alloidal Surfactants
Textbooks on colloid chemistry and most physical chemistry textbooks contain accounts of colloidal surfactants (colloidal electrolytes, detergents, etc.) but an examination of more than a dozen textbooks on experimental physical chemistry yielded only two experiments ( 1 ) on the physical properties of colloidal surfactant solutions. To increase this number suggestions are given below which can be developed as single 2-3 hr experiments, as series of experiments ranging from 5-20 hr in duration or as undergraduate research projects. Introduction
Colloidal surfactants are comprised of molecules possessing both lyophobic (solvent-hating) and lyophilic (solvent-liking) properties and may be anionic, cationic, non-ionic, or amphoteric. Typical examples are Anionic Cationic
Sodium dodecyl sulfate C,dLySO,-Na.+ Dodecyl trimethyl ammonium bromide
Non-Ionic
Palyoxyeth C,,HmO( Sodium d,
Amphoteric
This dual character of the snrfactant molecules gives rise to the phenomenon of micellization the onset of which is marked by an abrupt change in physical properties. As the concentration of a surfactant solution is increased a point is reached where aggregates (of colloidal dimensions) of the surfactant molecules are formed. These aggregates are termed micelles. Further increases in concentration of surfactant beyond this point increme the number of micelles while the concentration of simple surfactant species remains constant. The concentration a t which this behavior begins is referred to as the critical micelle concentration (cmc). Among the factors which affect the value of the m c in aqueous
solution and which can be investigated by the experiments outlined below are temperature, the electrostatic repulsion between polar head groups, the hydrocarbonwater interfacial energy, and the tendency of the solvent to hydrogen bond. The methods available for the determination of cmc values are many and varied (2) hut the methods suggested below are restricted to those involving apparatus that most reasonably well equipped laboratories will contain. The colloidal surfactant chosen for many of the exemplary experiments was sodium dodecyl sulfate (SDS) (BDH Chemicals Ltd., especially pure grade) because i t is readily available, inexpensive, has a convenient cmc value, and is well documented in the literature, but other surfactants (ionic and non-ionic) could be substituted and could form the basis for comparisons of the behavior of different types. Investigation I Determination of cmc in Water
Method A: Surface Tension Measurement Bemuse of their surface active nature sufactantnts lower the surface tension, 7, of water a t concentrations below the cmc. Above the crnc the concentration of "on-micellized surfactant remains constant resulting in little change in the surface tension. Figure 1 shows data obtained by measuring the surface tension of aqueous solutions of SDS with a du Nouy tensiometer (other methods, drop weight, etc., could be used). The concentration a t which the abrupt changein 7 occursis taken as the cmc. If the surfactant sample contains active impurities a minimum will beobtained (Fig. 1). Should this hehavim occur the student could be asked to investigate it further and (a)explain the reason for the minimum, or ( b ) follow the effectiveness of purification procedures, or (c) investigate the effect on the magnitude of the minimum by addition of known quantities of, say, dodecanol to pure samples of SDS (5). An alternative extension of this investigation of a pure surfact m t could be the application of the Gibb's adsorption isotherm to the surface tension data. to determine the area occupied per mole s t the surface of the solutions (4).
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WAVELENGTH
NM
Figure 3. Spectrum of methylsno blue I 1 X 10- M I in presence of sodium dodecyl rulfde. (11 4.28 mM SDS, 12) 5.36 mM, 13) 6.70 mM, (41 8.38 mM,(51 10.47 mM, (6) 13.10 mM,l7) 16.35 mM.
35
4
8
Figure 1. rulfote.
16
12
CONCENTRATION
mM
Surface tension, y, of aqueous solutions of - d i m pvriRed SDS, cmc = 8.1 mM.
dodscyl
0 impure SDS,
with an aqueaus dye solution, the dye concentration being the same in the two solutions. The m e i s t&en as that point where a chsnee in color is first noticed and is evaluated by knowing the orieind volume and concentration of titrand and-the volume of titFant addedup to this point. Typical systerhs are: for anionic surfactants: cationic dyes, such as pinacyanol chloride, Rhodamine 6G, Qninddine Red. for cstionio surfactants: anionic dyes such as eosin, fluorescein, Sky Blue FF (5) for non-ionies: pinacyanol chloride has been used. Speclrophotmet~icDetmnination. The spectral changes may also he followed spectraphotometrically, and this extends the range of dyes that may be used to include those whose color changes could not he observed visually. Figure 3 shows the changes brought about by diluting an aqueous SDS solution containing Methylene Blue (Hopkin and Williams Ltd), concentration 1 X 10" M , with rtn aqueous dye solution of the same conrent,mt.ion a ~Unicam SP 800 B Ultraviolet ~ ~ and~ recorded ~ with Spectrophotometer (1 em cells). Figure 4 shows how a plot of absorbance versus concentration data obtained from Figure 3 allows the m c to be determined. Longer investigations could study the same surfactant but different types of dyes to show that a dye of opposite charge to that on the micelles is required to produce sufficientlylarge effects. The spectral dye method has the advantage of simplicity but i t has been pointed out that the cmc values obtained are approximate vdues (6). ~~~~~
Figure 2. wlfate.
Specific conductance, K, of aqueous rolutionr of sodium dodecyl rmc = 8.3 mM.
~~
Investigation I1
Method B: Conductance Measurement
The Effect of the Composition o f the Surfactant Molecule on cmc
A widely used method for the determination of cmc values of ionic surfactants is that of conductance measurement. The specific conductance increases less rapidly above the ~c than below the m c mainly because of the inclusion of counter ions (Nat in the case of SDS) in the micelle, thereby reducing their contribution to the conductance, and because of the resulting decrease in effective micellsr charge. Typical specific conductance data for SDS a t 298°K are shown in Figure 2. (Conductance bridge used LKB model 3216B.) The cmc may also he obtained from equivalent conductance versus (con~entration)"~ plots.
Series of related surfactants could be studied by any of the three methods mentioned above to examine the dependency of the cmcan:
Method C: Spectral Change of a Dye
where A and B rtre constants for a homologous series, and m is the number of carbon stoms in the hydrocarbon chain.
Surfactants may cause the absorption spectrum of a dye solution to change in such a way that the m c o f the surfactant may he determined from the changes. Two treatments of this phenomenon are possible. Titrimelric Determination. Some spectral changes are sufficiently marked to allow a visual determination to be made of the m i n t of change and henee the m c . For e x a m ~ l eRhodamine 6G is orrtnee a n d hiehlv fluorescent in anionic s&factant solutions
being carried out by titrating an aqneous surfactant-dye solution
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the lyophohiv part t,f thc iurf?ctnnt mdec,~lc,c.g., h a i n lenqth the Iprphilic part, c.g, t h c k:nd of I\..qhilic grdllp, the type und vulc~rryof the w r ! m r i u w , the rnderulc shape, etv. In general it is found that the effect of chain length is the greatest and may he expressed by lag,, (m)= A
- Bm
Investigation Ill The Effect o f Temperature on Micellization
The increased thermal motion of surfactant molecules in solutions a t higher temperatures may generally be expected to decrease the tendency to form micelles and hence increase the cmc. All the above methods could be used to investigate this effect but the most convenient experimentally is the conductimetricmethod. From experimental data students may he asked to explain the observed effects and calculate and comment on the enthalpy,
4 8 12 16 CONCENTRATION mM
O
0
I0
20
30
/o CONCENTRATION
OF
40°/0
MaOH
Figure 4. Absorbance at 660 nm of methylene blue in the SDS solutionr listed under Figure 3. cmc = 8.6 mM.
Figure 5. Effect i f methonol on cmc of sodium dodosyl sulfate. 0 sonductmse, 0 rpestrol dye method.
AH,, and entropy, AS,, of micellization. For non-ionies or ionic surfactrunts in excess electrolyte (containing s. common ion) the relationship between m c and temperature is
simple electrolytes also causes a decrease in mc val~teluesof ionic surfactants, an effect which can be expressed by the equation
and since AG, = 0,then AS, = AH,/T. For ionic surfactants in the absence of excess electrolyte the relationships are more complex (8).
In (cmc) = D - K i n c where D and K are constants and c is the concentration of the counterions. Numerous factors may he studied including the concentration, the type, the valency of the counterions, and the sirnili-ions of the added electroly-ach effect throwing further light on the factors affecting micellization. With "on-ianics the effects are very small.
Investigation IV Summary
Effect o f a Third Component The effects on the m e of numerous additives have been studied. e.g., a second surfactrtnt, higher slcohols, lower alcohols, hydrocarbons, simple electrolytes, etc., and all these may he studied by the three methods outlined under Investieation I. An i m d v r ~ r m d i(11 t ~r~ h ~~R P N S01 a S P C U ~ nud ~ third) ~ u r f ~ c prrpnrcdsurlacranra taut in in.pwtmt i n 11191 moil ~~mmercially are mixtures of two or more homologs. Many such surfactants also contain long chain alcohols as impurities, small quantities of which can considerably affect the uro~ertiesof surfactant solutions. A study of the kffects of lower aicohols, methanol, ethanol, etc., helps in understanding some of the factors involved in micellization. For example Ward (7) explained the effect of ethanol on the m e of SDS in terms of the change in intelfacial energy as the concentration of ethanol is increased and the effect of dielectric constant changes on the repulsion between the changed head groups. Figure 5 shows students' results for the effect of methanol on the m e of SDS followed by conductance and spectral dye change (methylene blue). Extension to include ethanol.. ~. m o a n o l butanol. . , etc.. ~rovideathe student with sufficicnl i n b m r n t i ~ ~ton mmment ov. the r l f w of divlrctric nmitat.t 01 rllr wlvr!.r, 11,e *fleet ul the ~ l c o h d011~ the hydrogen b o d e d .\rrwtun. 01 warrr and the effect d rharlpcs in htcrfa~.ialenrrcy on micellization. The addition of benzene, toluene, heptane, eta., generally cause a decrease in m c . Such investigations give scope to the student to give explanations of these ohservrttions. The addition of
-
.
.
, . ~
~~
Students usually find a study of surfactants interesting firstly because these compounds form part of an important commodity and secondly the students are able by means of a simple physical picture to visualize micelles and the factors involved in their formation. To provide ideas for experiments so that they can investigate and discover some of these factors for themselves an outline of some of the factors and convenient methods for their investigation has been given. Further theoretical detail may be found in most treatments of colloidal surfactants and in particular the reader is referred to reference (2) for a very comprehensive survey of published experimental data. Literature Cited (1) JAM.;^, A ~ T H U RM.. "Practical Phvaioal Chemistry" (1st ed.). J. A. Churchill Ltd., London, 1061,p. 295. (2) Snrsoon. Koro, ET AL, " C ~ l l o i d d Surf~otmts," Aoademic Press. New York, 1963, Chapter 1. r , J . Phva. Cham.,48.57 (1944). (3) MILES. G.D.,nlio S n ~ o ~ o v s n L., (4) BRADI,A. P.. J . Phys. Coll. Chcm.. 53,56 (1949). (5) COBBIN.M. L., AND HARXINB.W . D.. J . Amcr. Chem. Soc., 69, 679 (1947). P.,, AND M r s m s , K. J., J . Amm. Chcm. Soc., 77, 2937 (6) M u ~ e w a ~ (1955). (7) WARD,A. F. H.,P m . Roy. SOE..A176 412 (1940).
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