The interactions between triblock copolymers of poly(ethylene oxide) and poly(propylene oxide), P103 and F108, EO(n)PO(m)EO(n), m = 56 and n = 17 and 132, respectively, and gemini surfactants (oligooxa)-alkanediyl-alpha,omega-bis(dimethyldodecylammonium bromide) (12-EO(x)-12), x = 0-3, have been studied in aqueous solution using isothermal titration calorimetry. The thermograms of F108 as a function of surfactant concentration show one broad peak at polymer concentrations, Cp, < or =0.50 wt %, below the critical micelle concentration (cmc) of the copolymer at 25 degrees C. It is attributed to interactions between the surfactant and the triblock copolymer monomer. The critical aggregation concentration (cac) remains constant while deltaHmax2 and the saturation concentration, C2, increase with increasing copolymer concentration. Analysis of the cac data offers semiquantitative support that the degree of ionization of the surfactant aggregates bound to polymers is likely to be larger than that at the surfactant cmc. In P103 solutions at Cp > or = 0.05 wt %, two peaks appear in the thermograms and they are attributed to the interactions between the gemini surfactant and the micelle or monomeric forms of the copolymer. An origin-based nonlinear fitting program was employed to deconvolute the two peaks and to obtain estimates of peak properties. An estimate of the fraction of copolymer in aggregated form was also obtained. The enthalpy change due to interactions between the surfactants and P103 aggregates is very large compared to values obtained for traditional surfactants. This suggests that extensive reorganization of copolymer aggregates and surrounding solvent occurs during the interaction. Dehydration of the copolymers by the surfactant may also play an important step in the interaction. The endothermic enthalpy change reflecting interactions between the surfactant and polymer decreases more rapidly as the length and hydrophilic character of the spacer increases, suggesting that more favorable interactions occur with the P103 monomers having shorter PEO segments.

Cationic gemini surfactants, N,N-bis(dimethylalkyl)-alpha,omega-alkanediammonium dibromide [C(m)H(2m+1)(CH(3))(2)N(+)(CH(2))(s)N(+)(CH(3))(2)C(m)H(2m+1) x 2 Br(-), or m-s-m], have proven to be effective synthetic vectors for gene delivery (transfection). Complexes (lipoplexes) of gemini compounds, where m = 12, s = 3, 12 and m = 18 : 1(oleyl), s = 2, 3, 6, with DNA have been investigated using isothermal titration calorimetry (ITC), dynamic light scattering (DLS), zeta potential, atomic force microscopy (AFM) and circular dichroism (CD) techniques. The results show that lipoplex properties depend on the structural properties of the gemini surfactants, the presence of the helper lipid dioleoylphosphatidylethanolamine (DOPE), and the titration sequence. ITC data show that the interaction between DNA and gemini surfactants is endothermic and the observed enthalpy vs. charge ratio profile depends upon the titration sequence. Isoelectric points (IP) of lipoplex formation were estimated from the zeta potential measurements and show good agreement with the reaction endpoints (RP) obtained from ITC. DLS data indicate that DNA is condensed in the lipoplex. AFM images suggest that the lipoplex morphology changes from isolated globular-like aggregated particles to larger-size aggregates with great diversity in morphology. This change is further accentuated by the presence of DOPE in the lipoplexes. The results are interpreted in terms of some current models of lipoplex formation.

Critical micelle concentration (cmc), degree of micelle ionization (R), headgroup area (a0), enthalpy of micellization (∆H°M), and volume of micellization (∆Vφ,M) properties are reported for a series of (oligooxa)alkanediyl-R,ω-bis(dimethyldodecylammonium bromide) gemini surfactants, 12-EOx-12 (x ) 0-3), having several ethylene oxide moieties within the spacer group. The cmc and headgroup areas are observed to increase with an increase in the number of EO groups in the spacer, in contrast to the complex behavior of gemini surfactants (m-s-m) having a normal hydrocarbon chain. Also, the ∆H°M increases (becomes less exothermic) and ∆VM decreases with an increase in the number of EO groups in the spacer. These observations are rationalized in terms of changes in the hydration and conformation of the spacer group at the micelle-bulk solution interface. Gibbs energies of micellization, ∆G°M, and entropies of micellization, ∆S°M, are calculated from the ∆H°M, cmc, and R values. The results indicate that the micellization process for these surfactants is entropy-driven.

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The water content-dependent supramolecular structure formation of polystyrene-block-poly(acrylic acid) (PS-b-PAA) copolymer in the presence of a fourth-generation amine-terminated poly(amido amine) dendrimer (PAMAM) is investigated by dynamic light scattering, turbidity measurements, and transmission electron microscopy. The solvent system for this study is a mixture of dioxane/THF and water. A very complex turbidity profile is observed with increasing water content in the system and is explained by the presence of various aggregated structures based on strong interactions between the amine-containing dendrimers and the poly(acrylic acid) blocks of the polymer. The onset of the self-assembly of single chains of PS-b-PAA (primary structure) into single and multiple dendrimer core inverse micelles (secondary structure) is detected as very low water contents of cw < 2% wt (cwc). These micelles consist of dendrimers coated with PAA blocks, which are connected to the corresponding PS chains that form the corona. Further addition of water leads to an association of these micelles into compound multiple dendrimer core inverse micelles (tertiary structure) in the range of cw = approximately 6 to approximately 10% wt. At still higher water content, some of the acrylic acid chains of the block copolymer move from the vicinity of the dendrimer to the outside of the aggregates, resulting in a decrease in the size of the formed structures and the acquisition of progressively increasing hydrophilic character of the aggregates. Multiple dendrimer core inverse onion micelles are formed, which agglomerate into compound multiple dendrimer core inverse onion micelles at cw = approximately 12 to approximately 18% wt. Above this water content, vesicular structures are formed. The complexity is unusual for block copolymer systems and illustrates the importance of strong interactions in structure formation.