Ionic complexes of poly(γ-glutamic acid) with alkyltrimethylphosphonium surfactants

Abstract

A series of ionic complexes with a comb-like architecture and a nearly stoichiometric composition were prepared from bacterial poly(DL-glutamic acid) (PGGA) and alkyltrimethyl phosphonium bromides (nATMP·Br) bearing long linear alkyl chains with even numbers of carbon atoms (n) ranging from 12 to 22. The nATMP·PGGA complexes were non-water soluble but readily soluble in organic solvents, and they displayed a high thermal stability. Combined DSC and XRD studies revealed that these complexes adopted an amphiphilic layered structure with the polypeptide chain and the alkyl chain separated in two differentiated phases with a nanoperiodicity that increased steadily with the length of the alkyl chain. The paraffinic phase was found to be partially crystallized in an extent that decreased with n, so that complexes with n = 12 and 14 did not show any sign of crystallinity whereas those with n from 16 to 22 showed crystalline melting in the ~30-70 ºC range. The structural transitions taking place by temperature effects were characterized by simultaneous SAXS/WAXS using synchrotron radiation at real time. In all cases, a shortening of the layer periodicity occurred upon heating with recovering of the initial structure after cooling. nATMP·PGGA with n ≤ 16 showed strong antimicrobial activity against both E. coli and S. aureus, a property that could be related to the weak dissociation of the complexes happening upon incubation in water. The structure and properties of these complexes where comparatively discussed taking as reference their analog complexes made from PGGA and alkyltrimethylammonium bromides already studied by us.

Nanocomposites of Microbial Polyglutamic Acid and Nanoclays Compatibilized by Organophosphonium Surfactants

“…The TGA traces recorded for the two nanocomposite series ( n = 12 and 20), including the two P MMT modified clays ( X = 100%) as well as the two n ATMP·PGGA complexes ( X = 0%), are compared in Figure , and the more relevant thermal decomposition parameters afforded by this analysis are listed in Table 1 . TGA results indicated that decomposition of these nanocomposites induced by heating was found to proceed along two steps following a pattern similar to that displayed by the n ATMP·PGGA complexes …”

“…The thermal behavior of nanocomposites regarding melting‐crystallization was examined by differential scanning calorimetry (DSC). It is known that n ATMP·PGGA complexes display crystallinity only for n = 16, 18, 20, and 22, which is associated to the ordered close packing adopted by the polymethylene chains . Accordingly, the DSC analysis of 12ATMP·PGGA· X % P MMT nanocomposites did not show melting for whichever composition due to the inability of dodecyl chains to crystallize.…”

“…According to the scheme depicted in Figure , firstly n ATMP·PGGA complexes were prepared by coupling of Na·PGGA with the corresponding n ATMP·Br. These complexes have been previously studied by us and characterized in great detail . The nanocomposites were prepared by mixing the chloroform solution of the complex with the corresponding P MMT suspended in the same solvent.…”

“…The preparation of nanocomposites was carried out in two steps. First, the n ATMP·PGGA complexes with n = 12 and 20, were synthetized by adding dropwise an aqueous solution of the corresponding n ATMP·Br salt to an aqueous solution of Na·PGGA according to the procedure previously described by Gamarra et al Second, the nanocomposites were prepared by mixing a 5 g L −1 dispersion of P MMT in chloroform with a 0.05 m solution of the complex in the same solvent. The mixture was stirred at room temperature until no sedimentation was detected upon standing.…”

“…PGGA has been subjected to diverse chemical modifications with the main purpose of obtaining materials with better processing properties . Coupling of PGGA with quaternary alkylammonium or alkylphosphonium surfactants leading to stoichiometric or nearly stoichiometric ionic complexes with a comb‐like molecular architecture has been the approach chosen reiteratively by us for such purpose . These complexes are well stable to heat, water‐resistant, and soluble in organic compounds, but their mechanical properties need to be improved if they are intended to be used in active packaging or coating applications .…”