Langmuir 1990,6, 334-331
334
Characteristic Solution Properties of Mono-, Di-, and Triglyceryl Alkyl Ethers: Lipophobicity of Hydrophilic Groups K. Shinoda,' M. Fukuda,+ and A. Carlssons Department of Applied Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya, Yokohama 240, Japan Received J u n e 26, 1989 The lipophobicity, i.e., the reciprocal of the saturation concentration of singly dispersed solute, x in saturated hydrocarbon in equilibrium with water phase, is an important property of amphiphiles. ?he lipophobicity of oligo(oxyethy1ene)nonionic surfactants increased about 2 times per oxyethylene group, and that of a secondary hydroxyl group, CH(OH), corresponded to about 4.6 oxyethylene groups (on weight basis 6.7 times higher lipophobicity). Due to the change from CH,O (ether) to COO (ester), the lipophobicity increased about 5 times. Although the lipophobicities of hexa(oxyethy1ene) dodecyl ether and monoglyceryl a-dodecyl ether are close to each other, the former is water-soluble whereas the latter is oil-soluble at 25 "C. The fact that the surfactants used in oil media are poly01 type (e.g., Span) and not oxyethylene type coincides with the strong lipophobicity of OH groups and their oil-soluble tendency, revealed in the present study. It is evident that monoglyceryl alkyl ether or ester is an excellent oleophilic cosurfactant. The effect of temperature on the solution properties of glyceryl alkyl compounds was much smaller than that of oligo(oxyethy1ene) alkyl ethers. Thus, polyol-type nonionic amphiphiles seem promising in practical applications due to their high lipophobicity and temperature insensitivity.
Introduction Lipophobicity is an important property of oil-soluble amphiphiles since lipophobic solutes form aggregates in dilute solution and are effectively adsorbed a t interfaces. The importance of lipophobicity of surfactants in oil media is neither well recognized nor quantitatively studied. The hydrophilic moiety of nonionic surfactants is commonly composed of oxyethylene groups. A series of nonionic surfactants whose hydrophilelipophile balance changes from lipophilic to hydrophilic can be obtained by varying the oxyethylene chain length. Since the lipophobicity of the oxyethylene group is weak, the saturation concentration of singly dispersed oligo(oxyethylene) surfactants in oil is relatively high.'-3 An efficient surfactant should be strongly amphiphobic as well as strongly a m p h i p h i l i ~ . ~When .~ the hydrophilic portion of the nonionic surfactant mainly consists of CH,OH or CH(OH), as in glyceryl alkyl compounds,6-8 both hydrophilicity and lipophobicity will increase drastically. Actually, the use of such cosurfactant has revealed a much better solubilizing property than that of the oligo(o~yethylene)-type~~~~ surfactant.
* To whom
correspondence should be addressed (Yokohama
National University).
'
Present address: Kao Corp., Tokyo Research Labs., Bunka 21-3, Sumida-ku, Tokyo 131, Japan. On leave from Physical Chemistry 1, Chemical Center, Lund University, P.O. Box 124, S-221 00 Lund, Sweden. (1) Crook, E. H.; Fordyce, D. B.; Trebbi, G . F. J . Phys. Chem. 1963,
*
67, 1987.
(2) Saito, H.; Shinoda, K. J . Colloid Interface Sci. 1971, 35,359. Harusawa, F.; Tanaka, M. J. Phys. Chem. 1981,85, 882. Shinoda, K. J . Phys. Chem. 1985,89, 2429. Shinoda, K. Progr. Colloid Polym. Sci. 1983, 68, 1. Suzuki, Y.; Tsutsumi, H. J . Jpn. Oil Chem. SOC.1984, 33, 786. Suzuki, Y.; Tsutsumi, H. J. Jpn. Oil Chem. SOC.1987, 36, 947. (8) Vanlerberghe, G. In Physics of Amphiphilie Layers; Meunier, J., Langevin, D., Boccara, N., Eds.; Proceedings in Physics, SpringerVerlag, 1987; Vol. 21, p 199. (9) Shinoda, K.; Kunieda, H.; Arai, T.; Saijo, H. J . Phys. Chem. (3) (4) (5) (6) (7)
1984,88,5126.
0143-7463/90/2406-0334502.50/0
The present investigation has been undertaken in order
to characterize the solution properties of glyceryl monoalkyl ethers, containing one, two, or three glyceryl units, and glyceryl monoalkyl esters. A comparison is made with the solution properties of oligo(oxyethy1ene) alkyl ethers.
Experimental Section Materials. Tri(oxyethy1ene) monooctyl ether (R,(OCH,CH,),OH; >98%) was obtained from Pola Corp., Yokohama. Tetra(oxyethy1ene) monooctyl ether (Rs(OCH,CHz),OH), tetra(oxyethy1ene)monododecyl ether (R,,(OCH,CH,),OH), penta(oxyethy1ene) monododecyl ether (R,2(0CH,CH,)50H), hexa(oxyethy1ene)monododecyl ether (R,,(OCH2CH,),0H), hepta(oxyethy1ene) monododecyl ether (R,,(OCH,CH,),OH), and octa(oxyethy1ene)monododecyl ether (R,,(OCH,CH,),OH), all with a purity higher than 98%, were obtained from Nikko Chemical Co., Tokyo. Monoglyceryl a-monooctyl ether (R,OCH,CH(OH)CH,OH), mono- and diglyceryl a-monoisostearyl ethers (iR,,(OCH,CH(OH)CH,),OH; n = 1 and 2), and monoglyceryl a-isostearate (i-R,,COOCH,CH(OH)CH,OH), all with a purity of about 95%,were supplied by Kao Corp., Tokyo. Almost 100% pure monoglyceryl a-monododecyl ether (R,,OCH,CH(OH)CH,OH) was obtained from Nikko Chemical Co., Tokyo. Diglyceryl a-monododecyl ether (R,,(OCH,CH(OH)CH,),OH); mp 68 "C, 99.7%) and triglyceryl a-monododecyl ether (R,,(OCH,CH(OH)CH,),OH; 93.4%) were supplied by Pola Corp., Yokohama. The latter substance was a mixture of straight and branched triglyceryls. Monoglyceryl a-dodecanoate (R,,COOCH2CH(OH)CH,0H);>%%I was obtained from Tokyo Kasei Kogyo Co. The organic solvents were all of extra pure grade. All surfactants were used without further purification. Water was doubly distilled. Procedures. In order to determine the saturation concentration of singly dispersed surfactant in oil phase saturated with water, series of ampules with various amounts of surfactant, oil, and water were well shaken in a thermostat to attain equilibria. When a surfactant was dissolved as singly dispersed molecules in the oil phase (0),namely, when the surfactant was water-soluble (Wm),the saturation concentration in oil was deter(10) Lindman, B.; Shinoda, K.; Jonstromer, M.; Shinohara, A. J . Phys. Chem. 1988,92,4702.
0 1990 American
Chemical Societv
Lipophobicity of Hydrophilic Groups C-CBHIZ
O,+W
Langmuir, Vol. 6, No. 2, 1990 335
( O i l Soluble1
RizIOCHzCHzl70H RsOCH2CHCHzOH
-5v
-5
1
I
-4
-3 log
1
-2
I
x2
Figure 1. Ordinate: saturation concentrations of singly dispersed nonionic surfactants in various oils (x, = mole fraction
of surfactant). Abscissa: the same concentrations with the locations of the dodecyl compounds in n-decane determined so as to make the saturation concentrations fall on the straight line of slope 1. Differences between these compounds and compounds with different alkyl groups in other oils are thus easily seen. Small x, value means strong lipophobicity. mined by high-performanceliquid chromatography (HPLC).The liquid chromatograph (Japan Spectroscopic Co., model BIP-1) was equipped with a Finepak SIL-C,, column with a methanolwater mixture as eluant. A differential refractometer (Showa Denko Co., Model RISE-11) was used as detector. When a surfactant is oil-soluble in the presence of water, the solubilization of a water-soluble dye (Rhodamine 6G, 0.2% (w/ v)) as a function of surfactant concentration was measured to determine the saturation concentration of singly dispersed surfactant in oil.llolz (HPLC cannot distinguish between aggregated and singly dispersed surfactant.) When Rhodamine 6G is solubilized in the interior region of the reversed micelles (Om), the system looks reddish. The absorbance of the 0, phases was measured on a spectrophotometer, either Shimadzu UV365 at 25 2 "C or Beckman DU-7, when the temperature effect was investigated. The surfactant concentration at which the absorbance started to increase linearly was taken as the critical (reversed) micelle concentration. An increase in dye concentration did not alter the cmc value. Phase inversion temperatures (PITS) were visually determined in oil-water mixtures (1:l by weight) from the emulsion types at varying temperatures. The total surfactant concentration was 1 wt %.
*
Results a n d Discussion Lipophobic Property of Hydrophilic Groups: S a t u r a t i o n C o n c e n t r a t i o n of S i n g l e D i s p e r s e d Amphiphile i n Hydrocarbon Saturated with Water. The saturation concentrations of singly dispersed nonionic surfactants in oil a t 25.0 "C are presented on the ordinate in Figure 1. According to the saturation concentrations of various dodecyl compounds in n-decane, the position on the abscissa was fixed, so the differences due to the alkyl groups and the types of oils are shown in comparison with them (see also Table I). The open circles in Figure 1 represent systems in which the surfactant is water-soluble, i.e., the phase inversion temperature (PIT) is higher than 25 "C, the filled circles stand for oil surfactants, i.e., the PIT is lower than 25 (11) Stearns, R. S.; Oppenheimer, H.; Simon, E.; Harkins, W. D. J. Chem. Phys. 1947,15,496. (12) Arkin, L.; Singleterry, C. R. J.Am. Chem. SOC.1948, 70,3965.
"C. The e symbol stands for three coexisting phases, Le., surfactant phase coexisting with water and oil phases. The data for Rs(OCH,CH,),OH and R,,(OCH,CH,),OH in n-decane have been extrapolated to 25 "C from 9 and 20 OC and from 5, 10, and 16 "C, respectively. At these temperatures, the surfactants were watersoluble, and therefore the saturation concentrations were determined with the HPLC technique. Let us first consider the effect of the structure and length of the polar head group. The saturation concentration of singly dispersed nonionics regularly decreases with increasing number of oxyethylene units, as expected. This reflects the increasing lipophobicity of the surfactants. For the water-soluble dodecyl ethers, the saturation concentration in decane is reduced by a factor of 2.0-2.4 per OCH,CH, unit, as shown in Table I. The value of R120CH,CH(OH)CH,0H lies between those of R,,(OCH,CH,),OH and R,,(OCH,CH,),OH; Le., one OCH,CH(OH)CH, unit corresponds to approximately 5.6 OCH,CH, units in lipophobicity. Although the lipophobicities are the same, the former compound is oil-soluble (lipophilic)whereas the latter are water-soluble (hydrophilic). This is an important advantage of nonionics based on hydroxyl groups, since they act as lipophilic surfactants with a high lipophobicity. From the comparison of lipophobicity of glyceryl dodecyl ether and dodecanoate, we notice that the lipophobicity is further increased by a factor of about 5. Thus, the saturation concentration of R,,COOCH,CH(OH)CH,OH roughly corresponds to that of R,,(OCH,CH,),.,OH, as seen in Figure 1. The molecular weights of the hydrophilic groups of these lipophobic and lipophilic surfactants are 119 and 347, respectively, which very well reflects the efficiency of the glyceryl ester in regard to lipophobicity, i.e., associativity. The saturation concentration of singly dispersed R12(0CH,CH(OH)CH,)20H is only 0.0055 wt % in ndecane a t 25 "C; the lipophobicity increases by 70-80 times per OCH,CH(OH)CH, group, which corresponds to 5.6 OCH,CH, groups. (Due to the very low solubility of R,,(OCH,CH(OH)CH,),OH in oil, the saturated oil phase had to be concentrated through evaporation for the HPLC analysis.) Again, it is interesting to note that the saturation concentration for the diglyceryl compound in n-decane is about 400 times lower than that for R,,(OCH,CH,),OH, whereas the molecular weights for the two compounds are almost the same (334 and 362, respectively). The saturation concentration of R12(0CH,CH(OH)CH,),0H in n-decane was so s m d that the value could not be accurately measured. In mxylene, x , was of the order of 5 X lo*, as seen in Table I. An increase in the alkyl chain length of the surfactants leads to a minor increase of the saturation concentration in oil. The monoglyceryl derivatives which have relatively large x , values (therefore determined with better accuracy) yielded more or less equal results. However, when increasing the chain length of the tetraoxyethylene and diglyceryl compounds from 8 to 12 and 12 to 18, respectively, the relative increase in the saturation concentration is somewhat bigger, and now the longer surfactants also are oil-soluble a t 25 "C. The effect of the chain length of oil is relatively small, especially when comparing the x , values of Rs(OCH,CH,),OH, R,(OCH,CH,),OH, and R,,(OCH,CH,),OH in n-decane and nhexadecane. Effect of Hydrophilic Groups of Amphiphile on Saturation Concentration in Water. The saturation
Shinoda et al.
336 Langmuir, Vol. 6, No. 2, 1990
Table I. Saturation Concentrations of Singly Dispersed Nonionic Surfactants in Various Oils at 25 "C surfactant oil wt% concn, mM XZ log x z 139 62 34.6 18.0 46 30 16.5 20 5.7 2.7 1.1 8.2 8.4 1.6 0.12 4.5 0.17 0.16
5.0 2.1 1.45 0.71 2.3 1.4 0.92 1.2 0.35 0.18 0.08 0.23 0.30 0.06 0.0055 0.22 0.01 0.002
Table 11. Saturation Concentrations of Nonionic Surfactants in Water (cmc) at 25 "C surfactant
cmc, M
log cmc
ref
R,OH R,OCH,CH,OH R60CHzCH(OH)CHzOH
0.0038 0.0049 0.0058 0.025 0.00023 0.0022 0.000015" 0.00019
-2.42 -2.31 -2.24 -1.60 -3.64 -2.66 -4.82 -3.72
13, 14 14 14 14,15 13 15 13 15
octyl glucoside RlOOH
decyl glucoside RlZOH
dodecyl glucoside
" Solubility of supercooled
I(
0.027 0.018 0.0068 0.0053 0.0091 0.0087 0.0032 0.0022 0.0011 0.00052 0.0002 0.0016 0.0017 0.0003 0.000023 0.00087 0.00003 0.000005
-1.57 -1.74 -2.17 -2.28 -2.04 -2.06 -2.49 -2.66 -2.96 -3.28 -3.70 -2.80 -2.77 -3.52 -4.64 -3.06 -4.52 -5.30
Temperature/ "C 50 30
70 I
IO
I
I
liquid state (solid at 25 "C).
concentrations of sin ly dispersed surfactants in water are listed in Table ILF3-" The saturation concentration in water is naturally strongly dependent on the alkyl chain length, while the type of the hydrophilic group has a minor effect. For example, adding one C H ( 0 H ) group to mono(oxyethy1ene) monooctyl ether causes the saturation concentration of OCH,CH(OH)CH,OH to increase only by a factor of 1.18.14 For the glucoside surfactants, which have four CH(0H) groups, the saturation concentration increases 1.6-1.9 times per CH(0H) unit.14 On the other hand, it is a well-known fact that the cmc is strongly dependent on the alkyl chain length of the s ~ r f a c t a n t ; ~ ' "for ~ example, the cmc of dodecyl P-D-glucoside is 132 times lower than that of octyl 0-Dglucoside; i.e., the cmc decreases by a factor of 3.4 per methylene group." Change of Saturation Concentration in Oil with Temperature. In Figure 2, the saturation concentrations of oligo(oxyethy1ene) dodecyl ethers and iRl,COOCH,CH(OH)CH20H are presented as a function of the reciprocal of temperature. There is a linear relationship between log x 2 and T-', and the slopes of the straight lines for the oligo(oxyethy1ene) compounds are all more or less parallel. However, the results for iRl,COOCH2CH(OH)CH,0H differ remarkably from the others: the slope for the monoglyceryl ester is approximately half. This clearly shows that the glyceryl-type surfactant is less influenced by temperature compared with the oligo(oxyethy1ene) compounds. The difference (13) Kinoshita, K.; Ishikawa, H.; Shinoda, K. Bull. Chem. SOC.Jpn.
1958,31, 1081.
(14) Shinoda, K.; Yamanaka, T.; Kinoshita, K. J . Phys. Chem. 1959, 63, 648. (15) Shinoda, K.; Yamaguchi, T.; Hori, R. Bull. Chem. SOC.Jpn. 1961, 34, 237.
(16) Shinoda. K. Colloidal Surfactants: Academic Press: New York, 1963; Chapter 1.
7
28
30
32 T-'/lO-' K - '
34
36
Figure 2. Effects of temperature on the saturation concentrations of singly dispersed nonionic surfactants in n-decane: (0) water soluble (W, + 0),(e)three phase system (0 D W), ( 0 )oil soluble (0, + W).
+ +
in alkyl chain length of the surfactants is insignificant, as seen in Table I. Effect of Temperature on Solubilization and Hydrophile-Lipophile Balance (HLB) of Diglyceryl Dodecyl Ether. In Figure 3, the phase diagram for 1 wt % Rl,(OCH,CH(OH)CH2),0H in water/ dodecane mixtures is presented. A t high weight fractions of dodecane (0.941, an isotropic one-phase region (I,) appears. At this point (57 "C), the solubilization of water in oil is at a maximum. The solubilization curve, i.e., the boundary between 0, + W and I,, extends to temperatures above 90 "C. This is much higher than that of oligo(oxyethy1ene) alkyl ethers; e.g., the corresponding curve for 5 wt % R12(OCH,CH,)50H in water/ tetradecane covers only a temperature range of 20 OC.' These findings display the smaller temperature sensitivity of the glyceryl alkyl ether. Hence, the HLB of monoglyceryl a-mono(2-ethylhexyl) ether and ionic surfactant mixtures is scarcely affected by t e m p e r a t ~ r e .This ~ makes monoglyceryl a-mono(2-ethylhexyl) ether superior to an oligo(oxyethy1ene) type of surfactant as cosurfactant in most applications. Phase Inversion Temperature (HLB Temperature) in H,0/R12(OCH2CH(OH)CH,),OH/Hydrocarbon. When the hydrophile-lipophile property of a surfactant balances, three phases appear in a water surfactant/oil system at low surfactant concentration."-
I
Langmuir, Vol. 6, No. 2, 1990 337
Lipophobicity of Hydrophilic Groups 90
I
I
I
I
I
I
I
I
Table 111. HLB Temperatures (PITS) for Water/Di- and Triglyceryl Dodecyl Ether/Hydrocarbon.
I
surfoctont I wt%/system
surfactant
t-
IW 30k'
I
WmtO
I Iw
oil R,,(OCH,CH(OH)CH,),OH hexane isooctane decane dodecane tetradecane hexadecane squalane R,,(OCH,CH(OH)CH,),OH~ toluene m-xylene p-xylene ethylbenzene mesitylene
HLB temp, o
C
10
22 29 45 58 68 90-110 23 33 34 40 53
1 wt % surfactant system; water:hydrocarbon = 1:l (wt).
* Mixture of straight and branched triglyceryls.
14 Hz0
I
0' 2
'
0I4 0 I6 weight fraction
0 i0
'
'
IO CizH26
F i g u r e 3. Phase diagram of a n-dodecanelwater system containing 1 wt % R,,(OCH CH(OH)CH,),OH: I,, isotropic aqueI,, isotropic nonaqueous micellar ous micellar solution solution (=Om); 111, three-phase region, surfactant phase with excess water and oil (0 D W).
(=5tm); + +
An emulsion is of w/o type when the surfactant is lipophilic and of o/w type when the surfactant is hydrophilic. The HLB temperature (PIT) of R,,(OCH,CH(OH)CH,),OH in a water/hexane system was 10 "C, whereas that of R1,(OCH,CH(OH)CH,),OH in the same system was approximately 140 "C. Mixtures of the surfactants give intermediate values. The change in HLB temperature when extending the hydrophilic headgroup with one glyceryl unit is much larger than the effect of 5.6 OCH,CH, units.20 In Table 111, HLB temperatures for aliphatic and aromatic hydrocarbons are presented. The HLB temperatures of aliphatic oils (n-hexane to nhexadecane) ranged from 10 to 60 "C when R12(0CH,CH(OH)CH,),OH was used as surfactant. For aromatic oils (toluene, m-xylene, p-xylene, ethylbenzene, and mesitylene) the HLB temperatures ranged from 20 to 55 "C when R,,(OCH,CH(OH)CH,)30H was used. Thus, oligoglyceryl alkyl ethers and their mixtures can conve(17) Shinoda, K.;Saito, H. J. Colloid Interface Sci. 1968,26, 70. (18) Shinoda, K.;Friberg, S. E. Emulsions and Solubilization; WileyInterscience: New York, 1986;pp 30-40. (19) Shinoda, K.; Lindman, B. Langmuir 1987,3, 135. (20) Reference 18,pp 102,113.
niently be used for preparation of o/w- or w/o-type emulsions according to the P I T method21'22regardless of whether the oil is aliphatic or aromatic. In the case of an emulsifier based on the usual oligo(oxyethylene) type, the oxyethylene chain length should not exceed four units in order to be oil-soluble. However, such a surfactant has a high saturation concentration in oil (as seen in Figure l),which makes it unfavorable to use as cosurfactant compared with a glyceryl type of surfactant. Furthermore, the solubilities of oligo(oxyethy1ene) alkyl ethers in aromatic oils are too high, which makes these surfactants unsuitable for emulsification of aromatics. On the other hand, triglyceryl dodecyl ether seems promising for this, implied by its very low solubility in m-xylene (cf. Table I).
Acknowledgment. The stay of A. Carlsson in Japan was supported by a grant from the Swedish National Board for Technical Development. We are grateful to Kao and Pola Corps. for the synthesis of the glyceryl alkyl ethers. The financial support from the Japan Society for the Promotion of Science is gratefully acknowledged. Registry No. Rs(OCH,CH,),OH, 19327-38-9;RJOCH,. CH,),OH, 19327-39-0;R1,(OCH,CH,),OH, 5274-68-0;R12(OCH,CH,),OH, 3055-95-6;R12(OCH2CH,)60H,3055-96-7; R12(OCH2CH2),0H, 3055-97-8;R1,(OCH,CH,)sOH, 3055-989; RsOCH,CH(OH)CH,OH, 10438-94-5;RlZOCH, CH(0H)CHZOH, 1561-07-5;R,1COOCHZCH(OH)CH,OH, 142-18-7; Rl,(OCH2CH(OH)CH~)20H, 85590-51-8;i-R1,COOCH,CH(OH)CH,OH, 67938-24-3;i-R16(OCH,CH(OH)CH,)2OH, 85621-13-2;R1,(OCH,CH(OH)CH,),OH, 123541-03-7. (21) Shinoda, K.;Saito, H. J. Colloid Interface Sci. 1969, 30, 258. (22) Reference 18,pp 129-135.
