Isothermal titration microcalorimetry has been applied to investigate the interactions between hydrophobically-modified water-soluble polymers and surfactants. The following polymers were used in this study:  poly(sodium acrylate-co-n-alkyl methacrylate) (A), where n-alkyl = C9H19, C12H25, and C18H37 (percentage of n-alkyl methacrylate to total monomer content ranging from 0 to 8), and poly(acrylamide-co-n-alkyl methacrylate) (B), where n-alkyl = C12H25 (percentage of lauryl methacrylate to total monomer content ranging from 0 to 5). The surfactants were a cyclic (mono-) n-dodecyl sodium phosphate (1) (CMP), a cyclic di-n-dodecyl sodium phosphate (2) (CDP), n-dodecyltrimethylammonium bromide (3) (DTAB), and di-n-dodecyldimethylammonium bromide (4) (DDAB). The following factors were found to influence the interactions between polymers and surfactants:  electrostatic forces, polymer hydrophobicity (both the length of the hydrophobic moiety and the degree of hydrophobic modification), and the aggregational states of the amphiphilic molecules, which are micellar for the single-tailed surfactants and vesicular for the double-tailed amphiphiles. We provide evidence that, in the case of the single-tailed surfactants, individual amphiphilic molecules adsorb onto existing polymeric microdomains. This is in strong contrast with ‘classical' polymer−surfactant interactions, where cooperative aggregation of single-tailed amphiphiles in the presence of homopolymers like poly(ethylene oxide) or poly(propylene oxide) was found at concentrations lower than the critical micelle concentration in pure water. In the case of vesicle-forming surfactants, the hydrophobic side chain of the polymer anchors into the bilayers of the vesicles. Non-hydrophobically-modified polymers do not interact at all with the vesicle bilayers. Interestingly, the interactions between single-tailed surfactants and hydrophobically-modified polymers are governed by different factors than the binding of hydrophobically-modified polymers to vesicular bilayers. In the former case, the number and strength of existing (inter)polymeric associations is of importance, and it is particularly the length of the hydrophobic moieties that is decisive. However, for favorable polymer−bilayer interactions it is sufficient that the hydrophobic moieties are long enough to be able to anchor. If this is the case, the number of hydrophobic anchors per polymer molecule further determines the effectiveness of the interaction. Finally, it appears that electrostatic repulsions can be easily overcome by hydrophobic interactions, but added salt facilitates the interactions between equally charged polymers and surfactants.

The self-association of copolymers of N-isopropylacrylamide and N-n-octadecylacrylamide (Pnipam-C18) in aqueous solutions was studied by means of time-resolved fluorescence quenching. The discrete domains consist of several polymer chains interacting through their hydrophobic side chains, since the number of aliphatic side chains involved in the microdomain formation (aggregation number) is larger than the number of aliphatic side chains per polymer. By means of titration microcalorimetry, the interaction of the copolymer with surfactants was studied. Strong association between the copolymer and the cationic surfactants N-cetylpyridinium chloride (C16PyCl) and cetyltrimethylammonium bromide (CTAB) occurs by partitioning of the surfactants in a noncooperative mechanism. Prior to mixed-micelle formation, individual surfactant molecules adsorb to collapsed polymer coils as can be seen from the large exothermal contribution in the enthalpy curves which result from microcalorimetric titration of surfactant into aqueous Pnipam-C18 solutions.

Recent studies on the self-assembly of novel catanionic, bolaform and gemini surfactants provide evidence that the Israelachvili packing parameter approach can often be successfully used to predict the morphology of surfactant aggregates on the basis of the geometrical properties of the surfactant molecules. Furthermore, combined theoretical and experimental efforts have provided a consistent picture of the requirements for spontaneous vesicle formation which, in addition to a favorable packing parameter of the individual surfactant molecules, calls for a nonideal mixing of the bilayer components in order to provide the bilayer with a nonzero spontaneous curvature.

Procedures are described for analysing enthalpograms characterising adsorption by macromolecules in solution recorded using a titration microcalorimeter. The experimental procedure involves injecting small aliquots of a solution containing adsorbate into a sample cell containing a solution of macromolecular adsorbent. Treatments based on both Langmuir and Frumkin adsorption isotherms are described. The procedures are illustrated by application of the derived equations to the interaction of micelles of sodium dodecylsulfate(aq) and of sodium decylsulfate(aq) with the water-soluble polymer, PVP. The dominant features in the recorded enthalpograms are described using equations developed from the Frumkin equation. In both cases the adsorption is endothermic attributed to hydrophobic interactions between polymer and surfactant. However, an important feature of the analysis is the characterisation of adsorbateÈadsorbate interactions using enthalpic interaction parameters. The enthalpograms are characterised by three composition ranges : (i) micelle deaggregation and weak interaction of monomers with polymer, (ii) micelle adsorption on to the soluble polymer up to surface saturation and (iii) micelle dilution.