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.

Titration microcalorimetry and steady-state fluorescence spectroscopy have been used to study the aggregation of mono-endcapped hydrophobically modified poly(sodium acrylate)s in aqueous solution. Polymers with molecular weights varying between 800 and 31,700 were synthesized by radical polymerization using an initiator and chain transfer agent. The resulting polymers form hydrophobic microdomains in aqueous solutions. The following conditions were applied: no salt and pH 5 and 9, respectively; 1 M sodium citrate and pH 9. At pH 5 the critical aggregation concentration (CAC, the concentration at which microdomains are formed) increases with increasing molecular weight of the polymers. The concentration range for aggregation is about 0.2-2.4 mM. At pH 9 the carboxylic acid groups are deprotonated and electrostatic repulsions are introduced; therefore the concentration for aggregation rises to about 80 mM. Interestingly, in case of polymers having M(n)<1400 the CAC decreases with increasing molecular weight due to a counterion-concentration gradient toward the hydrophobic microdomain. Near the microdomain the counterion binding is increased, reducing the electrostatic repulsions and allowing for lower aggregation concentrations. In the presence of 1 M sodium citrate this anomalous trend is suppressed to a large extent; since the overall counterion binding is increased and the CAC is lower. The concentration for aggregation is then in the same range as at pH 5 in the absence of salt. Copyright 2000 Academic Press.

Polymerization of the lipid headgroups inhibits calcium-induced fusion of small unilamellar vesicles of the lipid di-n-dodecyloxypropyl beta-nitrostyryl phosphate but does not influence vesicle aggregation. Addition of a copolymer of lauryl methacrylate and acrylamide (LMPAM) provides the vesicles with a steric shield that prevents both fusion and aggregation. Accurate microcalorimetric determination of the enthalpies of vesicle aggregation and fusion was possible by comparison of titrations of vesicles into CaCl2 in the absence and presence of LMPAM, both before and after polymerization of the lipids in the vesicles. Whereas calcium-induced aggregation is associated with an enthalpy of +2.6 ± 0.1 kJ/mol of lipid, fusion occurs with a minimal endothermic heat effect. We contend that the driving force of membrane fusion is of entropic origin.

Microstructure of dispersions of lamellar droplets carrying anchoring hydrophobically endcapped poly(sodium acrylate)s as novel steric stabilizers Kevelam, J; Martinucci, S.; Engberts, J.B.F.N.; Blokzijl, W.; van de Pas, J.C.; Blonk, H.; Versluis, P.; Visser, Antonie J.W.G. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. We have studied the influence of anchoring hydrophobically single-endcapped poly(sodium acrylate)s on the microstructure and colloidal stabilization of self-assembled lamellar droplets formed from a mixture of anionic and nonionic surfactants in concentrated aqueous electrolyte solutions. A fluorescently labeled hydrophobically endcapped poly(sodium acrylate) has been synthesized and characterized using timeresolved fluorescence spectroscopic techniques; it appears that the fluorophore has considerable freedom of internal rotation. Using this labeled poly(sodium acrylate), the presence of an adsorbed polymer layer bound to the surface of the droplets was imaged by confocal scanning laser microscopy, providing visual evidence that the droplets are sterically stabilized. Laser diffraction and refractive index measurements were employed to determine average particle sizes of the colloidal particles, and it was established that increasing the molecular weight of the hydrophilic (pendant) backbone at a constant (hydrophobic) anchor density, or increasing the concentration of polymer in the dispersion at constant molecular weight, results in a decrease of the average droplet size. This is in agreement with theoretical predictions that an increased lateral pressure in the adsorbed layer, due to a higher polymer segment density near the surface, is relieved by increasing the curvature of the lamellar droplets. Finally, the adsorption of hydrophobically endcapped polymers to lamellar droplets has been described in terms of a Freundlich isotherm, reflecting the degressive increase of the amount of polymer adsorbed onto the surface of the droplets with increasing polymer concentration. Again, an increase of lateral pressure with surface coverage is held responsible for this effect.

). Thermodynamics of micellization of nonionic saccharide-based N-acyl-N-alkylaldosylamine and Nacyl-N-alkylamino-1-deoxyalditol surfactants. Langmuir, 15(6), 2009Langmuir, 15(6), -2014 https://doi.org/10.1021/la981404w Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Eight homologous series of nonionic carbohydrate-derived surfactants in which the alkyl chains are linked through N-acylated amine bonds were synthesized, and their critical micelle concentrations (cmc's) and standard enthalpies of micellization were determined using titration microcalorimetry. Gibbs energies of micellization (∆micG°) were calculated from the critical micelle concentrations. N-Acyl-N-alkylaldosylamines (acyl ) acetyl/propionyl, aldosyl ) glucosyl/lactosyl) and N-acyl-N-alkylamino-1-deoxyalditols (acyl ) acetyl/propionyl, alditol ) glucitol/lactitol) with alkyl chain lengths of 8, 10, and 12 carbon atoms show a 10-fold decrease in cmc when the length of the chain is increased by two methylene groups. The enthalpograms for the C8 analogs are more complicated than those for the C10 and C12 analogs. Therefore, the enthalpograms were modeled using a computer-based program which takes account of the nonideal properties of the solutions, yielding enthalpies of micelle formation. Increments in the thermodynamic parameters show satisfactory self-consistency. Each CH2 group contributes -2.4 kJ mol -1 to ∆micH°, ∆micG°-(CH2) is -3.0 kJ mol -1 for each series, and T∆micS°(CH2) is 0.7 kJ mol -1 at 40°C. Although the change in entropy is the main driving force for micellization, the enthalpy of micellization may also contribute significantly to the Gibbs energy of micellization, particularly for longer alkyl chains.