774
RETSOLD C. MERRILL A S D R.LTJ1OSD G E T T l
necessary to find out whether anything similar occurs \\-ith Concord, Sisgara, Delaware, and Scuppernong grapes. 6. The pink-fleshed grapefruit apparently ov e5 its characteristics to somc dement or group of elements in the soil, but I have never been any scientific study of the subiect. 7. Under suitable laboratory conditions one vould gron Havana tobacro in This has never been tried and ptvhaps Connecticut and perhaps in Can&. never will be. 8. The grafting of floirerb and fruit3 is uhually a matter of heredity, but in the protection against phyllosera TI e seem t o have a ca5e n here there is a transmis4cn of characteristics through thc graft. Thi- may br a misunderstanding on my part. 9. The horticulturist ignores the question of environment \\hen possible, even though the composition and structure of the 3011 are of tremendous importance t o him. 10. It is startling to see how much is not yet knovn about the effect of environment on the colors of leaves, floners. :~ndfruit., and :ihcut thc biochemistry of plants. 11. Some day it will be possible to grou- blue appleq, blue stranberrie-, blue roses, and blue hollyhoclis; but nobody can b ~ i y11hcn. RE;Fl:RESVF,P (1) h D L U \ f A J l e m o t r on tire Ciiltualzon o j the 17Lmi n A4mc/?co,12 173 (1828) (2) See B ~ \ C R O F T J Pliys Colloid Vhc,iii 51, 10% ( 1 Y l T (3) P ~ L L . I D IBiocheiii Y J 18, 170 (1900) (4) S k \ D > , ~ I I L ~ EA \RD ,S I f E R V k Y ,J 13101 ('her11 109,201 (JOOS ( 5 ) S C O T r - ; \ I O S C R I E E F , Ro% Blochrm .I 24, 25j f10.30)
MICELLE FORMATIOX IS SOAP SOLUTiOIUS
775
mashing or detergent operations are done, so the assumption was made that thc equilibrium between free ions and micelles was of little practical interest. However, recent studies of long-chain compounds, including the soaps, have shown that the formation of micelles begins at about 0.01 N or 0.2 per cent for detergents containing twelve carbon atoms and around 0.001 S for those containing sixteen carbon atoms. For example, the “critical” concentration at which micelle formation becomes readily apparent is around 0.023 JI or 0.5 per cent for sodium laurate and less than 0.001 -11 or 0.020 per cent for potassium palmitate (11). Micelle formation for most soaps, therefore, begins in the range of concentrations used for practical detergent operations. Data presented recently by Preston indicate that a great increase in detergent artion occurs at about the concentration a t which micelle formation becomes apparent (22). Any material which lowers the minimum concentration necessary for micelle formation in a soap solution may then likewise decrease the concentration necessary for good detergency. Several investigation? have shown that various electrolytes decrease this critical concentration for micelle formation ( 4 , 9, 14.IG, 24). It !vas, therefore, of interest t o study the effect of various silicates and other electrolytes commonly used as soap builders on the concentration at which micelle formation first becomes readily apparent in soap solution.. This paper reports such a study. EXPEIIIMCSTAL
The ”critical” concentration for micelle format ion in soap solutions was determined by both the dye-titration technique of Corrin and Harkins (4) and the solubilization method. The first method depends on the fact that certain dyes, buch ab pinacyanol chloride, change color at the “critical” concentration. In thih work a stock solution of soap above the “critical ”roncentration containing 1 x 10-5 nioles of pinacyanol chloride per liter \vas titrated with thc same concentration of dye in tvater. The “critical” concentration n-aa determined as the point at which the blue hoap-dye solution fii .t acquired ii purplish tingc. (‘omparison xith an aliquot portion of the original wap-dye solution facilitntc(l detection of the end point, n-hich is otherwise rather difficult to cleteiminc ac*riir:itply. .Ill dye titrations u-ere run in duplicate. The solubilization method fur clet~rniiningu i t i r i i l voncrrit ration< depends o n the fact that solutions containing soap or detergent mic+cllescan solubilize \raterinsoluble materials, whereas yoap or detergent solution.: tielon- the ’bcritical”(’oncentration do not (17. 20). The solubility of the. \vatel-inwluhlc dye, Orange OT (F. D. and C . Orange S o . 2 . 1-o-tolylazo-2-naphthol)vas measured in varping concentration< of soap -elutions a t (i0”C. Thc t cchniqiw i i h d v-as similar to those previously described (15, 17). Series of mlutiotis of n r y i n g ioap concentration but with constant ialt contcnt nerc inndr .:1 (li!uting a definite volume of a stock soap solution with kiion n am6iints of a storlc salt or dicate solution and T\-ater. Thcbe solutions \I c:’c placcd in 50-nil gl:i\? bottles contain-
770
HEYPiOLD C. MMEHKILL AKD HATMOSD GETTY
ing a slight excess of dye sealed \\.it11 metal capb, and placed in an air oven at 60°C. At least several days were required for the solutions to dissolve the maximum amount of dye. _$iter allo\\-ing excess dye crystals t o settle, the dye concentration I\ a s nieasured by determining the percentage transmission of light from a I-)lue filter in L: photoelectric* colorimeter. Special care \ m a taken to avoid measuring suspended dye particl(1s. From the t rttnsmission reading, the amount of dye dissolved is obtained by cornptirison with a calibration c u r w of known amounts of dye dissolved in acetone
The Orungc 0‘1‘ (molecular \\eight, 2 6 2 ) was recrystallized from methyl alcohol and obtained in the form of small tiright orange-red crystals. The pinacyanol chloride was a commercial product arid \\.as used irithout further purifics‘tt’1011. Sodium laurate \vas prepared from a lauric acid obtaincd t)y recrystallizing the Eastman ICodak (’ompiny’s best product t \\ ice from acetonitrile. The recrystallized acid tiad a melting point of 4 3 . X ’ . The sodium soaps were made by neutralizing the ncid in uc~toncwlut ioii \\ it11 sodium rnetlioside t o the phenolphthalein end point. The soap \\.as n-iished with acetone and dried at 105°C‘. Potassium lauratc u a\ made 5i1nilarly~using an acetone solution of potassium hydroxide, from a recrystallized lauric acid v-ith a melting point of 42.2”C. and an equivalent weight of 200.8 (theoq-, 200.3). The palmitic acid from which the sodium palmitatc I\ i i h mnd(~I\ a- rrcrystallized from methyl alcohol and had a melting point of (i2.GY‘. :inti :in equivalent 11eight of 256.2 (theory, 256.4). Analyses for three of the silicate< iised \\ere given in a previous paper (19). The silicate with a silica-to-alkali ratio by \\eight of 1.6 \\-as a standard commercial product (“RJV”) of thc 1’hilulclphi:L Quartz (‘ompany. The other salts \I ere c.P., except foi the polypho>phatt, [“unadjusted (’algon”) and the carboxymethylcellulose, \rhic.fi I\ ere comnwrcial products. The latter \vas the medium viscosity grade of the Herculei l’o\vdei (’ompany. The moisture content of the C‘algon was determined itnd its formula assumed to lie (SaP03),c. The sodium content of the c ~ : i r t ~ o x y m c ~ t l i ~ l c e was l l ~ i ldetermined o~~~ by nshing the material, acidifying t hc ash with hydrochloric acid, evaporating t o dryness on a water bath, moistening t\\ icc with distilled water (evaporating to dryness each time), and titrating the ctis-olvrd i-e4duc with qtandard d v e r nitrate. iising potassium chroninte indicaioi*. l,Xl’k~Rl~lk,h‘r \ I A D.I‘l’ 3 r 7
l h e effects of varioiii d t h intliistiially important as soap builder6 011 the “critical” concAentration for micelle formation in sodium lnurate solutions at room temperaturr (-25O(‘.) are given in table 1 and illustrated in figure 1. These data were obtairicd by the titration technique. The “critical” concentration is s h o ~ nas a function of tlic ccjuimlent concentration of added salt based on the sodium content. When sufficient salt i-; ndded t o make the solution 0.065 -11,the critical concentration for sodium lauiate is reduced from 0.024 IIf or 0.53 per cent to 0.012 -11or 0.26 per cent. The reduction in critical concentration is essentially
777
MICELLE FORMATIOX I N SOAP SOLUTIOKS
TABLE 1 Thc “critzcal” concentralion for mzcelle forination of sodium lairralc salts at 25°C. _.
MOLARITY
or
SALT
’
lhc pwspncc o j a d d d
211
_____
~
_-__
~
D K c R ~ ~ ~ & ~ : “ , d “CRIT.” ~ ~ ~ ~_ ”
___
COSCN. P U R E SOAP
~
“CRITICAL” CONCENTRATION
“CRIT.”
_ _
CONCN. WITH WLT
Sodium chloride M
M ,
0 2.18 x 10-2 3 . 0 X 10-2 4.23 X 10-2 6.9
2.37 X 10-2 I .82 x 10-2 1.49 X 10-2 1.41 1.15
23 37 10 51
1.29 1.58 1.67 2.04
23 2 used :I> ioap builders. SU~IU1KT
Xt equal sodium-ion concentrations, solution3 of $odium chloride, sodium hydroxide, sodium ortho- or pyro-phosphat(1,sodium tetraborate, sodium metasilicate, and three sodium silicates with silica-to-soda (Sa20) ratios of 1.60, 2.46, and 3.93 lower the “critical” concentration for micelle formation in sodium laurate solutions to the same extent. The dat’a \yere obtained by the dye-titration method with pinacganol chloride. Measurement s of the solubilization of Orange O T in dilute sodium and potassium laurate solut,ions at 60°C. alone and with four sodium silicates confirm this result. The decrease in the bLcritical”concentration for micelle formation in soap solutions due to the salt is interpreted as a commonon effect, in accordance with the lun- of ni REFERESCES
(1) BROJVS, G . I,., GHIEGXH, P. F . , E V E R SE. . C . . ASD ‘fCH.%rrs, C I,:J. h m . Chem. So e . 69, 1835 (1947). (2) B U R T ,C. R . , ASD PARKY, C. A , : J . Chciii. Soc. 1935, 626. (3) CORRIK,AI. I,... ~ S D HARKISS.W.D . : J. = \ i n . Cliem. Soc. 69, 679 (1917). (4) C O R R I N11. , L.: A S D HARKISS, W.D . : J . -1111. C%ein. doc. 69, 683 119471. (5) DAVIS, J . I
