Abstract: Response surface method (RSM) was used to optimize technological parameters for digestion process of extraction of sodium alginate (SA) from Hainan wild sargassum.The optimum technical conditions were as follows:Na₂CO₃ mass fraction,2.49%;digestion temperature,61.42 ℃;digestion time,2.07 h.Under these conditions,the extraction yield of SA from Hainan wild sargassum could reach 34.82%,which is close to practical value 34.99%.The relative error was only 0.047 7%,showing that the model fits pretty well.¹HNMR was used to characterize and analyze the ratio of the mannuronic acid (M) to guluronic acid (G),the sequences and composition ratios of G,M,GG,GM,MM and MG of the prepared product and that of the market product.The F_M/F_G ratios of the prepared product and market product are 0.41 and 1.72 respectively,indicating a great difference of their structure.Meanwhile,molecular weight of the prepared product and the market product are 488 699 and 93 766 respectively,and all their polydispersity (PDI) are less than 3,indicating that the molecular weight distribution is rather uniform.Morphological structure of the prepared product and the market product was observed via TEM.And results showed that the prepared product with different structure and molecular weight is forming linear network crosslinking distribution in aqueous solution and self aggregating into spherical aggregates with a uniform dispersion and similar size of 40 nm in 0.15 mol/L NaCl aqueous solution.Its critical aggregating concentration was detected by photofluorimeter and the result value is 0.884 7 g/L.

Key words: additive of cosmetics, sodium alginate, sargassum, response surface method, structure