MICELLAR PROPERTIES OF DIMETHYLALKYLPHOSPHINE OXIDESOLUTIONS
2909
Micellar Properties and Critical Opalescence of Dimethyalkylphosphine Oxide Solutions
by K. W. Herrmann, J. G. Brushmiller, and W. L. Courchene Miami Valley Laboratories, The Procter & Gamble Company, Cincinnati, Ohio 46389 (Received March 8, 1966)
Phase diagrams and light-scattering measurements of a homologous series of dimethylalkylphosphine oxides are used to determine the micellar molecular weights and their relation to consolute phase boundaries in these systems. The temperature dependence of micellar molecular weights is discussed with regard to both light-scattering and solution theory, and it is concluded that, in systems which show consolute boundaries, the lightscattering behavior can be qualitatively explained on the basis of existing theory without invoking any large temperature or concentration dependence of micellar molecular weight. The observed light-scattering behavior can easily be accounted for by the nonideality of a micellar solution and the temperature dependence of micellar-water interactions. Lightscattering measurements at the critical concentration for phase separat,ion of the dimethyldodecylphosphine oxide-water system have also been made and show that, critical opalescence is observed in this system. These results are discussed in the light of existing theoretical treatments of this phenomenon, and the experimental results are shown to agree qualitatively with the theory.
Introduction
methylalkylphosphine oxide [CnH2.+~P(CH3)2O]. The three homologs will be abbreviated as DCBPO, DCIOPO, and DCEPO.
I n recent years there has been considerable work reported in the literature dealing with the micellar properties of nonionic surfactants, l-* particularly the (1) K. Shinoda, T . Nakagawa, B. Tamamushi, and T . Isemura, alkylethylene oxide surfactants, CH~(CH~),(OCHZ- “Colloidal Surfactants. Some Physicochemical Properties,” Academic Press Inc., New York, N. Y., 1963, Chapter 2. CH2).0H (m = 8-16, n = 6-9).334,9-18 Aqueous solu(2) L. M. Kushner, W. D. Hubbard, and A. S. Doan, J . Phys. Chem., tions of these surfactants show “cloud points”; i e . , 61, 371 (1957). relatively dilute solutions of these surfactants become (3) L. M. Kushner and W. D. Hubbard, {bid., 58, 1163 (1954). very turbid a t a well-defined temperature when the (4) M. J. Schick, 5. M. Atlas, and F. R. Eirich, ibid., 66, 1326 (1962). solution is heated.” .At temperatures above this (5) K. W. Herrmann, ibid., 66,295 (1962). “cloud point” the light-scattering turbidities of these (6) W. L. Courchene, ibid., 68, 1870 (1964). surfactant solutions are large and very temperature (7) L. Benjamin, ibid., 68,3575 (1964). dependent ; however, the solutions have dissymmetries (8) K. Kuriyama, Kolloid-Z., 180,55 (1962). which are close to The increase in solution (9) T. Nakagawa and K. Tori, ibid., 168, 132 (1960). turbidity with increasing temperature as the “cloud (10) J. M. Corkill, J. F. Goodman, and R. H. Ottewill, Trans. Faraday SOC.,57, 1627 (1961). point” is approached has been interpreted as meaning ( I f ) R. R. Balmbra, J. S.Clunie, J. M. Corkill, and J. F. Goodman, that the micelle molecular weight increases exponentiibzd., 60, 9 i 9 (1964). ally with increasing temperature in these sys(12) R. R. Balmbra, J. S.Clunie, J. M. Corkill, and J. F. Goodman, t e m ~ . ~ * We ~ , ~believe ~ , ~there ~ , ~may ~ be another exibid., 58, 1661 (1962). (13) J. M. Corkill, J. F. Goodman, and S.P.Harrold, ibid., 60,202 plana tion. (1964). This paper reports on the phase behavior and solu(14) P. H. Elworthy and C. B. Macfarlane, J . Chem. SOC.,537 tion properties of the CS, CK,, and Clz homologs of di(1962); 907 (1963). ~~
Volume 70,Number 9 September 1966
K. W. HERRMANN, J. G. BRUSHMILLER, AND W. L. COURCHENE
2910
The critical micelle concentrations (cmc) and micelle molecular weights (mmw) of these surfactants are presented and the effect of temperature on the scattering of light by dilute and concentrated DClzPO solutions is reported and discussed. “Cloud point” phenomena are discussed in terms of consolute solution theory, and light-scattering data obtained a t the critical concentration of DClzPO are examined using Debye’s critical opalescence theory. The solution properties of these surfactants are correlated with the phase boundaries shown by their respective systems, DC,POHzO.
Experimental Section Materials. The Cs, (210, and Clz homologs of dimethylalkylphosphine oxide were prepared by a twostep synthesis.
0
li
RP(OC6H&
+ 2CH3SIgBr -+ 0
/I
RP(CH&
+ 2C6HaOllgBr
All surfactants were greater than 99.5% pure as indicated by gas phase chromatography. Elemental analyses were as follows. Anal. Found for DCEPO: C, 63.2; H, 12.0; P, 15.9. Found for DCloPO: C, 65.6; H, 12.2; P, 13.5. Light Scattering. A Phoenix Precision Instrument Co. light-scattering photometer (Model lO00) provided with narrow slits was used for all investigations except that dealing with the critical opalescence shown by DCIZPOwhich will be discussed separately. RIeasurenients were made a t 30” using a cylindrical cell (Catalog KO. C-101) and using the blue line of mercury (X 4358 A). Calibration of the Phoenix instrument, temperature control, and the method of solution clarification were identical with those described previo~sly.~ Scattered intensities observed a t an angle of 90’ are expressed as turbidities for dilute solutions near the cmc; the intensities for the more concentrated solutions are given simply as Iga.Dissymmetry values (Zdj)were calculated from 24:
=
Id5(so1ution) - 145 (solvent) I135(solution)- Ila5(solvent)
Where the critical opalescence of DClzPO solutions was examined, a Sofica light-scattering photometer The Journal of Physical Chemistry
(Model 70142000) was employed with the standard measuring cell (25-mm i.d.), X 4358 A, and the special temperature-controlled vat was equipped with a variable-speed magnetic stirrer. Calibration of this instrument was similar to that described earlier for the Phoenix i n ~ t r u m e n t . ~For t,he critical opalescence study, the scattered intensity was measured as a function of angle a t several temperatures. The intensities reported were obtained by multiplying the measured galvanometer readings by an instrument constant and the appropriate (sin e)/(l cos2 0) correction factor. The temperatures of the solutions being examined were maintained constant within 0.05’. Refractive Index Increments. A Brice-Phoenix different’ial refractometer was used to det’ermine the refractive index increment (dnldc) required for the calculation of micelle molecular weights. The instrument was calibrated with sucrose solutions using light of X 4358 A. Average dnldc values from three or four solutions whose concentrat.ions were above, but near, the cmc are reported. Phase Studies. Fixed compositions of DC,POH 2 0 were sealed in small (4-ml) glass test tubes and were equilibrated in a controlled-temperature bath. Continuous agit’ation was supplied by a mechanical rocking device. The temperatures a t which phase separations occurred were determined on both heating and cooling over bhe range -5 to 200°, the system being under considerable pressure at, the higher temperatures. Separation of a second isotropic phase was indicated by a great increase in turbidit,y; marked birefringence, shown by the samples being equilibrated between crossed nicols, was observed when a mesomorphic phase formed. Surfactant crystals could be detected with the eye. At least two heating and cooling cycles were made to obtain the results reported. Identification of the mesomorphic phases was made using a polarizing-light microscope and the characteristic “textures” reported by Rosevear.’*
+
Results and Discussion Phase Behavior of DC,PO Surfactants. The phase diagrams for DCEPO, DCloPO, and DCIzPO in water are shown in Figures 1, 2, and 3, respectively. The middle and neat phases exhibited by these surfactants are typical mesomorphic * (liquid crystal) (15) P. H. Elworthy and C. -McDonald, Xolloid-Z., 195, 16 (19643. (16) C . W. Dwiggins. Jr., and R . J. Bolen. J. Phys. Chem., 6 5 , 1787 (1961). (17) W. N. Maclay, J . Colloid Sci., 11, 272 (1956). (18) F. B. Rosevear, J . A m . Oil Chemists’ Soc., 31, 628 (1954). (19) A. J. Mabis, Acta Cryst., 15, 1152 (1962). (20) V. Luzzati. H. Mustacchi, and A. Skoulios, Nature, 180, 600 (1957); Discussions Faraday Soc., 25, 43 (1958).
MICELLAR PROPERTIES OF DIMETHYLALKYLPHOSPHINE OXIDESOLUTIONS
291 1
160‘
140-
TWO ISOTROPIC PHASES
120-
-0
I 100J
- 0. Figure 1. Phase diagram for dimethyloctylphosphine oxide-water.
1.0
20
40
60
cC**PO
I ut
2.0
d0
LOO
”
Figure 3. Phase diagram for dimethyldodecylphosphine oxide-water. Insert shows consolute boundary a t low concentrations.
90-
IS0TH 0PIC PHASES
e
sC1OPO ( ut.
%
Figure 2. Phase diagram for dimethyldecylphosphine oxide-water.
and will not be considered any further in this paper. The part of the phase diagram of particular interest here is the region where two immiscible isotropic solutions coexist (Figures 2 and 3). In binary solution terminology,21 the two immiscible isotropic solutions are referred to as conjugate solutions. The temperature at which phase separation occurs is called an upper consolute temperature when a maximum occurs in the phase boundary. The surfactant concentration at which the maximum or minimum occurs is called the critical concentration. The temperature at which the maximum occurs in the consolute boundary is called an upper critical solution temperature, and the temperature at which the minimum occurs in the
consolute temperature is called a lower critical solution temperature. Thus, while DCEPO does not form immiscible isotropic solutions a t the temperature examined, both the DCloPO and DCJ’O binary systems do exhibit critical solution behavior. If one examines phase data for ethylene oxide surfactant-water systems published in the literature,11~12it can be seen that the term “cloud point,” as used by the authors, is really the lower consolute temperature of the system. Hence, we equate the term “cloud point” and lower consolute temperature. The temperature and composition ranges over which conjugate solutions exist in the dimethylalkylphosphine oxide-water systems decrease with decreased alkyl chain length in the surfactant (see Figures 1, 2, and 3). Only DClzPOexhibits a lower consolute boundary which extends over a wide range of temperatures and compositions; the critical solution temperature and concentration were visually estimated to be approximately 38.8” and about 0.7% DC12P0, respectively. The DCloPO system exhibits both an upper and lower consolute boundary with critical temperatures of about 177 and 124’) respectively, and critical concentrations between 10 and 15% DCloPO. Considerable pressure must have developed in the sealed phase tubes at the temperatures where the conjugate solutions of DClaPO were observed; the effect of pressure on this type of phase separation is unknown. The DCEPO system does not form conjugate solutions below 200”. General Light-Scattering Properties and Theory. (21) S. Glasstone, “Textbook of Physical Chemistry,” 2nd ed, D. Van Nostrand Co., Inc., New York, N. T., 1946, Chapter X.
Volume 70, Number 9 September 1966
K. W. HERRMANN, J. G. BRUSHMILLER, AND W. L. COURCHE:
2912
and Debye,24the excess turbidity given by r =
A i
of a solution
HcRT/(dII/dc)
where H is an optical constant equal to 32&zo2(d ~ C ) ~ / ~ NHere X ~ .no is the refractive index of the s vent, N is Avogadro's number, X is the waveleng of the incident radiation in vucuo, dnldc is the spec; refractive index increment of the solute, II is the osmo pressure of the solution, R and T are the gas constt and absolute temperature, respectively, and c is I concentration of solute. Equation 1 holds as long the scattering entities are small compared with t wavelength of the light and can be rearranged to
a
dII 1 - _ __ - Hc dc RT r
K ~ P O Coneenwatianlgms lo0mle)
Figure 4.
(r)
Scattered light intensity of DC,PO solutions.
Thus, as pointed out by D e b ~ e , Hc/r ~ * is intimati connected with the behavior of the osmotic pressure the solution and is directly proportional to the slope the II vs. c plot. If the light-scattering experiment carried out on very dilute solutions, the van't H expression (II = RTc/M) may adequately descr the concentration dependence of n. For higl concentrations a modified expression is often U I [II = RTc(l/M Bc)]. Figure 6 shows the expecl behavior of the osmotic pressure in such solutions a also illustrates the concentration dependence of dII/ which is equivalent in form to that expected for Hc When we are dealing with a binary solution exhibit a critical concentration and a critical consolute te perature, osmotic pressure, dII/dc, and HC/Twill have as shown in Figure 7z5 in the region of the criti point. Near the critical concentration dII/dc E HC/T are markedly dependent on concentration I decrease as the solution temperature approaches critical temperature; these values become zero a t critical concentration and a t the critical temperatt In our surfactant systems, and perhaps in 0th showing lower consolute boundaries, this means t turbidity data must be obtained as close to the crit micelle concentration (cmc) as possible if valid mic molecular weights are to be obtained by the us practice of extrapolating Hc/r curves to the CI This point will be discussed in greater detail in the n section.
+
% 45
Figure 5.
Dissymmetry of DC,PO solutions.
Figures 4 and 5 show the concentration dependence of scattered light intensity (190) and dissymmetry (Z,,) for solutions of DCsPO, DCIOPO,and DClzPO at 30" up to surfactant concentrations of 40 g/100 ml. All three surfactant systems exhibit a turbidity maximum in their respective turbidity vs. concentration curves. This behavior is similar to that observed for other nonionic surfactants such as the alkylethylene oxidesll,l z and dimethylalkylamine oxide,22 but is slightly different from that observed for anionic surfactant systems.23 Only DClzPO shows a dissymmetry maximum. To explain properly the light-scattering data obtained for both dilute and concentrated surfactant solutions in systems exhibiting a lower consolute boundary, it is necessary to review briefly the theoretical basis for light scattering by solutions. According to Einstein
(22)J. M. Corkill and K. W. Herrmann, J . Phys. Chem., 67, (1963). (23) J. N. Phillips and K. J. Mysels, ibid., 59, 325 (1955). (24) A. Einstein, Ann. Phyeik, 33, 1275 (1910); P.Debye, J . A Phys., 15,338 (1944); J . Phys. Colloid Chem., 51,18 (1947). (25) 0. K. Rice, ibid., 54, 1293 (1950).
MICELLAR PROPERTIES OF DIMETHYLALKYLPHOSPHINE OXIDESOLUTIONS
d"dc
2913
= RT M
H c I =1 M
d T1
I I
I8 I
I
I
- I
Concentration
Figure 6. Hypothetical osmotic-pressure and light-scattering curves in the absence of a consolute boundary.
I
I I
I
C~,,C12PO/loOml, which is well removed from the concentration at which the dissymmetry maximum occurs.
Acknowledgments. We gratefully acknowledge the assistance of Dr. R. G. Laughlin, who prepared the DC,PO surfactants, and that of Dr. R. C. Mast, who determined the surface tension cmc values. We are especially indebted to Professor P. Debye for his encouragement and for many helpful discussions regarding the applicability of light-scattering and consolutesolution theory to the surfactant solutions studied.
