Langmuir[kl_]:=Function[p,p kl/(1+p kl)]
Competing[k1_,k2_,x1_]:=Function[p,x1 p k1/(1+p k1)+(1-x1 )p k2/(1+p k2)]
Module[{th},
FFG[kl_,beta_]:=Function[p,th/.FindRoot[th/(1-th)==kl p Exp[beta th],{th,0.99}]]]
BET[kl_,km_]:=Function[p,If[p<1/km,p kl/(1+p kl-p km)*1/(1-km p)]]

Manipulate[
fiso=Switch[isotherm,
"Langmuir",
Langmuir[kl],
"Competitive",
Competing[kl,km,x],
"FFG",
FFG[kl,beta],
"BET",
BET[kl,km]
];
Plot[fiso[p],{p,0,5},PlotRange->{{0,5},{0,1.5}},Frame->True,FrameLabel->{"P","Θ"},PlotStyle->Directive[Red,Thick]]
,{isotherm,{"Langmuir","Competitive","FFG","BET"}},
PaneSelector[{
"Langmuir"->Pane[
Column[{
Control[{{kl,1,"(Subscript[k, L])"},0.1,5,Slider}]}]],
"FFG"->Pane[
Column[{
Control[{{kl,1,"(Subscript[k, L])"},0.1,5,Slider}],
Control[{{beta,1,"(Subscript[nϵ, b])/(Subscript[k, B])T"},0.1,5,Slider}],
}]],
"BET"->Pane[
Column[{
Control[{{kl,1,"(Subscript[k, L])"},0.1,5,Slider}],
Control[{{km,0.5,"(Subscript[k, M])"},0.1,1,Slider}]
}]],
"Competitive"->Pane[
Column[{
Control[{{kl,1,"(Subscript[k, 1])"},0.1,5,Slider}],
Control[{{km,1,"(Subscript[k, 2])"},0.1,5,Slider}],
Control[{{x,0.5,"(Subscript[x, 1])"},0,1,Slider}]
}]],
_->Row[{}]},Dynamic[isotherm]]
,Paneled->True,SaveDefinitions->True]