Patent Application: US-200913056858-A

Abstract:
an improved process for converting an oil suspension of nanoparticles into a water suspension of nps , wherein water and surfactant and a non - surfactant salt is used instead of merely water and surfactant , leading to greatly improved np aqueous suspensions .

Description:
the invention is a novel way of converting oil preparations of nps to nps dispersed in aqueous phase , using an additional ingredient — salt . phase - transfer by the newly proposed method results in dispersions that have following features : ( 1 ) high stability of nps from aggregation for several weeks / months , and ( 2 ) retention in functional properties of nps for several weeks / months . stability from aggregation and retention of np functionality is significantly better by this route when compared to dispersions obtained by conventional methods that phase - transfer nps to water . further , addition of salt also increases the fraction of particles that get transferred from oil to water . a preferred salt for use in the invention is nacl , but many other salts can be used , provided only that the salt function to lower the critical micellar concentration and to form compact micelles by charge screening of surfactants and does not otherwise alter the functionality of the np , oil or surfactant . in preferred embodiments , the salt comprises a cationic portion selected from the group consisting of alkali , alkaline earth metal , sodium , potassium , magnesium , calcium , and quaternary ammonium and an anionic portion selected from the group consisting of halide , acetate , malate , fumarate , citrate , succinate , phosphate , polyphosphates , hypochlorite , perchlorite , carbonate , bicarbonate , bisulphite , sulphite , sulphonate , and nitrate . salts that can be used include , but are not limited to , nacl , kcl , cacl 2 , mgcl 2 , na 2 so 4 , k 2 so 4 , caso 4 , na 3 po 4 , k 3 po 4 , ca 3 ( po 4 ) 2 , na 3 citrate ( sodium citrate ), nai , na 2 hpo 4 ( disodium hydrogen phosphate ), sodium acetate , edta and quarternary ammonium salts . preferred oils for use in the invention should be of low molecular weight so that the boiling point of the oil is lower than that of water . this constraint may be overcome , however , by a simple liquid extraction process of nps from higher molecular weight oils , through the use of low molecular weight organic solvents with higher selectivity towards nps . this requires an additional step in the phase - transfer process , however , and is less preferred . alternately , for the case where the nps are insensitive to high temperatures , the high molecular weight organic solvents can be boiled off and the nps may be precipitated and / or dried before being redissolved into lower molecular weight solvents . additional variations in the invention include other types of anionic surfactants , such as , but not limited to , sodium dodecyl benzene sulfonate ( sdbs ), sodium dodecyl sulfate ( sds ), sodium laureth sulfate ( sles ) or anionic lipids like phosphatidic acid , phosphotidyl glycerol , phosphotidylinositol , etc ., over and above herein demonstrated anionic aerosol - ot ( aot ). other categories of surfactants might also be employed in the invention , such as cationic ( positively charged ), nonionic ( neutral ), zwitterionic ( positive and negative charge ) and gemini surfactants ( 2 regular surfactants linked together by spacer molecules ). examples of cationic surfactants include cetyl trimethyl ammonium bromide ( ctab ), cetylpyridinium chloride ( cpc ), benzalkonium chloride ( bac ), benzethonium chloride ( bzt ), amine and imidazoline salts such as primary , secondary and tertiary amine hydrochlorides and imidazoline hydrochlorides . examples of nonionic surfactants include alkyl poly ( ethylene oxide ), copolymers of poly ( ethylene oxide ) and poly ( propylene oxide ), alkyl polyglucosides , alkyl phenol ethoxylate , polyoxyethylene esters of fatty acids , polyoxyethylene fatty acid amides , alkyl polysaccharides , ethoxylates of alkyl amine , castor oil , end capped synthetic alcohol , tallow amine and alkanol amine mercaptan . examples of zwitterionic surfactants include dodecyl betaine , dodecyl dimethylamine oxide , docamidopropyl betaine and coco ampho glycinate . alcohols and amines that can be used to stabilize the dispersions include , but are not limited to , methanol , ethanol , isopropanol , butanol , monoethanol amine , diethanol amine , triethanol amine , ethylenediamine . other embodiments of the invention include phase - transfer nps using bicontinuous / o / w microemulsions , o / w nanoemulsions / w / o nanoemulsions , in addition to herein demonstrated phase - transfer using w / o microemulsions . also , w / o emulsions , water - in - oil - in - water ( w / o / w ) multiple emulsions or oil - in - water - in - oil ( o / w / o ) multiple emulsions can advantageously be employed . in one embodiment , the nps are solubilized in oil , surfactant added to the np - oil blend and the resulting np - oil surfactant blend is phase - transferred at high temperature in salt - containing water . in another embodiment , the nps are incorporated in microemulsions ( w / o / bicontinuous / o / w ) or in emulsions ( w / o or o / w ) or in multiple emulsions ( w / o / w or o / w / o ), the oil boiled off and the resulting np surfactant mix phase - transferred in salt - containing water . in another embodiment , the water may contain different types of surfactants or their mixtures , containing different types of salts such as sodium citrate , sodium iodide , sodium hydrogen phosphate , sodium acetate , sodium bromide etc . or their mixtures and under conditions of different ph and phase - transfer temperatures . other salts included but not restricted to are weakly dissociating salts that act as salting - in electrolytes or water structure breakers , such as urea , guanidium chloride and 1 - 4 dioxane . in another embodiment , the method involves phase - transfer of mixtures of different nps from oil to water to produce dispersions with dual / multiple functionalities . for example , au and pd nanoparticles can be mixed in hexane and aot and phase - transferred to salt containing water to result in assemblies of au — pd nanoparticles that can catalyze aqueous phase reactions . alternately , the catalytic properties of au or pd could be combined with the optical properties of cdse tetrapods or quantum dots to give novel np assemblies with optically responsive catalytic properties . in another embodiment , the method allows control over self - assembly of np aggregates comprising of a single type of np or mixtures of different nps as a function of salt and / or alcohols . as an example , controlled crystallization of quantum dots could be obtained by phase - transferring them using oppositely charged surfactants ( cationic and anionic ) in salt water . in another embodiment , the method allows control over self - assembly of np aggregates comprising of a single type of np or mixtures of different nps and as a function of evaporation rates of oils that solubilize and host the nps . the self - assembled aggregates obtained through procedures described above would potentially have novel optical , electrical , catalytic and magnetic properties . in another embodiment , polymers are incorporated in phase - transfer solutions so as to add stability and functionality to np dispersions and as avenues to create new structures such as functional / smart capsules , micro / nanowires etc . as an example , controlled crystallization of quantum dots or formation of np assemblies / nanowires could be obtained by phase - transferring them using combinations of oppositely charged surfactants and polymers in salt water . in another embodiment , the method allows the formation of np dispersions stable across a wide range of ionic strength , ph and temperatures using mixed surfactants of ionic and non - ionic types , for high ionic - strength biological applications and as possible hydrocarbon / oil / gas sensing agents in hydrocarbon reservoirs . this example demonstrates benefits obtained by phase - transferring different types of nps into salt - containing water over conventional processes that phase - transfer nps to plain water . in all experiments , the oil used was hexane , surfactant used was aot at 1 . 5 g / l and salt used was nacl at 3 g / l , unless specified otherwise . table 2 summarizes the results of phase - transfer experiments carried out in plain water , and in salt containing water , featuring nps of different compositions and shapes . phase - transfer experiments were carried out as per the descriptions provided in table 1 ( bagaria , et al ., 2009 ). in all experiments for this section , the oil used was hexane , surfactant used was aerosol ot ( aot ) and salt used was sodium chloride ( nacl ). phase - transfer was carried out at 85 ° c . all resulting np dispersions were characterized by their “ dispersion ” and “ functionality ” attributes . dispersion refers to the settling of nps in time and was recorded visually or after centrifugation at 9000 rpm for 1 hour by uv measurements ( where applicable ). functionality refers to measurable physical properties that are specific to a type of np , such as photoluminescence etc . inorganic semiconductor nps ( cdse ) have a characteristic property called fluorescence , wherein nanoparticles upon exposure to uv light , emits light in the visible region ( appears orange / red / green etc .). photoluminescence ( pl ) is the quantitative measure of fluorescence recorded by a fluorometer that was used to differentiate between np dispersions of cdse , when phase - transferred to plain water and water containing salt . cdse is available in different shapes such as spheres , tetrapods and concentric spheres ( cdse / zns core - shell ), whose dispersibility in plain water , and in water containing salt were also characterized by pl measurements ( fig2 and fig5 - 8 ). retention in pl of cdse dispersions prepared in plain water and in water containing salt was investigated as a function of time for qds ( fig3 ), tetrapods ( fig6 ) and core - shell particles ( fig8 ). all time studies of pl are reported as a fraction of original pl value of respective cdse np types in hexane , so as to have a reference point for comparison . organic np ( c 60 ) transferred to plain water and to water containing salt was characterized by uv spectroscopy ( fig9 ). visual inspection showed that phase - transfer of c 60 to water that did not contain salt was a near transparent solution while that transferred to water containing salt had a light grayish / pink color . it is noteworthy that in the former case , a lot of c 60 particles were stuck to the magnetic stirrer rod and the glass walls of reaction vials after phase - transfer , while when transferred with salt , this was not observed . absorbance value is an indicator to the quantity of material present suggesting superior phase - transfer of c 60 nanoparticles in water containing salt . inorganic gold nps ( au ) transferred to water plain water and to water containing salt was characterized by uv spectroscopy ( fig1 ). visual inspection showed that phase - transfer of au to plain water was a near transparent solution while that transferred to water containing salt had a distinct pink color ( data not shown , see bagaria et al . 2009 ). as in the case of c 60 phase - transfer , many au particles stuck to the magnetic stirrer rod and the glass walls when phase transferred in plain water , while with salt this was not observed . absorbance peaks is an indicator to the quantity of material present , suggesting superior phase - transfer of au nanoparticles in water containing salt . visual inspection of phase - transferred inorganic metal - oxide nps ( iron - oxide spheres ) also showed that the fraction of nps transferred from oil to water was higher in salt - containing water ( fig1 ) as evidenced by the darker colored np dispersion . table 3 summarizes the results of phase - transfer experiments carried out in plain water with no salt , and in salt - containing water , featuring oils that are non - polar in nature such as hexane and polar oils such as chloroform . phase - transfer experiments were carried out as per the descriptions provided in table 1 ( baragia et al ., 2009 ). in all experiments for this section , the surfactant used was aot , salt used was nacl at 3 g / l and the nps phase - transferred were spherical cdse qds . phase - transfer was carried out at 85 ° c . pl measurements were used to characterize the dispersions ( fig1 ). the pl of cdse qds phase transferred from chloroform to salt - containing water was higher than that of cdse qds phase transferred to plain water . thus , the benefits of dispersing nps in salt - water is independent of the oil that initially stores the nps . table 4 summarizes the results of phase - transfer experiments carried out in water with salts other than sodium chloride — namely sodium sulfate ( na 2 so 4 , fig1 ), calcium chloride ( cacl 2 , fig1 ) and sodium citrate ( na 3 citrate , fig1 ). phase - transfer experiments were carried out as per the description provided in schematic 2 , shown in fig2 . in all experiments for this section , the oil used was hexane , the surfactant used was aot and the nps phase - transferred were spherical cdse qds . sodium sulfate solutions were prepared at concentrations of 0 . 19 , 0 . 38 and 0 . 72 g / l . levels of cacl 2 were fixed at 0 . 05 and 0 . 1 g / l , whereas the study with na 3 citrate was done at 1 g / l . phase - transfer was carried out at 85 ° c . uv and / or pl measurements were used to characterize the dispersions . a control run for the experiment is described as a footnote of table 4 . table 5 summarizes the results of phase transfer experiments carried out through the use of different surfactant types , namely ctab ( cationic ), ddab ( cationic ) and a mixture of aot ( anionic ) and polystep c - m4s ( c 9 - phenyl - eo 4 — so 3 — na + : non - ionic ) surfactants . for all the experiments , the oil used was hexane and the salt used was nacl . upon phase - transfer , the samples were centrifuged at 9000 rpm for 1 hour and the supernatant nps were retained . as shown by uv - vis spectra and / or photographs in fig1 ( ctab ), fig1 ( ddab ) and fig1 ( aot and polystep c - m4s ), the phase - transfer of cdse nps using the above surfactants was found to be superior in the presence of salt over plain water . + all phase transfer experiments were carried out from hexane to plain or salt - containing water using aot as surfactant at 1 . 5 g / l . cdse qds were the nps phase transferred and nacl was the salt used . unless mentioned otherwise , the concentration of nacl was 3 g / l as the surfactant and with cdse qds as nps . after nps are phase transferred with the surfactant aot , the dispersion may appear turbid due to liquid crystalline surfactant aggregates . the presence of these aggregates results in an overall high level of absorbance due to scattering , as shown in the uv - vis spectra of cdse nps in fig2 . these aggregates can be removed by centrifugation as evidenced by the near - zero absorbance at higher wavelengths in the curve indicated as ‘ after centrifugation ’ ( fig1 ). the photographs of cdse nps clearly show the removal of the turbidity from the dispersion upon centrifugation leading to clear cdse dispersions ( refer to bagaria et al . 2009 for color images ). the stability of nps to high ionic strength solutions is higher , when nps are phase transferred using surfactants in salt - water over plain - water . fig1 depicts the percentage of nps retained in high ionic strength solutions of kcl , after solutions of cdse qds that were phase transferred in plain water and salt ( nacl ) containing water , were added to varying levels of ionic - strength of kcl . a blend of aot and polystep c - m4s surfactants was used to phase transfer nps from hexane to plain or salt - containing water . upon phase transfer , the nps were centrifuged at 9000 rpm for 1 hour to remove non - dispersed nps and liquid crystalline aggregates of aot . prior to stability studies in high ionic strength solutions of kcl , the concentration of np solutions phase - transferred in salt - water was adjusted to match np concentrations in plain water by diluting with brine and measuring uv - absorbance . for stability studies in ionic strength solutions , kcl solutions were prepared at varying levels of concentration . nps in salt - containing water and plain water were added ( dilution 1 in 5 ) such that the final kcl concentration ranged between 3 to 10 g / l . the solutions were centrifuged at 9000 rpm for 1 hour after which uv measurements were made on the supernatant solutions . the percentage np retained is the ratio of concentration of nps ( measured from absorbance values at the first exciton — 542 nm ) contained in a specified ionic strength solution of kcl after centrifugation to the concentration of nps contained in the lowest ionic strength solution ( 3 g / l kcl ) after centrifugation , as a reference point . as a note , this reference point was selected since there was no apparent difference in np concentrations as measured by uv , before and after the centrifugation step . from fig1 , it is apparent that nps stabilized by compact micelles in salt - 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