Aqueous Solution Properties of a Fatty Dicarboxylic Acid Hydrotrope

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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on February 27, 2018 | https://pubs.acs.org Publication Date: May 21, 1984 | doi: 10.1021/bk-1984-0253.ch008

Aqueous Solution Properties of a Fatty Dicarboxylic Acid Hydrotrope A. B E L L Westvaco Corporation, Charleston Research Center, North Charleston, SC 29406 K. S. BIRDI Fysisk-Kemisk Institut, Technical University of Denmark, DK-2800 Lyngby, Denmark

Aqueous solution properties of the twenty-one carbon dicarboxylic acid 5(6)-carboxyl-4-hexyl2-cyclohexene-1-yl octanoic acid (C -DA) in salt form - alone and in the presence of a nonionic, anionic or cationic detergent - are reported. Membrane osmometry results indicate that C -DA alkali salt forms low molecular weight aggregates or micelles, its aggregation behavior appearing to resemble that of certain polyhydroxy bile salts. In the presence of detergent, small aggregates are also formed provided the weight fraction of C -DA salt in the micelles exceeds ca. 0.5. Phase equilibrium studies show that C -DA (as the dipotassium or full triethanolamine salt) acts as a hydrotrope above certain concentration levels in concentrated detergent solutions, retarding build-up of anisotropic aggregates responsible for mesophase formation, in accordance with previous investigations by Friberg and co-workers. 21

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21

21

In recent studies, Friberg and co-workers (J_,2) showed that the 21 carbon dicarboxylic acid 5(6)-carboxyl-4-hexyl-2-cyclohexene1-yl octanoic acid (C21-DA, see Figure 1) exhibited hydrotropic or solubilizing properties in the multicomponent system(s) sodium octanoate (decanoate)/n-octanol/C2i-DA aqueous disodium salt solutions. Hydrotropic action was observed in dilute solutions even at concentrations below the critical micelle concentration (CMC) of the alkanoate. Such action was also observed in concentrates containing pure nonionic and anionic surfactants and C21-DA salt. The function of the hydrotrope was to retard formation of a more ordered structure or mesophase (liquid crystalline phase). 0097-6156/84/0253-0117S06.00/0 © 1984 American Chemical Society

Rosen; Structure/Performance Relationships in Surfactants ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

S T R U C T U R E / P E R F O R M A N C E R E L A T I O N S H I P S IN S U R F A C T A N T S

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As i s well known, hydrotropes are incorporated in liquid detergent formulations in order to produce transparent (isotropic) solutions at high solids concentrations which w i l l be stable under varying conditions of temperature and composition. Depending upon i t s structure, the hydrotrope may also assist in solubilizing components added in small amounts (e.g., perfumes, colorants, bactericides) and, depending upon the nature of the s o i l , enhance detergency. In order to better understand the molecular basis of hydrotropic action, i t i s useful to investigate phase properties of multicomponent regions of interest, specifically the micellar or isotropic regions. Recently, a powerful technique has again emerged for characterizing micellar formation in detergent solutions - membrane osmometry (3-6). With this technique, the effect of additives (electrolytes, organic solutes, etc.) can also be explored (7,8). Here, u t i l i z i n g membrane osmometry, we report formation in solutions of C21-DA alone (in dilute and in the presence of surface active ingredients in commercial liquid detergent formulations. Phase 3-component blends (detergent/C2i-DA salt/H20) presented.

on micelle electrolyte) incorporated diagrams of are also

Experimental R

Materials. The dicarboxylic acid was Westvaco DIACID 1550, or H-240, i t s p a r t i a l l y neutralized 40Î solids aqueous potassium salt solution (9.>1Q.) · Linear fatty alcohol ethoxylate (LE-9), prepared from alcohols containing 12-15 carbon atoms and containing an average of 9 ethylene oxide groups per molecule, was obtained commercially. Sodium dodecylbenzenesulfonate (SDBS, technical grade) and cetyltrimethylammonium bromide (CTAB) were obtained from Fluka, triethanolamine (TEA) from Fisher. Inorganic chemicals were reagent grade. Water was Milli-Q deionized. R

Membrane Osmometry. The apparatus has been described in detail elsewhere (5)· The concentration of detergent in the solution compartment was many times the CMC (10 to 100) whereas the concentration in the solvent compartment was somewhat above (i.e., 5 to 10 times) the CMC. Under these conditions, the following limiting equation applies (3-6):


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A Fatty Dicarboxylic Acid Hydrotrope

where 7Γ i s the osmotic pressure, C the detergent concentration in the solution compartment, C the detergent concentration i n the solvent compartment, R the universal gas constant, Τ the absolute temperature, M the number average molecular weight, and Β the second v i r i a l coefficient. As shown previously (3-5), the term involving Β i s small compared to RT/M , and thus a plot of 7Γ /RT(C-C ) versus (C-C ) gives the value 1/M at the intercept. Recent fluorescent probe studies (JM_) indicate that in pure detergent solutions, M does not change appreciably i n dilute electrolyte medium as concentration i s increased well above the CMC. This suggests that i n aqueous micellar regions containing mixtures of C21-DA salt, detergent and dilute electrolyte, the micellar aggregation number should remain essentially constant (except perhaps near phase boundaries) at a given weight ratio of hydrotrope to detergent. A recent investigation by Kratohvil and co-workers (X2) on the concentration-dependent aggregation of conjugated bile salts (especially taurodeoxycholate) i n concentrated electrolyte media (> 0.1M NaCl), however, appears to indicate otherwise. CMC values were obtained from dye (azobenzene) solubilization and surface tension measurements. Values of if used in the above equation were obtained via extrapolation (4-5). For very low values of M , there i s concern about leakage through the membrane ( J 3 . ) . Assuming that leakage did occur within the timeframe of the measurements, M (and hence N ) values would be lower than those calculated. For this reason, a l l values of M and N are reported as being apparent. n

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Phase Regions. Phase regions were determined by visual inspection of blended components stored in tightly capped v i a l s . The blends were prepared by mixing the components together, with s t i r r i n g , at 60-100°C, followed by cooling i n a i r to 25°C. The C21-DA dipotassium salt was f u l l y neutralized (with 45$ KOH) Westvaco H-240 (acid value equivalent), the f i n a l solids content being ca. 45Î by weight. Higher concentrations of this salt were obtained via evaporation at 90-100°C. Anhydrous C21-DA f u l l triethanolamine salt i s an isotropic liquid at 25°C. Anisotropic regions were determined by viewing the samples between crossed polarizers (J.,2) or under a polarizing microscope. In the v i c i n i t y of phase boundaries, many weeks were often required before equilibrium was attained. Results CMC values i n 0.05M electrolyte at pH 10 are given i n Table I. Apparent number average molecular weights M of aqueous detergent mixtures in the same medium are l i s t e d i n Table I I . The maximum concentration of surface active agents i n any given system was 50 g/1. Figure 2 i s a plot of 77VRT(C-C ) versus (C-C ) for the system CTAB:C21-DA s a l t . Figure 3 shows the n

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variation of N , the apparent micellar aggregation number (calculated by dividing M by the average molecular weight of the detergent mixture), as a function of weight fraction of detergent in the micelle, assuming that the ratio of detergent to C21-DA salt in the micelle i s equal to that of the overall surfactant inventory. This assumption i s essentially valid well above the CMC of the mixture (14). Results of phase equilibrium studies are shown in Figures 4-6. n


n

Table I. Results of CMC Determinations (0.05M NaCl or NaBr, pH 10, 25°C) Weight Ratio Detergent in Solution 0.00 0.25 0.50 0.75 1.00

Table II.

CMC Value (g/1; obtained v i a extrapolation) LE-9 SDBS CTAB 0.4 0.4 0.4 0.4 0.25 0.3 0.3 0.15 0.2 0.25 0.25 0.15 0.2 0^2 0.1

Results of Membrane Osmometry (0.05M NaCl or NaBr, pH 10, 25°C)

Weight Ratio Detergent in Micelle 0.00 0.25 0.50 0.75 1.00

Apparent M Values (g/mol) SDBS 2400 3100 3000 10800 14000 n

LE-9 2400 3000 2600 7000 48000

CTAB 2400 2700 6800 14400 90000

Discussion The aggregation behavior of C21-DA salt in dilute electrolyte medium appears to resemble that of certain polyhydroxy b i l e salts (15,16). That C21-DA, with a structure quite different from bile acids, should possess solution properties similar to, e.g., cholic acid i s not entirely surprising i n light of recent conductivity and surface tension measurements on purified (i.e., essentially monocarboxylate free) disodium salt aqueous solutions, and of film balance studies on acidic substrates (J7.). The data in Figure 3 suggest that C21-DA salt micelles incorporate detergents - up to an approximate weight fraction of 0.5 -much like cholate incorporates lecithin or soluble


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CHCH„ CH„ CH. 3 2 2 2

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OH Figure 1. Structure of the dicarboxylic acid 5-carboxy-4-hexyl2-cyclohexene-1-octanoic acid (C21-DA).

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Figure 2. Plots of /RT(C-C ) vs. (C-C») for mixtures of cetyltrimethylammonium bromide (CTAB) and C21-DA a l k a l i salt i n 0.05M NaBr, pH 10, 25°C. Weight ratio CTAB:C

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Aqueous Solution Properties of a Fatty Dicarboxylic Acid Hydrotrope

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