according to at least one embodiment of the present invention , a demulsifier comprises the reaction product of : a ) an alkoxylated polyol or ester thereof ; and b ) a dicarboxylic acid . in accordance with at least one emdodiment , the demulsifier is obtainable by reacting : a ) an alkoxylated polyol or ester thereof ; and b ) a dicarboxylic acid . when used herein , the term “ dicarboxylic ” refers to an acid comprising two functional groups , i . e . carboxylate groups . the term dicarboxylic acid defines the group of compounds containing both bi - functional carboxylic acids and dimer acids . the demulsifier is operable to separate oil - in - water and / or water - in - oil emulsions . preferably , the demulsifier is operable to separate water - in - oil emulsions of the type commonly harvested in crude oil drilling . preferably , the demulsifier is operable to separate brine from crude oil . the polyol in the alkoxylated species preferably comprises at least 3 hydroxyl groups . preferably , the polyol in the alkoxylated species comprises up to 9 hydroxyl groups . desirably , the polyol has an average of 1 or 2 primary hydroxyl groups and at least 1 , preferably 1 to 4 secondary hydroxyl groups . the group r 1 is desirably an aliphatic hydrocarbyl group . preferably , the group r 1 is saturated . preferably , r 1 has from 3 to 10 carbon atoms , preferably from 3 to 8 , and especially from 3 to 6 carbon atoms . r 1 will usually be linear , though it may include branching . suitable polyols include glycerol , c 4 polyols such as threitol and erythritol , c 5 polyols such as inositol , arabitol and xylitol and c 6 polyols such as sorbitol , and compounds derived therefrom , for example sorbitan . the c 4 to c 6 polyols are commonly the reduced or hydrogenated forms of corresponding tetrose , pentose and hexose sugars . desirably the polyol is glycerol or a derivative thereof , particularly sorbitol or sorbitan ( usually derived in situ from sorbitol ) or a mixture or combination of these . the polyol may be present in the demulsifier in an esterified form . preferably , when the polyol is sorbitan , the sorbitan is present in the demulsifier in the form of an ester derived from the reaction of the sorbitan with a fatty acid or derivative thereof . preferred fatty acids or derivatives thereof comprise in the range from 6 to 24 , more preferably 8 to 20 , particularly 10 to 18 , and especially 12 to 16 carbon atoms . linear fatty acids are preferred . suitable fatty acids include capric , lauric , myristic , palmitic , stearic , and / or behenic acid . suitable fatty acids or derivatives thereof for reaction with the sorbitol or derivative thereof are preferably derived from natural sources , preferably from vegetable sources . for example , lauric acid is the main acid in coconut oil and in palm kernel oil . it may also be found in animal milk , for example cow &# 39 ; s milk and goat &# 39 ; s milk . the fatty acids or derivatives thereof may be derived from palm oil , american oil palm oil , nutmeg oil , peach palm seed oil , betel nut , date seed , macadamia nut oil , watermelon seed oil , pumpkin seed or flower oil , and other vegetable sources . in one embodiment , the polyol or ester thereof is a sorbitan compound , more specifically , a sorbitan ester . suitable sorbitan esters include sorbitan cocoate , sorbitan caprate , sorbitan laurate , sorbitan myristate , sorbitan palmitate and / or sorbitan stearate . preferred sorbitan esters are sorbitan caprate and / or sorbitan laurate , preferably sorbitan monolaurate . the polyol present in the demulsifier is alkoxylated . the alkoxylated polyol preferably comprises residues of an alkoxy group , preferably a univalent radical r 2 — o —, or anion r 2 — o − , where r 2 is an alkyl group , preferably a c 1 to c 6 alkyl group , preferably c 1 to c 4 and more preferably c 2 to c 3 . preferably , the alkoxy group is a methoxy group , ethoxy group or propoxy group , preferably ethoxy or propoxy . an increase in the presence of ethoxy groups in the alkoxylated polyol or ester thereof increases the solubility of the demulsifier in water , or in aqueous phases of oil - in - water and / or water - in - oil emulsions . an increase in the presence of propoxy groups in the alkoxylated polyol or ester thereof decreases the solubility of the demulsifier in water , or in aqueous phases of oil - in - water and / or water - in - oil emulsions . the presence of ethoxy groups in the alkoxylated species increases the hydrophilic - lipophillic balance ( hlb ) of the alkoxylated species . the presence of propoxy groups in the alkoxylated species lowers the hlb of the alkoxylated species . preferably , ethoxy groups are present in the alkoxylated polyol or ester thereof at a pre - determined concentration to provide the desired water solubility and / or hlb in the demulsifier . alternatively , a mixture of ethoxy and propoxy groups may be present to provide the desired water solubility and / or hlb in the demulsifier . preferably , the alkoxylated polyol or ester thereof is derived from the reaction of an alkylene oxide with the polyol or ester thereof . one or more equivalents of alkylene oxide may react with each polyol molecule or molecule of the ester thereof . preferably , the polyol is polyalkoxylated . preferably , the alkylene oxide is selected from the group comprising c 1 to c 6 alkylene oxides , preferably c 1 to c 4 and more preferably c 2 to c 3 alkylene oxides . preferably , the alkylene oxide is ethylene oxide or propylene oxide or a mixture thereof . preferably , the alkoxylated polyol or ester thereof comprises between 1 and 500 alkylene oxide equivalents per molecule , preferably between 1 and 400 , more preferably between 1 and 200 and most preferably between 2 and 100 alkylene oxide equivalents per molecule . where the number of equivalents of alkylene oxide is given in terms of per molecule , preferably , this is the average number of equivalents per molecule in a sample of the product . individual molecules in the sample may have fewer or greater than the stated number of equivalents of alkylene oxide , but on average , the sample will comprise molecules having an average of the stated number of equivalents of alkylene oxide . where the demulsifier comprises an alkoxylated polyol , there are preferably between 1 and 500 alkylene oxide equivalents per molecule , preferably between 2 and 400 , more preferably between 5 and 200 and most preferably between 10 and 100 alkylene oxide equivalents per polyol molecule . where the demulsifier comprises an alkoxylated ester of a polyol , there are preferably between 1 and 500 alkylene oxide equivalents per molecule , preferably between 2 and 300 , more preferably between 3 and 150 and most preferably between 5 and 50 alkylene oxide equivalents per molecule . in one embodiment , the alkoxylated polyol is preferably an alkoxylated sorbitol , more preferably an ethoxylated sorbitol . preferably , the alkoxylated polyol comprises between 1 and 500 alkylene oxide equivalents per molecule , preferably between 1 and 400 , more preferably between 1 and 200 and most preferably between 2 and 100 alkylene oxide equivalents per sorbitol molecule . preferably , the alkoxylated sorbitol has the general structure ( ii ): where a , b , c , d , e and f may each independently be any number between 0 and 100 ; ao is an alkylene oxide residue , preferably an ethylene oxide ( eo ) residue ; and where a + b + c + d + e + f is between 1 and 500 , preferably between 1 and 400 , more preferably between 1 and 200 , even more preferably between 2 and 100 and most preferably between 10 and 40 . preferably , in this embodiment , the alkoxylated polyol is an ethoxylated sorbitol , more preferably a polyoxyethylene ( x ) sorbitol , wherein x is a number between 1 and 40 , preferably polyoxyethylene ( 10 ) sorbitol or polyoxyethylene ( 40 ) sorbitol , where a + b + c + d + e + f in formula ( ii ) is 10 or 40 , most preferably polyoxyethylene ( 40 ) sorbitol , where a + b + c + d + e + f in formula ( ii ) is 40 . polyoxyethylene ( 40 ) sorbitol is available commercially from croda under the trade name atlas tm g2004 . in another embodiment , the alkoxylated polyol ester is preferably an alkoxylated sorbitan ester , more preferably an ethoxylated sorbitan ester . preferably , the alkoxylated polyol ester comprises between 1 and 500 alkylene oxide equivalents per molecule , preferably between 1 and 400 , more preferably between 1 and 200 and most preferably between 2 and 100 alkylene oxide equivalents per sorbitan ester molecule . preferably , the alkoxylated sorbitan ester has the general structure ( iii ): where w , x , y and z may each independently be any number between 0 and 100 ; ao is an alkylene oxide residue , preferably an ethylene oxide ( eo ) residue ; r is an alkyl group ; and where w + x + y + z is between 1 and 300 , preferably between 2 and 200 , more preferably between 3 and 100 and most preferably between 5 and 50 . in formula ( iii ), r may be saturated or unsaturated , preferably saturated . r preferably comprises between 1 and 29 carbon atoms , preferably between 5 and 25 , preferably between 9 and 21 , more preferably between 11 and 17 . preferably r is derived from a fatty acid , preferably selected from the group comprising lauric acid , palmitic acid , stearic acid and oleic acid . preferably , in this embodiment , the alkoxylated polyol ester is an ethoxylated sorbitan ester , preferably an ethoxylated sorbitan monolaurate , monopalmitate , monostearate or monooleate , more preferably an ethoxylated sorbitan monolaurate , and most preferably polyoxyethylene ( 20 ) sorbitan monolaurate , where w + x + y + z in formula ( iii ) is 20 . polyoxyethylene ( 20 ) sorbitan monolaurate is available commercially from croda under the trade name tween ™ 20 . the dicarboxylic acid present in the demulsifier preferably has from 4 to 40 carbon atoms . preferably , the dicarboxylic acid is aliphatic . typically , the dicarboxylic acid is of the formula ( iv ): where r 3 is a c 2 to c 36 hydrocarbyl group which can be saturated or unsaturated , linear or branched and can be aromatic e . g . a phenyl ring ( thus giving a phthalic , terephthalic or iso - phthalic dicarboxylic acid ) or , desirably , aliphatic e . g . an alkylene or alkenylene group , and may be cyclic though it is desirably open chain . commonly r 3 r is a group : —( ch 2 ) m -, where m is from 2 to 36 . suitable reactive derivatives of the dicarboxylic acids include lower e . g . c 1 to c 4 and particularly methyl , alkyl esters ( usually diesters ) and anhydrides , particularly cyclic anhydrides such as succinic , maleic and phthalic anhydrides . in one embodiment , the dicarboxylic acid has at least 4 carbon atoms , preferably at least 5 and more preferably at least 6 carbon atoms . in this embodiment , the dicarboxylic acid preferably comprises up to 36 carbon atoms , preferably up to 20 carbon atoms , more preferably up to 12 carbon atoms and most preferably up to 10 carbon atoms . in this embodiment , the dicarboxylic acid may be selected from the group comprising malonic acid , succinic acid , glutaric acid , adipic acid , pimelic acid , suberic acid , axelaic acid and sebacic acid , preferably adipic acid , suberic acid and sebacic acid , more preferably adipic acid . in another embodiment , the dicarboxylic acid is preferably a dimer acid . in this embodiment , the dimer acid preferably comprises from 24 to 52 carbon atoms , preferably from 28 to 48 carbon atoms , more preferably from 32 to 46 carbon atoms and most preferably from 36 to 44 carbon atoms . preferably the dimer acid is a c36 dimer acid . the term dimer fatty acid is well known in the art and refers to the dimerisation product of mono - or polyunsaturated fatty acids and / or esters thereof . preferred dimer acids are dimers of c 10 to c 30 , more preferably c 12 to c 24 , particularly c u to c 22 , and especially c 18 alkyl chains . suitable dimer fatty acids include the dimerisation products of oleic acid , linoleic acid , linolenic acid , palmitoleic acid , and elaidic acid . the dimerisation products of the unsaturated fatty acid mixtures obtained in the hydrolysis of natural fats and oils , e . g . sunflower oil , soybean oil , olive oil , rapeseed oil , cottonseed oil and tall oil , may also be used . hydrogenated , for example by using a nickel catalyst , dimer fatty acids may also be employed . in addition to the dimer fatty acids , dimerisation usually results in varying amounts of oligomeric fatty acids ( so - called “ trimer ”) and residues of monomeric fatty acids ( so - called “ monomer ”), or esters thereof , being present . the amount of monomer can , for example , be reduced by distillation . particularly preferred dimer fatty acids have a dicarboxylic ( or dimer ) content of greater than 70 %, more preferably greater than 85 %, and particularly greater than 94 % by weight . preferably , the molar ratio of alkoxylated polyol or ester thereof to dicarboxylic acid in the reaction product is at least 0 . 05 : 1 , preferably at least 0 . 1 : 1 , more preferably at least 0 . 5 : 1 and most preferably at least 1 : 1 . preferably , the molar ratio of alkoxylated polyol or ester thereof to dicarboxylic acid in the reaction product is up to 20 : 1 , preferably up to 10 : 1 , more preferably up to 5 : 1 and most preferably up to 3 : 1 . optionally , the reaction product of the alkoxylated polyol or ester thereof and the dicarboxylic acid may further comprise an end - cap . preferably , the end - cap comprises a monovalent radical . preferably , the end - cap comprises a monocarboxylic acid . preferably , the monocarboxylic acid is a fatty acid . preferably , the monocarboxylic acid comprises from 2 to 30 carbon atoms preferably between 12 and 26 , more preferably between 14 and 22 carbon atoms and most preferably between 18 and 22 carbon atoms . the monocarboxylic acid may be selected from the group comprising lauric acid , myristic acid , palmitic acid , stearic acid , arachidic acid and behenic acid , preferably stearic acid and behenic acid . fatty acids suitable for use herein can be obtained from natural sources such as , for example plant or animal esters . for example , the acids may be obtained from palm oil , rape seed oil , palm kernel oil , coconut oil , babassu oil , soybean oil , castor oil , sunflower oil , olive oil , linseed oil , cottonseed oil , safflower oil , tallow , whale or fish oils , grease , lard and mixtures thereof . the fatty acids can also be synthetically prepared . relatively pure unsaturated fatty acids such as oleic acid , linoleic acid , linolenic acid , palmitoleic acid , and elaidic acid may be isolated , or relatively crude unsaturated fatty acid mixtures employed . resin acids , such as those present in tall oil , may also be used . preferably , the monocarboxylic acid is saturated . the fatty acid may be either a branched fatty acid or a linear fatty acid . a mixture of fatty acids may be present . in this case , the mixture may comprise branched fatty acids , linear fatty acids , or a mixture thereof . preferably , the molar ratio of the end - cap to the reaction product of the alkoxylated polyol or ester thereof and the dicarboxylic acid is at least 0 . 05 : 1 , preferably at least 0 . 1 : 1 , more preferably at least 0 . 2 : 1 and most preferably at least 0 . 35 : 1 . preferably , molar ratio of the end - cap to the reaction product of the alkoxylated polyol or ester thereof and the dicarboxylic acid is up to 20 : 1 , preferably up to 10 : 1 , more preferably up to 5 : 1 and most preferably up to 2 : 1 . preferably , the molar ratio of the alkoxylated polyol or ester thereof to the dicarboxylic acid to the end - cap is at least 1 : 0 . 1 : 1 and more preferably at least 1 : 0 . 5 : 1 . preferably , molar ratio of the end - cap to the reaction product of the alkoxylated polyol or ester thereof and the dicarboxylic acid is up to 1 : 1 : 5 and more preferably up to 1 : 1 : 2 . preferably , the reaction product has a molecular weight of greater than 700 daltons , preferably greater than 1000 daltons , more preferably greater than 1500 daltons and most preferably greater than 2000 daltons . preferably , the reaction product has a molecular weight of less than 100000 daltons , preferably less than 80000 daltons , more preferably less than 50000 daltons and most preferably less than 20000 daltons . preferably , the reaction product has a relative solubility number ( rsn ) of at least 2 , preferably at least 4 , more preferably at least 6 and most preferably at least 8 . preferably , the reaction product has an rsn of up to 100 , preferably up to 60 , more preferably up to 30 and most preferably up to 20 . the rsn is a measure of the solubility of the demulsifier and corresponds to the hydrophilic - lipophilic balance of the demulsifier . the rsn can be determined according to the method set out in wu et al , colloids and surfaces : a , physicochemical and engineering aspects ; 2004 ; vol . 232 ( 2 - 3 ); pages 229 - 237 . preferably , the reaction product has a toxicity of less than 20 , 000mg / l , preferably less than 15 , 000 mg / l , preferably less than 10 , 000 mg / l and most preferably less than 5 , 500mg / l . the toxicity is determined according to the method set out below in experimental example 2 . preferably , the reaction product shows a biodegradation of at least 1 %, preferably at least 5 %, more preferably at least 8 % and most preferably at least 10 %. the biodegradation is determined according to the method set out below in experimental example 2 preferably , the reaction product has a viscosity at 25 ° c . of greater than 100 mpa · s , preferably greater than 300 mpa · s , more preferably greater than 500 mpa · s and most preferably greater than 900 mpa · s . the viscosity is measured at 25 ° c . on a brookfield viscometer using a 29 spindle at a shear rate of 0 . 25n . preferably , the reaction product has a pour point of less than 100 ° c ., preferably less than 80 ° c ., more preferably less than 50 ° c . and most preferably less than 30 ° c . preferably , the reaction product has a pour point of greater than 1 ° c ., preferably greater than 5 ° c ., more preferably greater than 10 ° c . preferably , the pour point is measured on an isl mpp 5gs automated pour point analyser according to the astm d97 standard method . preferably , the reaction product has a ph which is approximately neutral . preferably , the reaction product has a ph of between 3 and 12 , preferably between 4 and 10 , more preferably between 5 and 8 and most preferably between 6 and 7 . the ph of the reaction product is measured at a concentration of 1 % in an 85 % ipa solution using an hi 8424 portable ph probe . preferably , the reaction product has a density at 25 ° c . of at least 0 . 1 g / cm 3 , preferably at least 0 . 5 g / cm 3 , more preferably at least 0 . 8 g / cm 3 , and most preferably at least 1 . 0 g / cm 3 . preferably , the reaction product has a density at 25 ° c . of up to 10 g / cm 3 , preferably up to 5 g / cm 3 , more preferably up to 3 g / cm 3 , and most preferably up to 2 g / cm 3 . the density may be determined by pouring 10 ml of sample into a measuring cylinder and calculating the approximate density from the weight . preferably , the reaction product has good thermal stability in air and / or nitrogen . preferably , the reaction product is stable in air up to a temperature of at least 50 ° c ., preferably at least 100 ° c ., more preferably at least 150 ° c . and most preferably at least 200 ° c . before the product starts to degrade . preferably , the reaction product is stable in nitrogen up to a temperature of at least 50 ° c ., preferably at least 100 ° c ., more preferably at least 150 ° c . and most preferably at least 200 ° c . before the product starts to degrade . the thermal stability in air and nitrogen was measured according to the method set out in experimental example 1 below . preferably , the reaction product shows a mass loss in air over a period of 1 hour at 150 ° c . of less than 50 %, preferably less than 30 %, more preferably less than 15 % and most preferably of less than 7 %. preferably , the reaction product shows a mass loss in air over a period of 1 hour at 200 ° c . of less than 90 %, preferably less than 85 %, more preferably less than 80 % and most preferably of less than 75 %. the mass loss was measured according to the method set out in experimental example 1 below . preferably , the demulsifier is used at a dosage rate of between 0 . 01 and 1000 ppm , preferably between 0 . 05 and 800 ppm , more preferably between 0 . 1 and 500 ppm and most preferably between 0 . 5 and 100 ppm in the emulsion to be demulsified . it should be noted that particular demulsifiers can be extremely emulsion - specific . therefore , the failure of a demulsifier to work on one or two tests does not mean that the demulsifier is unsuitable everywhere . this fact makes it extremely difficult to judge the worth of a particular potential demulsifier based on a few negative results alone . positive results , however , may point to the worth not only of the demulsifier itself , but of the class of chemistry that that particular demulsifier represents . consequently , the existence of several cases of outstanding positive performance gives credibility to this invention as a whole . according to a second aspect of the present invention , there is provided a demulsification formulation comprising a demulsifier which is the reaction product of : a ) an alkoxylated sorbitol or sorbitan ester ; b ) a dicarboxylic acid ; and c ) optionally , an end - cap . preferably , the demulsification formulation is for demulsifying an oil - in - water or a water - in - oil emulsion . preferably , the demulsifier which is the reaction product of the alkoxylated polyol or ester thereof , dicarboxylic acid and optional end - cap is present in the demulsification formulation at a concentration of at least 2 % w / w based on the total weight of the demulsification formulation , preferably at least 5 % w / w , more preferably at least 10 % w / w and most preferably at least 15 % w / w . preferably , the demulsifier is present in the demulsification formulation at a concentration of up to 80 % w / w based on the total weight of the demulsification formulation , preferably up to 60 % w / w , more preferably up to 50 % w / w and most preferably up to 30 % w / w . preferably , the demulsification formulation also comprises a solvent . the solvent is preferably a derivative of the oil phase of the emulsion to be demulsified . for example , for crude oil , the solvent may be selected from xylene , heavy or light aromatic naphtha , ipa , methanol 2eh , diesel or toluene . preferably , the solvent is present in the demulsification formulation at a concentration of at least 20 % w / w based on the total weight of the demulsification formulation , preferably at least 30 % w / w , more preferably at least 40 % w / w and most preferably at least 50 % w / w . preferably , the solvent is present in the demulsification formulation at a concentration of up to 98 % w / w based on the total weight of the demulsification formulation , preferably up to 90 % w / w , more preferably up to 80 % w / w and most preferably up to 70 % w / w . preferably , the solvent is present in the demulsification formulation at a ratio to the demulsifier of up to 50 : 1 , preferably up to 30 : 1 , more preferably up to 20 : 1 and most preferably up to 10 : 1 . preferably , the solvent is present in the demulsification formulation at a ratio to the demulsifier of at least 1 : 10 , preferably at least 1 : 5 , more preferably at least 1 : 2 and most preferably at least 1 : 1 . the demulsification formulation may optionally further comprise a wetting agent . preferably , the wetting agent is a surfactant , preferably an anionic surfactant . preferably , the wetting agent is or comprises an ester . any suitable surfactant , particularly an ester - containing surfactant , or mixtures thereof may be used as the wetting agent in the present invention . examples of suitable wetting agents include , but are not limited to alkyl sulfates , such as ammonium lauryl sulfate , sodium lauryl sulfate ; alkyl ether sulfates , such as sodium laureth sulfate , also known as sodium lauryl ether sulfate ( sles ), sodium myreth sulfate ; sulfonates such as dioctyl sodium sulfosuccinate , perfluorooctanesulfonate ( pfos ), perfluorobutanesulfonate ; alkyl benzene sulfonates ; phosphates such as alkyl aryl ether phosphate , akyl ether phosphate , carboxylates such as alkyl carboxylates , i . e . fatty acid salts ( soaps ), sodium stearate , sodium lauroyl sarcosinate , carboxylate fluorosurfactants ( perfluorononanoate , perfluorooctanoate ( pfoa or pfo )) and synperonic ™ lf / 30 ( ex croda ). when present , the wetting agent is preferably present in the demulsification formulation at a concentration of at least 1 % w / w based on the total weight of the demulsification formulation , preferably at least 3 % w / w , more preferably at least 6 % w / w and most preferably at least 8 % w / w . preferably , when present , the wetting agent is present in the demulsification formulation at a concentration of up to 25 % w / w based on the total weight of the demulsification formulation , preferably up to 20 % w / w , more preferably up to 15 % w / w and most preferably up to 12 % w / w . the demulsification formulation may optionally further comprise an alkylene oxide block copolymer , for example an ethylene oxide ( eo )/ propylene oxide ( po ) block copolymer . the block copolymer may be either an eo - p0 copolymer , eo - po - eo copolymer or po - eo - po copolymer . preferably , the block copolymer has a molecular weight of between approximately 1000 and 10000 , preferably between 1500 and 8000 , more preferably between 2000 and 7000 . preferably , the block copolymer has an hlb of between 0 . 1 and 20 , preferably between 0 . 5 and 17 , and ore preferably between 1 and 15 . examples of suitable block copolymers include , but are not limited to surfonic ™ eo / po block copolymers ex huntsman and eo / po block copolymers from ineos oxide . when present , the alkylene oxide block copolymer is preferably present in the demulsification formulation at a concentration of at least 1 % w / w based on the total weight of the demulsification formulation , preferably at least 3 % w / w , more preferably at least 6 % w / w and most preferably at least 8 % w / w . preferably , when present , the alkylene oxide block copolymer is present in the demulsification formulation at a concentration of up to 25 % w / w based on the total weight of the demulsification formulation , preferably up to 20 % w / w , more preferably up to 15 % w / w and most preferably up to 12 % w / w . preferably , the demulsification formulation is anhydrous . preferably , the demulsification formulation comprises less than 5 % water , preferably less than 3 %, more preferably less than 2 % and most preferably less than 1 % water . it will be appreciated that the exact composition of demulsifying formulations will vary according to the particular emulsion it is to be used on , and even for crude oil obtained from the same well , over time , the optimum amount of demulsifier will vary as the production conditions change . for example , different temperature and pressure conditions , concentrations of naturally occurring emulsifiers , production techniques , etc ., make it impossible to predict in advance the demulsifier proportions required . preferably , the demulsification formulation is used at a dosage rate of between 1 and 1000 ppm , preferably between 5 and 800 ppm , more preferably between 15 and 500 ppm and most preferably between 20 and 200 ppm in the emulsion to be demulsified . according to a third aspect of the present invention , there is provided a method of demulsifying an oil - in - water or water - in - oil emulsion , the method comprising adding a reaction product of : a ) an alkoxylated sorbitol or sorbitan ester ; b ) a dicarboxylic acid ; and c ) optionally , an end - cap to the emulsion . preferably , the emulsion is a water - in - oil emulsion . preferably , the emulsion is an emulsion of water , preferably salted water , more preferably sea / ocean water in crude oil . according to a further aspect of the invention , there is provided the use of a reaction product of : a ) an alkoxylated sorbitol or sorbitan ester ; b ) a dicarboxylic acid ; and c ) optionally , an end - cap preferably , the reaction product is used in the demulsification of an oil - in - water or water - in - oil emulsion , preferably a crude oil emulsion . any of the above features of the invention may be combined in any combination and with any aspect of the invention . the present invention will now be described further , for illustrative purposes only , in the following examples . all parts and percentages are given by weight unless otherwise stated . a composition comprising sorbitan ( 20eo ) monolaurate and adipic acid in a 2 : 1 molar ratio was produced . 384 kg of sorbitan ( 20eo ) monolaurate was added to the reaction vessel and heated to 80 ° c . whilst stirring . 23 kg of adipic acid flake was slowly added to the warm stirred sorbitan ( 20eo ) monolaurate . the mixture was heated to 235 ° c . observing distillation water removal . the reaction was continued until an acid value of the less than 5 mgkoh / g was observed . a composition comprising sorbitan ( 20eo ) monolaurate , adipic acid and stearic acid in a 2 : 1 : 2 molar ratio was produced . 313 kg of sorbitan ( 20eo ) monolaurate was added to the reaction vessel and heated to 60 ° c . whilst stirring . 73 kg of stearic acid flake was slowly added to the warm stirred sorbitan ( 20eo ) monolaurate and the resulting mixture heated to 80 ° c . 19 kg of adipic acid flake was then added to the mixture in the reaction vessel . the mixture was heated to 235 ° c . observing distillation water removal . the reaction was continued until an acid value of the less than 5 mgkoh / g was observed . a composition comprising sorbitol ( 40eo ), adipic acid and stearic acid in a 1 : 1 : 2 molar ratio was produced . 299 kg of sorbitol ( 40eo ) was added to the reaction vessel and heated to 60 ° c . whilst stirring . 86 kg of stearic acid flake was slowly added to the warm stirred sorbitol ( 40eo ) and the resulting mixture heated to 80 ° c . 23 kg of adipic acid flake was then added to the mixture in the reaction vessel . the mixture was heated to 235 ° c . observing distillation water removal . the reaction was continued until an acid value of the less than 5mgkoh / g was observed . a composition comprising sorbitol ( 40eo ) and c 36 dimer acid in a 4 : 3 molar ratio was produced . 72 kg of c 36 dimer acid was added to the reaction vessel and heated to 80 ° c . whilst stirring . 335 kg of sorbitol ( 40eo ) was slowly added to the warm stirred c 36 dimer acid . the mixture was heated to 235 ° c . observing distillation water removal . the reaction was continued until an acid value of the less than 5mgkoh / g was observed a ) the physical properties of the demulsifiers 1 - 4 were tested . the results and standard testing methods are detailed in table 1 below . between 10 and 15 mg of the sample to be tested was weighed into a 70 μl alumina crucible put into the thermogravimetric analyser &# 39 ; s ( mettler tg50 ) furnace and run under air at the following conditions : gas ( flow rate ): air ( 200 ml / min ) temperature range : 30 - 150 ° c . at 50 ° c ./ min then 150 ° c . for one hour then 150 - 600 ° c . at 50 ° c ./ min then 600 ° c . for five minutes . the method as described above was performed , but over the below temperature range : 30 - 200 ° c . at 50 ° c ./ min then 200 ° c . for one hour then 200 - 600 ° c . at 50 ° c ./ min then 600 ° c . for five minutes . the percentage mass lost was calculated by step horizontal analysis using stare software ( version 9 . 2 ) on the results of the two methods described above . the results are shown in table 2 below . the biodegradation of the demulsifiers was tested according to the guidelines set out in oecd 306 “ biodegradability in sea water ” ( adopted : 17 . 07 . 92 ). the method set out on pages 10 to 17 of the guidelines , i . e . the closed bottle method , was performed . the results are given in table 3 below . the toxicity of the demulsifiers was tested according to iso 10253 ( second edition , 15 . 04 . 2006 ). the results are given in table 3 below . samples of crude oil used was classified using the american petroleum institute ( api ) test guidelines . the oil was obtained from a uk onshore source at star energy located near to lincoln in april 2011 . the crude oil samples were cut with water at the percentages described below in table 4 . tests were performed on the oil samples from table 4 to determine the effectiveness of the demulsifiers described above . the tests were carried out using a turbiscan tlab thermo manufactured by formulaction . the demulsifiers were added to the crude oil at a concentration of 100 ppm ( in xylene ) and analysed in the turbiscan at 1 scan per minute for 60 minutes . the temperature of the turbiscan was 60 ° c . the turbiscan monitors the duration of the demulsification , along with the transmission level and clarity of the resulting water phase and the quality of the interface between the resulting water and oil phases . the results of the turbiscan tests are given in table 5 . the present invention , therefore , provides a demulsifier which shows the same or superior properties and efficacy as the standard production chemicals , but which is environmentally friendly and reaches or surpasses the rules of the ospar . the demulsifier of the present invention shows good stability , so it can easily be stored onsite at off - shore drilling locations until required for use . when used , the demulsifier shows good efficacy in demulsifying crude oil emulsions , and has superior biodegradability and toxicity properties meaning it can safely be discharged into the sea / ocean without further treatment of the aqueous phase of the demulsified emulsion being required . any or all of the disclosed features , and / or any or all of the steps of any method or process described , may be combined in any combination . each feature disclosed herein may be replaced by alternative features serving the same , equivalent or similar purpose . therefore , each feature disclosed is one example only of a generic series of equivalent or similar features . the above statements apply unless expressly stated otherwise . the term specification , for these purposes , includes the description and any accompanying claims , abstract and drawings .