Chapter 5 Wetting and its Modification by Surfactants

Chapter 5 Wetting and its Modification by Surfactants 2006.4.12.

§1. Wetting process and contact angle. • Wetting • Wetting is a familiar surface phenomena; e.g. Polyester and cellulose acetate films Plant leaves and pesticides Wetting and waterproofing of fabrics cleaning and antirust of metals (2) Wetting process is application of adsorption on solid/fluid medium;

(3) What is the Wetting process ? One fluid(L or G) is replaced with another on solid(S)/fluid(L or G). 2. Contact Angle接触角 () and Young’ 杨氏equation • Definition –when a fluid drop reach spreading equilibrium on the interface of solid/fluid, the angle  rotated to pass through the fluid from the tangent of G/L interface to interface of solid/fluid, used the point o as a centre of a circle, is the contact angle of this fluid on this interface.

(2)  and wetting <90º wetting =0º full wetting >90º no wetting =180º full no wetting (3) Young’s eq. – 1805’ Young found SV = SL + LV cos or cos = (SV - SL)/ LV SV, cos ,   ; LS, cos  ,   3 Wetting process • Adhesional wetting process(沾湿) (a) G/S + G/L  L/S -G = SV + LV - SL = WSL 0 WSL - Adhesional wetting work (b) Relation with 

-G = WSL = LV(1+cos )  0 cos   -1,   180º Adhesional wetting   180º no Adhesional wetting (2) Immesional wetting process (浸湿) • Hard solid surface • G/C  L/S -G = SV - SL = Wi= A  0 Wi - Immesional wetting work; A - adhesional tension, driving force of spreading process • Relation with  -G = LVcos  = Wi= A  0   90º immesional wetting;   90º no immesional wetting

(b) Soft (porous) solid surface • G/C  L/S capillarity penetration pressure or addition pressure P = 2 LV/R = 2LVcos /r = 2(SV - SL)/r R – curvature radius of surface;r – Radius of capillary • > 90º, P < 0 no penetration;  < 90º, P >0 penetration • r , SV, SL, or if LV, then , P 

(3) Spreading wetting process (铺展) • G/C  L/S + new G/L -G = SV - SL - LV = S  0 S – Spreading Coefficient S = SV - SL - LV = SV - SL + LV - 2 LV = WSL – WAA WAA= 2 LV the cohesion work of liquid (b) Relation with  -G = LV (cos-1) = S  0   0º Spreading wetting   0º no Spreading wetting

(4) Adhesional tension and three type of wetting • Adhesional tension A = SV - SL Power replacing gas with liquid on solid(S)/fluid(L or G). WSL = SV - SL +LV = A +LV ,   180º Wi = SV - SL = A ,   90º S = SV - SL - LV = A - LV,   0º (b) driving force of spreading process SV, SL , A  wetting  (c) LV – surface tension of liquid LV, spreading wetting; LV , Adhesional wetting ; cos = A /LV, if A>0 , LV, cos ,  

4. Mensuration of contact angle (1) Angle measure

(2) Sloping plate(斜板法)

(3) Height measurement (高度测量法) sin = 2hr/(h2+r2) or tg(/2) = h/r

(4) Suspender(吊杆法) Sin  = 1 - gh2/2LV

(5) powder method h2 = CrLVcos t/2 Cr – constant t – time (s)  - viscosity of liquid

5. Contact angle hysteresis (1) Hysteresis 2- 1  0 (2 > 1 )

(2) Factors affecting the hysteresis • Surface roughness (粗糙度) • Young’s eq. SV - SL = LV cos0 • Surface roughness r(SV - SL) = LV cosW • Wentzel eq. roughness r = cosW/cos0> 1 • Wetting solid surface: 0 < 0 < 90° cosW > cos0, W < 0 r, W  • No Wetting solid surface: 90 ° <0< 180° r, W 

(B) Surface non-uniformity • Advancing angle - the wettability of the lower surface energy area. • Reversing angle - the wettability of the higher surface energy area. The curves about  of water on the mixed monofilm of TiO2 and C18H37N(CH3)3Cl, and rate of TiO2 overlay

(C) Other • Temperature T = 10  =0.20° • Viscosity equilibrium time  §2. Wettability of solid surface From Young’s eq. Cos  = (SV - SL)/ LV If SV , and SL , then , but the measurement of SV is very difficult Generally LV 100mJ/m2 (water: 72.8mJ/m2) • Low energy surface SV 100mJ/m2 • High energy surface SV = 100-5000mJ/m2

1.Low energy surface(LES) and critical surface tension(c) (1) The solid of LES: PP, PET, PTFE etc. (2) c – the maximum surface tension of the liquid spreading on low energy solid surface. c = lim LVcos 0 Zisman plot of Zisman plot of homologous compound non-homologous compound

(3) c , wettability , if c  LV then can spreading • (4) Facts of affecting c • Element of materials • F < H <Cl < Br < I < O < N • Surface mono-film of materials (to see table 5-2)

2. High energy surface(HES) and self-repel(自憎) (1)The solids of HES: wool, cotton, glass, metal oxide, sulfide(硫化物) ect. SV>100mJ/m2 > LV, generally 0 of liquids on it (2) Self-repel liquids – e.g. octanol (辛醇), n-octanoic acid (正辛酸) can not spread on high energy surface 3. The wetting of surfactants (1)Low energy surface SL, LV (LV < C), then spread aq. on surface

(2) High energy surface (a) same charge with surface – orientation adsorption of hydrophobic groups – wetting (b) opposition charge with surface – • orientation adsorption of hydrophilic groups – no wetting • adsorption of hydrophobic interaction – wetting

4. Evaluation of surfacial energy • Fowkes model  = d + h + m +  + I d – dispersion force h – hydrogen bond m – metallic bond  -  electric interaction dispersion force is a interaction : ubiquitous, long distance, attract force

(2) L/L interfacial tension ab ab= a+ b – 2(ad bd)1/2– 2(ap bp)1/2 a, b – surface tension of liquid a and b ad, bd – contribute of dispersion force e.g. Hg/W= HgLV+ WLV – 2(Hgd Wd)1/2 =484 + 72.8 – 2(21.8200)1/2 = 424.8mJ/m2~ 426mJ/M2 (experimental value ) (3) S/L interfacial tension SL SL= S+ LV – 2(Sd Ld)1/2

5. Wetting agents • Characters of Structure • branched chain e.g. Surfynol 104, AOT • short straight chain e.g. JFc C7-9OEO5-6 • Anionics and Nonionics (2) Application of wetting process • Ore floatation(矿石浮选):collectors & foaming agents • Waterproof & Oil repel of fabics • Antirust & Corrosion mitigation

Chapter 5 Wetting and its Modification by Surfactants