Thermodynamic aspects of polymer–surfactant interactions: Gemini (16-5-16)-PVP-water system

2.1 Materials and instruments

The cationic gemini surfactant with a five-methylene spacer group (16-5-16), was synthesized in our laboratory, according to usual method (De et al., 1996). For synthesis, 1,5-dibromopentane (⩾98%, Fluka, Switzerland) and N,Ndimethylhexadecylamine (⩾95%, Fluka, Switzerland) were used without further purification. The investigated polyvinylpyrrolidone, PVP K30 (M.W. ∼40,000, 99.8% Fluka, Switzerland) used without further purification.

2.2 Synthesis of gemini surfactant

The molar ratio was 2.1:1 and the mixture was refluxed in ethanol for 48 h. The solvent was removed and then the raw material was recrystallized in ethanol–ethyl acetate mixtures. The crystallization was normally repeated four times. The cationic gemini of hexadecyl series with methyl spacer –(CH₂)₅– was prepared according to Scheme 1. All products were checked by ¹H NMR spectroscopy using CDCl₃ as solvent. The overall yield was 70–80%.

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Scheme 1

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2.3 Conductivity measurements

Jenway (model 4510) bridge was employed to perform the conductivity measurements. The experiments were carried out by adding progressively concentrated surfactant stock solution (pure gemini or gemini + PVP) into the thermostated solvent (dimineralized double-distilled water) at temperature ranges from 30 to 45 °C. The critical micellar concentration of the pure surfactant used was obtained from the plots of specific conductivity κ as a function of the surfactant concentration. The cmc values were taken from the intersection of the two straight lines. As in case of the 16-5-16 + PVP mixtures the plots of κ versus [surfactant] showed two breaks, the cac was determined by the intersection of first and second linear parts and the cmc in this case was the intersection point of the second and third linear parts.

3.1 Critical aggregation concentration (cac) and critical micelle concentration (cmc)

It is clear from the above figure (Fig. 2) that for gemini surfactant, the specific conductivity–concentration plots give a sharp change from the pre-micellar to the post-micellar regions at all temperatures studied. However for gemini and polyvinylpyrrolidone (PVP) systems there are two break points in conductivity–concentration profile. In case of pure surfactant, when we increase the surfactant concentration in solution then the activity of surfactant reaches a constant “monomer solubility” limit called cmc. If the total surfactant concentration exceeds the cmc, the monomer activity remains almost constant and the excess amount of surfactant separates into the micellar phase. When ionic surfactants interact with neutral polymers as surfactant aggregates, that is, the surfactant interaction is cooperative similar to the micelle formation. Because this cooperative nature of surfactant/polymer interaction is characterized by critical interaction concentration of the surfactant, it is called the critical aggregation concentration (cac). There is no interaction between surfactant and polymer below the cac. As the surfactant concentration exceeds the cac, the surfactant starts to bind the polymers. As the total surfactant concentration increases, the bound amount rapidly increases, which is accompanied with a slow increase of equilibrium surfactant activity. At the cmc, the polymer may be saturated with surfactant; however, the binding process is not necessarily completed at this surfactant activity. The cac is not a monomer solubility limit, because with increasing total surfactant concentration the equilibrium monomer concentration rises from the cac to the cmc. This means that with decreasing the number of available polymer binding sites, the concentration of the surfactant monomer must increase.

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Figure 2

Plots of specific conductivity, κ of 16-5-16 in the absence of PVP (●) and in the presence (0.03 w/v%) of PVP (■) versus total surfactant concentration at 30 °C.

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The cac and cmc values of pure gemini as well as with PVP are summarized in following table (Table 1).

Data represented in this table show that the cac/cmc values increase with increasing temperature. At higher temperature, the dielectric constant of solution decreases resulting in greater repulsion among the ionic head groups of surfactant molecules, which leads to increased cmc values. It is also clear from table 1 that the values of cac increase as the concentration of PVP increases, because of the amount of polymer increase, there is increase in the binding sites available to the surfactant monomer or micelle-like aggregates. Therefore, more amount of surfactant is required to bind to the polymer. After the total binding sites are occupied, the surfactant monomers become free, to form micelles.

3.2 Degree of counterion binding of micelles (β)

The specific binding of counterions to the micelles is a prerequisite for an understanding not only of micellization but also of all kinds of aggregation in aqueous solutions. The electrical conductivity of gemini + PVP aqueous solution is measured to study the aggregation behavior. The degree of micelle ionization was calculated by taking the ratio between slopes of the linear portion above and below the break point in the conductivity profiles. Hence two values, i.e., α₁ and α₂ were obtained in our systems (Table 1). It is clear from the table that the values of α₂ are larger; indicate increased degree of dissociation as a result of the interaction of surfactant with polymer.

3.3 Thermodynamic of micellization

The thermodynamic parameters of micellization, Gibbs energy of aggregation and Gibbs energy of micellization can be evaluated by using the values of cac and cmc at various temperatures (Zana, 1996). The Gibbs energy of aggregation and Gibbs energy of micellization can be calculated from the pseudophase separation model for ionic surfactants according to the following equations

The standard energy of micellization of gemini surfactant in the presence of PVP was calculated at temperature range 303–318 K (Table 2). Apparently the standard free energy change of micellization of gemini surfactant in the absence and presence of PVP has negative values at all temperatures, showing that the process of micellization is a spontaneous process. The energy of transfer, ΔG_tran, associated with binary interaction between surfactant and polymer is given by

The standard enthalpy change for micelle aggregation process, ΔH_agg, can be determined by Gibbs–Helmholtz Eqs. (4) and (5) for aqueous solution (Fig. 3),

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Figure 3

Variation of ln X_cac with temperature: at 0.02 w/v% PVP.

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The values of standard enthalpy change for micellization are negative, inferring that the micelle formation process is exothermic. The entropy changed is positive in all cases. The positive values of $\mathrm{\Delta }{S}_{\text{agg}}^{\circ }$ are due to the melting of “flickering cluster” around the hydrocarbon tails of the surfactant and the increased randomness of hydrocarbon chains in the micellar core. The lower values of $\mathrm{\Delta }{S}_{\text{agg}}^{\circ }$ indicate that the restriction of free motion for micelle.