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Chitosan/sodium dodecylsulfate interactions

Calorimetric titration and consequences on the behaviour of solutions and hydrogel beads

Journal of Thermal Analysis and Calorimetry
Authors: R. Barreiro-Iglesias, C. Alvarez-Lorenzo, and A. Concheiro

The thermodynamics of the interaction of chitosan and sodium dodecylsulfate, SDS, was characterised by titration microcalorimetry to gain an insight into the binding process of amphiphilic molecules to this biocompatible polymer and its consequences on the behaviour of the solutions and chemically cross-linked hydrogels of chitosan. 0.2 M acetic acid was used as solvent medium, without or with 0.9% NaCl, in order to evaluate the influence of the ionic and hydrophobic interactions with two chitosans of different molecular mass and degree of deacetylation, DD. The critical micellar concentration, CMC, of SDS was ten times lower in the presence of the salt (0.35 vs. 3.5 mM, as estimated by surface tension measurements). Binding to chitosan (at 0.25%) began at concentrations significantly lower than CMC (critical aggregation concentration, CAC=0.035-0.17 mM) and saturation was reached at around 10 mM SDS, which corresponds to a positive/negative charges ratio of about 1. The process was in all cases enthalpy-driven (strongly exothermic) and, in the absence of the salt, also entropically favourable. The Gibbs free energy of interaction values were slightly greater for the chitosan with lower DD but greater molecular mass. The addition of increasing amounts of SDS resulted in a continuous decrease in the viscosity of chitosan solutions above the CAC, which ended in a macroscopic coacervation when around 1/3 of the positive charges were neutralised. In the same range of SDS concentrations, the hydrogel beads showed a continuous decrease in the swelling degree and a final collapsed state. The scarce tendency to redissolution or hydrogel reswelling in the presence of greater SDS concentrations can be attributed to that the binding process is mainly caused by the ionic interaction and did not go beyond the neutralisation point.

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Abstract  

Tetronic® comprises X-shaped copolymers formed by four poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEO) block chains bonded to an ethylene diamine central group. Micellization behaviour of three representative Tetronics (T304, T904 and T1307) was characterized to gain an insight into the interactions between the copolymer unimers and the state of water in their solutions. The enthalpy of demicellization, recorded at 37°C in an isoperibol microcalorimeter, indicated that the process was in all cases exothermic and the enthalpy ranked in the order T1307≥T904>>T304. Micellization is entropy-driven owing to hydrophobic interactions between the PPO chains. DSC analysis showed that the crystallization and melting peaks of the free water remaining in T304 and T904 solutions were progressively shifted toward lower temperatures as the surfactant proportion increased, owing to a colligative effect. Bound water corresponded to 3 water molecules per EO repeating unit. In the case of T1307, which has longer PEO chains, a splitting of the melting peak was observed, one peak appearing around 0°C due to free water and another at –15°C due to interfacial water. As T1307 proportion raised, the enthalpy of the former decreased, whilst the enthalpy of the latter increased. In 40% T1307 solutions, interfacial water overcame the proportion of free water; there being 1 interfacial and 3 bound water molecules per EO repeating unit. Gaussian deconvolution of FTIR spectra also enabled to characterize the evolution of free water as a function of Tetronic proportion. The dependence of micellization and water interaction behaviour on Tetronics structure should be taken into account to use these copolymers as drug solubilizers and micellar carriers.

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The miscibility of poly(N-isopropylacrylamide) (PNIPA) with poly(vinyl pyrrolidone) (PVP) and a cross-linked poly(acrylic acid) (Carbopol 971P) was evaluated from the rheological data of aqueous dispersions and the temperature of glass transitions of films made of binary mixtures. PNIPA has a low critical solubility temperature (LCST) of about 33C, below which 1% dispersion behaves as a viscous system. At temperatures above LCST, the hydrophobic interactions among the isopropyl groups initially provide transient networks of greater elasticity. The LCST of PNIPA as well as its T g (144C, estimated by DSC and MTDSC of films) were not modified by the presence of PVP. The immiscibility of PNIPA and PVP was confirmed by the absence of interaction between both polymers as shown by FTIR analysis of the films. In contrast, PNIPA and carbopol were miscible and the behaviour of their mixtures differed significantly from that of the parent polymers; i.e. a strong synergistic effect on the viscoelasticity of the dispersions was observed below the LCST. As temperature increased, the blends showed a decrease in the loss and storage moduli, especially those with greater PNIPA proportions. The fall was smoother as the PNIPA proportion decreased. This behaviour may be explained as the result of the balance between PNIPA/carbopol hydrogen bonding interactions (as shown in the shift of C=O stretch in FTIR spectra) and PNIPA/PNIPA hydrophobic interactions. The T g values of the films of the blends showed a positive deviation from the additivity rule; the mixtures containing more than 1:1 amide:carboxylic acid groups have a notably high Tg (up to 181C). This increase is related to the stiffness induced in the films by the PNIPA/carbopol interactions.

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Abstract  

Glass transitions of several non-ionic cellulose ethers differing in molecular mass and nature and amount of substituents were analyzed (as compressed probes) by differential scanning calorimetry (DSC), modulated temperature differential scanning calorimetry (TMDSC@®), and oscillatory rheometry. In general, the low energy transitions accompanying the Tg of methylcellulose (MC), hydroxypropyl methylcellulose (HPMC), and hydroxypropylcelluloses of low (L-HPC) or medium-high (HPC) degree of substitution were difficult to characterize using DSC. Non-reversing heat flow signals obtained in TMDSC experiments were more sensitive. However, the best resolution was obtained using oscillatory rheometry since these cellulose ethers undergo considerable changes in their storage and loss moduli when reaching the Tg. Oscillatory rheometry also appears as a useful technique to characterize the viscoelastic behavior and thermal stability of pharmaceutical tablets. Tg values followed the order HPC (105°C)<HPMC (170-198°C)<MC (184-197°C)<L-HPC (220°C). For HPMCs, the Tg increases as the methoxyl/hydroxypropoxyl content ratio decreases. The results indicate that Tg depends strongly on the structure of the cellulose ethers. In general, increasing the degree of substitution of cellulosic hydroxyls, the hydrogen bonding network of cellulose decreases (especially when the substituents cannot form hydrogen bonds) and, in consequence, Tg also decreases.

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