METHOD FOR PREPARING A FATTY AMIDOALKYLDIALKYLAMINE The present invent ion concerns a method for preparing a fat t y amidoalkyldialkylamine by enzymat ic synthesis. Fatty amidoalkyldialkylamines such as for example lauramidopropyl dimethylamine are well known chemical compounds which are widely used in industry in particular as intermediat e compounds for the preparation o f betaïnes and sultaïnes which are extensively used in personal care, home care and agrochemical composit ions. Fatty amidoalkyldialkylamines are commonly prepared by pure chemical route, as described for example in WO 98/47860 which discloses a method for producing carboxylic amides by reacting a carboxylic acid and amine. The reaction is performed under nit rogen, at high temperatures (about 180°C or above). There is a cont inuous need for providing new methods fo r preparing these useful chemical materials, which br ing improvement s in terms of efficiency, costs and environmental impact. The inventors of the present applicat ion have now discovered that fat ty amidoalkyldialkylamines could be prepared in an efficient manner by reacting a fatty acid with a dialkylaminoalkylamine in the presence of a part icu lar enzyme as catalyst. The present invent ion thus concerns a method for preparing at least one fatty amidoalkyldialkylamine by reacting at least one fatty acid containing from 8 to 24 carbon atoms with at least one dialkylaminoalkylamine o f formula (I): in which: R1 represents a divalent alkyl group containing fro m 2 to 6 carbon atoms; R2 and R3 represent, independent ly of each other, monovalent alkyl groups containing from 1 to 4 carbon atoms, using a molar ratio of said dialkylaminoalkylamine to said fatty acid of more than 1 and up to 1.5, in the presence o f Candida antarctica lipase as catalyst. The present invent ion provides an enzymat ic route for synthet izing fatt y amidoalkyldialkylamines which leads to excellent yie lds, while requiring lower excess o f the dialkylaminoalkylamine starting material and therefore consuming fewer amounts if this product and generat ing less effluents. The synthesis can also be performed in milder condit ions, in particular at lower temperatures, when compared to the prior art pure chemical routes and therefore allows significant energy savings (such as about 45%). Other subjects, characterist ics, aspects and advantages of the invent ion will emerge even more clearly on reading the descript ion and the examples that follow. In the present description, and unless otherwise indicated: - the expression "at least one" is equivalent to the expression "one or more" and can be replaced therewith; - the expression "between" is equivalent to the expressio n "ranging from" and can be replaced therewith and implies that the limit s of the range are included; - the term “compound in CX” designates in a manner known per se a compound having X atoms of carbon in it s mo lecule. The start ing materials: The method of the invent ion uses two starting materials as described below. The first starting material is a fatty ac id having 8 to 24 carbon atoms. Such compound corresponds to the general formula R-COOH with R represent ing a linear or branched, saturated or unsaturated hydrocarbon group containing from 7 to 23 carbon atoms. Fatty acids wherein the R group is linear, saturated or unsaturated, are preferred. Preferred fatt y acids are those containing from 10 to 20 and more preferably from 12 to 18 carbon atoms. Particularly preferred fat t y acids are lauric acid, stearic acid, oleic acid, and mixtures thereo f, as well as mixtures o f fa t ty acids comprising from 8 to 18 carbon atoms and preferably from 12 to 18 carbon atoms. Lauric acid and stearic acid are particularly preferred. The second starting material is a dialkylaminoalkylamine o f formula (I): R1 represents a divalent alkyl group containing from 2 to 6 carbon atoms, preferably 3 or 4 carbon atoms and most preferably 3 carbon atoms which corresponds to R1 represent ing a propyl group. R2 and R3 may be ident ical or different and represent monovalent alkyl groups containing from 1 to 4 carbon atoms, preferably 1 or 2 carbon atoms. According to a preferred embodiment, the R2 and R3 groups are ident ical, and preferably represent methyl or ethyl groups, most preferably R2 and R3 both represent methyl groups. According to a most preferred embodiment, the dialkylaminoalkylamine o f formula (I) is dimethylaminopropyl amine, which corresponds to R1 represent ing a propyl group and R2 and R3 both represent ing methyl groups. In the present invent ion, these two starting materials are combined with a mo lar rat io o f said dialkylaminoalkylamine to said fatty acid of more than 1, that is to say strict ly greater than 1. The molar rat io preferably ranges from 1.05 to 1.3, and most preferably from 1.1 to 1.2. The fatty amidoalkyldialkylamines: The present invent ion provides an improved method of synthesis of fatty amidoalkyldialkylamines having the general formula (II): with R, R ₁ , R ₂ and R ₃ having the definit ions given above, inc luding the preferred embodiment s described above. A part icu larly preferred compound of formula (II) is lauramidopropyl dimethylamine, which corresponds to R being a linear alkyl group in C ₁₁ , R ₁ being a propyl group and R ₂ and R ₃ both being methyl groups. This compound is obtained by reacting lauric acid and dimethylaminopropylamine as starting materials, according to the fo llowing react ion scheme: Other preferred compounds are amidopropyl d imethylamines obtained from stearic acid, o leic acid, as well as from mixtures o f fatty acids comprising from 8 to 18 carbon atoms and preferably from 12 to 18 carbon atoms. The enzyme catalyst: The present invent ion uses a specific lipase, namely Candida antarctica lipase, as catalyst. The catalyst is preferably under the form o f a so lid catalyst containing Candida Antarctica l ipase grafted onto a polymeric support , preferably an anionic resin such as an acrylic resin. Such catalyst s made of enzymes grafted onto appropriate so lid supports are well known in t he art and commercially available. A part icular commercial catalyst which can be used in the present invent ion contains Candida Antarctica lipase in an amount of 21% by weight, grafted onto an acrylic resin and is so ld under the name Novozym 435 by the company Novozymes. The catalyst is preferably used in an amount ranging from 0.2 to 2% by weight, preferably from 0.5 to 1.5% by weight, and most preferably in an amount o f 0.9 to 1.1% by weight, with regard to the total weight of fatty acid(s). The above amounts correspond to the amount of catalyst active matter, ie the amount of Candida antarctica lipase. Such amounts do not include the possible support which is inert. According to a preferred embodiment, Candida Antarctica l ipase is the sole catalyst used in the process of the invention. The operat ing condit ions: The reaction between the fatt y acid(s) and the dialkylaminoalkylamine(s) is advantageously carried out at a temperature within the range from 75 to 120°C, preferably from 85 to 105°C, and most preferably from 90 to 100°C. The reaction is preferably carried out under vacuum, at a pressure ranging from 25.102 to 200.102 Pa (25 to 200 mbar) preferably from 50.102 to 100.102 Pa (50 to 100 mbar). The reaction is preferably carried out in a batch reactor, as described hereafter. The fatty acid(s) is (are) first loaded into the rector and the catalyst is added. The fatty acid(s) can be melted before being int roduced into the reactor, or it (they) can be melted directly within the reactor, preferably before adding the catalyst . The mixture is heated up to the reaction temperature and placed under vacuum, and the dialkylaminoalkylamine(s) is(are) then added progressively with cont inuous mechanical st irr ing of the reaction medium. The reaction is generally completed within a few hours, rang ing from 8 to 40 hours, preferably from 15 to 30 hours. In these condit ions, the molar rate of conversion o f fatt y acid into fat t y amidoalkyldialkylamines is advantageously above 99%, such as about 99,5% or even more - At the end o f the react ion, the agit at ion is turned o ff and the enzymat ic catalyst is separated by decantat ion or filtrat ion. Alt ernat ively, the catalyst can be separated by centrifugation. According to a preferred embodiment, after a first reaction cycle the catalyst is recycled to the reactor where it is reused for the next reaction batch. The catalyst can advantageously be recycled at least three times and up to twenty times, preferably up to ten t imes. The fatty amidoalkyldialkylamine is then recovered. When the react ion is performed under vacuum, water and residual amine have already been removed from the reactor, and there is no need of further purificat ion. The examples o f implementat ion o f the invent ion below are given purely by way o f il lustration and shall not be interpreted at limit ing the scope thereof.

EXAMPLES Example 1: Synthesis o f lauramidopropyldimethylamine by react ing lauric acid with dimethylaminopropylamine The syntheses were carried out in a jacketed reactor having a vo lume o f 250 mL coupled with a vacuum pump, a condenser, a mechanic st irrer and a peristalt ic pump for amine feeding. The reactions were performed at 95°C, under vacuum at 50-100 mbar, mechanical agit at ion, and with a fed-batch o f amine. First, the lauric acid was int roduced into the reactor. It can be melted in t he reactor or be melt ed previously to the addit ion. Then, the enzymat ic catalyst was added. The catalyst used was t he commercial product Novozym 435 which contains 21% by weight of Candida Antarctica lipase or iginat ing from Candida Antarctica B , grafted onto an acrylic resin. The amount of enzyme (active matter, ie excluding the support) used corresponds to 1% w/w based on lauric acid. The stoichiometr ic amount of dimethylaminopropylamine (DMAPA) plus 10% of excess was added cont inuously into the reactor for a period of 7 hours. After 7 hours of feeding, the acid index was monitored for each hour. When it became constant, amine excess was added, divided into portions of 1%. For the fir st react ion batch, a rate of 99.5% of conversion was achieved with 11% (molar) o f amine excess. The total react ion t ime was around 15 hours. After the reaction, the agita t ion was turned o ff for the enzymat ic catalyst to go to the bottom of the reactor. The product was removed from the top. Acid index and residual amine were measured to cert ify that all specificat ions were achieved. After removal o f the product (end o f cycle 1), a new reaction cycle was performed using the catalyst remaining in the reactor: a new charge o f lauric acid was added, and the feeding o f dimethylaminopropylamine was started. Three successive reaction cycles were achieved using the same load of catalyst . The table hereunder summarizes the condit ions and results o f the react ion for the three successive cycles. ( contains 21% by weight of Candida Antarctica lipase Example 2: Synthesis o f stearamidopropyldimethylamine by react ing stearic acid with dimethylaminopropylamine The synthesis was carried out in a jacketed reactor having a vo lume o f 250 mL coupled with a vacuum pump, a condenser, a mechanic st irrer and a peristalt ic pump for amine feeding. The reaction was performed at 95°C, under vacuum at 50-100 mbar, mechanical agit at ion, and with a fed-batch o f amine. First, the stearic acid was int roduced into the reactor. It can be melt ed in the reactor or be melt ed previously to the addit ion. Then, the enzymat ic catalyst was added. The catalyst used was t he commercial product Novozym 435 which contains 21% by weight of Candida Antarctica lipase or iginat ing from Candida Antarctica B , grafted onto an acrylic resin. The amount of enzyme (active matter, ie excluding the support) used corresponds to 1% w/w based on lauric acid. The stoichiometric amount of dimethylaminopropylamine (DMAPA) plus 10% of excess was added cont inuously into the reactor for a period of 7 hours. After 7 hours of feeding, the acid index was monitored for each hour. When it became constant, amine excess was added, divided into portions of 1%. A rate of 99.2% of conversion was achieved with 18% (molar) of amine excess. The total react ion time was around 20 hours. After the reaction, the agita t ion was turned o ff for the enzymat ic catalyst to go to the bottom of the reactor. The product was removed from the top. Acid index and residual amine are measured to cert ify that all specificat ions were achieved. The table hereunder summarizes the condit ions and results o f the react ion for the synthesis.

( *) contains 21% by weight of Candida Antarctica lipase The examples above show that the method of t he invent io n provides an efficient route for the synthesis o f fatt y amidoalkyldialkylamines. The benefits o f the enzymat ic process of the invent ion in particular versus chemical synthesis are the reduction o f dialkylaminoalkylamine raw material from 30 to 10-13% and the reduct ion o f the temperature. Both parameters have a significant impact on the process costs. Furthermore in the chemical synthesis, after specifying acidity content, another step is required in order to remove residues o f DMAPA, which is not needed in the method of the invent ion.