NOTES
Feb., 1959
317
The compound resembles carnallite in that it is an incongruently saturating type of double salt. Water decomposes it into ammonium bromide a i d a solution richer in mngnesium bromide.
THE EFFECT O F EXCESS SALT ON MINIMA OBSERVED I N ?/LOG C CURVES FOR SURFACE ACTIVE AGENTS BYS. P. HARROLD Basic Research Department, Thomas HedleU Le. Co. Ltd., A’ewcaslle upon Tune, 1 , England Rcceizmf Julu 25, 1955
The minimum which occurs in the surface tension (r)/concentration (C) curves of surface active agents has been variously explained as due to metal ions in the water71impurities in the chemical compounds examined2 or to time effect^.^ The most likely explanation is the presence of other components since it is to remove the minimum by successive purification processes, and to produce one by the addition of a third component.6 20 It has been observed that the addition of excess salt -3 -2 with a common ion removes the minimum in the log molarity. ?/log C curve for several anionic detergents. Fig. 1.-Surface tension of p-nonylbenxene sulfonate: I, d o n e in distilled water at 20”; 11, plus 0.5% bv weight Experimental Surface tensions Rere measured by the tlu S o u y tensiometer wit,h appropriate cor~ections.~Sodium alkylbenzene sulfonate (ABS) was prepared by direct sulfonation of tetmpropylene benzene, boiling range 284-2!34’, with S03.8 The material was found to be 96.797, pure (dry weight basis) by the method of Eptong based on cetylpyridinium broniide referred t,o dichromate. Sodium lauryl sulfiite (SDS) was synthesized by condensation of lauryl alcohol with chlorosulfonic acid in ether. (Calcd. for C,zHn&WrNa: C, 50.0; H, 8.7; S, 11.1. Found: C, 49.7; H, 8.5; S, 11.2). Soxhlet extraction with light petroleum mas used to reduce t,he minimum in the curve of surface tension with concentration. Sodium p-nonylbenzene sulfonate was prepnred by synthesis from pelargonyl chloride by Friedel-Crafts condensation with benzene followed by reduction with hjdrazine and sulfonation. (Calcd.: C, 58.8; H, 7 . 5 . Found: C, 57.95; H, 8.3.) It was purified by recrystallization from ethanol. A.R. grade NaCl was used at a constant concentration of 0.2 AI, and solutions were made in distilled water which had been passed over a fresh bed of Bio-deminrolit.
Results and Discussion Sodium p-nonylbenzene sulfonate had no minimum iii its ?/log C curve after purification (Fig. 1, I). The addition of 0.5% of a third component, lauryl alcohol, resulted in the appearance of a pronounced minimum (Fig. 1, 11). Repetition of the cxperiment with added alcohol in 0.2 ilP sodium chloride essentially removed the minimum (Fig. 1 111). The absence of a miiiimum in the preseiice of salt ( I ) C. Robinson in, “ F e t t i n g R. Detergency,” Harvey, London, 1937. p. 137. (2) D. Reiehenberg. Trans. Faraday SOC.,43, 407 (1047). (3) E. J. Clayfield and J. B. hIatthems, “Proc. 2nd Cong. Surface Activity,” Butternorth, London, 1957. Vol. 1, p. 172. (4) A. P. Brady, THIS JOURNAL, 53, 56 (1949). ( 5 ) E. F. W i l l i ~ ~ i i N. s , T. Woodberiy and J. IC. Dixon, J . Colloid Sci.. 12, 452 (0) J. D. Miles and L. Shedlovsky. TIIISJOURNAL, 48, 57 (1944). (7) W. D. Harkins and H. F. Jordan, J . A m . Chem. Soc.. 52, 1751
Internat.
(1857).
(1930). (8) 0 . K. Ashforth. prirntr roniiiiunication. (9) S. R. E p t o n . l’rons. Faraday SOL’., 4 4 , 22G (19.18).
lauryl alcohol; 111, plus 0.5% lauryl alcohol in 0.2-M NaCl
3G
34 I
i 32 0
n
Bx
r:
p 30
2s
26
-3 -2 log mo1:irity. Fig 2.-Surface tcnsion of sodium tetrxpropj lene benzene sulfonate: I, in distilled water a t 20”; 11, in 0.2 M XaC1. -4
wncl also found for ABS (Fig. 2) whcre the minimum without salt is probably due to the presence of sei.eral closely related alkyl benzenes, and for SDS (Fig. 3) where the minimum without salt is proba l ~ l ydue to traces of lauryl alcohol. An explanation is offered based on the treatment of adsorption a t the fluid/water interface given by Reichenberg. 2. No Salt.-The Gibbs equation for adsorption a t the air/water interface for an ionized surface active agent (det.) containing a small amount of impurity, e.g., lauryl alcohol (alc.) may be written - d r = rdet-dwet- 4- h a + d m n +4- raiodplnio 4hf-thofl-
rHaO+dfiHlo+
(1)
where y is the surface teiisioii aiid I? the surface
318
NOTES
Vol. 63
- d r = rxa+dpNa+-b rcl-dpclrHao+dpHaO+
+
FoH-dpoH- -/rdet-dwdetralodplalc ( 5 )
+
As before, d p ~ ~= o0; dpcl- will approximate to zero too since no more will be added and dpNa+ can be neglected since the coiicentration of Na+ will already be determined by the excess salt. Theref ore - d r = rdet-dpdet- -/-
ralcdPnlo
(6)
and
In this case n = 1, as shown by Pethica,'O and - will be constant in the presence of excess salt with a coinnioii i011.l~ The first term on the R.H.S. of eq. 7 does not decrease. The experiments show dy/d In C d e t - does not change sign, hence the contribution of the activity term to the equilibrium must be significantly greater than that due to the change in chemical potential and surface excess of the additive. It is well known that the presence of excess salt lowers the critical micelle concentration and thus the bulk concentration a t which r d e t - approaches saturation (reduction in double layer repulsion and solvation effects). It may be that this will increase the proportion of det- in the surface at the expense of alc so that Pale may be less, reducing the contribution of the second term in this instance. Acknowledgment.-The a u h r wishes gratefully to acknowledge the advice of his colleagues and in particular Dr. J. F. Goodman.
.fdet.
-3 -2 log molarity. Fig. 3.-Surface tension of sodium dodecyl sulfate: I, in distilled water a t 20"; 11, in 0.2 ill NaC1. -4
excess of the individual conipoiients, chemical potential p dpoH-
+ dpH,o+ =
dp
0
in the dilute solutioiis we are consideriiig. It is iiot generally true, as Pethica'O poiiits out, that Fdet.- = r N a + , so we must write
- dr
=
Fdetdpdet -t r a i c d P s ~ c
where d/*det = nRT d 111 f =t
