Langmuir 1992,8, 2623-2628
2623
Aqueous Phase Diagram and Crystal Structure of Sodium Dioctylphosphinate Pablo C . Schulz' Department of Chemistry and Chemical Engineering, Universidad Nacional del Sur, Bahia Blanca, Argentina
Jorge E.Puig Faculty of Chemical Sciences, Uniuersidad de Guadalajara, Guadalajara, Jalisco, Mkxico Received August 2,1991. In Final Form: July 23, 1992 The sodium di-n-octylphosphinate-watersystem was studied by several methods and a phase diagram drawn. A micellar solutionwas found to existat very low concentrations. There are two lamellar mesophaees, Lam 1 and Lam 2; the former is present at temperatures below a 3 3 0 K and the latter at high temperatures. Lam 2 liquid crystals become isotropic liquid at about 380 K. The anhydrous solid SDOP is monoclinic. The elementary cell parameters are a = 1.7221 f 0.0013 nm, b = 1.0584 f 0.0012 nm, c = 2.345 f 0.017 nm, and B = 61.15 f 0.45O, the number of surfactant moleculesper elementary cell is 12, the crystallographic density is 1667 f 12 kg.m9, and the chains are tilted 47.63 f 0.39O with respect to the (001) plane. Three kinds of water are present: two water molecules are bound to a surfactant molecule so tightly that it is impossible to detect them by differential scanning calorimetry; about 9.6 water molecules are bound to the polar group and the Na+ counterion in such a way that their melting point is about 263 K, and the remaining water in the system behaves as pure water.
Introduction There is great interest in elucidating various phases which occur in mixtures of twin-tailed surfactants and water. The applications of these self-organized structures include processes as diverse as production of polymers with unusual structural and morphologycal characteristics,' enhancement of analytical methods? and enhanced oil rec0ve1-y.~They are also of possible use in understanding the formation of organized systems4 and in modeling biological membranes. Sodium dioctylphosphinate Na(CHdCH2)7)902(SDOP) is a little-studied twin-tailed surfactant. Because of ita short hydrocarbon tails and large polar group volume, it was assumed that a variety of different phases would be formed in the SDOP-water system at low temperatures. This conjecture is confirmed by the studies we are reporting. Experimental Section Dioctylphosphiuicacid was synthesizedand purified according to the method in the literatureP The melting point was measured with a KBffler thermic microscope. Elemental analysis was carried out in the OrganicChemistryLaboratory of the University of Buenos Aires. IR and proton NMR spectra were employed to confirm the structure. The acid was neutralized with a concentrated NaOH solution and then dried in vtcuum to constant weight. The density of the crystalline powder was measured with a 25-cm3pycnometer and a Sartorius analytical balance to 0.01 mg. Filling liquid was petroleum ether, fraction 80-100 O C . To avoid evaporation, a rubber stopper was employed. Powder X-ray diffractograms were run in a Rigaku Denki diffractometer with a horizontal goniometer, Cu Ka radiation, and a nickel filter. Wavelength was 0.154 05 nm, goniometer (1) Puig, J. E.; Corona-Galvh, S.; Maldonado, A.; Schulz, P. C.; R o d h e z , B.; Kaler, E. W. J. Colloid Interface Sci., in preas. (2).Schulz,P. C; Femhdez-Band, B. S.;Palomeque, M.; Allan, A. L. Colloids Surf. 1990, 49, 321. (3) Latil, M. Enhanced Oil Recouery; Gulf Publishing Co.; Houston, Tx,1980. (4) Schulz, P. C. Colloid Polym. Sci. 1991,269, 612. (6)Williams,D.; Hamilton, P. J. Am. Chem. SOC.1955, 77,3411.
rate 2OImin, 35 kV, time constant 2 8, current intensity 12 mA, divergence slit lo,reception slit 1mm, exploration slit lo,and temperature 298 K. Diffractogramswere indexedusingthe Vand method!.? Allsolutionsampleswereprepared with doubly-distilledwater. Surface tension measurements were performed with a K r h ring tensiometer thermostatized to 298.1 f 0.1 K. pH measurements were performed with an Orion 601-A millivoltmeter and an Orion glass electrode. Solutions were maintained at 298.1 f 0.1 K. Solubilization measurements were performed leaving the different concentrations of SDOP solution with solid Sudan I11 for 1 week, with periodic shaking. The solutions were then analyzed with a Beckman DB spectrophotometer at 455 nm. Conductivity measurements were carried out at lo00 Hz and 298.1 h 0.1 K with an Orion conductometer and a Yellow Springs Instrument immersion cell, whose constant is 1.12 cm-l. The specificconductivityof the pure water was subtractedfrom that of the solutions. Differential scanning calorimetry (DSC) samples were made by weighing appropriate amounts of SDOP and water in vials. The vials were hermetically sealed and heated for 1h at 373 K, and then left for 1week. Before the run, the samples were stirred in order to homogenize them. Runs were made with a PerkmElmer DSC-4 calorimeter with an Intracooler I refrigeration system. Heating and cooling rates were 10 K/min. Sealed aluminum pane for volatile samples were employed, to avoid evaporation. Samples were weighed before and after the run and discarded if any weight loss was detected. The calorimeter was calibrated with indium, water, n-octane and n-tetradecane. Samples were cooled to -30 O C . Microscopeobservationswere made in a Leitz crystallographic microscopewith thermic plate and a digitaltemperature control. All percenta are weight to weight. Confidence intervals were estimatedby means of the Student t distribution and a confidence level of 0.90.
Results Dioctylphosphinic Acid. The melting point was 357.7-358.2 K, very close to the 358.2 K cited in the literat~re.~ The elementary analysis yielded 66.23 & 0.31 % (6) Vand, V. Acta Crystallogr. 1948, 1, 109. (7) Vand, V. Acta Crystallogr. 1948,1, 290.
0743-746319212408-2623$03.00/0 Q 1992 American Chemical Society
Schulz and P u g
2624 Langmuir, Vol. 8,No. 11, 1992
i'
100
0
1
2
3
4
c IO^,
5
6
7
8
Figure 1. Electric conductivity vs the molar concentration of SDOP. T = 298 K.
e
5 6 -log Figure 3. Surface tension vs the negative logarithm of the SDOP molar concentration. T = 298 K. 1
7l
2
3
6
2
3
-log
4
c
5
6
7
Figure 2. pH vs the negative logarithm of the SDOP molar concentration. T = 298 K. of carbon (66.23theoretical yield) and 12.275 f 0.060% of hydrogen (12.15theoretical yield). The neutralization equivalent was 286 g (the figure in the literature is 290.4
g5)*
Sodium Dioctylphosphinate. Figure 1 shows the conductivity resulta. A slope break can be observed a t M (0.25%) concentration. The resulta of the the 8 x pH analysis are plotted in Figure 2;here a sudden rise in pH and then in hydrolysis (to form HDOP and OH-) is apparent at lo-*M (0.31%), corresponding to the critical micelle concentration ( C ~ C ) . ~ !A~ further slope break M (0.7%). Because the studied occurs at 2.23 X concentration range covers several orders of magnitude, this and successive figures have been drawn with logarithmic concentration scales. Figure 3 shows surface tension results. A plateau can be seen between 5.6 X M (0.18%)and 3.16 X lo4 M (0.99%) and a strong drop of surface tension is evident after the last concentration. It does not show any minimum. This indicates the abscence of surface-active impurities. Figure 4 shows Sudan I11 solubilization. No solubilization was found until a concentration of 3.2 x 10-5 M (0.099%)was reached. A t 5.6 X lo4 M (1.7%)there is a strong rise in solubilization. Figure 5 is an X-ray diffractogram of anhydrous SDOP at room temperature. Figure 6 shows some representative thermograms of the SDOP-water system. With the exception of the two lowtemperature peaks,which can beattributed to water fusion, (8) Staineby, G.;Alexander, A. E.Trans. Faraday Soe. 1949,54,585. (9) Schulz, P. C. Anales Asoc. Qulm. Argentina 1984, 76 (2), 529.
1
2
4
3
5
6
-logC Figure 4. Solubility of Sudan I11 vs the negative logarithm of the SDOP molar concentration. T = 298 K. 66
c
1 I
I
I
40
30 20
10 5
I
I
,
I
1U
15
20
25
1
e
35 (DEGREES)
30
40
45
Y)
55
60
Figure 5. X-ray diffraction pattern of anhydroue solid SDOP. the curves are not sharp, suggesting that they correspond to gradual transitions. This was confirmed with normal and polarized light microscope observations. Figure 7a shows the melting enthalpy of the water in the system, per gram of sample; it ranges from that for pure water (320Jsg-l) to zero at 90% SDOP concentration. Curve 7b represents the dissolution enthalpy of the SDOP crystals per gram of sample, reaching zero at a17 % SDOP.
Study of Sodium Dioctylphosphinate
Langmuir,
IIL
Table I. Transition Temperature (V)*
rr: SOOP
concentration (% (W/W))
TI
TZ
9.94 19.89 25.05
-9.5
-2.5
30.13
-9.5
-2
40.02
-9.5
-2.5
49.55 I \ 47,
-9
-2
58.33
-9
-2
69.01
-8.5
30 35.5 36* 31 34.5 341
80.50
-9
35 35*
9.94
d
4.98
L s . 3 0
100
Figure 6. Some representative water mixtures.
DSC thermograms of SDOP-
3 00
23 23* 23 23* 24.5 26* 28 31* 355 37 355
52 50* 47*
89.01
TS
T4
T3
-2 -3.4 -8.5 -2 -9 -2.5
lJ---
Vol. 8, No.11,1992 2625
58.5 57*
T6
TI
8 6 8 6 89 89*
32.5* 36 36* 37* 41.5 41* 445 43.5 43* 45* 50.1 50* 48.5* 57* 60 601 65* 90 80* 95* 112 112*
56.5 601 52.5 58* 61* 61 58* 59 64 62
75 78 80 70 75
93 95 95*
90
100 97*
89 93 94 100
100.5
62
123*
132*
63.5
a Asterisk indicates we of a polarizing microscope. No asterisk indicates value obtained by DSC.
2 00 h
Table 11. Transition Enthaby ( J / a of samde)
OI 7
Y
r a J
U
I
5
100
W
4.98 9.94 19.89 30.05
318 312.5 308 276.5
40.02 49.55
225.8 164 163.8 140.1
69.01
92
80.50 89.01
41.5
58.33 I ! I
I
I
I
1
50 WT X SDOP
I
I
100
Figure 7. Enthalpy per gram of sample, for (a) water and (b) dissolution of SDOP crystals. The microscope observations will be discussed in the following section. Transition temperatures and enthalpies are shown in Tables I and 11.
Discussion Phase Diagram. DSC data and polarized light microscope observations enabled us to draw the phase diagram shown in Figure 8. There are two melting points for water, shown by curves TI and T2; curve T2 is close on 273 K and corresponds to "free water", whose properties are those of pure water. This water does not associate with surfactant molecules. Curve Tz finishes at about 60% SDOP. Curve TI is -10 K below curve T2and finishes at 90% SDOP. This point was determined by extrapolation to zero of curve a in Figure 7. From curves TI and T2 it may be concluded that 10% of the water (about two water molecules per surfactant molecule) is so tightly bound to the polar heads and counterions that it is impossible to freeze or detect them calorimetrically. This defines the
52.8 53.2 58.7
16.5 49.5 58 75
9.5 25 34 34 52.4
18 20.2
88.2
52.8
24.8
10
93
62
10.1
106 149
75 85.1
35 25.1 34
8.86
13
hydrated surfactant phase. Another 9.6 water molecules per surfactant molecule are bound in some way to the polar heads and counterions, so they melt at -263 K. All other water molecules in this system behave as pure water. The presence of several kinds of water in similar systems has been reported in In the low concentration zone, above the melting point of water, there exists an isotropic molecular solution. The critical micelle concentration (cmc) is (6.7f 3.5) X M (0.31 0.11%) a t 298 K. At the concentration (3.7 2.9) X 10-4M (1.14 0.91 %) and 298 K, some dramatic changes occur: there is a sharp increase in the solubility
*
*
*
(10)Tokita,M.;Terakawa, K.; Ikeda,t.;Hikichi, K. Polym. Commun.
1990, 31,38.
(11) Schulz,P.C.;Puig,J.E. MemoriaeIVColoquiode Termodin4mica, M6xico, D.F., 1989;p 81. (12)Torres, L.A.;Barreiro, G.; Schulz, P. C.; Puig, J. E. Memorias IV Coloquio de Termodidmica, Mexico, D.F., 1989;p 227. (13)Senatra, D.;Zhou, 2. B o g . Colloid Polym. Sci. 1988, 76, 106. (14)Casillas, N.;Puig, J. E., Olayo, R.; Hart, T. J.; Frames, E. I. Langmuw 1989,5, 384.
Schulz and Puig
2626 Langmuir, Vol. 8, No. 11, 1992
ISOTROPIC LlOUlb
0
50
/
+O0
100
WTo/e SDOP
Figure 8. Phase diagram for the SDOP-water system: curves TI and T2, water melting points; curve T3, crystals start the dissolution giving mesophase: cuxveT4,end of crystal dissolution; zone T g , transformation of Lam 1to Lam 2 mesophases; curve Ts, the mesophase Lam 2 begins the transformation to isotropic liquid; curve T,,the transformation begun in Ts is completed; point A, fusion of the hydrocarbon network of anhydrous SDOP crystals; 1, solid SDOP.13.6H20; 2, solid SDOP-PH20.
of Sudan 111, a marked drop in surface tension, and a reduction in SDOP hydrolysis. Observation with a polarized light microscope showed the fine emulsion of a birefringent golden mesophase. Under magnificationthese emulsion particles were seen to be mosaic texture, with the small positive and negative units and oily streaks typical of lamellar mesophases. The curve in Figure 3 may be explained by the model of Mitchell and Ninham.15 A t C < 5.6 X M,the ionic atmosphere size large enough to overlap a significant part of the SDOP tails. This prevents micelleformation. Since the twin-tailed surfactants efficiency in surface tension lowering is smaller than the efficiency of a surfactant with a single straight-chain with the same total number of carbon atoms,I6the surface tension decreases slowly with an increasing concentration. M, the size of the ionic atmosphere At C = 5.6 X has decreased to an extent which permits micelles formation, giving the plateau. M, the reduction of the ionic A t C 2 3.16 X atmosphere size gives a cylindrical shape to the SDOP molecule. This causes the formation of a lamellar mesophase. The compression of the electrical double layer by the increase of ionic strength and the consequent decrease in repulsion between the hydrophilic heads results in a closer packing at the interphase, approaching the surfactant surface activity to that of the nonionics.16 As a consequence, there is a strong drop in surface tension. Alexopouloset al.l7 also reported a very low surface tension (
