Langmuir 1991, 7,2021-2027
2021
Dynamic Behavior and Detergency in Systems Containing Nonionic Surfactants and Mixtures of Polar and Nonpolar Oils Jong-Choo Lim and Clarence A. Miller' Department of Chemical Engineering, Rice University, P.O.Box 1892, Houston, Texas 77251 Received October 9,1990. In Final Form: February 15, 1991 Videomicroscopy was used to observe the dynamic behavior that occurred when aqueous solutions of nonionic Surfactants were brought into contact with oils containing various proportions of n-hexadecane and oleyl alcohol. A new contacting technique was used in which a single oil drop was injected into a surfactant solution. Under lipophilic conditions where the surfactant is preferentially soluble in the nonaqueousphase, the drop experienced significant swelling as it tookup surfactant and water. Eventually, it reached a composition where the lamellar liquid crystalline phase began to develop at the interface as myelinic figures growing toward the aqueous solution. A quasi-steady-state analysis was developed that was able to explain the effects of such variables as initial drop size and initial surfactant concentration on the time until liquid crystal formation began. The relationship of the results to detergency is discussed. It appears that a major mechanism of detergency for oils containing more than about 10% polar material under lipophilic conditions is solubilization into an intermediate lamellar phase which is dispersed into the washing bath as agitation breaks up the myelinic figures.
Introduction Previous papers from this laboratory have described the dynamic behavior observed by videomicroscopy when pure hydrocarbons were carefully brought into contact with water containing pure nonionic surfactants,1p2 mixtures of such surfactants,3 and mixtures of nonionic and anionic surfactants.' Prominent among the phenomena seen in many of the experimentswas growth near the initial surface of contact of intermediate phases which were not present initially, e.g., microemulsions and liquid crystals. Recently, results of a similar study were presented where the pure triglyceride triolein was used instead of pure hydrocarbon^.^ Here too, intermediate phases were frequently observed. Mixtures of triolein and n-hexadecane were investigated as well.6 These papers also addressed the relationship between the observed dynamic behavior and the removal of the same oils from polyester/cotton fabric by the same surfactants during washing. Basically, it was found that detergency was best when the oil was solubilized most rapidly into intermediate phases and/or rather concentrated phases such as liquid crystals initially present in the washing bath.2-s For instance, it was found that maximum removal of hydrocarbons occurred near the phase inversion temperature (PIT).2J It is well-known that microemulsionswith high oil solubilization form near the PIT and that low interfacial tensions occur therees The latter would facilitate emulsification of microemulsions formed during washing. For the systems studied, the PIT was well above the surfactant cloud point. In practical washing situations removal of oilysoils which are mixtures of nonpolar compoundssuch as hydrocarbons or triglycerides and polar compounds such as long-chain alcohols or fatty acids is of great interest. Because the (1) Raney, K. H.; Benton, W. J.; Miller, C. A. J. Colloid Interface Sci. 1986,110,363. (2) Raney, K. H.; Benton, W. J.; Miller, C. A. J. Colloid Interface Sci. 1987,117,282. (3) Ran~y,K. H.; Miller, C. A. J. Colloid Interface Sci. 1987,119,539. (4) Mon,F.; Lim, J. C.; Miller, C. A. h o g . Colloid Polym. Sci. 1990,
--
82. --,114._.
(5) Mori, F.; Lim, J. C.; h e y , 0. G.; Elsik, C. M.; Miller, C. A. Colloids surf. 1989, 40, 323. (6) Shinode, K.; Friberg, S.Adu. Colloid Interface Sci. 1976,4, 281.
polar compounds produce large reductions in the PIT, excellent removal of these mixed soils can frequently be achieved at temperatures below the surfactant cloud point, in contrast to the results summarized above for nonpolar soils. h e y and Benson recently presented quantitative information on removal of mixed n-hexadecane-oleyl alcohol and n-hexadecane-oleic acid soils by pure nonionic surfactants below their cloud points.' They found soil removal to be excellent over a wide range of temperatures. The lower limit of this range was the PIT of the surfactant under conditions where the excess oil phase in equilibrium with the microemulsion and water had the same composition as the soil. In this paper we describe videomicroscopy observations of dynamicbehavior in some of the same mixed soil systems in order to provide further information about the mechanism of soil removal. Because variation of soil composition with time during the washing process is an important factor in these systems, we have utilized a new technique in which observations are made of asingle oil drop brought into contact with a much larger volume of surfactant solution. A theoretical analysis based on a quasi-steadystate solution of the relevant transport equations has also been developed to interpret the results. Our chief observation is that for the lipophilic systems which exhibit good soil removal, Le., those above the PIT, an intermediate lamellar liquid crystalline phase develops as myelinic figures, beginning at a definite time after initial contact. The dependence of this time on such factors as initial drop diameter and surfactant concentration can be explained by the theoretical analysis. It is significant that use of the analysis requires only limited information on phase behavior, an important consideration if it is to be applied to various four-component or even more complex systems. It appears that a major mechanism of detergency in such systems is dispersion into the washing bath of liquid crystalline particles incorporating soil which are broken from the myelinic figures by agitation. In contrast, microemulsion phases are evidently the chief vehicle of the solubilization and dispersion (emulsification) process for pure nonpolar soils. (7) Raney, K. H.; Beneon, H.L. J. Am. Oil Chem. SOC.1990,67,722.
0743-7463/91/2407-2021$02.50/0 0 1991 American Chemical Society
2022 Langmuir, Vol. 7, No.
Lim and Miller
IO, 1991 oleic ohase
(I) Oil Drop ( wi )
Figure 1. Schematic illustration of contacting experiment where a small oil drop is injected into an aqueous surfactant solution.
(11) Surfactant Solution (
wi' )
Experimental Section 1. Materials. The pure nonionic surfactants n-dodecyl hexa-,
hepta-, and octaoxyethylenemonoethers (CI*&,&E,, and C12Ee) were obtained from Nikko Chemical Co., Japan, and were used as received. The purity of the neat surfactants was typically greater than 91 7;. Oleyl alcohol was obtained from Sigma and had a reported purity of 99%. The hydrocarbon, n-hexadecane, was analytical grade and was obtained from Humphrey Chemical Co. Water used for solution preparation was deionized and double distilled. 2. Methods. Solutions for phase behavior studies were prepared in Teflon-capped, 13-mm-i.d.,flat-bottomed test tubes and mixed for 20 s by vortex mixing. Typically, sample volume was about 10 mL with comparable volumes of the initial oil phase and the aqueous surfactant solution. All solutionswere blanketed with nitrogen. The polarized light screening (PLS) system described previouslf was used for determining macroscopic phase behavior. This technique provides information about the isotropy, anisotropy, and scattering of solutions. Liquid crystaline phases were easily distinguished by the birefringence displayed with polarized light. The test tubes were calibrated, and the phase volumes were measured with an accuracy of 1% of the test tube volume. A new contacting technique was developed to observe the dynamic behavior of small individual drops of oily soils immersed in a large quantity of surfactant solution. With this technique, which differs from that of our previous contacting studies where comparable volumes of oil and surfactant solution were employed,lv*psthe latter stages of solubilization, a significant part of the detergency process in the systems of interest here, can be observed. Moreover,partitioning of various components between phases, an important phenomenon in systems containing mixed soils and/or surfactants and one dependent on the relative amounts of surfactant solution and soil contacted, can be made close to that expected in practical systems. The basic videomicroscopyequipment used to observe dynamic behavior accompanying the contact of the dilute mixtures of surfactant and water with oil has been described previously.1J However, the microscope used here was a Nikon Metallurgical Microscope, Optiphot, with the usual horizonatalstage, in contrast to our previous experiments which employed a microscope with a vertical stage. Samples of the surfactant solutions to be studied were introduced into rectangular optical glass capillaries (Vitrodynamics, Inc.) by capillary action. The capillaries 50 mm long and 4 mm wide and had an optical path length of 400 pm. For cleaning they were soaked in chromic acid solution for at least 24 h, rinsed several times with distilled water, and then dried before use. After filling,they were sealed at one end and attached to standard microscope slides with an ultraviolet light sensitive polymer adhesive which was cured with an ultraviolet fiber optic gun. A very thin hypodermic needle (0.21 mm o.d., Hamilton Co.) was used to inject individual oil drops into the surfactant solution (see Figure 1). This procedure gave good control of drop size. Drops about 100 pm in diameter were injected. Temperature control for the samples was obtained with a Mettler FP-52 controller and FP-5 microscopehot stage. The thermal (8) Benton, W. J.; Miller, C. A. J. Phys. Chem. 1983,87, 4981.
i-1
Nonpolar Oil
i-2
Surfactant
i-3
Water
i-4
Polar Oil
Figure 2. Schematic illustration of a spherical drop (I)immersed in a large quantity of aqueous surfactant solution (11). stage was modified and placed on top of a new X-Y positioner (Nikon, Inc.) so that drops could be observed during and immediately after injection while the same was maintained at constant temperature. Interfacial tensions were measured by using a University of Texas Model 500 spinning drop tensiometer.
Theory Our previous contacting experiments with pure surfact a n t ~ ~could * ~ -be ~ interpreted by using diffusion path theory. A diffusion path is a plot on the equilibrium phase diagram of the compositions which develop as a result of diffusion when two semiinfinite phases of uniform composition are brought into contact in the absence of convection. With suitable assumptions, e.g., constant diffusion coefficients and equilibrium at all interfaces, a similarity solution to the governing differential equations exists for these conditions, and it can be shown that the resulting diffusion path is independent of time.9910 The similarity solution applies when both phases are semiinfinite and hence cannot be used to describe the oil drop experimentsused in this study. Accordingly, we have developed a theory to interpret our results based on the assumption of quasi steady state. That is, compositions within the oil drop and the surroundingsurfactant solution are assumed to change slowly with time. The validity of this approach is discussed below. Shown in Figure 2 is a schematic illustration of a spherical oil drop (I) immersed in a large quantity of aqueous surfactant solution (11). A quaternary system is assumed with i = 1, 2, 3, and 4 denoting nonpolar oil, surfactant, water, and polar oil, respectively. With the assumptions of constant diffusion coefficients and no convection or cross diffusion effects, the mass balance for each species simplifies in the quasi steady state approximation to
where wi is the mass fraction of species i. Since the mass (9) Ruschnk, K. J.; Miller, C. A. Ind. Eng. Chem. Fundam. 1972,II, 534. (10)Miller, C. A.; Neogi, P. Interfacial Phenomena: Equilibrium and Dynamic Effects;Marcel Dekker: New York, 1985.
Nonionic Surfactants and Polar and Nonpolar Oils
Langmuir, Vol. 7, No. 10, 1991 2023 equation can be easily solved to obtain
OIL
NONIONIC
HATER
We expect that surfactant concentration w'2 in the L1 phase is very small at the interface for mixed soils above the system PIT. After all, with pure polar oils it is wellknown that surfactant concentration at the L1 end of the limiting tie-line, usually called the limiting association concentration or LAC, is well below the surfactant critical micelle concentration (cmc). For nonionic surfactants the cmc is itself low. With this assumption the surfactant concentration distribution given by eq 5 becomes w'2 = W t 2 4 1- R
);
SURFACTANT EF
is
limiting tie-line
Figure 3. Schematicpseudoternary phase diagram of nonionic surfactant-water-cetane-oleyl alcohol mixture showing change in oil drop composition with time.
As a further simplification, we fit the (L1 coexistence curve by a polynomial
+
L2)
Two boundary conditions which must be satisfied are wti wti= as r
w2 = A, + A2w, + A3w12 (10) where A,, A2, and A3 are constants to be determined empirically from experimental phase equilibrium data. Then dw, ,--=dw2dw, -dwl (11) ) dt dw, dt (A2 + 2 A 3 ~ 1 dt Substituting eqs 9 and 11 into eq 6 applied to the surfactant, simplifying by setting w'2 = 0, and invoking eq 8 to eliminate R, we find
dwi/dr = 0 at r = 0 (i = 2,3,4) (3) where W'i and w+ are the local and bulk mass fractions of species i in the aqueous solution. Thus, inside the drop (1)
Integration of this equation with w1 = WIO at t = 0 leads to
fractions must sum to unity, this equation need be solved for only three of the four species present. The general solution of eq 1 is wi = bi
+ ai-r
--
-
wi
= Wi(t)
(4)
and outside the drop (11) W'i
= wtim
+r
(5)
The result after performing the integration may be arranged as follows:
Equations 4 and 5 can be solved with appropriate boundaryconditions at the interfacer = R(t). The species mass balance equations are given by
where
dw'i dR ?rR3wi)= DJidr ( 4 ~ r ~ ) I+, =4=R2 ~ w',(R) (6) dt 3 dt where D'i is the diffusion coefficient of species i in the aqueous solution. We also require local equilibrium at the interface. This condition assumes that diffusion through the bulk phases is slow in comparison toadsorption and desorption at the interfaces. That is, the concentrations at the interface lie on the ends of a tie-line joining equilibrium compositions of the aqueous (L1) and nonaqueous (Lz)phases. The above equations can be simplified with the assumption that mass transfer of alcohol and hydrocarbon into the L1 phase is negligible. For rather lipophilic conditions, i.e., above the system PIT, there is very little solubility of either long-chain alcohol or hydrocarbon in the L1 phase, and the assumption should be reasonable. It implies that the alcohol-hydrocarbon mixture may be treated as a single component. If we let this oil mixture be component 1, its mass balance equation (6) simplies to
Equation 14 can be used to predict the time required for drop composition to follow the coexistence curve as indicated by the arrows in Figure 3 until it reaches the composition of the limiting tie-line (W1F). Beyond this point, further transfer of water and surfactant into the drop is incompatible with two-phase coexistence, and an intermediate phase begins to form. For the systems of interest here, the third phase is the lamellar liquid crystal, as indicated in Figure 3, although it may be some other phase such as a microemulsion in other systems. Equation 14 predicts that the time required to initiate intermediate phase formation is proportional to the square of the initial drop size and inversely proportional to the initial surfactant concentration and the surfactant diffusion coefficient. K is a constant that includes the effects of the shape of the L1-L2 coexistence curve and the location of the limiting tie-line.
(7) Initial conditions are R = Ro and w1 = w10 at t = 0. This
Results Oil drop contacting experiments were conducted with three pure nonionic surfactants and various mixtures of
Lim and Miller
2024 Langmuir, Vol. 7,No. 10,1991 Table 1. Results of Contacting Experiments for Nonionic Surfactants and Mixed Soils (0.05 wt % surfactant was used unless otherwiee stated) oil temp, 2R,, time of PIT,' 7% n-Cle Mm first LC, min "C removal7 surfactant comp O C 12.5 10 56 30 35 37.0 67 40 70 82.0 60 50 62 d
