Performance Relationships in Surfactants - American

16 Linear Sodium Alkylbenzene Sulfonate Homologs

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Comparison of Detergency Performance with Experimental and Thermodynamic Wetting Theories J A M E S A. WINGRAVE

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Conoco, Inc., Chemical Research Division, Ponca City, OK 74603

Pure homologs of linear alkylbenzene sulfonate sodium salts (LAS) were evalulated for detergency performance. The surface tensions of the wash liquors used for these detergencies were then measured and used in conjunction with a wetting model to calculate a theoretical detergency per formance. The theoretical and experimental deter gency results were compared. The molecular struc ture effects of the LAS homologs on detergency performance were calculated by incorporating into the detergency equation several molecular struc ture theories such as the cohesive energy ratio concept, molar-attraction constants, internal liquid pressure, and liquid thermal properties. The assets and deficiencies of these approaches are discussed.

The process of detergency involves the complete separation of two substances by means of a detersive bath. The success of such a process requires a knowledge of the chemistry between the two substances to be separated (henceforth referred to as soil and substrate). From this knowledge, a detergency bath can be designed with chemical characteristics necessary to separate the soil and substrate by overcoming the attractive forces between them. However, stating the principles of detergency and achiev ing them in practice are different; the latter being far more difficult, as noted by the voluminous literature on the subject (1-10). It will, therefore, be the purpose of this paper to develop a better understanding of how surfactant structure 1

Current address: Ε. I. du Pont de Nemours, Inc., Chambers Works, Deepwater, NJ 08023 0097-6156/ 84/0253-0241 $08.00/0 © 1984 American Chemical Society

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

STRUCTURE/PERFORMANCE RELATIONSHIPS IN SURFACTANTS


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e f f e c t s detergency performance by using a homologous s e r i e s of l i n e a r alkylbenzene sulfonate sodium s a l t (LAS) d e r i v a t i v e s i n the detergency b a t h . The k i n e t i c e f f e c t s of detergency w i l l not be explored i n t h i s study, but review a r t i c l e s on t h i s t o p i c can be found e l s e where ( 1 - 5 ) . Instead, the a g i t a t i o n w i l l be h e l d constant (see Experimental Section) so that the e q u i l i b r i u m (or near e q u i l i b rium) processes can be observed. E q u i l i b r i u m was achieved for s i m i l a r s o i l / s u b s t r a t e systems w i t h i n 5-10 minutes i n previous studies (3). For t h i s study, the s o i l w i l l be a mineral o i l c o n s t i t u t e d as shown i n F i g u r e 1. The c l o t h used w i l l be a c o t t o n / p o l y e s t e r blend with a polyethylene g l y c o l f a b r i c f i n i s h . Since the s o i l w i l l be a l i q u i d , e f f e c t i v e detergency w i l l r e q u i r e a bath that can overcome the p h y s i c a l forces between the s o i l and f a b r i c ( i . e . , the f a b r i c f i n i s h ) . For p h y s i c a l f o r c e s , the problem be comes one of developing a model of the detergency system, then combining t h i s model with wetting theory i n order to produce a detergency equation for detergency performance. Discussion S o i l - F a b r i c Morphology. In F i g u r e 2, scanning e l e c t r o n m i c r o graphs of the f a b r i c used i n t h i s study are shown. In the threads of the f a b r i c are f i b e r s which run very n e a r l y p a r a l l e l . When s o i l e d with a l i q u i d s o i l , a pendular drop of s o i l should form between contiguous f i b e r s as shown i n cross s e c t i o n i n F i g u r e 3. The b a t h / s o i l i n t e r f a c e i s shown planar and p a r a l l e l with the plane passing through the f i b e r centers (Figure 4 a ) . When one chooses a p a r t i c u l a r detergent b a t h / s o i l / f i b e r com b i n a t i o n , the contact angle, ψ (measured through the s o i l phase), at the l i n e where a l l three phases meet w i l l s t r i v e to a t t a i n a s p e c i f i c value based on the p r o p e r t i e s of the three phases, as shown i n F i g u r e 4 b . T h e r e f o r e , the t r i p l e - p h a s e l i n e (TPL) w i l l move to assume a value of ψ . T h i s , i n t u r n , w i l l cause a curvature J i n the s o i l / b a t h i n t e r f a c e . This movement of the TPL i s the process by which l a r g e s o i l drops can be formed o n / i n the f a b r i c as ψ approaches values of 9 0 ° and g r e a t e r , as shown i n F i g u r e 4 c . This process i s r e l a t i v e l y r a p i d , o c c u r r i n g w i t h i n 10 minutes for most systems, and i s gen e r a l l y r e f e r r e d to as the r o l l - u p mechanism of s o i l removal. From t h i s d e s c r i p t i o n , i t i s obvious how a g i t a t i o n and buoyant e f f e c t s of the s o i l could speed up t h i s mechanism ( i n f a c t , r o l l up cannot occur at a l l unless buoyant or a g i t a t i o n forces act on the s o i l ) , but s o i l - r e m o v a l r a t e i s a k i n e t i c question and w i l l not be pursued f u r t h e r here. Another s o i l - r e m o v a l mechanism i s a l s o working, s i m u l t a neously, to remove s o i l a l b e i t a much slower process ( i . e . , up to s e v e r a l h o u r s ) . When the b a t h / s o i l i n t e r f a c e becomes curved due to a change i n ψ (see F i g u r e 4 d ) , i t s chemical p o t e n t i a l i s



WINGRAVE

LAS: Detergency and Wetting

25 30 35 40 42 NUMBER OF CARBONS IN MINERAL OIL CONSTITUENTS Figure 1.

Constitution of the Mineral Oil Soil.


Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 12, 2016 | http://pubs.acs.org Publication Date: May 21, 1984 | doi: 10.1021/bk-1984-0253.ch016 Figure 2.

Scanning Studies.

Electron

Micrographs

of

Fabric

Used for


Detergency


16.

WINGRAVE

Figure 3.

LAS: Detergency and Wetting

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Cross-Sectional View of Soil Drop Between Contiguous Fibers.

Figure 4.

Mechanism for Soiled Fabric Fiber Detergency.


STRUCTURE/PERFORMANCE RELATIONSHIPS IN SURFACTANTS

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greater than a f l a t , b a t h / s o i l i n t e r f a c e . The instantaneous curvature J w i l l change through s o i l d i s s o l u t i o n u n t i l i t achieves a smaller curvature ( f l a t t e r i n t e r f a c e ) J . The s o i l removed i n t h i s way w i l l e i t h e r s o l u b i l i z e i n the b a t h , c o l l e c t as small macroscopic o i l droplets at the b a t h / a i r i n t e r f a c e , or emulsify i f a g i t a t i o n i s present. Hence, i n a detergency system that has a t t a i n e d e q u i l i b r i u m , s o i l lenses or small (nominally 1 mm diameter) s o i l droplets are commonly seen f l o a t i n g at the air/bath interface. Under these circumstances, the curvature i s very small (nominally 1 mm diameter), and when a p p l i e d to the s c a l e of pendular o i l droplets between f i b e r s (nominally 1 to 10 Urn diameter) the b a t h / s o i l i n t e r f a c e w i l l be f l a t for a l l p r a c t i c a l purposes. Therefore, the model i n F i g u r e 3, showing a f l a t b a t h / s o i l i n t e r f a c e , should be v a l i d f o r a l l d e t e r s i v e s y s tems where bulk s o i l i s observed and f a b r i c f i b e r s are small ( $fw i- parameter type in a detergency system is film pressure of the two liquids on the solid fiber: ïïf and TTf . an
ΰ ^ ! 7 /

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γ


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Performance Relationships in Surfactants - American

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