in the second association step, the increase in K2 with increasing NaCl concentration can be mainly attributed to short range hydrophobic interactions.

Conductometric investigation of dye–surfactant ion pair formation in aqueous solution

Abstract The formation of a dye–surfactant ion pair was studied using a conductometric method in which the conductance of aqueous solutions of C.I. Acid Orange 7 was measured in the presence of the cationic surfactants dodecylpyridinium chloride or hexadecylpyridinium chloride at four different temperatures. Two theoretical models to calculate the relevant equilibrium constants were derived. Both methods of calculation led to similar results that were in good agreement with other methods of investigation of dye–surfactant association. The results have shown that an increase in temperature lowers the tendency for ion pair formation as the equilibrium constants decrease with increasing temperature. A comparison of the behaviour of both surfactants revealed that the surfactant which contained the longer hydrophobic chain had a stronger tendency to associate with the dye and the corresponding equilibrium constants were considerably higher than those recorded for the surfactant with the shorter aliphatic chain. Such findings indicate that not only long range electrostatic forces but also short range, non-electrostatic interactions have a significant influence on dye–surfactant ion pair formation.

Solvation of β-lactoglobulin and cyhmotrypsinogen A in aqueous urea solutions

Abstract The preferential interaction of the solvent components with β-lactoglobulin and chymotrypsinogen A, respectively, was determined in aqueous urea solutions by means of equilibrium dialysis and differential refractometry. It has been found that at all concentration studied urea was preferentially bound. Using a modified equilibrium dialysis technique the total urea binding to the two proteins was also obtained. Total urea binding increases with increasing urea concentration. In 8 M urea where the two proteins are completely unfolded there is about one urea molecule per three amino acid residues. The results obtained strongly suggest that the extent of urea interaction with protein is the decisive factor in bringing about the unfolding of protein molecules.

A study of the self-association of simple azo dyes using the potentiometric method

Abstract The thermodynamic aspects of the self-association of three monoazo dyes, viz. C.I. Acid Red 88, C.I. Acid Orange 7 and C.I. Acid Orange 8, in aqueous solution have been studied by means of a potentiometric method. An ion-selective membrane electrode, selective to a dye onion, has been constructed. This electrode permits a direct determination of the dye monomer concentration. Measurements of e.m.f. versus dye concentration were carried out and the results show that the e.m.f. response is not linear over the whole measured concentration range. Because of dye association at higher concentrations the measured e.m.f. values start to deviate from the theoretical linear relationship. From these deviations the dye monomer concentration at a given total dye concentration was determined. Using an appropriate stepwise association model the dimerization constants and concentrations of the dimers and higher multimers were calculated. It is concluded that C.I. Acid Red 88 aggregates more readily than C.I. Acid Orange 7. C.I. Acid Orange 8, the structure of which includes a charged sulphonic acid group in the central part of the molecule, does not aggregate in the measured concentration range. The effects of temperature and added salt on the association of C.I. Acid Red 88 have also been investigated. From the results, it can be seen that the association of C.I. Acid Red 88 decreases with increasing temperature, and strongly increases with increasing salt concentration. Thermodynamic functions of dimerization of C.I. Acid Red 88 in aqueous solutions at different temperatures were also determined. It appears that the dimerization of C.I. Acid Red 88 in aqueous solution is governed mainly by dispersive interactions between the dye molecules.

Osmotic coefficients of C.I. Acid Orange 7 in aqueous solution and in the presence of simple electrolyte

The aggregation of a monoazo acid dye was studied with osmometry. The osmotic coefficients of the dye were measured in aqueous solution and in 0.01 M potassium chloride. The mean aggregation numbers were calculated on the basis of the spherical cell model of dye in the solution. The results showed that the aggregation process is more distinctive in the presence of a simple electrolyte, probably because of the screening effect of the salt ions.

Electrical transport in polystyrenesulfonate solutions

SummaryElectric conductance and transport numbers of polyions were measured in aqueous polystyrenesulfonate solutions having different counterions, at concentrations from 2×10−3 to ∼ 10−1 basemol dm−3. From experimental data the extent of atmospheric binding of small ions to polyions was derived. The resulting fraction of free counterions was found to agree within ±5% with the coefficient of self diffusion calculated according to the theory of Jackson, Lifson, and Coriell. The molar conductance and the effective charge of the polyion were applied for the calculation of the hydrodynamic friction coefficient related to a monomer unit of the polyion.

Acid red 88 dye anion-selective electrode

Abstract An ion-selective membrane electrode selective to the Acid Red 88 (C.I. 15620) dye anion has been constructed. This electrode, with the Acid Red 88 dye anion-cetylpyridinium cation carrier complex incorporated within a poly(vinyl chloride) gel membrane, permits direct determination of the dye monomer concentration. In this work its fabrication, general performance and selectivity are described. The selectivities have been obtained by a mixed solution method for the Acid Orange 7 (C.I. 15510), Acid Red 13 (C.I. 16045) and Acid Blue 25 (C.I. 62055) interfering onions. The selectivity coefficients suggest that the dye electrode is not sensitive to interfering dye anions of similar structure and different charge or size.