1. INTRODUCTION:

Polymer-ligand interaction studies are of fundamental as well as theoretical interest (Kim et al., 2003; Seguel et al. 2005; Rivas & Novas, 2005; Grubbs, 2007, Shen, 2011). Several divergent views exist regarding the nature of surfactant-polymer interaction. While Murata and Arai(I973) inferred that at low concentrations, a surfactant (e.g. sodium dodecyl sulphate, SDS) adsorbs at polymer (PVP) coils, Fishman and Eirich(1971), on the contrary, claimed that there is a little evidence of SDS adsorption at the PVP coils. Further, whereas, Rudd and Jennings(1974) postulated that polymer (PVP) molecules undergo no conformational change while interacting with the surfactant (SDS) molecules, Fishman and Eirich(1971) proposed that the SDS+PVP system can be characterized by PVP-SDS micelles of complex poly-electrolyte nature. To reach to a definite realistic conclusion regarding the mechanism of interaction in surfactant-polymer systems, a systematic study of their physico-chemical properties is needed. We present here the results on the binding of an anionic surfactant, sodium dodecylbenzene sulfonate (SDBS) and a cationic surfactant, cetyltrimethylammonium bromide (CTAB) with poly(N-vinyl-2-pyrrolidone (PVP) and bovine serum albumin (BSA) usung equilibrium dialysis method.

2. MATERIALS AND METHODS:

2.1. Materials

Sodium dodecylbenzene sulfonate (LR.grade; BDH), cetyltrimethyl ammonium bromide (LR grade;Wilson Laboratories), poly-(N-vinyl-2-pyrrolidone) (mol. wt. 3,60,000; Sigma) and bovine serum albumin (L.R.,CDH) were used as such. Double distilled water (specific conductivity: 0.5x10^-5 S.cm^-1) was used for preparing aqueous surfactant-polymer solutions.

2.2. Methods

2.2.1 Equilibrium Dialysis

Dialysis tube, initially kept in boiling-water for 15 minutes, was rinsed thoroughly with distilled water. Five ml aqueous polymer solution of known concentration taken in the dialysis tube, was immersed in a 20 ml surfactant solution of desired concentration taken in an outer glass tube. At least ten such sets were prepared covering the surfactant concentrations both on the lower as well as higher sides of its critical micelle concentration (CMC). The glass tubes, sealed at the top, were shaken for 48 hrs in a water thermostat (308.15 ± 0.01 K). After the equilibrium was reached, the solutions present both at the inside as well as at the outside of the dialysis tube were analyzed for their surfactant concentration, spectro-photometrically, using the following procedure-

One ml 0.1 % methylene blue dye (in 1 % aqueous Na₂SO₄ solution) was mixed with 10 ml chloroform and 0.1 ml surfactant solution. The dye-surfactant complex extracted into the chloroform layer was diluted 10 times using chloroform and its absorbance was measured at λ_max= 650nm.

3. RESULTS AND DISCUSSION:

3.1. Equilibrium dialysis:

Surfactant-polymer binding ratio (r), defined as the moles of surfactant bound per mole of polymer-monomer, was obtained using the relation-

r = ( [S]_in - [S]_out) / P -------------------- (3.1)

where [S]_in and [S]_out) represent the surfactant molar concentration at the inside and at the outside of the dialysis tube, respectively, at the equilibrium state; P = molar concentration of polymer-monomer.

Plot of binding ratio (r) as a function of surfactant concentration at equilibrium for SDBS+PVP(1%)+H₂O system is presented in Fig. 1., During SDBS-PVP interaction, three distinct regions of surfactant concentrations can be marked-

1) Below a critical surfactant concentration, T_1,(<10^-3M) rate of binding of surfactant with the polymer (PVP) is low, indicating a weak inter-molecular interaction between surfactant and polymer molecules, in this region. It may be because in this region of low surfactant concentration, due to the unfolded structure of the polymer, the number of available binding sites on PVP is only limited.

2) In the surfactant concentration range between T₁ (3.0x10^-3M) to T₂ (5.0x10^-3M), there is a rapid increase in the binding ratio (r) suggesting a co-operative binding of surfactant molecules with the polymer (PVP). In this region, already polymer-bound surfactant molecules interact, co-operatively, with the fresh incumbent through hydrophobic-hydrophobic interactions between their hydrocarbon chains. Furter, when ligand-polymer binding is in progress, the unfolding of the polymer (PVP) molecules occurs, thus, exposing more and more binding sites at the polymer molecular surface, thereby, enhancing the rate of binding of surfactant with the polymer.

3) In the surfactant concentration region from T₂ onwards (> 5.0x10^-3M) the binding ratio (r) becomes independent of surfactant concentration indicating that in this region, further binding of surfactant with the polymer ceases due to the exhaustion of the active binding sites at polymer molecules. In fact, in this region, since the binding sites on the polymer are exhausted, on further increasing surfactant concentration, merely the usual micelles may be formed.

A comparison of the plots of binding ratio (r) versus [Surfactant] for the systems: SDBS + PVP (1 %) + H₂O (Fig. 1) and SDBS + BSA (0.6%) + H₂O (Fig.2) revealed that-

a) At low surfactant concentrations (< 3.0x10^-3 M), the rate of binding of the surfactant (SDBS) molecules with PVP is lower compared to its rate of binding with the BSA. It may be due to a slower unfolding rate of PVP molecular chain, during surfactant-polymer interaction, in comparison to BSA. Upon unfolding of a macromolecule fresh binding sites are exposed for further interaction with the surfactant molecules.

At higher concentration of the surfactant (>5.0x10^-3M), a saturation stage in surfactant –macromolecule binding is reached in case of SDBS+PVP+H₂O system. On the contrary, no such saturation stage is achieved in case of SDBS+BSA+H₂O system. In fact, in the later system, after attaining maxima in the plot a decrease in the binding ratio is observed at higher surfactant concentration (Fig. 2).

This may be due to the partial desorption of previously BSA-bound surfactant (SDBS) molecules owing to their collision against the free micelles in the bulk. This also suggests that the surfactant molecules bound to BSA molecules remain barely exposed in the bulk solution and are thus prone to their collision with free micelles. From this it can be inferred that the SDBS-BSA polyelectrolyte complex has a structure in which the BSA molecule acts as a nucleating agent for the interacting surfactant molecules, in accordance with a model, earlier, proposed by Fishman and Eirich (1971). In case of SDBS + PVP + H₂O system, however, the binding ratio, above 5.0x10-3 M SDBS concentration becomes independent of surfactant concentration. This can be explained if one assumes for the SDBS-PVP polyelectrolyte complex a structure where polymer molecule wraps around the bound pre-micellar aggregate (Cabane, 1977). This would avoid direct impact of free micelles with the previously Polymer-bound surfactant monomers, hence, no desorption of already bound surfactant molecules.

b) From the binding ratio (r) versus surfactant concentration plots (Fig. 2.), it can be seen that the interaction of cationic surfactant (CTAB) with BSA is much weaker than of the anionic surfactant (SDBS). Observed higher binding ratio (r) for SDBS-BSA system may be due to the strong electrostatic attraction between the negatively charged SDBS head group and positively charged binding sites (e.g. protonated amino residual groups) at BSA molecule. If such electrostatic interaction were the only cause for the binding of a surfactant with the macromolecule such as BSA then the cationic surfactant (CTAB), with positively charged head group, would not bind with BSA. The observed binding of CTAB with the BSA, though of small magnitude, may be due the hydrophobic-hydrophobic interaction between the hydrocarbon chain of CTAB and the non-polar polypeptide chain of BSA molecule.

The binding data for the studied surfactant-polymer systems have been examined in the light of following models for ligand-polymer interaction:

3.2 Identical and independent site model

This model assumes the existence of a number of binding sites per monomer of polymer molecule with each site having similar affinity for the binding ligand (e.g. surfactant) (Tinoco et al.1978). Besides it, a ligand occupying a binding site is considered not to influence the binding of ligand molecule to other sites. Assuming that the surfactant pre-micellar aggregates bind to the polymer, the formation of such 'associate' is represented by:

n.S Û S_n ............................ (3.2.1)

Where 'n' is the number of ligand-monomers in each premicellar aggregate and 'S' represents the. ligand (e.g. surfactant) monomer. The equilibrium constant, K, for the above process is given by:

K = [S_n] / [S]ⁿ ............................ (3.2.2)

Now, if a polymer molecule contains 'p' binding sites per monomer unit, and 'r' is the binding ratio of bound entity (S), then the Scatchard equation is given by:

l/r = 1/p + 1/{p.C.[S]ⁿ} ............................ (3.2.3)

Where C is a constant and [S] = free surfactant concentration at equilibrium state of ligand-polymer interaction.

It was found that the plot of 1/r versus l/[S]ⁿ, for n = 1, is linear. This suggests that SDBS molecules bind to the polymer as monomers. The intercept value in the plot of l/r versus l/[SDBS] for the system SDBS + PVP(1%) + H₂O yielded p = 0.077, which implies that when the ligand (SDBS) molecules binds with the PVP, nearly thirteen monomer units of PVP molecule constitute one binding site. Further, since the PVP molecule, having molecular mass 3,60,000 and monomer-unit' mass 111, contains about 3240 monomer units, the observed value of p=0.077 in the present analysis also suggests that PVP contains nearly 250 binding sites per PVP molecule.

3.3. Co-operative binding model / Hill Isotherm

This model assumes that the ligand-polymer binding involves the simultaneous attachment of two or more ligand molecules onto a corresponding number of binding sites which may or may not be located adjacent to each other on the polymer molecule (Hill 1910)

The semi-empirical equation for this model is given as:

Log {q/( 1-q)} = h_H. log C_F + log K_H .............. (3.3.1)

Where q = fractional saturation of binding sites on each polymer molecule represented by: q = r/s, where 'r' is the binding ratio, defined earlier, at any free surfactant concentration and s = binding ratio at full saturation; h_H = Hill coefficient; K_H = apparent binding constant and C_F= free ligand (surfactant) concentration.

Upon substituting, q = r/s, equation :(3.3.1) becomes:

Log [r/(s-r)] = h_H log C_F + log K_H ................. (3.3.2)

A plot of log (r/(s-r)) versus log C_F is found to be linear. The slope of this plot gave the value for h_H = 2.79; since h_H > 1, it suggests that SDBS binds with PVP in a co-operative manner i.e. an already polymer-bound ligand (surfactant) molecule facilitates the binding of the next approaching ligand molecule at a neighboring site through the hydrophobic-hydrophobic interaction of their hydrocarbon chains.

3.4. Smith and Muller Model

In this treatment of ligand-polymer interaction, it is assumed that each polymer molecule, P, contains a number of "effective segments". Each segment is able to bind a cluster of 'n' surfactant (S) molecules in a single step as per the equilibrium (Smith and Muller 1975)

P + n.S Û PS_n ............................ (3.4.1)

The equilibrium constant, K, for such binding process is given by the relation:

K = [PS_n] / {[P] [S]ⁿ} ............................ (3.4.2)

Where, [P] = molar concentration of polymer monomer (=90 mM, in case of the PVP in the present studies) and [PS_n] is given by:

[PS_n] = [S]_T - [S]_F ............................ (3.4.3)

Where [S]_T and [S]_F represent the total and the free molar concentrations of the surfactant respectively, at equilibrium. Equation (3.4.2) may thus be written as:

Log [PS_n] = Log K[P] + n log [S]...................... (3.4.4)

The slope of the linear plot .between log [PS_n] and log [S] gave the value of 'n' equal to unity for each of the systems: SDBS + PVP (I %) + H₂O; SDBS + BSA (0.6%) + H₂O (pH = 5.8) and CTAB + BSA (0.6%) + H₂O (pH = 5.8). It conforms with the inference drawn above from the identical and independent site model that the surfactant molecules, in the studied systems, bind at the polymer binding sites in their monomeric state.

4. CONCLUSION:

Ligand-polymer interaction studies in SDBS+PVP+H₂O, SDBS+BSA+H₂O and CTAB+BSA+H₂O systems have been carried out using equilibrium dialysis method. On gradually increasing SDBS concentration, the process of binding of the surfactant with PVP passes through three distinct stages viz. (a) a weak interaction region, (b) a strong cooperative binding region and (c) a no binding region. The observed surfactant-polymer binding data analyzed in the light of various theoretical models suggests that SDBS molecules bind with PVP as monomers, in a co-operative manner. About thirteen PVP monomer units of PVP molecule constitute one binding site while interacting with the ligand (SDBS) molecules. Anionic surfactant (SDBS) exhibits stronger interaction with BSA compared to the cationic surfactant (CTAB) suggesting the existence of positively charged binding sites at BSA.

5. REFERENCES:

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Received on 31.01.2012 Modified on 26.02.2012

Asian J. Research Chem. 5(4): April 2012; Page 552-555