Extracted ulvans from green algae Ulva linza of Lebanese origin and amphiphilic derivatives: evaluation of their physico-chemical and rheological properties

Over the last years, different seaweeds have raised considerable interest as sources of valuable and biodegradable materials from renewable and environmental sources in order to limit the use of the traditional non-renewable petroleum/chemical-based ones. For instance, the exploitation of algae as a source of carbohydrate polymers has been the target of special attention and specifically research on the unexploited green algae, including their polysaccharide ulvan. In this study, a simple method for extraction of ulvan from the green alga Ulva linza of Lebanese origin is demonstrated successfully without enzymatic treatment. The isolated ulvan (yield = 21.7%) was characterized in terms of composition and structure by elemental analysis, turbidimetric method of barium sulfate, size exclusion chromatography, HPLC after acidic methanolysis of ulvan, FTIR, and NMR spectroscopy. An ulvan with a relatively low content of protein (2.56%), high amount of sulfate (17.2%), and a molecular weight of 565,100gmol(-1) was identified as a sole polysaccharide. This ulvan was rich in rhamnose (26.2%) and glucuronic acid (11.5%) while a low amount of iduronic acid (3.5%), xylose (5.8%), and glucose (1.2%) was detected. Rheological properties of ulvan revealed a shear-thinning pseudoplastic behavior for a concentration range between 0.5 and 3% in a solution of 7mM CaCl2 and 7mM H3BO3. Also, storage (G) and loss (G) moduli were studied for ulvan solution (1%). Besides, an original hydrophobically modified uronamide ulvan (HM-ulvan) was elaborated in aqueous medium using the coupling agent EDC-HCl in the presence of octylamine. The linkage between carboxylic groups of ulvan uronic acid and octylamine via amide functions was demonstrated chemically by FTIR and NMR (1D and 2D). In conclusion, the introduction of octyl fatty chains on the ulvan backbone increased the dynamic viscosity and improved the rheological properties by creating hydrophobic associations in the solution of HM-ulvan. The low degree of substitution (0.42) influenced dramatically the critical aggregation concentration (CAC) and no foam properties of HM-ulvan were detected.