Using Polymer/Surfactant Complex As A Probe To Study Small Organic Molecule-surfactant Interactions

Interactions between polymer and surfactant in aqueous solutions have attracted significant interest for their widespread applications and relatively complex behaviors. Several aspects are discussed in this thesis about the utility of polymer/surfactant complex to study the interaction between small organic molecules with surfactants. The main findings are as follows:1. Data presented in viscosity, conductivity, surface tension and fluorescence studies on the interaction between Poly(vinyl pyrrolidone)(PVP) and Sodium Dodecyl Sulfate(SDS) in aqueous solution indicates that the interaction between PVP and SDS can be separated into two stages. In the first stage the anionic headgroups of SDS individually bind to the cationic side groups of PVP chain due to electrostatic attraction. In the second stage when the surfactant concentration reaches the second critical aggregation concentration cac2, the micellization of polymer-bound surfactant occurs.2. Using PVP/SDS complex as a probe the inclusion complexation betweenβ-cyclodextrin(β-CD) and SDS has been studied. Viscosity measurements show that the inclusion complexation betweenβ-CD and SDS may cause the SDS molecules being stripped off the PVP chains, resulting in the decrease of the solution viscosity due to the decaease of electrostatic repulsion between polymer-bound SDS molecules. The viscosity minimum at Cβ-CD/CSDS=1 indicate the molecular ratio of host molecule to guest molecule is 1:1 in theβ-CD/SDS inclusion complex. Data from conductivity measurements indicate that there is the 2:1 inclusion complexation betweenβ-CD and SDS after the 1:1 inclusion complexation being finished.3. The effect of benzyl alcohol on the micellization of SDS has been studied using PVP/SDS complex as a probe. When the concentration of benzyl alcohol is low benzyl alcohol behave as a cosurfactant. Through hydrophobic interaction the mixed micellization between benzyl alcohol and SDS occurs. As a result the number of polymer-bound SDS increasing. Thus the relative viscosity of PVP/SDS/benzyl alcohol solutions increases due to the stronger electrostatic repulsion and the conductivity decreases because polymer-bound SDS has lower mobility than free SDS molecule. At high concentrations benzyl alcohol behave as a cosolvent. The properties and structure of the aqueous solvent mixture are modified in such a manner that the solubility of an ionic surfactant increases, that is, the solution is more hydrophobic and the hydrophobic interaction between the SDS molecules or between PVP and SDS decrease. These factors cause the SDS molecules being stripped off the PVP chains, resulting in the increase of the conductivity and the decrease of the solution viscosity due to the decrease of electrostatic repulsion within PVP chains. In the presence of benzyl alcohol the amount of surfactant bound to the polymer viz. Cbound at saturation decreases. At different benzyl alcohol concentrations the ionization degree of PVP-bound SDS micelles does not change much, which is about 30% larger than that in the absence of benzyl alcohol. The decrease of electrostatic repulsion within PVP chains resulting from the decrease of the amount of SDS bound to PVP chains can be compensated by the increase of the ionization degree of PVP-bound SDS micelles.4. The effect of n-butyric acid, n-butylamine and n-butanol on the micellization of SDS has been studied using PVP/SDS complex as a probe. Viscosity data indicates that the electrostatic character of the headgroups has strong effect on the response of PVP/SDS complex upon the addition of different additives. The headgroup of n-butyric acid has a negative charge, same as SDS molecule, but it has a lower ionization degree. The initiation of adding n-butyric acid cause PVP chains have more positive charge, resulting in the decrease of dimension of PVP chain which is bound to the SDS micelle. As the n-butyric acid concentration increases the mixed micellization of n-butyric acid and SDS make more SDS micelles associate with PVP chains. Thus the relative viscosity of PVP/SDS/n-butyric acid solutions increases due to the stronger electrostatic repulsion. Adding more n-butyric acid the screening effect upon the electrostatic interaction make the PVP chains shrink again. The headgroup of n-butylamine has a positive charge which is oppositely to SDS molecules. Mixed micellization is promoted by the hydrophobic interaction between the hydrocarbon chains and the electrostatic attraction between the oppositely charged headgroups. The negative charge of SDS is neutralized and there is the repulsion force between positively charged n-butylamine and PVP. As a result the interaction between PVP and SDS/n-butylamine mixed micelles decreases sharply, resulting in PVP-bound SDS being stripped off the PVP chains. At low concentrations n-butanol behaves as a cosurfactant. Through hydrophobic interaction the mixed micellization between n-butanol and polymer-bound SDS occurs. As a result the number of polymer-bound SDS micelles increasing. Thus the relative viscosity of PVP/SDS/n-butanol solutions increases due to the stronger electrostatic repulsion and the conductivity decreases because polymer-bound SDS has lower mobility than free SDS molecule. At high concentrations n-butanol behaves as a cosolvent. The solution is more hydrophobic and the hydrophobic interaction between the SDS molecules or between PVP and SDS decrease. These factors make the PVP-bound SDS micelles being stripped off the PVP chains, resulting in the increase of the conductivity and the decrease of the solution viscosity due to the decrease of electrostatic repulsion within PVP chains. In the presence of n-butanol the amount of surfactant bound to the polymer viz. Cbound at saturation decreases. As n-butanol concentration increases the ionization degree of PVP-bound SDS micelles increases distinctively. The character of the hydrophobic group has strong effect on the response of PVP/SDS complex upon the addition of different additives. The volume of hydrophobic group of n-butanol is smaller than benzyl alcohol. As a result SDS molecules are more closing in SDS/n-butanol mixed micelles than that in SDS/benzyl alcohol mixed micelles. And the solubilization amount of n-butanol is more than benzyl alcohol. Thereby the ionization degree of PVP-bound SDS micelle increases with the n-butanol concentration. The hydrophobic group of benzyl alcohol is larger. As a result the partitioning of benzyl alcohol in the PVP-bound SDS micelles blocks the closing between SDS molecules. The solubilization amount of benzyl alcohol is relatively small. Thereby the ionization degree of PVP-bound SDS micelle does not change much with the benzyl alcohol concentration.5. Interaction between gelatin and cationic gemini surfactant 1,2-ethane bis(N, N, N-dimethyl (Z-13-docosenyl) quaternary ammonium bromide) (22-2-22) has been studied by conductivity, viscosity and fluorescence spectroscopy. In the initial binding of surfactant to gelatin molecule the hydrophobic interaction between the surfactant and the hydrophobic segments of gelatin causes the breaking of the hydrophobic microdomain on polymer chains, resulting in the extension of polymer chains, that is, the partially denaturing of gelatin. With the further addition of 22-2-22 at low surfactant concentrations inter-polymer association mediated by the surfactant aggregates dominates. One surfactant aggregates may bind more than one polymer strands. While at high surfactant concentrations the intra-polymer association dominates. Then with the increase of C22-2-22 one polymer strand may accommodate several surfactant micelles forming a pearl-necklace complex. By using fluorescence method a critical concentration of 22-2-22 can be defined, viz. cac, beyond which the micellization of gelatin-bound 22-2-22 occurs.