Solution relates to the preparation of new N-alkylamides of alginic acid of formula, in which are those the modification attending in the carboxylic acid groups by esterification followed by an amino-de-alkoxylation and by direct amidation or amino de alkoxylation of an ester to aid microwave heating. This modification results in attachment of various long chain alkyl (depending on the amine used) and the obtained compound obtained hydrophobic character. Of the amines used was selected n-butylamine, n-hexylamine, n-octylamine, n-dodecylamine and n-octadecylamine. Hydrophobic amidated derivatives containing n-alkyl (C4 to C18) are able to function as sorbents nonpolar compounds and thereby decrease the in vivo levels of neutral sterols and triglycerides. Alginates are extracted from brown algae (Phaeophyceae) and have a wide industrial application. This procedure is a modification extends the possibility of recovery.