Missing:
/SSN /li71i-3632, Russian Jomnai ofGeneral Chemistry. 2li12, Vol, 82, 11'0,12, pp. 1916-1927. © Pleiades Pnbl tshmg, LId, 2li12.
(Jn ginal Russian Text ,j) I. G, Asadov, G,A. Akhmedova, A.D. Ag a-Zad eh, ShM Nasibova. I.A. Zarbaheva. A.kl Bagirova, RA, RagIIJlO1', 2li12, published
III Zlrurnal Obshchei Khiniii, 2li12, Vol. 82, No. 12, pp. 1961i-1972.
Ionic Liq uid Su rfactants Z. G. Asa dov, G. A. A khmedova, A . D. Ag a-Zadc h, Sh. M. Nasihova , 1. A. Za rbalicva, A. M. Ba gi rovn , an d R. A. Ragimov lnstitute of Petrochemical Processes of National Academy ofSciences ofAzerbaijan.
PI', t lodzhalv 3D, Baku, .'/Z ID25 Azerbaijan
e-mail.a_gulnara(d)lUtl71ai/.col71
Received November 1,20 I I
Abstract-For the first time an overview' of the available latest publications from the scientific and technical
literature on surfactant ionic liquids is compiled. The methods of preparation, structural features, the most important physicochemical properties, and the areas of application of these substances are considered. Particular attention is paid to the data related to the properties of ionic liquids manifested at the interfaces. Applied properties of ionic liquid surfaciants 'lIT demonstrated.
DOl: IO.1134/S1070363212120043 Ionic liquids consisting of a large-sized organic cation (mostly the nitrogen-containing one) and a considerably smaller inorganic anion are compounds with a melting point below ISO°C. Various com binations of cations and anions are described capable to form ionic liquids [1-5]. Variety of properties of ionic liquids, including the ability to dissolve a large number of inorganic and organic, including polymeric substances, their non corrosive properties, low viscosity, and insignificant vapor pressure attract an increasing interest to their use in multiphase homogenous catalytic reactions [6-IIJ, when one phase chosen to dissol ve a catalyst does not mix with another phase, which contains the initial and final products. Similar catalysis, as expected, occurs at the boundary of the ionic liquids and the upper organic phase. A clearer understanding of the mechanisms of these catalytic processes requires the knowledge of surface properties of the ionic liquids. For this reason,
n
=
the data concerning the surface activity of ionic liquids, incl uding the I iterature data on surface acti ve ionic liquids, are considered in the present article separately. An European patent [12J discloses a new class of ionic liquids and methods of their preparation. The se ionic liquids can be obtained on the basis of such anionic surfactants as alkylarylsulfonates and alkyl sulfate, alkylalkoxysulfonate and alkylarylsulfate derivatives branched in the middle of the chain. Similar ionic liquids can be produced also from the sui ronates like al ky I glycero I ethers, methy I sui tonates, and sulfonates of c-olefins The anions me combined with various cations supplying the final product with a variety of useful, including surfactant, properties. Kimizuka et al. [13J synthesized a series of surface active ionic liquid substances composed of fragments of ethers and esters:
1,2
1916
191 7
ION IC LIQ UID SURFAC TA NTS
OH
H
OH
I
CH J ( C H2)I IO(CH2H~ HC CI H ./ NI~O H CH, CH J(C H 2)I IO(CH 2)/',1
(~
~
OH
1
OH
I~dl-I: o
These ionic liquids are capab le of disso lving carbohydrates like D-glucose, cyclodextrin, amylose, aga rose, glucoprotein, oxidized glu cose, When glycol lipides are dispersed in an ionic liquid having affinity to carbohydrates, a stable double-layer membra ne is formed. In such membranes a reversible thermal transformati on was obse rved that incl udes the struc tures in the range from fib rillary aggregates to vesicles. Physical gelation of ionic liquids is due to the disso lution of glyco lipids enriched with lipid g roups, The authors mar ked that the mention ed prope rties of the self-formi ng ge ls (capability of disso lving carbohyd rates, forming two-layer membrane) provide the oppo rtunity for the deve lopmen t of new directions in research of ionic liquids. Unfortu nately, the paper does not provide spec ific da ta on the surface prope rties of the synthesized compounds, Merrigan et a l. [14, 15J report on the sy nthesis of imidazolium ion with a long "tail" cha in, which forms a part of the low-me lting fl uoride ionic liquids, The formatio n of a liquid-crys talline phase was obse rved, and it was therefore sugges ted that the association of fluorocarbon chains occ urred in these ionic liqu id cations , In [14J the synthesis was described of four new fluorinat ed ionic liquids that were significantly different from the know n fluoride ionic Iiquids [16-19J by the presence of a long fluorinated alky l chain in the cation part. [
R-~
A
t (C H 2!2(CF2),CFJ
R = C11 1 , X = 5 (u): R = I1-C4 [-[ 9 , = I1-C 4 H9 , X = 7 (tl).
X
] ~ [PF t 6
= 5 (b); R = CI-I), x = 7
(e): R
Melting points of these sa lts are 61, 88, 87, and 97°C respective ly, the tempera ture of appearance of trans parency 109, 134, 130, and 145°C, respectively. All four salts are surfac tants and promote formation and stabil ization of perfluor inated hydrocarbon disper sions in the medium ofa common ionic liquids [hmirn]
[PF61 ( 1-hexyl-3-methy lilmidazol ium hexafl uorophos phatc). To obtain these fluorin ated ionic liqu ids [14J imidazole derivatives, name ly l-rneth yl- and 1 buty limidazo le were disso lved in toluene, treated with a sma ll molar deficit of I, I, I,2,2,3,3,4,4,5,5,6,6-tri deca fluoro-8- iodooc tane or 1, I, I,2,2,3,3,4,4 ,5,5,6,6; 7,7,8,8-heptadecafluoro- 1O-iodododeca ne, and the mix ture was heated, After cooling to room temperature followed by remo val of the so lvent in a vacuum, a gray -wh ite substance was obtained, These iodide salts readily undergo metath esis with Ag PFc, in acetone to form the ionic liquids in good yields, These com pounds show a number of the surfactant prop ert ies. Measurements by the meth od o f the cap illary rise of level showed that the surface tension of the saturated solution (0.3 wt %, namely, ~ 1 -4 mill imola r solution) in [hmimJ[PF6 J was by 10-1 5% less than with the ionic Iiquid proper. The viscometric measur ements showed that, as in the case of conve ntional surfactants [20J, the add ition of these fluorine-containing surfactant ionic liquids to [hm imJ[PF 6 ] increased the viscosity of the latter. But the most surprising feature of these fluorinated surfactant ionic liquids is their ability to form and stabilize the perfluorohexane disper sions in usual ionic liquids, namely, in [hmimJ[P F6 l In [21J a quaterni zation reac tion was used with microwave radiation for the synthes is of a ser ies o f ionic liquid salts, which inc lude triethy lhexy lammo nium bromide, triethyldodecylam mon ium brom ide, tri ethy loctadecylammonium bromide , l-m ethyl-3-tetra decylimidazo lium bromi de, l -methyl -3-octadecy l imidazo lium brom ide, I-t etradecylpiridinium bromide, I-octadecylpiridinium bromi de, tripentylmethylammo nium chloride, trihexyimeth ylammoniurn chloride , and triocrylmerhylammonium chloride, These salts were identified using the I H and I J C NMR spectroscopy, All these ionic liquids at concentration 1000 ppm and SO°C show good demulsifyi ng capa city towards the typical crude oi l. The demul sifying capacity of eac h of the salts dec reases with incre asing density of the oil. The import ant role of the alkyl g roup at demulsificati on
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 12 2012
A SAD OV
191 8
was demonstrated. Best resu lts were obtained when the chain cont ained 12 or 14 carbo n atoms . Synthesis of the surface-active ionic liquids consist ing of imidazol ium cation and a hydrophobic chain was reported [22].
~
o N (;'N
-
"
t: ,
N (;'N
IO~ ~ B r - ~o" ~/ ~ s.: I
II
They are used to stab ilize the microemu lsion. The first ionic liquid surfacta nt [Cl rmim ]Br (I ) was synthesized by the reaction of l-bromdodecanoi with I-methyl imidazole. In the synthesis of 1-(2-acryl oyloxyundec yl)-3- methylimidazo lium bromide (II) , initially the acry loyl chloride reac ted with I I-bro mo undecano l-I in equimo lar amounts in retrahydrofuran to obtain bromoalkyl acrylate. In a reaction with 1 methylimidazole this compound forms bromide II , which is capab le to polymerize. Both products are ionic liquids at room temperature . The critical micelle-fo rmation concentration of bromide 1 in water determined by the measurement of the surface tension by the drop method is 10.31 mM at 34°e. The microernu lsion po lymerization in the c-Br methyl meracrylare- HjO sys tem (we ight ratio of 0.04:0.04:0.92) was initiated with azobisisobutyro nitrile at 60°e. During polyme rization turbidity occurred due to the aggrega tion of the particles. The resulting polymer latexes are monodi sperse , with the particle diameter about 50 nm. The critical micelle-formation concentra tion of bromide II found by the abov e method is 15.35 mM at 24°e. Like the non-po lymerizing bromide I, bromide " can be used for the stabilization of the H 20 - methyl metacrylate system to form a transparen t and stable microernu lsion . The observed benefi t of the poly merizab le surfactants in comp arison with non-poly merizing one is that it allo ws to obtain micro emulsions of nanosca le and nanos tructur e as a result of polymerization [22]. The polymerization of the microernulsion system of If-methyl metacry late-HjO (weight ratio 0.07:0_07:0.86) leads to a po lymer latex with no appa rent aggregat ion. The co polymerization of methyl metacry late with ionic liquids surfactant JI to fo rm the microemu lsio n was confirmed by the FTIR spectroscopy. The average diameter of the nanoparticle is - 30 nm as determ ined by the method of trans mission electron microscopy. Since bromide II is more
et
al.
polar than methyl metacry late, it likely forms a polyrner izable polar shell around the formed partic les, which increases the stabi lity of the particles and allows easy redispe rsing of the part icles in water and NaBr so lution without" noticeable aggre gation. This indicates that the surface of the latex is a hydrop hilic one. These latexes begin to aggre gate in NaBF 4 • A strong aggregation occu rs in KPF(, solut ion. This aggregation is caused by the conversion of the hydrop hilic surface into hydroph obic when the anion Br is replaced by BF4" or PF6- [23, 24]. Zhang et a l. [25] synthesized a series of new surface-active ionic liquids based on N-methy l-2-pyr rolidone and alky l brom ide. The physical propertie s of these salts were described: they have the characteristics of ionic liquids, the su rface activi ty and biocorn patibility. In v iew of this circumstance the authors classified these salts as the "g reen" ionic liquids. In [26] a new modu lar architecture of surfactants was deve loped based on compounds related to RTIL (room temperature ionic liquid s). These surfactants can be transfo rmed to either polymerizable, or non polyrneriza b le form, therewith the molecules form a phase of lyotrop ic liquid crystal with a dual cont inuity. These phases include three-dime nsiona l interconnected periodic nanopores when mixed with wate r or other RTIL as a solvent. The polymerizab le analog s of these surfacrants can be transfo rmed into a stable nanopor e organic materia ls by in situ cross-linking (polymeriza tion of the monomers in water and lyotropic liquid crystalline phase s based on RT IL). Th is tech nology allows to obtain a set of polymer nanomate rials based on the RTIL with superior accessibi lity of pores and the prope rties of molecu lar transpo rt (or exclusion according to size) in compari son with the known liquid-crystall ine and polyme r RTIL materia ls. The high accessibil ity of pores of these mater ials and selectivity with respect to the molecu lar size in the cubi c structure with double cont inuity in comb ination with the chemica l propertie s of water or RTIL in channe ls makes it possible to achieve high per fonn ance in several app lications (nanofiltration, gas separation, mate rials with barrier to vapor, the ionic condu ct ivity, preventing pollution and damage of protei ns, electrochem istry, catalysis). The catanionic (cation-anionic) surfactants com prise a special class of surfacranrs, in which both ions are organ ic. If the hydropho bic hydroc arbon cha ins of the two organic ions have the same length (strictly
RUSSI/\ NIOUR N/\L
or
C,ENER/\ L CHEM ISTRY
Vol. 82 No. 12 2012
191 9
IONI C LI QUI D SURF A CT ANTS
speaki ng, two orga nic ions should have the same surface activity), the catanionic surfactant is classified as symmetric [27]. In aqu eous soluti on both the surfac e-active ions roll up. In the mice lles and the adsorpti on layer they are in appro ximately equal amounts, that is, the mo le fraction 0 f each ion is eq ual to 0.5 [28]. Eventu ally , the surface tension -Go (be fore adsorp tion) is chan ged to G (after adsorp tion). Accordi ng to the mode l of mas s action , or model of the division of the charged pseudophase [28J, a change in the stand ard free ene rgy of micelle formation (L-,C,vl) of a symmetric catanionic sur factant is desc ribed by the equation L-,C M = 2R T In CCMcThe analy sis of changes in the free energy transfer at the micelle format ion (L-, C M ) and s urface adsorpt ion (L-,C A ) of the catanionic surfactants resulted in two impo rtant conc lusions: ( I) The va lues of MfS(C H 2 ) calculated from either the mass action (or div ision of the charged phases) model or the model of the free energy of the hydroca rbon/water interface are in good agreeme nt with eac h other (~ -3 kJ mol-I) [29J, the same is true for Mf,~I ( C H 2 ) ' (2) From the values of L-,C\ (or L-, C1)) and c\ G,S (HC) [or L-, C1\HC)J [M;\ ~ can be divided into LiCS(HC ) and L-,0S(W) for the hyd rocarbon chain and hydrophi lic head , respecti velyJ fairly constant values of L-,CS(W) and MJ ~(W) can be obtained for homological catan ionic surfactants. The spontaneous folding of the surfactants hydrocarbon chain in aqueous solution was also confirmed. If the hydrocarbon chain were in a fully extended conformation in aqueo us so lution , the va lue of molecular area of the hydrocarbon chain of surfactant
/
in contact with water wou ld be muc h larger than for the spontaneously folded conformation, and the value of L-, CS (C H2) or L-,CJ (CI12) would be equal not to - 3 kJ mor l , but to about - 6.5 kJ mol", which is obviou sly impossibl e, showi ng that the extended con formation of the hydrocarbon chain does not ex ist. It is noteworthy that the value of L-,C i~(W ) is much larger than L-,CS(W). Thi s may be d ue to a very dense packing and, therefore, muc h grea ter dehydration of the ionic head on the surface of the so lution during adsorption . For example, for CsNM '- CsS - (NM' is trimethy lammonium cation, and S- is sulfate anion ) the average surface-molec ular area of the orientation hydrophob ic chain is 29 N , wh ile the aver age area occ upied by surfactant mol ecule s on the surface of the mice lle eq uals ~69 N . Aq ueous so lutions of CnNM '- C/I -, whe re n = 8, 10, and 12, become turbid afte r reac hing critical mice lle-formation concentration, and phase se paration occurs [30]. It was sugge sted that it was not a true critic a l micelle-form ation conce ntration but the so lub ility limit, as the micel les are so large that they form a new concentration 0 f phase of the surfactants. However, the results are interesting and show that in this case the critica l micelle-formation conc entration is real and that the mice lles are of the s pherica l shape, at least at the criti cal micell e-formarion con centration. The fact that C~NE ' - C /I S -, where NE' is triethylammonium cation, can form a clear micell ar so lution and its stand ard free energy is similar to that of C'"NM 1- C"S , can be considered as a paral lel evidence of this statemen t. OH
I
\
R I_Ny ;-~
-Cl-12CHC H 2 0 I-1 CI
CH:;
RI = c I,I-b, CII [129 , C 1(,H 31
Dorj narnj in et al. [31J sy nthesized a series of cationic imidazo lium ionic liqu ids of the surfactant nature (the formulas are show n abo ve) for the use as
reducers of silver and the protector of the formed
nanoparticles of the metal. For the synthesis a quaterni zation was used with l-chloroethanol and [
ch loropropane-2,3-dioI.
For the prior introduction of a long alky l substituent in the imidazole ring an exchange reaction was carried out.
+ R1X -HX
Y = H, CH,: X - halogen.
It is shown that ion ic liqu ids form com plexes with silver partic les, ther eby stabiliz ing the metal nano partic les.
RUSSIAN JOURNAL OF GEN[R AL CHEMISTR Y Vol. 82 No. 12 20 12
1920
;\SADOV et al.
ln [32] the representati ves of non-tradi tiona l class of liquid crysta ls the ionic meta l mesogens were investigated, which are the salts of meso gens, the salts of higher alkanoic acids forming thermotropic and lyo tropic bilayer smectic meso-phase. The ion proper con ductivity , high so lvating power wit h respect to sub stances of different chemical nature, and a pronounced tendency to vitrification of the meso phases in the a lkanoates of a number of metal s makes them promis ing for the creation of new liquid crysta lline mater ials for the optical electroni cs and laser techn ology . A rule of the ionic mesogene ity was formulated whereby the mesogenic propertie s can be displayed only by the metal alkanoates in which the metal is mono -, bi-, or tri-valent. On basis of the study of the phase equilibria in binary systems of metal alkanoates with a co mmon anion or cation (more than 100 systems) the regula rities of the formation of liquid crysta lline solutions (con tinuous, boundary, and intermed iate), and the optically anisotropic (mesomorph ic) and isotropic glasses were revealed. 18 Major topological types of phase diagrams of binary systems of mesomorphic metal alkanoates were identified. It was found that mesomorphic glass of rare-earth meta l alkanoates may be of non-porous structure and they may be a directionally-organized environment for the stab ilization and orientation of nanopartic les of different che mica l nature (fullerenes , nanotubes, organic dyes, metals, metal oxides, and chalco genides). It was shown that the smectic structure 0 f the alkanoate mesomorphic glasses provide d a possibility to carry out high-rate holographic recording stipul ating actuality of developing ionic liquid crystal materials for ultrafast optical sw itches in the telecom -munication optical-wave systems. In [33-35] the surface-acti ve propertie s of a series of salts with the l-alkyl-3-methyl imidazo liuIll (rnirn ) cation (alkyl is C4 , C8 or Cu) and anion PFi" BFi , CI-, and Br" were investigated . The dependence of surface activity from the temperatu re was studied using a ring tensiometer . Also, expe riments were performed for revealing the structure and orien tation of the ionic liquids surface. As estab lished, the surface tension comprises an unusua lly wide range in the compo unds of similar structure, namely, from 45 mJ m- 2 for [bnim][PF6] to 24 mJ m-2 for [C lr mim][PF6 ] at 336 K (bmim is l-buty l-3- met hylimidazo lium). All of these ionic liquid salts exhib it a linear variation of surface tension with temperature, which allows the division of the surface indicators according to the excess entrop y and energy. The surface tension (0) of the studied ionic liquids, as revealed, is well described by a function decreasing linearly with the temperature:
c;( T)
= a-bl
whe re a and b are the constants depen ding on the nature of the sa lt. The values of excess parameters on the surface depend on the lengt h of the alky l chain . Both surface entropy (Ss) and energy (E s ) decrease with e longation of the alky l chain of R I (position I) in the cation when the an ion is constant. For the same cat ion, the reduction in the size of the anion leads to lowering the surface entropy excess. Jn the same way the excess of the surface energ y chan ges with decrea sing anion size in the case of the same salts with shorter a lkyl chains. Wide range of surfac e prope rties of these compound s probabl y reflect s changes in the surface orientation of the cation . The compa rison of data on E, and S, for different substances shows that the measured va lues for the above ionic liquid salts are comparable with the corresponding values for some of neutral organic liquid s, n-a lkanes and heter ocycles [36,37]. The ionic liquid [bmim ][PF6 ] is comparab le with the imidazole, and its surface ener gy is similar to that of such heterocycl es as pyrrole and quinolin e, but like the case of imidazole, its free surface entrop y is much less. The surfac e entropy of the fluo rine-con taining ionic liquids salts excep t for [bm im][PF 6 ] , is lower, and decrea ses with the lengthening of the a lky l chain . This decrea se in surface ent ropy with incre asing chain length is also obse rved in n-alkane s and is usua lly taken as an indicator of the increased degree of orient ation at the surface [36]. The studi ed fluorin ated ionic liquids salts exhibit surface energies close to those of the n-a lkanes that are in the bottom of the range for organic liquids. These ionic liquids con sist of large ions, which are suffi cie ntly distant from each other. Consequ ently , we can expect that the Cou lomb forces between them are relatively weak. When comparing ionic liquids sa lts with similar length of a lkyl chain, it was found that surface energy was lower when the anion s were smaller (BFi compared with PF(," or Cl"compared with Br). The diffe rence in surface ener gy decreases with increasing alky l chain length and practically disappe ars at reaching the C I2H 25 chain . Thi s suggests that with increasing size of the alky l chain the effe'ct of anion on the surface energ y is quick ly sup pressed . Thus, in the case of ionic liq uid sa lt with the catio n [C1 rmi mf the surface energy is determin ed mainly by the cation, probably d ue to the increas ed van del' Waals forces between the alkyl chains.
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IONIC L1Q UJD SURFAC TANT S
In [38] electro conductometr ic measurements of surface-active ionic liquid salts like [bmim][BTFMSI ], [bmim][triflate] and [om imj -jtriflare] {[bmim] = 1 butyl-3-meth yl imidazoliu m, [BTFM SI] == bis(trifl uoro methylsulfonyl)imide, [orn im] 1-octy l-3-meth yl irnidazoliurn } were carr ied o ut. The measurements were performed in the temp eratu re ra nge from 20 to I50a C. The spec ific co nductivi ty of the last sa lt compared with the other two is found to be much lower, due to the large size of octy l chain in comparison with the butyl group. The calculated activation energy of spec ific conductivity increases when going from the first salt to the seco nd and third. Raising the tem peratur e within the abo ve limits leads to a decrease in the activation energy of spec ifie conductivity, which is assoc iated w ith a decr ease in viscos ity of ionic liqu ids at heating. The ca lculations show that the differ ence between the acti vation energies of specific elec trical co nductivity of the first and second salts is 2.5- 3.0 kJ mor l . The difference between the values for the third and second salts is much larger (5.0-7.0 kJ mol-I). The influence of the alkyl chain length in the cation and anion of l- alk.yl-3-m ethylim idazole (rnim) alkylsulfonare salts of ionic liquids type with the formula [C"H2". I-mim][C",H2",_ ISO,] (in the first bracket l-alkyl-3-methylim idazolium), where n == 8, 10, or 12, In == 1 and n = 4 or 8, 111 = 4 or 8, on the self aggregation either in pure form or separat ely, in aqueous solution was studied [39]. Some of these ionic liquids represent a new family of ionic liquid surfactanrs with doubl e cha in. In the case of pure compounds, their strong tendency to se lf-agg regation led to the discovery of a liquid-crystalline phase in one of the representatives. Molecular modeli ng com plements the study of the pure comp onents and points to their self-separ ation at the nanosca le into clear-cut non-polar and polar domains in the liquid non crystalline phase. In aq ueous so lutions of such ionic liquid surfactants the salts that contain an intermediate that includes long alkyl chains both in the cation and anion form pure cationic surfactants and behave like their real and effect ive representatives. Such surfa ctants are rare and diffe r from both the mixed cation anion sur factant (fo rmed by mixing of the two sa lts), and from the surfactant of the gemini type, in which there is an entropy limitation due to the chains attached to the same polar head group. FIuorescence spectroscopy and interfac ial tension meas urements allowed to determine the critical micelle-formation RUSSIAN JOURN A L OF G[' NER AL C HEM ISTRY
1921
concentration, the surface ac nv rty, and to assess the effec t of alkyl substitution in cations and anions on the surface properti es of these salts. 1n the case of relatively sma ll methylsulfon ate anions (n = 8, 10 and 12, m == 1) the salts behave J ike normal cationic surfactant with one chain. At the same time a decrease in the critical micelle-for mation concentration is obse rved with increasin g alky l chain length in the cat ion, that is, with increas ing n. When the length of alky l chains is the same in the cation and anion (n == 4, m == 4 and n = 8, tn ,= 8), new cationic surfactants are formed with lower critica l micelle-formation concentra tion values than for the co rrespo nding cationic analogs, and whose surface tension is ex tremely lowered. Investigations of the thermotropic phase behav ior of ionic liquids with the form ula [CsHl rmim-H] [CSH I 7S0 3] (n == m == 8) using the methods of X-ray scattering at variable tem peratur es, polarizing optical microscop y, and differential scanning ca lorimetry showed the formation of a smectic liquid crystalline phase in a wide range of the tem perature. In [40] the dynamic surface tensio n of octa decyld imethylarnmoniurn chloride was measured , and it was shown that with increas ing concentrations of ionic liquid surfactants the adsorp tio n barrier increases. Similar resu lts were descri bed in [4 1]. These experimental facts can be expl ained as follows. At sufficiently low surfactant concentration its molecules can eas ily take up the space on the interface and the adso rption is instantaneo us, that is, the limiting stage is diffusion. With increasing con centr ation, the number of the surfactant molecules adso rbed on the surface gradually increases and the surface press ure increases, which leads to appearance o f the energy barrier [42, 43]. At this stage , mixed mechanism o f diffusion contro lled adso rption do mina tes. Consequently, at high concentr ations of surfac tants, init ially the adsorption is co ntrolled by diffusion, and later this mechanism transform s to a mixed diffu sion- adsorption [40-42]. In [44, 45] three surfactants of the ionic liquids nature namely, l-d cd ecyl- f -m ethyl- im idazotiurn hexa fl uoropho sp hate ([C 12- mim][PF6 ]) , l-tetradecyl-3 methyl imidazolium-hexa fI uoroph osphate ([C w ll1 iIll] [PF6 ]) , and l-hexadecyl-j -me thylimid azolium hexa fl uorophos phate ([C I 6 -milll][PF6 ]) were used to form inclusion comp lexes with ~- c y cl o d e xtl·in . Measure ments of surface tension showed that there are two types of inclusion co mp lexes, with the components ratios ~-cyc lo dex t r i n :i o nic liquids I: I and 1:2 for ~ cyc lodex trin-[C l r mill1 ][PF6 ] and ~ - c y cl o d e xtr in -
Vol. 82 No. 12 20 12
;\SA DOV e t a l.
m][PF 6 ] , respecti vely, and only one type ( 1: 1) comple x p-cyclodextrin-[C wmim ][PF 6 ] , T hese unds were cha rac terized by X-ray diffractio n, .d I H nuclear Overh auser effec t, N M R spec py, and TGA . T he res ults obta ined showed that .omplexes are fine-crystalline powders. A channel structure was de tected , It is sugges ted that the iphobicity is cr ucial for the form ation of these sion complexes , The temp eratu re of decornpo si of the complexes is lower than that o f the or iginal ponents. The ability of the ionic liqui ds to form usion struct ures ca n be very useful for ex trac tion of .c Iiquids at the prepa rat ion.
The distribution- separation chromatographic columns ated with two alky lim idazo lium ionic liqu id sa lts .th long alky l chains ([Clrmim]Br and [C w mim ]Br) ere used to per fo rm se paration of conve ntiona l iorgan ic anions [46]. These two sa lts are new catio nic urfactants for ion ch romatog raphy . With the use of ihtha late buffer so lut ion as a mobile phase and detec .ion without su ppress ing the co nduc tivity a high efficie ncy and exce llent se lectivity of co lumns were achieved for the se para tion of inorganic anions. Calculated chromatographic parame ters and the results show that the colum ns with such coati ngs have great potential for the analysis of an ions like CH1CO O-, 10 3, cr. Br0 1, NOl , NO:;, sor. 1-, BF4 - and scu On a column coated with [C lrmi m] Br the influence of the pH of the e luent on the se paration of anions was studied, Also the sta bility of co lumns with suc h coati ng was studied.
Borisova et al. [47] investigated the surface -active ionic liquids sa lts of alkyl imidazo lium ty pe, wh ich contain alkyl groups of the linear structure C I 2 and C 14 • Their aggregation in wate r and in aqueo us phos phate buffer was studied spectrophoromerrically. The values of the cr itical micelle- form at ion concentration for these sa lts were dete rmi ned and com pa red with those of the cation ic surface-ac tive sa lts commonly used in capi llary elec trophoresis, like tetradecy l- and dodecy l trimethylam monium chlorides , The propo sed sa lts are pract ically useful as a pseudo-stationary phase at the chromatographic separation of isomers of methylresor cy nol and some benzene der ivat ives. On e of the authors of that pape r also has stud ied [48] the physico chemical properties of so me ionic liquid sa lts conta ining the dialky limidazo lium cation.
The co mpound [Cwm im]Br, which is also an ionic liqui d, was used as a new ca tionic surfactant for the
-, :
separation of phenolic compounds like hydro qu inone, phlorog lucino l, resorci no l, pheno l, p-cresol, and m nitrophenol by micellar electrokinetic capillary chromato graphy [49] . The effect of t he buffer co ncen tration and pH, the [C w mim]Br concent ration, and the appl ied potential on the effecti venes s of such separation was studied. Application of optimal buffer (NaH2P04 , 25 mlvl), at the concent ratio n of [C wm im]B r " '0 mM and the poten tia l 15 kV al lowed the optimum sepa ration in terms of reso lution and migra tio n time , Pheno lic compounds are detected at 2 14 nrn. At the ir se paratio n by this me thod the micelles of the used s urface-active ionic liquids salt wit h a hexadecy l s ubstituent act as a pse udo-stationary phase , 1n [50 ] microemuls ions were described with the ionic liquids serving as a surfactants, and the room tempe rature ion ic liquids (RTIL) , as po lar pseudo phase . T he mic roemu lsio ns obta ined contai ned t he ion ic liqu ids with a long alky l chain, in particular [C wmi m]CI, as a su rfac tant, decanol, as a co surfactant, dodecane , as a continuo us phase , and RT IL, as a polar microsphere. As RT IL, ethy lammo nium nitrate and [bmim][ BF,, ] we re taken separate ly, The phase diagra ms of both systems were bui lt at co nstan t mo lar ratio of surfactant/co-surfactant. T he method of elec trocond utome try allowed to co nfirm the presence of the ethy lammonium nitrate microareas in the oil. Zech et al. point to the possibi lity of app lication of the dyna mic perco lation model in the presence of ethylammonium nitrate . Meas urements of dynamic light scattering ev idence the swe lling of the formed nanostructures with the increase in the ethy lammonium nitrate content. A linear depend ence was foun d of the hydrodynamic radi us of these structures from the mass fraction of ethy lammo nium nit rate was found . Both system s exhib it a broad peak in the spectrum of small ang le X-ray scattering, The characteris tic depend ence of the scattering intensity (J) from the wave length (J ~ 1-:,4) for large I.. va lues is obse rved , The Te ubner-S tray mode l was used to determine the amphi philic factorj, and two characteristic va lues for the microemu lsions, namely , the frequency d and the corre latio n length L, In addi tion, it was possib le to determ ine the interface specific area. The amphip hilic factor clear ly demonstrated the structural diffe rences betwee n the two systems, suggesting that the nature of the polar ionic liquids played an important role in the rigid ity of the interfacial film. The app licabil ity of three diffe rent model s was ver ified, na me ly, the mode l of a sp her ical ion ic liqui ds in oil, the model o f repulsive s pheres, and the mode l of doub le con tinuou s structures.
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IONI C LIQ UID SURFAC l i\ N T S
The use of micel les based on the s urface -ac tive ionic liquids In the sta tionary phases for gas chromatography was estim ate d using equ ation s derived for the " three-p hase model" [5 1]. This model a llows the deter mination of a ll three d istribution coe fficients in this sys tem and reveal ing the micellar contribution to the reten tion and se lectivity. In the study exami ned exami ned fou r types of micellar-ionic liqu ids columns are exa mined: [bmi m]C I with sodium dodecy l sulfate or d iocty l sulfos ucc inate and [bmim] [PF6 ] with steary l ether PEG -I 00 (that is, 100 units of ethylene ox ide) or lauryl ether PEG- 23 (that is, 23 ethylene ox ide units). T he distribution coe fficients were measured for a wide range of the molecul es that are prone to various types of interactions in d ifferent degrees. In ge neral, mos t of the examined mo lecules are distributed mainly in the mice llar pseudo phase located abov e the main part of the ionic liquid co mponent of the stationary phase . Hence, the addit ion of the surfactant in the stationary phase usual ly leads to a greater retention of so lute. Also it was shown that the se lectivity of the stationary phase varies con siderab ly in the presence of micelles due to eithe r inten sifying or s lowing of the sepa ration. The surfactant action on the interaction parameters in the stationary phase was determined using the A braham' s model which takes into account the solvation para meter. Adding sodium dodecy \ sulfate or dioc ty l sulfo succinate to the stationary phase with [bm im]C I in general increases the basicity of the hydrogen bond of the phase and increases the level of dispersion inter action. However, PEG- IO O sreary l eth er and PEG-23 lauryl ether being surfacta nts increase IT-IT and n - IT interact ion, polarizability, and po larity of the di pole and the interactions of [bm im][ PF6J as a result of the basicity of the hydrogen bond to a greater extent than ionic surfactants on the basis of [bmim ]C I. These non ionic surfactants reduce the abi Iity of the stationary phase to interact with so lutes through dispersion forces. Consequently, it is possible to predict re liab ly which of the analyzed s ubstances wi1I be retained particu lar ly strongly by these micellar ionic liqu id stationary phases. Baghdadi et a l. [52] have developed a new technique for microextraction with surface- active ionic liquids named a micro extraction with the so lvent formed in situ. A small amount of sod ium hexa fluorophos phate (I laPFo) used as an agent for the ion pairing is added to the sa mple so lution con taining a very small amount of l- hexyl-3- methyl imidazo lium tetrafluo roborate ([hmimJ[BF 4D as a hydrophilic
1923
surface-active ionic liqui d. A turbid so lution is formed due to the formatio n of small dro plets of [hmim J [BF~J . After centrifugation, the fine droplets of extractant phase fall to the bottom. Thi s is a simp le and fast ex tract ion method for the pre-co ncen tration of metal ions from aq ueous samples. It can a lso be applied to a so lution co ntaining a sa lt in a very high co nce ntration. In addition, the method is much sa fer co mpared to the extraction with an organic so lvent. The rel iab ility of the de veloped method was estimated by analyzi ng a standard aqueo us so lution. The method was successfully used for the determination of Hg(!!) in se veral wate r sa mples. The type and am oun t of surface active ion ic liquids, tem perature and othe r parameters were optim ized. Under optima l cond itions, the limit of detec tion was found to be 0.7 ng ml- I and relati ve stand ard dev iation 1.94% at a con centration of Hg 40 ng ml- I . In the Institute of Petrochemical Processes of National Academy of Sciences of Azerbaijan a series of new ionic liquids surfactants was synthesized. The quaternary salts were prepared from the higher aliphatic alcohols (C9-CI~), epichlorohydrin, and (alkanol)amines. In the first stage of the catalytic condensation of alcohol and epichlorohydrin the respective oligomeric chloromethyl-substituted poly ether is formed which gives a quaternary sa lt in the quaternization reaction with a compou nd containing an amino group [53-59]:
where R '" C 9- C I4 , n is the degree of o ligo mer ization, R 1, R2 and R:; '" H, CH:;, C 2Hs, C 2 ]j ~ O H , C:; Hr,O H (depend ing on the used a lkano lamine and amine): R, RJ may together form a cy cl ic system (as in the case of pyr idine and piperidine). Among the a lkanolamines mono-, di-, and tri ethanolamine, methylarnin oetanol and propa nolamine, as amines die thy lamine, pyrid ine, and piperidine were used. The quatern ary salts were characterized by physicochem ical para meters like density, re fractive index, and ex tinction coefficient (E), hydroph ilic lipophilic balance calc ulate d by the sys te m of Davies, the s peci fic conductivity (X,) , hydroxyl numbe r, the surface tension (0-) for the water- air and the water kerose ne boundar ies.
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ASADOV e t al.
The most surface-active on the kerosene-water boundary (without a surfactant 0 = 46 .0-46.5 tnN rn " ) is the oligoethe r (n = 2.5) from ep ichloro hydrin and decanol quaternized with triethylamine (00 S- IO% = 5.5 mN m" ) as well as methylarn inoethano l salt of the oligoether (n = 7. 1) from ep ichloro hydrin and nona noI (ao s-I O% = 5.5 mN m- I) and oligoethe r (n = 4 .6) from epic hlorohydrin and dodeca nol quaternized with diethanolarnine (aIO % = 5.5 mN m- I ) . The E parameter of the quaternary salts is 1.3-1 .9 times highe r than that of the initial chlorornethyl deri vative . The X parameter of the 0.5 wt % aqueous solutions of the sa lts is in the range from 0.0 100 to 1.9 100 ,(r l m- I that is much higher than for disti lled water itself (0.001 0.000 I Q-I tn-I). Identifica tion of the composi tion and structure of the obtained compounds was carried out by 1[-1 NMR, IR, and UV spect roscop y. Most o f the quaternary salts and thei r 5 wt % aqueous solu tions (disper sions) show the oil-co llecting or dispersing properties on the water surface [60, 6 1]. The ionic liquid salts were obtained [56, 62-64] also on the basis of aliphatic po lyo!s. epichlorohydrin, and (ethano1)amines. As the polyo!s ethylene glyco l, 1,2-propylene glycol, l,3 -butylen e g lycol, and glycerol were used, arnines were dieth ano Jamine, pyridine and piperidine, ethanolarn ines co mprise rnono-, di-, and triethanolamine. In the first stage by the cata lytic reaction of alcoho l with epichlor ohydri n oligomeri c ethers were synthesized. In the second stage the chloromethy1- polyether reacted with the nit rogen base accordi ng to the quate rnization react ion. As a resu lt the ammonium salts of the ionic liqu ids type were synthesized.
Study of physicoc hemica l parame ters of the syn thesized ammonium salts showed that the ir X parameters (hom 0.0023 to 0.1830 Q- I m I) are much larger than X of distilled water. The compos ition and structure of synthesized ionic liquids sa lts were investigated by various spectroscopic metho ds. Stalag mometric investigation s of the water-ke rosene interface with the synthes ized ionic liquids salts have revealed high surface activity of the latters. At the 5% concentration of triethylarn rnonium salt of the chloromethylate polyester' with glycero l (n = 26.2) it is found that a = 2.0 mN m- I. Laboratory tests [60, 6 1] revealed the abi lity of ionic liquid ammonium sa lts
synthesized from C 2-C q polyo ls, ep ichlorohydrin and ' ethanolamines to s low do wn evaporat ion of gasoline at storage . On the basis of indi vidual saturated monocar boxy lic ac ids, epic hlorohydri n, arnines, and alkano l amines (incl uding rnono-, di-, and trietha nolam ine, methy laminoethano l, propanola rnine, diethylamine, pyridine , and piperidine) the respective quaternar y salts were produced [53,55-57,65-70].
Here R = C 6-C IO, CIS, C 17 ; n is the degree of chlo roxypropylation, R 1, R2 and RJ are H, CI-h, C2 Hs, C2 Hq OH and C,H I,OH; R1 -R, together may form a cyclic system. As the saturated monoca rboxy \ic acids, the linear monoc arboxylic acid s C7-C II, C II) and C 1R were used. The Xparameters of aq ueous solutions of most of these salts are much higher than this quantity for water , indicating their ionic liquid structu re. Compo sition and struc ture of these sa lts were st udied with the above mentioned spectral methods. The values of 0 of 0.5 wt % aqueous solutions are illustrative of the ir large surface activit y on the kerosene- water border (a is equa l to 0.8-15 .1 rnN m- I). The resu lting quaternar y sa lts have the properties of oil collection and dispersing ability. Catio nic surfactants were obtained [65, 69, 71 , 72] also on the basis of ethanolamine and chlororn ethyl derivati ves o f esters of the acid fractions isolated from a number of vegetable oils by alkaline hydro lysis descr ibed in deta il in [73-75]. As the oils castor, o live, linseed, cottonseed, and soybean oi ls were used, and ethanol amine s were mono -, d i-, and triethanolamine. High va lues of X of the 0.5% water so lutions as comp ared with water indicate the ionic structure of the resu lting produc ts. The hig hest surface activity on the kerosene- water border was shown by diethanolarnine salt of the acid fractio n of casto r oi I and tri ethanolamine salt of the acid fraction of olive oil (a = 3.6 mN m- I at a co ncen tration of 0.5 wt %). The identification of the composition and structure of the resulting prod ucts was ca rried out by the above considered spectrosc opic methods. The resulti ng salts exhibit the oil-co llecting and dispersing properti es.
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The complexes of natural fractions of the carboxylic acids extracted from natural natural lipids with mono-, di-, and triethanolamine as well as quaternary salts obtained from these fractions, epichlorohydrin and the ethanolamines, which are the substances of ionic liquid nature, show strong inhibitory properties with respect to the steel corrosion in acidic medium [71]. In the reaction of capric and tridecanoic acids as well as the acidic fractions of castor, cotton, corn, linseed, and sunflower oils with the above ethanolamines the ethanolaminocarboxylate complexes of ionic liquid type were synthesi zed [72]. They were identified by the methods of 11-1 NM R, IR, and UV spectroscopy and characterized by various physicochemical parameters. Complexes of individual monocarboxylic acids with ethanolamines reduce the o parameter on the kerosene water boundary from 46.0--46.5 to 1.8-2.7 mN m- I, and the complexes of acidic fractions with ethanol amines up to 1.5-2.7 mN m- I. These complexes exhibit fairly high oil-collecting and dispersing abilities. The complexes of ethanolamines with acid fractions of castor, sunflower, and soybean oils were found to have the property to slow down the steel corrosion in acid medium considerably. In addition, these complexes of the acidic fractions and the quaternary salts obtained by the modification of the chlorornethyl derivatives of polyesters obtained from these fractions through quarter nization reaction of chloromethyl groups in the oligo meric matrix with ethanolamines enable to collect or disperse the oil film on the water surface [53, 71, 72]. The ability of the ionic liquids derived from ethanol amine and acid fractions of castor and sunflower oils to the extraction of aromaticcompounds [72J was also revealed. The quaternary salts of the type of cationic surfactants were synthesized on the basis of ethanol amines and alkyl halides [55, 56, 65,76-80]. The ethanolamines used were mono-, di- and triethanol amines and dimethylaminoethanol. The alkyl halides were n-decyl bromide, »-heptyl iodide, and n-pentyl bromide. Quaternization reactions resulted in the compounds of general formula (R I)(R 2)(R O)(R) N ' X , where R is alkyl group (C s, C 7 and CIO); R I, R 2 , and RO are 1-1, CHo• and C 2H40H groups, and X- is Br- and [ anions. The values of X (XOS% = 0.0075-D.092 n-I m- I) for the synthesized salts in combination with the measured freezing points are a tangible proof of their ionic liquid structure. These compounds have good surface activ ity at the water-kerosene boundary, but the salts based on the decyl bromide and heptyl iodide RUSSIAN JOURNAL OF GENERAL CH EMISTRY
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are especially active: they reduce the interfacial tension to 1-2 mN m- I, and even at very low con centrations (0.05 wt %) to ultra-low values (practically to zero). Noteworthy is the fact of very good soJubility in water of the decyl bromide salts with mono-, di-, and trietanolamines (even 50% aqueous solution can be obtained). The composition and structure of the. quaternary salts obtained were confirmed by the IH NMR, IR, and UV spectroscopy. Studies of the oil-collecting and dispersing ability of these salts showed that the salts with decyl bromide and in part with heptyl iodide are mostly active. Comparison of the data obtained indicates the importance of the length of the alkyl radical. However, this condition is not sufficient. Experiments with a specially synthesized salt of diethylamine with decyl bromide I IN "(C 2H s) 2 loH2113 1'- showed lack of the oil collecting and dispersing properties. This is explained by the absence of a hydroxy group in the salt, which apparently plays an important role in the collecting or dispersing action. Such action is inextricably associated with the ability of surfactants to the micelle formation in particular conditions of its application with accounting for its working concentration. Some cationic surfactants were synthesized by the quaternization reaction from octadecylamine and alkyl halides, with the formula ClsH o7NH2(R)X, where R is alkyl, X- is halide (iodide or bromide) anion [55, 56, 81]. As the alkyl halide methyl iodide, propyl iodide, butyl iodide, pentyl bromide, and heptyl iodide were used. The composition and structure of the quaternary salts obtained were confirmed by IR and UV spec troscopy. The methy!-, propyl-, butyl-, and heptyl octadecyl-ammonium iodides at ambient conditions are pale-yellow solid substances that can be easily pulverized to form powder. The pentyloctadecyl ammonium bromide is a milky-white solid (at ambient conditions), also capable to be pulverized. The methyl derivative is dispersed in water and dissolved under heating. Propyl, butyl, heptyl, and pentyl derivatives are insoluble in water but soluble in ethanol. The pentyloctadecyl-ammoniurn bromide melts at 55SC, the heptyl derivative, at 65°C. Due to the presence ofa very long alkyl radical (CIS) and a polar NH 2 fragment (the analog of Of-[ groups in the salts of the fourth group), these products exhibit fairly high efficiency in collecting and dispersing the oil films. From the foregoing it is clear that the interest in the ionic liquids with a surface-active properties both from
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sc ie ntific an d practica l v iew po ints was g ro w ing in recent years. Th is is due to a large var iety of their composition (a nio nic, ca t io n ic , and catan io nic ionic liquid s) an d a wide range o f their pr actical properties.
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